Surging Tach and Car when overdrive engages

This has been an ongoing quirk with my car since I got it. It doesn't seem to get any worse, but why it's doing it is driving me insane....

Essentially when the car shifts into overdrive anywhere around 40-50 the tach will show a drop in RPM, suddenly it'll increase in RPM and you'll feel a surge in the car, before the tach drops and the overdrive gear engages again. If you're over 50 when the overdrive gear engages, it won't surge, it just tends to clunk into overdrive fairly hard compared to normally. What could be causing this, and is it something that I need to be worried about, or just one of those quirks?

I'm not a transmission expert, but it sounds like your problem is the Torque Converter Clutch losing its grip...

Someone else around here can probably say whether it's more likely to be a hydraulic pressure problem or the clutch lining itself. Either way, the tranny would have to come out to fix it, so in the meantime I'd leave it in Drive (3rd) unless I knew I'd be cruising above 50 for a while.

By the way, you DO have full, fresh tranny fluid, right?

Eldo wrote:

I'm not a transmission expert, but it sounds like your problem is the Torque Converter Clutch losing its grip...

Someone else around here can probably say whether it's more likely to be a hydraulic pressure problem or the clutch lining itself. Either way, the tranny would have to come out to fix it, so in the meantime I'd leave it in Drive (3rd) unless I knew I'd be cruising above 50 for a while.

By the way, you DO have full, fresh tranny fluid, right?

I knew someone was going to ask me that. I haven't changed the fluid. I checked it and it's clear and not grimy or anything and smells good.

About how much would this repair run?

GMFreak8 wrote:

I knew someone was going to ask me that. I haven't changed the fluid. I checked it and it's clear and not grimy or anything and smells good.

About how much would this repair run?

Well, if it's clear and sweet-smelling, then you probably don't need to change it.

Hopefully someone who has had this kind of surgery will see this thread, and can answer your question better than I. The problem is that even if it were just a bad TCC solenoid valve, just like the poorly designed boost & reverse pressure valves, the part is cheap, but with the valve body no longer on the bottom of the transmission, you have to drop the whole thing... I asked a friend who owns a tranny shop for an off-the-cuff estimate (he wasn't at work at the time) and he said probably 10 hours labor just to do the R&R.

I imagine a bad clutch means a new converter. Either way, whenever you have the thing out, you want to research this board and the Net for anything else that should be done at the same time. e.g. if your tranny has any soft shifts, or takes too long to engage Reverse or Drive when you shift, you wanna' replace that boost/reverse pressure valve assembly with an improved aftermarket design, because the factory used steel valves inside of an aluminum sleeve, even in the "HD" 4T65... A recipe for wear and pressure leakage.

In the meantime, follow the old rule: Leave it in 3rd until you get on the highway.

Eldo wrote:

I'm not a transmission expert, but it sounds like your problem is the Torque Converter Clutch losing its grip...

Someone else around here can probably say whether it's more likely to be a hydraulic pressure problem or the clutch lining itself. Either way, the tranny would have to come out to fix it, so in the meantime I'd leave it in Drive (3rd) unless I knew I'd be cruising above 50 for a while.

By the way, you DO have full, fresh tranny fluid, right?

I had my transmission overhauled by a local mechanic over 100,00 miles ago. What happened was the TCC was stuck off. What the rebuilder (who, as it turns out, maintains fleets under contract and builds racing transmissions) did, however was to replace the known problmeatic parts with better ones - the bands/cluthes etc., as well as replacing certain plastic parts with parts made of billet aluminum of similar weight but much higher durability.

The interesting thing to me is that if the 4T65E tranny was made in such a way that it used those parts at factory, and there was some way to make sure the build was clean in the first place, people would likely long ago have formed a much better opinion of GM cars. I have over 100,000 miles on the rebuild and none of the transmission issues that I read about on this forum.

Albertj

You're totally right, Albert. Just like those steel pressure valves in the aluminum sleeve. There's a bunch of companies that make aftermarket parts for the 4T65 that fix stupid mistakes made, despite huge engineering resources, by the former largest corporation in the world...

