I have a 1985 928 w/ automatic transmission.

After about 11 years of FL driving w/o a/c, my mechanic and I have gone through the HVAC system, replacing the pods, vacuum connectors, flap foam, evaporator, resistor, switches, controller and other parts. The a/c is now ice cold and works perfectly.

However, since fixing the vacuum leaks, the transmission shifts really rough from 2nd to 3rd gear. It feels and sounds as if a large boulder is being dropped on the rear deck as it shifts.

The mechanic went back thru all the vacuum connections and is certain they are connected properly, including the one above the torque tube to the transmission.

The mechanic is a knowledgeable Porsche mechanic with the WSM, but not an expert in AT's. He is uncertain what is causing the new shift problem. He is wondering if any connections in the aft section of the transmission may cause the problem, or if the problem could be internal to the tranny.

It shifted fine before the many vacuum leaks were fixed.

All connections are by the book except that the heater valve is tied off and the vac line plugged, and the air recirculating flap that allows outside air to enter the system has been sealed off and the line has been plugged to prevent outside air and dirt from entering the system.

I would greatly appreciate any information you might have regarding this problem.

Thanks,
Gary

 

Your vacuum line from the green pressure modulator on the Left side of your transmission (right above the middle of the pan) will have a vacuum line that feeds off your manifold. I'm not sure of the 'T' piece hook up on your MY, but I would start here. Isolate this line and measure the vacuum of the manifold connection. Post the result. Then isolate and check the line. The vacuum line as it travel from the engine to the modulator is not a single piece - there are places for vacuum loss. Measure it at the distal point at the modulator. Post it. Then hook a piece of good line and see if your modulator holds vacuum. It should. Also note if you have a black cap on top of the modulator. If you don't, or it's cracked, the modulator won't hold vacuum. Start here.

 

It does sound like the vac source line is disconnected (2nd that ^^^).

Or, if you are fully confident that the vac system is now correct, perhaps someone previously used the modulator adjustment to compensate for no or poor vacuum and it is now out of adjustment. Check the pressures against reference values.

 

Maybe these will help

 

Also make sure that the Bowden Cable & Throttle cable are correctly adjusted, many are not.

 

He should probably inspect (anyway) and exclude the vacuum system first. The Cable can come next.

 

You guys are great. Thanks so much for your help.

 

Mercedes has an updated vacuum modulator (that green dick that has the vacuum line hooked to it) and and updated spring/piston kit for the 2-3 shift. The updated modulator is around $50 and the spring/piston is around $10 at my dealer.

 

Is this something most should look into?

87 s4 AT

MJ

 

Do you have MB parts numbers for those items?

 

Note that loss of vacuum to the modulator will not affect full throttle shifts, only partial throttle, as there is no vacuum signal at full or close to full throttle. So, if the shifts are harsh with your foot into it, be sure to look at the other items in the bulletin that Larry provided.

 

I do plan on doing an adjustment soon, but on mine, it appears that the exhaust and a heat shield block any easy access to the line and/or modulator, exhaust is welded one piece from cat back to pumpkin. Not sure how to run it and adjust pressure?

 

My 86.5 (sometimes) has the same harsh shift at light throttle but I am inclined to avoid green dick with vacuum.....

 

DISK.....lol Fn' typo.

 

I would think so.

The modulator Mercedes Benz number is 1262709279. The spring kit for the 2-3 flare is 1262704477.