4spd auto trans questions
10-24-2009, 12:46 AM
#1
Burning Brakes
Thread Starter
4spd auto trans questions
It's very convenient how many of the main parts of my 1985 Mercedes found their way into the 928. I've got the 722.3 TC/trans, basically the same thing the 4sp 928s ran, just not as a transaxle.
One thing to know is that I have the vacuum modulator set for firmer shifts than factory spec - making about twice as much power as stock, I had to do this to keep the full throttle upshifts nice and tight. Also, this valve body does 1st gear start on every start.
Here's the issue I have -
If I set the bowden cable to be shorter than it is currently, things work as one would expect in all ways except that I run into the fuel-pump-shutoff-rev-limiter on the 2-3 shift.
The way it's set currently, it runs right up to redline but doesn't hit the fuel-pump-shutoff. The problem is, if I floor it (even hitting the kickdown) at a speed that should send it to 2nd gear, it doesn't go further down than 3rd. In this scenario, I can move the shifter into 2nd by hand without issue. If I floor it/kickdown at a speed that should send it down to 1st gear, it does in fact go to 1st, shifts into 2nd at redline, and will hold 2nd all the way to redline and shift into 3rd. It also seems to me that when I drive with a normal amount of throttle, the car spends very little time in 2nd gear, though this may be proper.
Any ideas?
Another thing - I had to replace the vacuum modulator. Is it possible that the vacuum modulator is working in reverse? I know it's working, because counter-clockwise makes it shift softer and clockwise makes it firmer, but it seems like it shifts more firmly in high vacuum situations than in low vacuum (it may just be that in low vacuum I am putting enough power through the trans that it feels softer - that's why I ask if this is even possible).
Thanks for the help my fellow fans of Stuttgart. If it helps any, I've always wanted a 928GTS.
One thing to know is that I have the vacuum modulator set for firmer shifts than factory spec - making about twice as much power as stock, I had to do this to keep the full throttle upshifts nice and tight. Also, this valve body does 1st gear start on every start.
Here's the issue I have -
If I set the bowden cable to be shorter than it is currently, things work as one would expect in all ways except that I run into the fuel-pump-shutoff-rev-limiter on the 2-3 shift.
The way it's set currently, it runs right up to redline but doesn't hit the fuel-pump-shutoff. The problem is, if I floor it (even hitting the kickdown) at a speed that should send it to 2nd gear, it doesn't go further down than 3rd. In this scenario, I can move the shifter into 2nd by hand without issue. If I floor it/kickdown at a speed that should send it down to 1st gear, it does in fact go to 1st, shifts into 2nd at redline, and will hold 2nd all the way to redline and shift into 3rd. It also seems to me that when I drive with a normal amount of throttle, the car spends very little time in 2nd gear, though this may be proper.
Any ideas?
Another thing - I had to replace the vacuum modulator. Is it possible that the vacuum modulator is working in reverse? I know it's working, because counter-clockwise makes it shift softer and clockwise makes it firmer, but it seems like it shifts more firmly in high vacuum situations than in low vacuum (it may just be that in low vacuum I am putting enough power through the trans that it feels softer - that's why I ask if this is even possible).
Thanks for the help my fellow fans of Stuttgart. If it helps any, I've always wanted a 928GTS.
10-24-2009, 06:22 AM
#2
Rennlist Member
The main difference between the MB unit and the 928 version is the valve body that was designed by Porsche to give sharper shifts I believe. If you have the MB valve body then you may be losing out somehow. The vacuum modulator obviously needs vacuum to do its job and as you clearly understand, the adjuster regulates the amount of vacuum applied to the diaphragm. Less vacuum gives a sharper but harsher shift [try disconnecting the vac line to see the difference if you have not done so already].
I have not managed to change the shift pattern and tightening the kickdown cable does not in my experience change the shift point. I find optimal enjoyment of the transmission's performance by shorting out the kickdown switch and tightening the kickdown cable until it is such that when cruising at around 70 mph it changes unto 3rd with just a small amount of throttle pressure.
The thing that I have not been able to defeat or change is the way it changes down 3/2 or 2/1. It seems there are just certain rpm points and no matter what it will not change down leaving the transmission feeling bogged down too low in the rev range. Manaul shifting tends to solve the 3/2 problem but not the 2/1 shift sometimes needed in low speed autocross type events.
If you are bouncing off the rev limiter in auto mode then I suspect something is not quite right as it is not designed to do that. A hard charge in auto usually results in a shift at around 6200 rpm [as indicated in my ST2 data logs] which is probably past peak HP anyway.
Regards
Fred
I have not managed to change the shift pattern and tightening the kickdown cable does not in my experience change the shift point. I find optimal enjoyment of the transmission's performance by shorting out the kickdown switch and tightening the kickdown cable until it is such that when cruising at around 70 mph it changes unto 3rd with just a small amount of throttle pressure.
The thing that I have not been able to defeat or change is the way it changes down 3/2 or 2/1. It seems there are just certain rpm points and no matter what it will not change down leaving the transmission feeling bogged down too low in the rev range. Manaul shifting tends to solve the 3/2 problem but not the 2/1 shift sometimes needed in low speed autocross type events.
If you are bouncing off the rev limiter in auto mode then I suspect something is not quite right as it is not designed to do that. A hard charge in auto usually results in a shift at around 6200 rpm [as indicated in my ST2 data logs] which is probably past peak HP anyway.
Regards
Fred
10-24-2009, 09:36 AM
#3
Burning Brakes
Thread Starter
Is there a separate kickdown cable? MB has great documentation...for everything but the trans which the service manuals don't address other than to say how to change the fluid. The Bowden cable I'm talking about is properly called (I think) the throttle position cable. It definitely changes the shift points. Car left the factory with a 5500rpm redline. The new engine can rev much higher and presently has a 6000rpm redline (the power curve is such that 6250rpm would be ideal, but MB only made V8 fuel pump relays up to 6000rpm - I'm eventually going to bypass it and come up with an MSD setup). The way we moved the shift point to get it to go past 5500rpm with the new fuel pump relay was to simply shorten that cable.
I think part of my problem is with lumpy cams I'm low-vacuum in a lot of situations where a factory engine wouldn't be, and that might just be confusing the system a bit.
As an aside, anybody want to take a pic of 928 w/CIS fuel pump relay for me? It'd be nice if they used the same design - another 200rpm would let me shorten the cable back to where it was working how I wanted it to work and match my power band better.
I think part of my problem is with lumpy cams I'm low-vacuum in a lot of situations where a factory engine wouldn't be, and that might just be confusing the system a bit.
As an aside, anybody want to take a pic of 928 w/CIS fuel pump relay for me? It'd be nice if they used the same design - another 200rpm would let me shorten the cable back to where it was working how I wanted it to work and match my power band better.
Last edited by marlinspike; 10-24-2009 at 09:57 AM.
10-24-2009, 09:56 AM
#4
Rennlist Member
I just know that my cable was too tight and kickdown switch too high.
Last week I did a couple of double downshifts at 70mph, unintentionally, and now I'm shuddering in every forward gear. I bet too much stress on B2 pistion or band. Now I'll have 2 down for repair. They don't seem to be bulletproof.
Last week I did a couple of double downshifts at 70mph, unintentionally, and now I'm shuddering in every forward gear. I bet too much stress on B2 pistion or band. Now I'll have 2 down for repair. They don't seem to be bulletproof.
