rear diff lock sticking on

Gaz · 10-05-15, 09:53 AM

The rear diff lock light on my cq will stay on after turning the switch off. And i have to get under the rear of the car and give the actuator a wiggle (if that's the technical term) in order to get the rear lock to disengage and the light to go out.

It seems to move a little bit on its own, but just not enough.

It also looks very oily.. Could it just need a clean? Or new vacuum lines? Or could it be the diff it self that is the problem?

Cheers.

yeti · 10-05-15, 01:12 PM

My limited understanding of it is that the lock is applied by the presence of a vacuum, and disengaged by the vac being released, so that would discount a vac leak for me, the lock wouldn't engage.

Im guessing its just gunked up.

msh · 11-05-15, 03:16 AM

Not really. Actuator has two sides, and lock is engaged or disengaged depending on which side has been connected to vacuum system. In this case it sound like the side, which disengages is not receiving any vacuum.

Hanuman · 11-05-15, 05:27 AM

The rear diff lock arm is also prone to a lot of corrosion due to build up of crud and road grime. They can get caked. Rusty actuator rods that get so thin they snap, and the pin and yoke that link the actuator to the arm, impinging the movement.

Another frequent problem is the actuators themselves, which are suffering perished diaphragms internally, so they are not so flexible, and the plastic body of the actuator getting brittle, so that the hose connections crack, resulting in partial loss of vacuum and eventually total failure. As mentioned, they receive vacuum to either chamber depending on whether being locked or unlocked.
They are easily damaged anyway.

A Mercedes part has been found to be almost an exact match, with some small modification.

I find that both my diff-locks occasionally stick; Either not engaging immediately, but they do engage as soon as the car moves, and likewise disengaging. In the worst case, going into reverse and moving backwards a short distance is enough to disengage. A lot of cars experience that.

M4rkp · 12-05-15, 12:20 PM

As The Real MSH says - it needs vacuum to engage AND disengage.

You need to check the pipes from the diff lock control to the actuator ( yellow and blue IIRC) with a MityVac - Its more likely to be the actuator itself, probably the sealing ring around the actuator shaft that is leaking. In some cases the retaining screws swell up with rust and crack the casing.

Its easy enough to take off - they are NLA from Audi but there is a Mercedes part that works.

I had the same issues last year - have a look at the link below.

http://www.quattroforum.com/index.ph...ctuator.24641/

Mark

kat1127 · 25-08-15, 05:05 PM

Hi, any more info on the merc part, the link doesn't seem to work t.i.a

jimh · 25-08-15, 05:21 PM

http://www.quattroforum.com/index.ph...14/#post-82528

You can usually find the Merc part on Ebay.

ChrisG · 25-08-15, 06:28 PM

I had a similar problem many years ago ( 1991 iirc) and after checking the freeness of movement I checked all the vacuum lines.
Under the bonnet the lines were breaking up ( a fairly rigid plastic) , probably age - although at that time only about five years old, and aided no doubt by heat from the engine.
The rest of the lines were OK.
Replaced with some tough piping and everything was fine !

Colin Aitchison · 03-12-15, 07:10 AM

Hi Gaz,
The only problem with any of my diff locks was that they came on by there self when the engine was revved on load over 3000rpm, there is a one way black and white valve on the vacuum pipe work behind the diff lock knob in the center consul, when this one way valve lets the vacuum pass both ways it draws the diff lock on, this was on a 1984 WR UR quattro.