GMFreak8 wrote:

This has been an ongoing quirk with my car since I got it. It doesn't seem to get any worse, but why it's doing it is driving me insane....

Essentially when the car shifts into overdrive anywhere around 40-50 the tach will show a drop in RPM, suddenly it'll increase in RPM and you'll feel a surge in the car, before the tach drops and the overdrive gear engages again. If you're over 50 when the overdrive gear engages, it won't surge, it just tends to clunk into overdrive fairly hard compared to normally. What could be causing this, and is it something that I need to be worried about, or just one of those quirks?

I don't think there is anything wrong with your transmission. The TCC will gradually apply under certain throttle positions. My car does the same thing. I'm not worried about it at all.

The higher RPMs in third won't hurt it will it? I actually drove it in third for a while when I first got it. Overdrive on my other car meant I'd lose 15 MPH if someone farted in the car ahead of me so I always drove it in third until a rod came through the hood at 330,000 miles while idling at the demo derby.

It doesn't seem to be getting worse or better. So maybe I should just leave it alone. Afterall my last car went 160,000 miles with weird TCC behavior and could have still gone more if the engine would have survived.

And yes the fluid is clear and sweet smelling no burnt smell.

I swear I love this car so much it's going to put me to an early grave. If it was a kid I'd be running in the room every time I heard a cough or sneeze or hiccup.

LARRY70GS wrote:

GMFreak8 wrote:

This has been an ongoing quirk with my car since I got it. It doesn't seem to get any worse, but why it's doing it is driving me insane....

Essentially when the car shifts into overdrive anywhere around 40-50 the tach will show a drop in RPM, suddenly it'll increase in RPM and you'll feel a surge in the car, before the tach drops and the overdrive gear engages again. If you're over 50 when the overdrive gear engages, it won't surge, it just tends to clunk into overdrive fairly hard compared to normally. What could be causing this, and is it something that I need to be worried about, or just one of those quirks?

I don't think there is anything wrong with your transmission. The TCC will gradually apply under certain throttle positions. My car does the same thing. I'm not worried about it at all.

That made me feel tons better. How long has it done it? Your tach will drop then raise and you'll feel the surge too? I've never owned a car with this particular transmission so I was wondering and hoping that it may just be a quirk of the transmission and not much to worry about.

GMFreak8 wrote:

LARRY70GS wrote:

GMFreak8 wrote:

This has been an ongoing quirk with my car since I got it. It doesn't seem to get any worse, but why it's doing it is driving me insane....

Essentially when the car shifts into overdrive anywhere around 40-50 the tach will show a drop in RPM, suddenly it'll increase in RPM and you'll feel a surge in the car, before the tach drops and the overdrive gear engages again. If you're over 50 when the overdrive gear engages, it won't surge, it just tends to clunk into overdrive fairly hard compared to normally. What could be causing this, and is it something that I need to be worried about, or just one of those quirks?

I don't think there is anything wrong with your transmission. The TCC will gradually apply under certain throttle positions. My car does the same thing. I'm not worried about it at all.

That made me feel tons better. How long has it done it? Your tach will drop then raise and you'll feel the surge too? I've never owned a car with this particular transmission so I was wondering and hoping that it may just be a quirk of the transmission and not much to worry about.

It's momentary, just a split second or so. Doesn't happen at higher speeds.

Here's what I think - read carefully, then tell me if it makes sense compared to your situation.

Let's first define OD and TC lock-up, as they are different things technically, although in function they can be separate, or overlapping "phenomenas" during driving. They can cause some confusion if you use the terms interchangeably (which I often do, for different reasons).

OD - OverDrive. This is also known as 4th gear. Technically that's all OD means. If you are in "OD" the transmission can shift up to 4th gear.

TCC - Torque Converter Clutch. This is what locks up your torque converter, coupling your engine's output with the drive shafts to the wheels. TC lock-up is what actually enables true overdrive (OD) by definition: transaxle output speed being greater than the input speed.

TC - just means Torque Converter. When the TCC is engaged, the TC is said to be "locked-up". It's normally an on/off thing.

