Modulator Valve replacement
 

86 automatiic is shifting harshly.

Vacuum check showed expected high vac on hose at modulator valve.
But, modulator valve, when checked with mighty vac unit, wouldn't hold vac.

Here are a few pictures of replacement.
Mounting bolts are 5mm hex allen.
Shift acutation rod was popped off stud for access.
Exhaust was dropped a bit.
Heat shield was removed.

Will need to get a shift rod boot, as this one broke-up.

In my S4 I just swapped the cracked rubber cap and it really helped. I will replace the whole assembly later on when I can afford to have the car down for a while if something goes wrong.

I didn't see that until after loading the pictures!

What was the part number and cost of the part, as I am thinking of replacing this part myself? Thanks.

Don't remember the exact price, but Roger has it and it is quite inexpensive.

Did you need to drain transmission fluid?
I only have harsh shift from 1st to 2nd and 2nd to 1st but not always, other gears are okay.

No need to drain / no leaking.
Similar shift behavior here before switching (haven't tried it yet / might need to adjust).
The heavier the acceleration and throttle, the less noticeable, but at light throttle 1-2 had become very abrupt. Valve itself wouldn't hold vac, maybe the cap as pointed out by Karl.

Check with Roger or others on price / part #. Reasonable.

If the valve is leaking vacuum it will shift harshly in all gears. You can replace the valve without draining the transmission.

Could this be my source of vacuum leak?? I used my mityvac to check vacuum at front and was pulling 21 inches

Excellent - thanks, Chris. Have to be doing this soon.

If it is holding vacuum at the T at the back of the motor then no. I had a crack in my T that I fixed and it would only hold vacuum for 3 seconds!

You said your car shifts hard on 1-2, is that all the time?

I am trying to find a decent check valve to put on that vacuum hose. On a NA car the modulator sets the maximum line pressure at 0 vacuum. On the boosted car it continues to increase the line pressure with manifold pressure, but I still have 18 hg at idle and vacuum under light throttle. If I adjust the modulator to have a smooth shift at WOT, I get shift flares at light throttle. If I set so it shifts properly at light throttle it shifts really hard at WOT. I have tried a vacuum check valve and it just messes up the shifting, they must accumulate vacuum or they don't react fast enough :(.

What position did you set the t-handle to on the new valve? Ideally, you'd measure and set the correct modulating pressure when replacing the valve.

I found a leak where the rubber hose from the modulator connects to the metal line on the torque tube. The hose end was frayed and missing a chunk of rubber. Cut off the bad end, reattached and held vacuum fine. YMMV.

Cosmo, do you still even have vacume there? The boost must change things quite a bit.

I have vacuum until about 1/4 throttle then it is positive pressure. The positive pressure really makes it shift hard under boost. I called Steve Cattaneo at Master Tech today and he suggested getting a check valve from a 1987 300D Mercedes as they are a turbo diesel and are fitted with one from the factory. Unfortunately Mercedes says the part is NLA.

I am going to try hooking my check valve on a T fitting with the check valve venting to the atmosphere under pressure and closing under vacuum. This should stabilize the vacuum signal and make it work a lot better.

Chris, that wasn't a trans you got from me was it?

Bill,

This is the 86 car that we found abandoned behind the Porsche dealer, has its original trans.

The 89 automatic from that broken-engine car is now doing DD service on I-81 up near Roanoke every day. Its installed in the red Stan Shaw '84. My daughter refuses to bring it back home.

Today I drove the 80 manual on the highway for the first time. Solid. Thank you.

When do we finish the race car? Pictures? You need a thread on that!

I checked my modulator value with mityvac today. Pumped it to 15" and slowly, very slowly leaked. How long should it hold constant pressure? The cap looked good with no visible cracks.

Mine leaked-down inside of 10 seconds.

Sounds normal to me.