Rear locking differential 1988 90Q

Louis-Alain Richard laraa at
Tue Aug 7 10:47:52 PDT 2012

The shorter version of the adress...



-----Message d'origine-----
De : quattro-bounces at [mailto:quattro-bounces at] De la part de Louis-Alain Richard
Envoyé : 7 août 2012 13:44
À : 'Radek'
Cc : quattro at
Objet : RE: Rear locking differential 1988 90Q

Just found my old post from december 2007... on a French Audi forum !

You also need a die to retread the metal shaft...


-----Message d'origine-----
De : Radek [mailto:radek at]
Envoyé : 7 août 2012 13:01
À : Louis-Alain Richard
Objet : Re: Rear locking differential 1988 90Q

Super, thanks!  Two questions:
1. Are screw openings (3) spaced the same as in the Audi?
2. Can the actuating arm (transparent plastic in the MB, metal with rubber boot in the Audi) be pulled out of the actuator without
breaking something inside?
Ottawa, Canada.

----- Original Message -----
From: "Louis-Alain Richard" <laraa at>
To: "'Radek'" <radek at>; <quattro at>
Sent: Tuesday, August 07, 2012 12:08 PM
Subject: RE: Rear locking differential 1988 90Q

The leak is often in the rubber boot over the actuating arm, hence the partial failure. The diaphragm is OK.

Repair is difficult since the plastic is very brittle and the boot is not available separately. I use some Mercedes-Benz door lock
actuator from the mid-80s on my urQ. Almost identical, except for the color (MB black, Audi beige) and probably plastic material.
You'll have to swap the plastic connector from the old one. Otherwise, they work well.

40$ here :


-----Message d'origine-----
De : quattro-bounces at [mailto:quattro-bounces at] De la part de Radek Envoyé : 7 août 2012 11:29 À :
quattro at Objet : Rear locking differential 1988 90Q

So now that the diff is out, we can clearly see the cause of it being locked all the time.  As I suspected, the plastic vacuum
actuator has broken off at one of the three screws and is moving around a bit.  I used a vacuum pump:  it seems to be holding vacuum
when the "on" side (yellow striped hose) is activated, but doesn't hold it when the "off" side (blue striped hose) has vacuum
applied.  This is odd, because I would expect a single membrane inside, so if it's broken there would be no vacuum held on both
sides.  To put things differently, the actuator has enough strength to engage the lock, but not enough to disengage it.

I would like to take it off the diff to clean, inspect and repair/replace. 
Here's the question:  how do I separate it from the diff?  The actuator arm has a pin at the pivot point, but it seems to be
permanent.  Can I pull the arm out of the actuator or out of the differential perhaps?  Bentley does not offer any help (or I
haven't found it).

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