Here is where it can get confusing. TC can lock up in both 3rd and 4th gears, and when lock-up occurs, it sort of feels like an upshift to the next gear, which may or may not be the case.

Our transmissions ('98 models) are programed to shift up or down according to the shift line maps. The curves on these maps are a series of MPH values spaced in equal throttle steps. There is a different map for each upshift & downshift. For example, on a stock map, if you are in 2nd gear applying steady 25% throttle, it will shift into 3rd at 39 MPH. However, if you are in 2nd gear and change to 75% throttle, it will wait and shift into 3rd at 72 MPH, if you don't command a downshift (downshifts happen more easily at lower speeds).

Using the same idea, when you are in 3rd gear applying 25% throttle, it will shift into 4th (OD) at 65 MPH. Applying only 12% throttle in 3rd gear will cause the upshift early at 42 MPH. So you can see how this works. If it doesn't make sense, keep in mind MPH is always increasing when you apply throttle.

Downshifting works the same way, but because it usually occurs because of a sudden increase in throttle, it helps to think about it in reverse. At a certain speed, it takes X% throttle to downshift to a lower gear. Example, cruising at 65 MPH in 4th gear, it will hold that gear until throttle is 50%, at which time it will downshift into 3rd gear.

The interesting thing is that TC lock-up occurs independently, based on its own set of maps, also based on MPH vs. throttle %. In 4th gear (OD) with 20% throttle, the PCM commands the TCC to apply at about 47 MPH. Any less throttle, it will apply at 40 MPH. When this occurs, you'll notice a drop in RPM and a loss of torque. This feels to some like the engine is bogging, especially if you're climbing a hill.

As long as you are in 4th gear at 47 MPH, the TCC will not release unless you apply at least 36% throttle. When this happens, you'll be in 4th gear (still in "OD") without TC lock-up, and RPMs will increase, causing a slight surge in power. Based on this, I think what you are experiencing is normally what happens when you try to mildly accelerate in 4rd gear at under 50 MPH.

FYI, those of us with PCM tuners have almost forgotten what it's like to experience what you're describing, as our maps have been optimized for our driving style preferences. My TCC is programed to release near the instant I touch the throttle to accelerate, so that I can have more power in 3rd and 4th gears. This isn't everyone's cup of tea, but it makes the car feel much lighter and responsive, imo.

AA wrote:

Here's what I think - read carefully, then tell me if it makes sense compared to your situation.

Let's first define OD and TC lock-up, as they are different things technically, although in function they can be separate, or overlapping "phenomenas" during driving. They can cause some confusion if you use the terms interchangeably (which I often do, for different reasons).

OD - OverDrive. This is also known as 4th gear. Technically that's all OD means. If you are in "OD" the transmission can shift up to 4th gear.

TCC - Torque Converter Clutch. This is what locks up your torque converter, coupling your engine's output with the drive shafts to the wheels. TC lock-up is what actually enables true overdrive (OD) by definition: transaxle output speed being greater than the input speed.

TC - just means Torque Converter. When the TCC is engaged, the TC is said to be "locked-up". It's normally an on/off thing.

Here is where it can get confusing. TC can lock up in both 3rd and 4th gears, and when lock-up occurs, it sort of feels like an upshift to the next gear, which may or may not be the case.

Our transmissions ('98 models) are programed to shift up or down according to the shift line maps. The curves on these maps are a series of MPH values spaced in equal throttle steps. There is a different map for each upshift & downshift. For example, on a stock map, if you are in 2nd gear applying steady 25% throttle, it will shift into 3rd at 39 MPH. However, if you are in 2nd gear and change to 75% throttle, it will wait and shift into 3rd at 72 MPH, if you don't command a downshift (downshifts happen more easily at lower speeds).

Using the same idea, when you are in 3rd gear applying 25% throttle, it will shift into 4th (OD) at 65 MPH. Applying only 12% throttle in 3rd gear will cause the upshift early at 42 MPH. So you can see how this works. If it doesn't make sense, keep in mind MPH is always increasing when you apply throttle.