10-05-15, 01:12 PM	#2
yeti Senior Member Classic Audi Club Member Join Date: Oct 2009 Location: Derby Posts: 1,166	My limited understanding of it is that the lock is applied by the presence of a vacuum, and disengaged by the vac being released, so that would discount a vac leak for me, the lock wouldn't engage. Im guessing its just gunked up. __________________ 1990 Coupe Quattro (WTF did I buy that for?) Half a 1983 Coupe FI (long term parts shelf) 2003 Cors-arrrgh More Polos that you'd ever believe No time, no money, all the wrong tools and a workshop that's too far away

11-05-15, 05:27 AM	#4
Hanuman Trickster Classic Audi Club Member Join Date: May 2009 Location: Heckling from the cheap seats, Phnom Penh, KoW Posts: 7,016	The rear diff lock arm is also prone to a lot of corrosion due to build up of crud and road grime. They can get caked. Rusty actuator rods that get so thin they snap, and the pin and yoke that link the actuator to the arm, impinging the movement. Another frequent problem is the actuators themselves, which are suffering perished diaphragms internally, so they are not so flexible, and the plastic body of the actuator getting brittle, so that the hose connections crack, resulting in partial loss of vacuum and eventually total failure. As mentioned, they receive vacuum to either chamber depending on whether being locked or unlocked. They are easily damaged anyway. A Mercedes part has been found to be almost an exact match, with some small modification. I find that both my diff-locks occasionally stick; Either not engaging immediately, but they do engage as soon as the car moves, and likewise disengaging. In the worst case, going into reverse and moving backwards a short distance is enough to disengage. A lot of cars experience that. __________________ I wish they would keep the damned Chinese away now that I can go home, so that I can enjoy Fish amok and a draught Angkor Last edited by Hanuman; 11-05-15 at 05:34 AM.

12-05-15, 12:20 PM	#5
M4rkp Grown up member Classic Audi Club Member Join Date: Feb 2013 Location: Sometimes Roberttown - sometimes Surrey Posts: 194	As The Real MSH says - it needs vacuum to engage AND disengage. You need to check the pipes from the diff lock control to the actuator ( yellow and blue IIRC) with a MityVac - Its more likely to be the actuator itself, probably the sealing ring around the actuator shaft that is leaking. In some cases the retaining screws swell up with rust and crack the casing. Its easy enough to take off - they are NLA from Audi but there is a Mercedes part that works. I had the same issues last year - have a look at the link below. http://www.quattroforum.com/index.ph...ctuator.24641/ Mark Last edited by M4rkp; 12-05-15 at 12:24 PM.

11-05-15, 03:16 AM	#3
msh 4 ring whore! Join Date: Jun 2008 Location: Rucava, Latvija Posts: 3,816	Not really. Actuator has two sides, and lock is engaged or disengaged depending on which side has been connected to vacuum system. In this case it sound like the side, which disengages is not receiving any vacuum.

25-08-15, 05:05 PM	#6
kat1127 Grown up member Join Date: Dec 2009 Posts: 24	Hi, any more info on the merc part, the link doesn't seem to work t.i.a

25-08-15, 05:21 PM	#7
jimh Grown up member Classic Audi Club Member Join Date: Dec 2008 Posts: 122	http://www.quattroforum.com/index.ph...14/#post-82528 You can usually find the Merc part on Ebay.

25-08-15, 06:28 PM	#8
ChrisG Senior Member Classic Audi Club Member Join Date: Dec 2007 Location: Bracknell, Berks. Posts: 1,465	I had a similar problem many years ago ( 1991 iirc) and after checking the freeness of movement I checked all the vacuum lines. Under the bonnet the lines were breaking up ( a fairly rigid plastic) , probably age - although at that time only about five years old, and aided no doubt by heat from the engine. The rest of the lines were OK. Replaced with some tough piping and everything was fine !

03-12-15, 07:10 AM	#9
Colin Aitchison Senior Member Classic Audi Club Member Join Date: Dec 2013 Location: Back home in Tillicoultry Posts: 1,315	Hi Gaz, The only problem with any of my diff locks was that they came on by there self when the engine was revved on load over 3000rpm, there is a one way black and white valve on the vacuum pipe work behind the diff lock knob in the center consul, when this one way valve lets the vacuum pass both ways it draws the diff lock on, this was on a 1984 WR UR quattro.