What makes the auto box change gears?
 

The slow changing of my autobox is annoying me. I have a 1985 S2 4.7 (think it's Euro spec).

When in D, the shift from 2nd to 3rd is fine, but 3rd to 4th is nearly always (but not always) slow.

When in 3, the shift from 1st to 2nd is fine, but 2nd to 3rd is slow.

I don't understand why 2nd to 3rd is fine in D but not in 3.

Is it vacuum that makes the box change gear? Could pipes be leaking or cracked?

I've had the oil and filter changed by a Porsche specialist, the changes improved slightly but not a lot. The oil that came out was black apparently. I hope this doesn't mean the problems are unfixable.

Thanks in advance
Matt (UK)

The auto box in the 928 is the same as Mercedes sedans of that era, so any shop that repairs a MB AT should have no issues with the one in the 928.

A vacuum control line and a throttle linkage both go back to the tranny as well as some electrical signals. Inside the tranny there is a valve body that does the actual shifting via hydraulic pressure to solenoids etc. inside the tranny. The valve body has valves with springs that set the pressure various actions have.

Did you have the engine oil and filter changed or the transmission fluid and filter?

That said, I really know very little about the specifics of the 928 auto. Makes me wonder how much your Porsche specialist knows given the results. I would look for a good transmission specialist familiar with 80's Mercedes Benz transmissions.

Better and more informed comments should follow. ;)

Just the autobox oil/filter. I had changed the engine oil myself a while before so that didn't need doing.
It wasn't a full service, I wanted them to take a look and change the oil/filter properly as I wasn't quite sure where the torque converter fits in all of this.

I'll keep an eye out for a merc or transmission specialist, I didn't want to leave the car with the porsche garage for them to play around with as they aren't cheap!

See what the next round of "informed" advice is, could be all you really need is some kind of adjustment.

I also would appreciate any info on how the 4 speed auto box works. I haven't been driving my new car hard but today I floored it. it seems to go past 5800rpm and into the red-line and not change up unless I ease off the pedal and it then slips up a gear. Im not to use to auto trans as most cars in Europe are manuals. I need to change the trans fluid but could anything else cause this problem.

I looked around a bit a while back, as mine was very 'light' with down-shifting - i.e. even before I hit the kick-down. Now I have a '84 Euro S, so was told that's not possible at all - but I swear if I drive it hard it changes faster for the next few weeks, until I have done quite a lot of 'slow' driving.


Anyhow, check this out for tips on how to use the automatic like a manual - works for me.

http://www.nichols.nu/tip199.htm

or just put it into permanent kick-down mode:

http://www.nichols.nu/tip047.htm


Good luck & let us know how it goes.