Downshifting works the same way, but because it usually occurs because of a sudden increase in throttle, it helps to think about it in reverse. At a certain speed, it takes X% throttle to downshift to a lower gear. Example, cruising at 65 MPH in 4th gear, it will hold that gear until throttle is 50%, at which time it will downshift into 3rd gear.

The interesting thing is that TC lock-up occurs independently, based on its own set of maps, also based on MPH vs. throttle %. In 4th gear (OD) with 20% throttle, the PCM commands the TCC to apply at about 47 MPH. Any less throttle, it will apply at 40 MPH. When this occurs, you'll notice a drop in RPM and a loss of torque. This feels to some like the engine is bogging, especially if you're climbing a hill.

As long as you are in 4th gear at 47 MPH, the TCC will not release unless you apply at least 36% throttle. When this happens, you'll be in 4th gear (still in "OD") without TC lock-up, and RPMs will increase, causing a slight surge in power. Based on this, I think what you are experiencing is normally what happens when you try to mildly accelerate in 4rd gear at under 50 MPH.

FYI, those of us with PCM tuners have almost forgotten what it's like to experience what you're describing, as our maps have been optimized for our driving style preferences. My TCC is programed to release near the instant I touch the throttle to accelerate, so that I can have more power in 3rd and 4th gears. This isn't everyone's cup of tea, but it makes the car feel much lighter and responsive, imo.

Wow, thank you for the detailed explanation about the whole process.

I experience the tach dropping (which is when I can feel whatever it is engaging. Here is where I'd expect the process to stop, but instead it continues for a few more steps. Next the tach will rise (it's more of a short jump and immediately back down) and with it you can feel the car sort of surge forward ever so slightly, almost as if the car changed it's mind and wants to reverse course and stop the od or tc lockup from engaging. Afterwards the tach settles back down to where it dropped to in the first step of the process and the car continues on as normal. This occurs most prominently around 40-45 MPH. If it's above 50, it doesn't do the tach jumping, it just seems to engage od or tc lockup with a harder than normal "clunk".

Hopefully this explains the situation in more detail. I really appreciate you taking the time to define the difference between OD and TC Lockup though. I've never fully understood the complete difference.

LARRY70GS wrote:

GMFreak8 wrote:

LARRY70GS wrote:

GMFreak8 wrote:

This has been an ongoing quirk with my car since I got it. It doesn't seem to get any worse, but why it's doing it is driving me insane....

Essentially when the car shifts into overdrive anywhere around 40-50 the tach will show a drop in RPM, suddenly it'll increase in RPM and you'll feel a surge in the car, before the tach drops and the overdrive gear engages again. If you're over 50 when the overdrive gear engages, it won't surge, it just tends to clunk into overdrive fairly hard compared to normally. What could be causing this, and is it something that I need to be worried about, or just one of those quirks?

I don't think there is anything wrong with your transmission. The TCC will gradually apply under certain throttle positions. My car does the same thing. I'm not worried about it at all.

That made me feel tons better. How long has it done it? Your tach will drop then raise and you'll feel the surge too? I've never owned a car with this particular transmission so I was wondering and hoping that it may just be a quirk of the transmission and not much to worry about.

It's momentary, just a split second or so. Doesn't happen at higher speeds.

Same. I just get a harder "clunk" when it engages at higher than 50 MPH.

Does it shift normally if you are accelerating rapidly?

My 95 PA used to have the same symptoms you are describing but instead of a clunk when it shifted into OD it felt like you were running over some small rumble strips.

I used some additive from NAPA called "Shudder Stop" and it helped reduce the shudder.

Bert

Bert made think of something I forgot: Lucas makes a good Auto Trans additive. It can't hurt to put some in, and I think they make it in a small bottle as well as the quart with the nozzle-tip.

Aaron put a lot of good info in his post, and it explains why you can't get a good feel for these trannies. My 40 year old Turbo 400 always up & downshifts when I want it to, and knocks off each shift at the perfect point of quick & soft but not slippery, not only because it was built right, but because it uses a vacuum modulator that senses the actual load the engine and tranny are under, instead of trying to interpret a fixed chart of TP vs speed... What now takes a hideously expensive scanner and a lot of trial and error in the computer tables, we used to do with an allen wrench and $50 or 60 in parts.

