Hi all, I'm haveing problems with my 84MY 4 speed auto. I have just had it rebuilt by a Mercedes specialist. I removed the box and delivered it to him and have now refitted it to the car. Now I took it for a test drive and it seems to slip into neutral on load. If I take my foot of the gas it seems to get back into gear but will slip into neutral again under heavy load. I have checked oil level several times with engine running and is all good (new oil). I have checked vacume from manifold to modulator and it holds vacume with no reduction. I tested vacume at the modulator and there is a small reduction in vacume. Vacume at the manifold is steady at 17.5 Also adjusted the modulator 2 full turns clockwise but I am yet to roadtest it just yet. While I had the cover off the modulator I could see oil in there. Does this mean that the modulator needs replacing and could this cause the problem of slipping into neutral??? Kickdown cable and selector cable have been installed as per the manual. The oil I'm useing is Castrol Transmax Z reccomended by Mercedes.

Many thanks in advance Ben

 

It's not the vacuum modulator valve.

You should feel a heavy detent for each gear on the gear selector. Could be the mechanic left a part out on that mechanism, which would allow free movement of the gear selector. But the shifter mechanism at the console also has a detent, so you shouldn't see the selector move on its own.

I suspect it is something in the transmission's valve body, and that it can be fixed by your mercedes mechanic that did the rebuild work, without removing the tranny from the car.

 

I guess it would be worth checking that the selector cable is adjusted the way it should be but does sound like something internal. You could do some more checking to narrow it down. Try driving in 2nd, 3rd etc and see what happens.

 

Good point Jason, I'll give it a try and see how it goes. I was wondering if it could be a poorly adjusted band? I went to see the mechanic who did the rebuild but he's on holiday until next week so I'll have to wait until then

Cheers Ben

 

Brake bands are adjusted by different length pins, and these can be fitted with the auto box in the car.

+1 on bowden cable and indent for gear selection.

You need to do a full set of adjustment as well as the location of the selector lever on the box matching the gear lever and the neutral positon on the safety switch on the box.

Do you have the WSMs or Jim Moorehouse's Tech CDs? Roger at 928sRus was going to have these CDs up on his website for free, so it would be worthwhile checking.

Read up on the Manual relating to the Auto boxes contained in the CDs and you will have a good 'heads up" for when you mechanic comes back from holidays.

Tails 1990 928S4 Auto