rhylan1961

I have a 87 S4 928 auto trans.
I shut it down for about a month to change the timing belt and water pump.
Trans. shifted fine before I shut it down.
Now since it is running again I have a clunk in 2nd and in 3rd gear @ slow speeds.
It shift great @ full acceleration.
I have checked both vacuum connections and topped off trans fluid.
Does anyone have knowledge of this trouble.
Thanks for the help.

UncleMaz

My guess is low modulating pressure/bad modulator.

rhylan1961

Where do I find it?
Does the trans have to come out to service?

UncleMaz

"Does the trans have to come out to service?" No. PM me with you email address and I will send you info for diagnosing and possibly fixing your issues. Hard shifting, in my experience, is usually caused by improper modulating pressure. You can adjust the modulating pressure on the trans, on the car.

G Man

My trans was clunking like that at light throttle before I changed the modulator. I used a vac testor on mine prior to replacing and found it would not hold any vac.

UncleMaz

Exactly. You will need a pressure tester like the one found here: https://rennlist.com/forums/6774742-post10.html. Checking the pressure is highlighted in the docs I will send you.

rhylan1961

153 BigBlock
My email is rhylan1961@embarqmail.com.
Can the adjustment be made and then just driven to test without the tester?

UncleMaz

I know people who have done that successfully, but I would take the guess work out with a gauge and a banjo fitting. Those gauges are around $10 at Harbor Freight.

UncleMaz

I would also check the archives here. I am sure there are some good threads on adjusting mod pressures.

rhylan1961

Thanks for all your help.

Tails

WSM fault finding for Auto Transmission Group 38 page 114 states:
Condition:
Strong jolts when changing gears.
Causes and correction:
1. Check modulating pressure, adjusting if necessary.
2. Check vacuum line and connections for leaks.

As previous stated the modulating pressure can be adjusted via the "T" adjuster located under the rubber cover on the modulating valve.

Modulating pressure should be adjusted to 4.2 +/- 0.05 bar with ATF at approx 80 degrees C. Selector lever at "D", and a road speed of approx 30mph. Vacuum disconnected from modulating pressure nozzel.

This is quite difficult to do unless you have the special gauges and extension piping and the correct union plug to connet to the transmission case, however it can be done by adjusting the "T" handled adjusting screw by itself. 1 turn adjusts the pressure by approximately 0.4 bar, anticlockwise reduces the modulating pressure and turning clockwise increases the modulating pressure.

If you adjust it without a pressure gauge it will be a hit or miss procedure, because if you screw the adjusting screw out to far the gear changes will start to slip and to far in and the changes will clunk into gear more harshly. After each turn it is advisible to check the shifts, so be prepared to jack the car up and down a few times.

I adjusted mine by making a small nipple union and fitted my pressure gauge direct to the union.

I place my car on ramps and aft jackstands, installed the pressure gauge into the modulator pressure port, run the car until the auto box was up to temperature, discconnected the vacuum line, started the car, put it in drive "D" at idle and slid under the car and adjusted the screw. I had harsh shifts and discovered the the modulating pressure was over 9 bar. I turned the screw anticlockwise until the pressure was the 4.2 bar. (Quite a few turns).

While you are under the car get your vacuum pump out and check the vacuum line for leaks and then start the car and measure the vacuum on the modulating vacuum line at idle in "D" gear to ensure that you do have a vacuum when system all connected.

When finished take the car for a drive and you will find a fantastic difference to the gear changes. My 928 gear changes are extremely smooth now. I you like a harsher shift pattern you can adjust the screw to achieve this, but for myself I prefered to set it to Porsche's specifications.

WYAIT I would also check the gear change selector safety switch operation the bowden cable to the auto box from the throttle assembly, carry out a stall test on the torque converters, and also check the auto transmission for leaks and the ATF level within the reservoir. Section 38 of the WSM fully describes how to carry out all the setting and adjustments.

Let us know how you go.

Tails 1990 928S4 Auto

rhylan1961

Thank you. For your help. I think I can do all that.

G Man

Be prepared for some work to get at the modulator. When I did mine I found it necessary to remove the rear half of the exhaust system and the heat shield for the muffler. Once that is out the rest is easy.

UncleMaz

Tails is the man.

WallyP

Tails, excellent post! One very small correction...

"While you are under the car get your vacuum pump out and check the vacuum line for leaks and then start the car and measure the vacuum on the modulating vacuum line at idle in "D" gear to ensure that you do have a vacuum when system all connected."

I don't think that you can check the vacuum line from the rear with a pump - the front end of the line will be open. If you change one word, it might work better.

"While you are under the car get your vacuum pump out and check the vacuum MODULATOR for leaks and then start the car and measure the vacuum on the modulating vacuum line at idle in "D" gear to ensure that you do have a vacuum when system all connected."