I don't think any of us here think that this problem is the tranny actually shifting 3-4-3-4, because that would be a lot more obvious that what you've described. The only reason I think something isn't right, is because I live 2 miles out of town, on a flat 40MPH road. So, I've seen a LOT of 42 MPH upshifts to 4th, and then the soft, too-slow-really TCC engagement that they programmed into these things.

There are 3 conditions under which the TCC should release again: As Aaron said, if you really lean on the gas; If you back so far off the throttle that you reach the minimum TP % for engagement, and at 45MPH you're barely turning 1100 RPM, so that can happen a lot - but if that were the case you wouldn't feel & see an RPM rise 'cuz you'd virtually be off the gas; Finally, the TCC disengages whenever the brake is applied... In all cases, it should never really grab or clunk.

If you're a two-footed driver, I suppose that last one could be your problem, or mayyyyyybe a badly adjusted brake-light switch...

As for your earlier question about driving in 3rd being too high RPM, don't worry about it. The car won't go into 4th until 42 anyway, and if you're not going to be exceeding 50 for awhile, neither the engine nor the mileage are going to really care. And if you've got hills at that speed, you're definitely better off in 3rd-Lock instead of 4th-hunting-between-lock & unlock

I totally dis-like the factory PCM programming from GM on TCC lock-up.
It was the same with my Blazer, only then I didn't know I could change it with some software.
Thanks Derek.
The Riv is much better since some re-programming.

Quote :

As Aaron said, if you really lean on the gas; If you back so far off the throttle that you reach the minimum TP % for engagement, and at 45MPH you're barely turning 1100 RPM, so that can happen a lot - but if that were the case you wouldn't feel & see an RPM rise 'cuz you'd virtually be off the gas

To clarify, what I said was TCC can release if more throttle is applied, not less. It takes 36% throttle at 47 mph to release - you do feel a surge in RPM - it's quite noticeable.

Backing off throttle does not release TCC until 0%, or until MPH goes down far enough to release it.

I'm not a fan of driving long distances in 3rd gear. At high speeds, you will not be in TC lock-up, which means the transmission gets really hot. Once on a 400 mile trip in 4th gear without TC lock-up, I totally destroyed my transmission. The torque converter was blued from the heat. I can't imagine what would happen in 3rd gear. DO NOT DRIVE LONGER THAN AVERAGE DISTANCES IN 3RD GEAR AT FREEWAY SPEED. If you do, monitor trans fluid temp with a scanner. Shut down if it gets much above 230ºF.

To clarify:

-I wrote: "as Aaron said, if you really lean on the gas," which IS more throttle.

-All of my other posts about leaving it in third (i.e. Direct Drive, or 1:1, just like the old days) were specifically for this 40-50MPH zone that he was talking about, such as when I said "In the meantime, follow the old rule: Leave it in 3rd until you get on the highway." After you're cruising at your (higher) speed, drop it into 4th. I really doubt 3rd could hurt anyway, because we're all running a tall, 3:1ish final drive ratio, and the engine and the tranny are working less hard in a shorter gear.

-Even at high speeds, I don't know why the TCC would disengage in third. Also, and I wasn't going to get this deep with the newby, there are plenty of times when a lower gear, with or without TCC, makes more sense than a higher gear when the load won't let the clutch engage... It's at different speeds for different loads, but with the high-stall-speed designs of these TCC-equipped converters, you can actually be climbing a hill in one gear, and drop down a gear and still be spinning the same RPM - because in the higher gear, the converter was slipping to give you the extra power, and in the lower gear, the ratio takes care of it. This is roughly analogous to your hi-speed run in OD, without the clutch locked... If you'd been in Drive, the converter wouldn't have been slipping so much and generating so much deadly heat.

I agree with Eldo here - anytime I manually shift into 3rd gear (D), my TCC is locked except for heavy throttle.

Sorry, Mark. I misunderstood you. When speaking of fuel, the word "lean" naturally makes me think there's less of it.

You are absolutely right about the TCC staying applied at higher speeds. I got it wrong there. However, the RPM surge would occur as the TCC is released, a result of increased throttle %. Only a moderate amount is required at 40-45 mph (about 35-40% throttle).

