bfellows

All advice welcomed on this one. I have just finished servicing my transmission after the box started slipping in 1st and 2nd (gaskets, filter, fluid, good clean up etc). When i checked the fluid level prior to servicing it was very low, presumably causing the slipping to occur (low level caused by a leak from the reservoir 'o' ring). Prior to the slipping occuring and servicing work the tranny has always performed OK.

I have followed all the procedures regarding ensuring correct fluid level within the box. The only alteration I have made has been to turn up the pressure on the vacuum modulator adjuster by 2 full turns.

During the test drive the car seemed to change from 1st to 2nd but very quickly, then changed into higher gears more smoothly than it has before. When i stamped on the throttle from a standing start the car briefly spun up in 1st (damp road and SC!) then jumped smoothly, but quickly to 2nd. I can't be sure but think my kickdown didn't kick in so to speak when I went WOT, but i couldn't tell if the car had changed itself down a gear back to 2nd? At standstill with partial throttle the box is changing from 1st to 2nd (2nd to 3rd?) at little over walking pace, if however i put my foot down it seems to pick up as normal.

As I said, all i have done is serviced the tranny and upped the vacuum modulator pressure by two turns.

IcemanG17

Hmm....the vacuum modulator is designed to provide SOFTER shifts at higher vacuum levels (think low throttle applications)....then as vacuum diminishes (think full throttle) the shifts get firmer....at least thats how its supposed to work....

An easy test is to pull the vac line and plug it....which should put the tranny in a "full throttle-zero vacuum" situation so all the shifts will be hard....

I've ridden in 928's with the vac line disconnected....it makes a noticeable difference when just cruising around...

bfellows

Just to clarify, car is changing up at low revs, but only 1st to 2nd (2nd - 3rd? initial change)

IcemanG17

Bert
Okay...I don't think the modulator does anything to the RPM shift point.....thats a combo of things including if the kickdown is engaged, the tightness of the bowden cable....etc...are you sure the kickdown is engaging? If I have my throttle anything less than completely floored.....it will shift somewhere under redline in every gear.....with it stomped it will hold to 6000-6200 everytime....

Mrmerlin

are the operating pressures correct this will affect the harshness of the shifts, it can be adjusted by removing the cap on the modulator pull out the T handle and turn it CCW for less pressure only go a turn or less at a time you will notice a difference, but you should check the trans pump pressures with the gauge set

bfellows

OK, so now i'm confused......spotted this whilst browsing the WSM in preparation for my latest attempt at sorting out my autobox. Does this apply to my box? Is my WSM woefully out of date (has '88 update) Thoughts?

Bill Ball

This depends on the tranny model. With 4-speed A28, there is no effect on shift points. Bert, 88 would have an A28.

bfellows

Thanks Bill, any ideas as to the best way to contact Jim about a more up to date manual? My WSM is incomplete by the looks of things

Bill Ball

Bert:

Get the WSM and tech documents CD pack from Jim Morehouse. (see the newbie sticky thread at the top).

Also, the AT information in the WSM is split in 2 differrent volumes. The early AT is covered in Vol. 2 and the later AT is in Vol. 3, although this specefic information tidbit is not there.

Tails

The modulation pressure should be adjusted via the installation of a pressure gauge to the test point, I would not advocate just carrying out the adjustment without the pressure gauge. The test pressure with the vacuum pipe removed from the modulation pressure valve is 4.2 bar =/- 0.05 bar at idle and at various speed ranges for the A28.16 model transmission. The WSM have the pressure listed to cover various transmission models. One turn of the modulation valve equates to approximatley 0.4 bar. Turning anti-clockwise reduces pressure and clockwise increases pressure. Increase pressure gives firmer shifts and lower pressure give smoother shifts, but if the pressure is reduced too far it will cause flaring of the shifts.

There have been numerous post over the last couple of weeks on adjustment of the modulating pressure valve.

Tails 1990 928S4 Auto