FAQ: Transmission Shift Modulator

Probably the tranny. The modulator wouldn't have fluid flowing through it , since the only connection it has to the tranny is through the vacuum line.


Better check mine tomorrow.....

The modulator has a vacuum diaphragm inside it, converts vacuum to actuating a pressure regulator of some sort inside. There could easily be fluid against the inside of the modulator, but it shouldn't be getting through.

thanks for the info. for $12 i'm just going to pick up a new unit and replace and check again in a few days.

ok i got a new modulator and installed it. so far it is not leaking through.
no difference in the way it drives, but again i didnt have a problem to begin with i just happen upon it.
thanks for the help guys.

Mr.Riviera wrote:

thanks for the info. for $12 i'm just going to pick up a new unit and replace and check again in a few days.

I recently paid $115 for a new Delco vacuum modulator 8662967.

ouch! return it and go to autozone
part # mv303

Mr.Riviera wrote:

ouch! return it and go to autozone
part # mv303

Yeah definitely get the new one from autozone.

Would a bad shift modulator give you an SES light? I don't have a light, but i am wondering if mine is bad.

i dont believe so. when mine went bad i never got a light.
its a $15 part and it super easy to replace but one way to tell if they are bad is to pull the vac line off it and see if there is transmission fluid dripping out or residue in the line

The part you are talking about is called a vacuum modulator. It is the sensor for engine load, and it uses engine vacuum to gauge that. High vacuum = low load, low vacuum = high load. The vacuum modulator is just one input the valve body uses to adjust line pressure, and shift point. The adjustment mainly affects minimum, and part throttle shift points. Turning the adjustment screw in (clockwise) gives you later shifts, turning it back the other way results in earlier,. softer shifts. The adjustment can be abused, if you back it out too much, it can lower line pressure to the point where clutches will slip. I think the last year on our Rivieras to use the vacuum modulator was 1996. After that, the transmission was totally electronic.

My shifts are a little hard, but tolerable. The only shift i'm worried about is my shift into 4th gear. It seems to vibrate and then go into gear a bit late. I hope my transmission isn't shot.

Having it tightened all the way will that harm the trans?

flyineagle96 wrote:

Having it tightened all the way will that harm the trans?

yes.
it will break the diaphragm inside the modulator and it will work the same as a broken one.

The instructions say to turn in or out 1/4 to 3/4 turn based on the transmission you have (these are made for the TH400 series transmission not the 4T60)
I have found that 3/4 turn clockwise works well. but there really isnt too much difference between all the way out and all the way in. it will only adjust the trans line pressure so much

I just bought one for my 95 and i put it in exactly as it came. It seems to shift a little harsh. It also slams a little when i put it into reverse, but it has always done that. Should i turn the screw clockwise or counter clockwise to make it a little smoother?

Counter-clockwise.

Thank you Karma!

Just installed mine and the car shifts a ton better.Never knew mine was and has been fucked for the longest.I've got mine set at the mid point.

I just installed mine yesterday and it looked like it come adjusted all the way out. I think i should turn mine in some, but Karma says that counterclockwise will make the shifts smoother.

LARRY70GS wrote:

Turning the adjustment screw in (clockwise) gives you later shifts, turning it back the other way results in earlier,. softer shifts. The adjustment can be abused, if you back it out too much, it can lower line pressure to the point where clutches will slip.

This is very good info for adjusting it.
By later shifts, it means that the gear will be dropped in faster which feels like the shift occurs slightly later in the RPM band.



Keep in mine that some of the hard shifting into reverse with the modulator will occur at lower temps. Nature of the beast. Also the line pressure will still always be related to engine vacuum or lack there of, so engine speed at shift matters too and will change along the RPM curve. You can run higher pressures in the summer and then back it off in the winter. I prefer my shifts pretty brisk, and it puts less wear on the clutches.

Hmm maybe i should turn mine a little clockwise then, maybe it it too far out and the line pressure is a little too low.

Thanks. I hope this solves my rough shifting between 1 and 2.

Does it just screw in by hand or is that bracket I see holding it down? Sorry, I've never replaced one before.

The bracket holds it in. Take off bracket, modulator just pops out.

Any long term reviews or lessons learned on this at this point? I see most of these installs are a few years old now.

I am about to change out the fluid, and this was going to be on the list.

Thanks,
Al

Mine has been on for almost two years now, and I've adjusted it once after the initial installation. My ATP unit is functioning great still, after about 3,000 miles.

Just installed mine and after 4 or 5 full turns ( i did it in increments) I still have not noticed much of a difference, I'll play with it some more. But I have another question: How many people broke the rubber "T" that connects the modulator to the throttle body and supercharger?? mine crumbled and now its pretty rigged up with all kinds of other vacuum hoses and zip ties. I hate having to rig stuff up like this. Is there a part number for this?? I tried searching to no avail.