[s-cars] "Antilock off" caused by diff lock vacuum failure

Wayne Dohnal wd43 at hevanet.com
Sat Nov 7 14:12:16 PST 2009

I've been chasing an intermittent "antilock off" light for a while and 
haven't noticed any postings about what failed in mine.  The first 
breakthrough was noticing the light seemed related to acceleration.  The 2nd 
was noticing that it was directly related to vacuum and boost levels, and 
the 3rd was that the diff lock's green light went came on and went off with 
the antilock off light.  I actually had 2 failures: (1) The vacuum check 
valve under the back seat was open in both directions, and (2) the hose 
nipple had broken off of the vacuum resevoir in the trunk.

I replaced the check valve with a $4 generic version from the parts store. 
It's very difficult to get to the clamp on the original check valve so I 
just added the new valve in line with the old one.  I glued the nipple back 
onto the vacuum resevoir, but this is a type of plastic that glue doesn't 
stick to very well so it may or may not stand the test of time.  Besides 
fixing the antilock off problem I'm noticing a smoother application of boost 
pressure.  Before the fix, the boost would climb to about 20 psi then 
abrubtly back off to 10 psi or so.  Now it climbs to 15 psi, smoothly tapers 
off, and holds at 15.  I assume the leak was causing the ECU to be getting 
corrupted vacuum/boost data.

1994 S4 

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