Hello Everyone.

About 17 Years ago I bought a '86 4000CS Quattro without really knowing anything about Audi's. I had some money burning a hole in
my pocket at the time when I came across it at a used car lot. I liked the way it looked and decided to take it for a test drive.
Having never driven a AWD car before it took all of 2 minutes to relize I was going to buy it. I loved that car like no other before
but being young and stupid I only had it a year before selling it because I couldn't afford a handful of major repairs...about 2k total.

Fast forward 17 years later and here I am looking to replace My '94 Volvo 850 turbo wagon that My Wife and I are giving to
one of our girls which also replaced My 01 AWD Subaru Forester I gave one of our other girls last year. Anyway while looking off and on
for a month at all kinds of cars I came across a identical '86 4000CS Quattro (minus a sunroof and heated seats).

Here is a car I've been talking about for 17 years...one of the best cars of the 80's. Although this has a little bit of purely surface rust here and there the undercarriage pictures (on a lift) are spotless. In person I found the interior is almost mint and while sitting down in the driver's seat I get a whiff of that old Audi interior smell. Well I pretty much bought the car before I turned the ignition key but that didn't stop me from taking a 1/2 hour test drive, the 4kq ran flawlessly.

Now 17 years I'm a little older and a little more confused but at least I now have the mech. experience, tools or financial means to fix most problems which brings me to my question.

I just noticed the center diff lock light stays on regardless of the switch postion. The rear diff light goes on and off depending on
wether it is engaged or not but the center diff lock light which always remains on even with both diff unlocked....It doesn't seem like the front is
always locked?? and I'm pretty sure it is only engaged when selected and I can brief hear the (air)actuator if I turn the dial slow enough...but that is kinda hard to test or is it?

Anyway I was wondering if anyone else has run into this problem.
.

Thanks in advance -Christian Jackson

 

I don't know about the rest of it, but I do know the front diff can not lock. I've been reading about differentials and the only locking front difs i've heard of are on WRC cars. someone else will have to answer your other questions.

 

You'll know if the center differential is locked or not if you can drive normally on pavement. If the lock is stuck on, the car will shake pretty violently and feel like it's about to fall apart due to the back wheels skipping on the road. The lock is mainly for getting out of tough places and for some ambitious fun in the snow, where road grip doesn't come into play so much. If the center AND rear differentials are locked, the shaking will be even more violent.

If the lock is not engaged though, and the indicator light is on, it may be due to corrosion of the indicator terminals on the transmission...but it seems like that would cause the light to be off permanently, rather than on permanently, since it's not making a complete circuit. A short maybe? Try pulling out the radio and look behind the differential lock console to see if there's any wires touching that shouldn't be.

 

If you're center differential is locked, it won't cause the rear wheels to skip down the road, but if you take a sharp turn in the car say in a parking lot, and the car is a pain to drive, and skips a bit, it's stuck on.

There is a check valve behind the center console, it's most lilkely stuck. It attaches directly to the diff.lock switch.
The differential locks are actuated by the cars vacuum system, and there's a check valve somewhere that's stuck not letting the vacuum release, and not letting the actuator release.
Or vice-versa, the calve is stuck open, not letting the actuator close.

I'll try to find some diagrams displaying where the check valves are.

 

Looks like I might be o.k. I have the console torn apart to install some audio and check out why the volt and oil temp gauges are dead.

Well I managed to fix the gauges...on the back of the housing was all plastic except for one broken filament/ribbon-wire from the gauge harness to a soldered termination. I used a little piece of bare 22awg wire to bridge the broken connection and what do you know My gauges both now work.

I had to disconnect the air lines for the diff-switch to do it and to my surprise the center diff light stayed on with no air lines connected to it while I was testing the unmounted cluster....so maybe I'm o.k. after all.


Thanks -CJ

 

With the line disconnected, and with the center console off, try to located the check valve.
Remove the valve from the line, and turn the car on to activate the vacuum pump.
The light will either stay on, or turn off.
You might have to drive the car a little bit and usually when you take your first turn, the light should go out.
If not, leave the valve off, and plug the vacuum line, and continue with a quick drive.

 

Thanks Sepp,

I actually went to a dry parking lot which hard to find in New England right now and tested the Audi with the switch in off and then in the on #1 position. The difference was pretty obvious. You think I would have thought of that between all of the 4WD trucks, AWD vehicles and tractors I've had. So the verdict is the LED is toast but the Diff switch definately works.

Thanks again....CJ

 

Good, it must just be a stuck indicator switch as one of the above posts mention.
Where in New England are you?
there's a large population of older Audi's up here.

 

I'm just west of Concord, NH (and you?)...only a 15 minute drive to Force-Five Automotive. I guess they are a great source for Audi parts. Going to pick up a replacement diff-gauge cluster and a Bentley factory manual for the 4kq tomorrow after work.

On a side note We had a Nasty 8-10 inches of snow last night which changed over to rain in the AM (all day). Needless to say I took the Audi out for some spirited backroad (since work was cancelled) driving early this morning. We had a good 2-3 inches on solid slush on all of the backroads with no traffic....nothing but a ear to ear grin here. The other cars We have '07 AWD Outback, '01 AWD Forester and '98 AWD Cross Country(RIP) couldn't hold a candle next to the '86 Audi with the center diff locked with today's weather and road conditions.

-Christian J.