After I recently changes the fluid and filter in my tranny it started to very randomly slip between shifts. It only happens when I accelerate slow it is never an issue under full throttle. It only does this from 2nd to 3rd gear. The fluid is ATF Dexron 3 and after the filter change the tranny shifts overall much smoother. The described behauviour is really random (probably once a week). Is there anything I can check, tranny fluid level is OK by the way.

 

Maybe you have a B2 piston issue... Do a search on the list, and you will see many posts.. Mine is on the way out, and a new Trans & torque Tube is going in very shortly.....

Cheers,

 

Schocki unfortunanlty my 1986.5 S is doing the same thing but its random and always when driving slow and easy. I almost wouldnt even call it a slip as much as a delay... I dunno hopefully not the beginning of the end of my tranny.

 

Yes that is exactly what mine is doing sometimes
With the exception that it is smooth even under very light load and at slow speed for days. When it slips it only does it once and it is fine afterwards for many shift cycles. It however never slipped so far under full throttle. The shift is still very smooth even when slipping occurs there is no hard engaging of the gear.

 

I experienced the same things days before my tranny failed. When it did fail, the car had no gears, forward or reverse.

I believe the actual culprit was the valve body, but once the transmission was out, it only made sense to rebuild it. I went with a new one from 928 International.

 

Hadn't someone advertised that they would rebuild the valve body, and make it a 1st gear start?

 

I would try tightening up the TV cable a touch. My 3 speed went from no kickdown to beautiful response and kickdown with a small tightening of the cable. Normal wear and tear and a stretched cable may be confusing the box as to what to do. At least the 4 speed TV cables are adjustable up at the engine end, not underneath.
jp 83 Euro S AT 48k

 

I was going to say the same thing as JP. I would try this before anything, cable stretches and loosens up.The same thing happnes to the throttle cable. You would be surprised at how many 928 owners out there have loose cables. I have been playing with the TV cable on my 82 3 spd. I am trying to get the shifts just right. It doesn't take a whole lot of adjustment, just a turn or two when the cable is taught. Too much adjustment and the tranny hangs in the lower gear too long.

I would say that you do not have a "slip" if it is going into a higher gear, you just need an adjustment.

 

I'm absolutely shure that the cable was never tightened after the car left the factory. I will try this first, seems to be an easy adjustment.

 

Schocki:

When you say "slipping" from 2nd to 3rd, do you mean the engine itself seems to be slipping...as in the engine's rpm is temporarily surging (almost like slipping into neutral for a moment) when the tranny shifts from 2nd to 3rd? If so, I doubt it's something that can be corrected by tightening the cable. That kind of slippage points to the valve body which, luckily, can be serviced without dropping the whole transmission.

 

The engine revs increase momentarily by about 500-600 and then it shifts into 3rd gear. Shifting however is still very smooth and it is happening VERY infrequent.
Where is the valve located that you talk about. I thightened the cable and if it still slips occasionally I will take a look at the valve.

 

Read up on adjustment to the vacuum modulator valve: a small adjustment clockwise on the retractable 'T-handle' ( passenger side of trans) may be in order - this decreases vacuum seen by the trans: worth a try, easy to do - and is reversible.

edit: You are correct re. the modulator controlling the firmness of the shifts; however, if there is too much vacuum seen, the shifts will be far too soft = slow, like a slip. Clockwise turning on the 'T' closes/restricts vacuum seen IIRC: some actually report plugging the line and modulator input for full firm/slam dunk shifts.

 

I thought that the vacuum modulator is responsible for the firmness of the shifts? But if the cable adjustment didn't do anything it is probably worth a try.

What I just remember now is when I replaced the ATF filter and fluid, that there was a regulating valve (according to PET) in the valve body(PET#126 270 057 0) That had a broken spring. The spring (PET#722 993 001 0) was connected on the outside and must be responsibel to pull the piston back to it's position. The piston moved freely and the car never had any tranny issues. So I forgot about it, sorry I don't have a picture but hopefully I can find one online.
Any ideas what this "regulating valve" does?

 

Found a picture online, circled is the regulating valve and the small spring that is broken on mine.

 

C'mon guys, no automatic gurus out there that know what the function of the red cirled piston on the valve body is?