As for 4th gear slipping more at high speed than 3rd gear, this is interesting to me. They did in fact include a 4th gear, which gets used mostly under acceleration at higher speeds. I'd think there would be more slippage during acceleration than at cruise. Please explain more about why 3rd gear slips more than 4th. Using this logic, wouldn't it be even safer to use 2nd gear?

i had the same problem too. i just changed the tranny fluid and filter and added a bottle of lucas transmission fix and the tranny is as quiet as she has ever been. taking a road trip Friday and lets see if she wants to stay in 3rd now...

AA wrote:

Sorry, Mark. I misunderstood you. When speaking of fuel, the word "lean" naturally makes me think there's less of it.

You are absolutely right about the TCC staying applied at higher speeds. I got it wrong there. However, the RPM surge would occur as the TCC is released, a result of increased throttle %. Only a moderate amount is required at 40-45 mph (about 35-40% throttle).

As for 4th gear slipping more at high speed than 3rd gear, this is interesting to me. They did in fact include a 4th gear, which gets used mostly under acceleration at higher speeds. I'd think there would be more slippage during acceleration than at cruise. Please explain more about why 4th gear slips more than 3rd. Using this logic, wouldn't it be even safer to use 2nd gear?

[[Well this got longer than I'd planned, but I think it is somewhat comprehensible... Oh, and I made a correction at the underlined section of your quote above, you'd reversed your gears at the end of the paragraph.]]

=Oh I get it, you saw "lean out the gas", instead of "lean on the gas"...

=Yes, the RPM would surge when demanding it with more throttle, just like a forced downshift. But the way I've been reading Kyle's complaint is that it's surging right after the upshift, when he's just accelerating normally, and not making large throttle displacements. Perhaps he can confirm or clarify that.

=Simply put, whether you're climbing a hill (gravity), accelerating (inertia), or highway cruising (wind resistance), you're doing the same thing, fighting load with torque, as opposed to HP. Horsepower is how fast you can make torque, but grunt is what changes the state of things...
To fight that load with an engine that has certain torque & power curves, you use gear ratios or slippage. A torque converter multiplies torque when there is slippage, which is why I said you can sometimes do the same job, tackle the same load, with the same RPM in an unlocked higher gear or a locked or unlocked lower gear. In any case, all that slippage/turbulence/friction causes heat, which is inefficient and unhealthy. As some of you know, GM built transmissions in the 50's & 60's that had 'switch-pitch' converters which would give high-stall on demand for acceleration, then revert to low-stall for efficiency. But with the adoption of the converter clutch, they could build a higher-stall-speed converter to being with, more of a 'racing' converter, to provide better performance, and then lock up the converter into direct drive for efficiency. If you're driving against a 60+MPH wind in OD with a tall axle ratio, you've got a fair amount of load, like riding a 10-speed bike against a headwind in 10th gear... If that converter isn't locking for some reason, you are in effect cruising with a racing converter, and the whole gearbox is going to get hot from the slippage and fluid agitation. If you're in Drive/3rd there is less torque required from the drivetrain, like dropping your 10-speed to 5th, and the mechanical advantage of the gear ratio and the higher revs from the motor take care of the load, allowing the converter to reach "hydraulic lockup", minimum slippage or 'coupling' mode.

I just know this stuff in my gut from a life spent around cars & mechanics, but you should be able to prove it out with your scanner, reading out the input/output speeds, % slip, torque request, etc...

One final note on "the old rule" about leaving it in 3rd until you get on the freeway. The tranny shops have been telling us this on most 4-speed trannies since the 700-R4 came out. In fact, up though the 4T60-E, GM still labeled the quadrant P-R-N-OD-D-2-1... There were a few reasons, including higher line pressure in D/3rd, so shifts were less slippery, and the input clutch staying engaged on decel, so less braking is needed around town. I saw the pressure tables for the 4T65, and I don't think the former applies to these, but you can still feel the latter for yourself. Just cruise along in OD at 35. You're only really "in" 3rd gear below 42, but if you pull the shifter back to 3rd, you feel the compression braking kick in, as though you had really dropped a gear...

ibmoses wrote:

Does it shift normally if you are accelerating rapidly?

