GregBBRD

Transmissions don't need to be rebuilt from hard shifting...they need to be rebuilt from slipping or internal leakage.
You just need someone that knows 928's....which will probably end up being you...experts (or even competent people) are few and far between.

Lifting off the throttle to get the transmission to shift indicates that the bowden cable (kickdown cable) is too tight...or there is no vacuum to the modulator. (But even a complete rookie would check for vacuum, before changing a modulator.)

Perform tasks from post #6 to post #12.
Check that the modulator holds vacuum...it's probably an updated unit with an external plastic adjusting lever, with limits the amount you can turn it (without removing the lever), but it has an internal O-ring that can get dislodged. It must hold vacuum.
Check the hose to the modulator, right at the modulator. Check vacuum at idle. ~16 inches.
For now, reset the modulator 2-3 complete turns (yes, that new style cap will need to be removed), you can come back and tune the modulator.
Perform full throttle/bowden/kickdown cable adjustments.

GuardsRedHammerhead

I checked the Bowden cable tonight. I popped it off of the ball, and it rests perfectly on top of the ball. It does not retract or advance at all.

Speedtoys

Perfect, the next step is easier with the transmission cold..so back it up onto ramps, let it sit..then next AM put vacuum gauge on the end of the modulator vacuum hose, turn on car, see what guage says.

https://rennlist.com/forums/928-foru...-pressure.html

All you wanna do is get a reading from the hose at the shows vacuum nipple.

Get this from any auto parts store

GregBBRD

That would be fantastic, if that was the way to check it....

Remove the cable ball socket (for the kickdown cable) off of the ball.
Have someone push the throttle pedal to the floor.
Make sure that you have full throttle...you may have to disconnect the throttle rod going back to the engine to determine this.
Adjust things until you have full throttle.
Once you have full throttle, have someone hold the throttle pedal to the floor, again.
Pull on the kickdown cable with your fingers as tight as you can pull it (with your fingers.)
The ball socket needs to be able to pop onto the ball...perfectly aligned.

or...even easier, for now:
Simply loosen the 8mm headed nut on the kickdown cable, pop it off the ball, and lengthen it 3 turns. Snap it back on and go drive the car to see what effect the change had.

GuardsRedHammerhead

I have a nice vacuum gauge set. When I get some time for work this week, I will check this. I am putting new wheels and tires on the back this weekend, so should be a perfect opportunity.

I greatly appreciate everyone's input. It really means a lot.

GuardsRedHammerhead

Greg, thank you for taking the time to detail all of this for me. It means a lot.
My Bowden cable is very deteriorated in some areas. The sheathing has the steel spring looking thing rubbing through in many areas along the cable, and up towards the air filter area. I guess I am going to need to call and see what the damage is on replacing that first.
By the way, love your Teflon fuel lines. Have them on a few of my cars.

GuardsRedHammerhead

Ok, so I had time home from work, finally, to adjust my Bowden cable, and there has been SIGNIFICANT improvement to where the cars shifts. No more lifting off pedal to get it to shift. Best it has been in years.

However, I am still "FANNING" second to third, which never occurred until my mechanic adjusted modulator to soften it up. Can you tell me how I know which way to turn to to reset it? I have the new type modulator with white plastic cap for fine tuning.. Which way do I turn to reset it. Thanks Greg.

GregBBRD

Clockwise. You probably will have to remove the fine tuning cap, in order to get it back into its proper range. The cap is required to test shifting, BTW.
The new design is kinda a pain...

GuardsRedHammerhead

You just turn it so many times and it stops, or turn it 2 or 3 times, and it is considered a reset? Thanks again, All of this advice is curing a longstanding issue that I have paid to have fixed. Twice.

Speedtoys

Remember, your mechanic was a mercedes transmission expert..

Glad its better!

GuardsRedHammerhead

Well, I did say that he did lots of Mercedes stuff, so I ASSUMED he knew these trans. I use him primarily for hot weather, summertime parts hanging. Things like shocks, etc. He treats me right, but he freaks out over certain things that are generally 928 specific. I have more than once fixed problems that he had the car for. Got home, frustrated, and checked in with you folks here. Two days later, and BAM. Fixed. Like chasing a performance issue, to no avail. Roger thought it sounded suspicious, sends me a MAF loaner, and sure enough that was the fix.

Greg hit the nail on the head when he said that we many times have to become our own experts. I am an average, "SAE Shadetree". Not an expert in anything car related. But, with my tools, basic mech experience and people like you guys that virtually hold my hand, I always seem to get it done, and done PROPERLY. Mechanics anymore are outright refusing to look at my car. I even had one say " I don't want that thing in here". You would think that with me living in Jacksonville(Brumos country) I could find a decent 928 guy. No such luck. The Porsche guys all turn me away. As time passes more and more, these are becoming more of a enthusiast with a wrench type of vehicle, unless you are lucky to live near some of the experts that are sprinkled around the country.

This partial throttle shifting was driving me nuts. It is like night and day now. I am ecstatic. THANK YOU again!

GregBBRD

No reset. You have to find the sweet spot. Adjust. Test drive. Adjust. Test drive. Adjust.
I'm just giving you a place to start, from your description of how it is shifting into third. You have to evaluate the change and decide if it needs to be adjusted further.

GuardsRedHammerhead

First of all, thanks for the previous advice. The partial throttle shifting has been resolved. Maybe needs a bit more fine tuning, but it is night and day difference after lengthening the Bowden. However, I still need to adjust the modulator. I have significant 2nd to 3rd Flaring that wasn't there prior to the previous guy "adjusting it softer".

I have nothing more than ramps and jack stands, and am having a hard time reaching the modulator cap. I have moved the exhaust heat shield aside, and that helps a bit, but still cannot reach it. I can only "fine tune" it with the external plastic dial that comes on these newer modulator types. Two questions.

Do you normally remove the exhaust or drop it down to reach the modulator? Do you have to replace the plastic cap each time you remove it? I have seen some Mercedes articles that say the white cap is one time use. Any advice in those areas would be appreciated.

GuardsRedHammerhead

Do you normally remove the exhaust or drop it down to reach the modulator? Do you have to replace the plastic cap each time you remove it? I have seen some Mercedes articles that say the white cap is one time use. Any advice in those areas would be appreciated.

GuardsRedHammerhead

I am following up on this thread because I hate people that never update when they fix a problem. I have resolved my 1985 transmission issues, and the car is like a new car. Not even a figure of speech.

So, as surmised, the vacuum hose was crumbled and totally degraded. We got that replaced. While we were at it, we got into the trans pan, replaced the Mercedes flare spring with the upgraded spring, new filter and fluid. Modulator was already a recent one. Put it all back together, adjusted the vacuum, and it is better than ever. Incredible. No more hard shifts, and no more 2 to 3 flare. Also, replaced the AC compressor, expansion valve, dryer, and refilled. Finally A\C.

This car was bought for a song because it had "transmission problems". Not any more.

Thank you Greg, and thank you all!