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?

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...

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.

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.

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