My 95 PA used to have the same symptoms you are describing but instead of a clunk when it shifted into OD it felt like you were running over some small rumble strips.

I used some additive from NAPA called "Shudder Stop" and it helped reduce the shudder.

Bert

It shifts perfectly accelerating rapidly. It's just like my other car. The harder you pound on it the better it seems to run. Go figure.

Eldo wrote:

Bert made think of something I forgot: Lucas makes a good Auto Trans additive. It can't hurt to put some in, and I think they make it in a small bottle as well as the quart with the nozzle-tip.

I was thinking of doing that. I want to get some of the lucas additive for my fuel too. I think I have a dirty injector causing a slight ticking noise. I'll maybe pick up some for the transmission too. I firmly believe the lucas stuff is great; I truly believe the lucas oil additive I put in my last buick allowed me to drive it 30K without oil pressure before throwing a rod.

Quote :

Aaron put a lot of good info in his post, and it explains why you can't get a good feel for these trannies. My 40 year old Turbo 400 always up & downshifts when I want it to, and knocks off each shift at the perfect point of quick & soft but not slippery, not only because it was built right, but because it uses a vacuum modulator that senses the actual load the engine and tranny are under, instead of trying to interpret a fixed chart of TP vs speed... What now takes a hideously expensive scanner and a lot of trial and error in the computer tables, we used to do with an allen wrench and $50 or 60 in parts.

Exactly. My last Buick wouldn't shift after doing some work under the hood. I couldn't figure it out, but then realized there was a cable leading into the transmission that was kinked. I researched it, and found out it was actually the throttle positioning cable. I unkinked it and it shifted like a charm. I could actually adjust the shift points just by making the cable shorter or longer. I miss the days of being able to fix things on a car without a computer and electrical engineering degree.

Quote :

I don't think any of us here think that this problem is the tranny actually shifting 3-4-3-4, because that would be a lot more obvious that what you've described. The only reason I think something isn't right, is because I live 2 miles out of town, on a flat 40MPH road. So, I've seen a LOT of 42 MPH upshifts to 4th, and then the soft, too-slow-really TCC engagement that they programmed into these things.

There are 3 conditions under which the TCC should release again: As Aaron said, if you really lean on the gas; If you back so far off the throttle that you reach the minimum TP % for engagement, and at 45MPH you're barely turning 1100 RPM, so that can happen a lot - but if that were the case you wouldn't feel & see an RPM rise 'cuz you'd virtually be off the gas; Finally, the TCC disengages whenever the brake is applied... In all cases, it should never really grab or clunk.

If you're a two-footed driver, I suppose that last one could be your problem, or mayyyyyybe a badly adjusted brake-light switch...

I think the clunk when in the throttle more is actually fourth gear engaging, whereas the soft engagement is the tcc engaging while just cruising along lightly on the throttle, which causes the RPM fluctuation and the surging. The harder I get on the throttle the "better" it shifts.

I'm beginning to think it may just be a normal operation of this particular transmission. I'm gonna have to do some driving and see if I can notice the TCC engaging and then disengaging when I step on the brake.

The only oddities I notice is when going down a steep grade the RPMs will go up 300, then down 300, then up 300, and so on until I come to a stop or step on the gas. What could cause this? Doesn't happen often enough for me to be worried about it, and the car doesn't do anything weird. I wouldn't even know it unless I was watching the tach.

Quote :

As for your earlier question about driving in 3rd being too high RPM, don't worry about it. The car won't go into 4th until 42 anyway, and if you're not going to be exceeding 50 for awhile, neither the engine nor the mileage are going to really care. And if you've got hills at that speed, you're definitely better off in 3rd-Lock instead of 4th-hunting-between-lock & unlock

I used to drive my old car is third all the time without much of an issue. I just don't like the idea of the engine turning 2-2.2K all the time. I guess I just watch the tach too much.

And thanks guys for the conversations. I'm learning a lot just reading the dialog back and forth.