kevin2012

My 91 S4 diff (with PSD) has whine for a while already, I can find a good used 92GTS (auto) trans and diff unit locally (722 270 21 01 722 326 02 908750). Can I swap this 92 GTS into my 91 S4? Can I just swap the diff or I will be better off to swap the whole thing( TC+ trans + diff)?

I had seen from some place that the diff on the GTS uses lass diff fluid than my 91 S4 diff. Are they interchangeable?

Thanks

mark kibort

I just posted some of the effects on gearing with just putting a GTS diff on a S4 automatic trans. may or may not be what you want.

kevin2012

So how about the whole thing? From the TC ,trans and the diff.?
Will this be a straight swap?
Cheers.

Rob Edwards

'91 vs. GTS transmissions have the same exact gearing, only difference is a slight bump in stall speed in the later trans to match the GTS's power band.



Big list of changes between A28-16 and A28-18, I honestly have no clue whether you can plug and play between them- knee-jerk would be yes given you're swapping a '92, but hopefully someone who knows will chime in.

GregBBRD

Swapping just the differential means setting up the pinion from the GTS (the ring and pinion are matched pairs and can only be swapped together) into your transmission.

Very few people have the skills required to accomplish this....it is very involved and there are several extremely critical dimensions that need to be measured and adjusted. Reading the factory workshop manual offers very little help...whoever wrote that section apparently had no idea of what was required and the instructions are very confusing/impossible to logic through.

Change the entire transmission.

BTW....differential noises are very rare. Not sure how many miles are on your '91S4, but I'd be certain to make sure what I was hearing wasn't something else....tires, wheel bearings, axles, etc.

James Bailey

As I recall differential noise is more common for the GTS.... no idea why that is. Porsche did start hardening them by shot peening with the GTS

ROG100

Hi Kevin - check out these two threads by Schocki
https://rennlist.com/forums/928-foru...ight-load.html
https://rennlist.com/forums/928-foru...n-s4-unit.html

kevin2012

Thanks for all of your comments and appreciated.

Yes, I also prefer to change the entire transmission (TC+trans+diff). No one here locally can set the pinion right.

My 91 S4 has around 86K km genuine. I am surprise also that the diff need replace or rebuilt so early. I had old Mercedes with 100K km more mileage but the diff still working very strongly.
Many things had been changed, all wheel bearings, diff fluid, Torque Tube, tires...
https://rennlist.com/forums/928-foru...ing-noise.html

My theory is that my PSD slave cylinder was leaking brake fluid into the diff and ruin the gears inside. I was losing PSD fluid after flushing, I couldn't find any external leak but the slave cylinder was leaking and the pin was corroded.
https://rennlist.com/forums/928-foru...luid-leak.html

Cheers

kevin2012

Hi all,

If I exchange the entire transmission to this 92GTS unit, will it be a straight swap? How about things like speedometer pickup, trans cooler lines, shift cable...Do I need to do any modification?

Thanks in advance for your advices.
Cheers.

MainePorsche

Kevin, I took a look at PET and all looks the same. The ATF lines don't change. The auto lock control looks the same. The range selector is the same (what your shift cable attaches to). I couldn't verify speedo sensor, but I believe it is the same magnetic sensor on the differential on all S4's and later.

James Bailey

all the PSD 90 later do not have the differential mounted alloy wheel with the magnets...speedo signal comes off the ABS sensors. Some years left front spindle other years off rear hub.

MainePorsche

Thanks Jim.

Vilhuer

There is drain hole in slave cylinder casting which should prevent this.

kevin2012

Thanks everyone for the information and advices.

Yes, I also saw the drain hole in PSD slave cylinder but it was very tiny and was covered with dirt. I doubt that any excess fluid can be drained out from that. Why didn't they make it bigger.

Also, when I was having that PSD fluid leak, the PSD warning was on. When I checked the PSD unit in the rear fender, the fluid was dropped to a very low level. I couldn't find any visible leakage or wetness around the slave cylinder or anywhere else, that's why I think that it was a internal leakage into the diff. BTW, I had changed the diff fluid few times in couple of months (within 500km), the changed out fluid was very dark every time.

https://rennlist.com/forums/928-foru...luid-leak.html

Cheers.

Vilhuer

To keep dirt out. In ideal world its more than enough to allow leaking fluid to drain out. The is so little fluid in PSD that error light should come on long before it causes any major damage.