Bertrand Daoust

Hello,

I have few questions regarding testing the pressure modulator on the trans.

While trying to get the trans to shift smoother, I checked at the trans. modulator:

1) The vacuum on the line from the manifold at idle and got 18. Wich is good I think.

2) The vacuum on the modulator itself. When I pump some pressure with the Mighty Vac, IT DOES NOT HOLD PRESSURE. It falls rapidelly to 0. I think it should hold vacuum. Right or wrong?

If it should hold vacuum, do I need a new modulator or only part of it and if so, what part?

Also, I notice that the rubber cap on the modulator has a crack. Could it be a problem?

Thank you.

Cosmo Kramer

Yes it should hold vacuum, but it might not be the modulater it could be the line going to it. Pull the vac line of the modulator and plug it and redo the test with the vac tool. If it still drops quickly then you have a leak in the line somewhere. There is a hard metal line running above the torque tube and 2 lines running off each end of it, one going up the bellhousing to the manifold splitter and one that goes to the modulator on the tranny. Both are a bitch to replace many people just run a new line all the way back to the transmission.

The I am not sure if the rubber cap is supposed to hold vacuum or whether it just holds the adjuster in place and keeps dirt out.

Bertrand Daoust

I did the test at the trans. and had a steady 18 (with the Myghty Vac) at idle on the line witch is good I think.
I did the test on the modulator itself and it does not hold vacuum. So I guest the modulator is bad.

I was wondering too if the rubber cap is suppose to hold vaccuum or not.

Cosmo Kramer

Here is a post from Steve Cattano from the same question a few years ago on an older thread, looks like the cap holds vacuum.

Bertrand Daoust

Thank you very mutch Ted.
Maybe I need only a new rubber cap.
Easier to change.

rawky

Can the modulator be adjusted and connected without removing any other parts?

If so, can it be removed completely without removing any other parts?

Cheers

G Man

I had a bad modulator on my 84'. A new one made a big difference. The rubber cap is just a dust and crud cover for the adjustment handle that is under it. Try the vac test at the modulator to verify it is the modulator that will not hold a vac. On my 84' I had to drop the rear part of the exhaust in order to remove the heat shield to gain access to the modulator. Have fun.

Bertrand Daoust

I can tell that YES the rubber cap does influence the vacuum on the modulator.

Here's what I did:
First: Tried the vacuum test on the modulator without the cap and it does not hold vaccum.
Second: Tried it with the old rubber cap, does not hold vac. either. The old one was cracked all around.
Third: Tried it with the same old cap but wrap with electrical tape. Now, it does hold vacuum.

From there, I did ajust the modulator two full turn counter clockwise to smooth the shifting but it was a little bit too much - a little bit of slip from 2 to 3.
So turn it one turn clock wise and WOW! Every shift is now the way it should be. Not too harsh, not too smooth either. Just perfect.

So, maybe before changing the modulator, check the rubber cap first. Easier job for sure.

Cosmo Kramer

Glad to hear you got it fixed, tape will do for now until you get a new cap.

I just adjusted my modulator last weekend. After I supercharged the car my 2-3 full throttle upshifts where hard enough to knock my fillings out (chirping the tires shifting into 3rd at 145 km/h) . I turned mine counterclockwise one turn and now it shifts firm but not jarring like before, so all is good.

If you have any questions on these automatics, search for any posts by Steve Cattaneo, he has a transmission shop and he knows these automatics inside and out. He doesn't frequent the board as often anymore but he has lots of useful information in the archives.

G Man

Nice fix. I had no idea that the cap was involved in the vacume circuit. Easy and low cost fixes are the best kind!