I have an '85 auto that, now that the transmission fluid level is up to snuff shifts very hard/abrupt at light throttle. Not bad under heavy accelleration.

Based on my search results I suspect that it a vacuum problem - such as no vacuum signal to the tansmission.

I replaced all the vacuum lines in the engine area last year so I suspect the problem is farther back.

I crawled around under car yesterday trying to figure out where the vacuum module is located on the transmission without success. Looked at the WSM - didn't seem to provide anything that helped.

I have a standard exhaust and the heat shield is fairly close to the transmission making visibility on that side hard but unless it is tucked in I must be blind.

The only thing I found I thought might be it was actually an electrical connection at the rear of the main box near the fliud reservoir.

Tried to locate the vaccum line coming back from the engine without success.

Can someone please point an apparently blind old man in the right direction?

Thanks

 

Go to your "Search" tab above. Click "Advanced search" and type in "How To Adjust Modulator Pressure" (w/o quotation marks of course) under "Title only" and select the "928 forum". This will bring up a post submitted by JP Rodkey that features a great photo. The Modulator is a plastic piece he has circled. The vacuum point you are looking for he refers to as the "Vacuum Nipple". In the very center of the Modulator is a black cap. You might consider replacing it as it can be a source for vacuum leaks because the cap tends to crack with age - check it out. Sorry, don't know how to get the link to you here in this body of text.

I hope this helps.

 

Once you locate the modulator, look at the center. There is a rubber cap. Remove it. Underneath there will be a small silver "T" handle - not bigger than a dime.

Pull the "T" handle out and turn it counter clockwise several turns. Test drive the car. If it shifts smoother but still too harsh repeat the adjustment until you get the shifting the way you want it. There is a vacuum measurement in the manual for adjusting it, but setting to taste works well too.

Be careful not to set the adjustment too tight. The shifting can become so harsh that it will snap the torque tube.

Dan the Pod Guy

 

Thank you for the input. Now all I need is a chance to get back under the car - hopefully next weekend.

 

Update - Fixed

Confirmed that there was no vacuum at the transmission so checked the engine end and it was connected. Spent a while trying to figure out where the line goes once it starts down the fire wall in the engine compartment I could not find any hint of the line until the transmission end. I seem to recall seeing that someone had dropped their exhaust to access the line above the torque tube area but decided that was to much of a hassle.

I ran a new steel brake line from the engine compartment to the transmission area and tied it in with flex tubing on each end.

What a difference - transmission shift very smooth now.

Of course while I was checking vacuum connections I found that the lines I replaced about a year ago were not in very good shape. The rubber T connectors apparently could not take the under hood heat as several were scracked or splitting. I don't think they were actually leaking yet but it would not have been very long.

 

Glad to see you've obtained good results. Mine is next.

 

Dan...its a counter clockwise adjustment that softens the shifts? Just making sure before I begin the trial and error process. A shop had softened the shifting from the very abrupt action. A vacuum leak was the real culprit and I fixed that. Now I fear that the modulator has been over compensated on the soft side and I would like return the adjustment closer to what it was before it was tweaked. So...turning clockwise will firm the shifts?

Mike

 

Mike,

I believe this is the case. There is not much to loose by just turning the thing. It takes a couple of turns to make much difference if I rememebr from when I did it.

On the Mercedes they cut the vacuum to the control when the car is cold to keep it in gear and warm up the engine faster. I always thought this was a strange feature.

Reading the manual there is an adjustment on the valve body to change the responsiveness of the transmission. I assume it affects the kick down points, but I have not fooled with it. If you do not get the car where you want it, take a look at this adjustment. I do know there is a world of difference between 928s based on how the transmission is set up.

Dan the Pod Guy.

 

Roger,

nice job! I was wondering, could you give a little more info on what brake line you used? Is it just one long piece or several spliced together? Where did you get it? Hose ID, etc?

Also, where/how did you route the new line?

Sorry for the questions, but inquiring minds want to know.

 

Like Roger, I had to run an alternate vacuum line from the multi-T connector behind the MAF to the Transmission vacuum modulator. I used flexible high-temp silicon hose that was easy to secure along the brake line path on the driver's side of the car. Strain relief loops in the vacuum hose were placed at strategic points along the route and the setup works perfectly. *IF* the vacuum line is somehow compromised, I will experience the physical manifestation immediately. But, I don't see any compromise to this setup.

Mike

 

Mike,

Thanks for the info. What ID line did you use?

 

Chris:

I used 3/16 steel brake lines from the local auto supply. Two sections 60 inches long.

There is a metal brake line that runns front to back under the driver side that is held in place with plastic clips. The clips have a spot for a second line on the other side so I used that to hold the line in place under the chassis. I removed th mouting screws for the rear muffler heat shield to allow the steel ine to route over the top of the shield and connected it to the transmission module with a local purchase vacuum line with enough loop to it so that transmission movement would not stress the connection.

At the engine end I ran the line up the fire wall near the brake booster and then across the firewall so I could use a piece of local purchase vacuum line to connect from the new steel line to my vacuum tee under the air cleaner.

Where the two steel lines connect under the car I used an in brass fitting that connected to the two original brake line fittings.

Hope that makes sense. If not let me know and I can try again.

Roger

 

Got it!

Thanks.

 

Chris;

I used 3mm ID silicon hose. I set out initially to duplicate what Roger had done, which seemed like a good idea. The run between the engine compartment and the vacuum modulator was such that strain-relief and obstacles could be avoided with proper loops and cable ties. Getting the car up off the ground as usual took most of the time. Routing and joining the two 6' lengths of silicon hose took less than an hour. As Roger mentioned, securing the lines or hose along the driver's side is almost too straight-forward. The difference in shifting with good vacuum was night and day.

Mike

 

Thanks Mike.

I found an online source for silicone hose. I am looking at placing an order for about 15' and trim off what I don't need. Looks like the price will be about $25 with shipping.

The procedure looks very straight-forward. Once I get the line, I'll get to it and see if I can improve poor shifting performance.

Thanks again.