Two approaches to getting your differential lock working
The differential lock is prone to failures due to two primary causes: (1) a leak in the vacuum lines, and/or (2) stuck components from non-use or dirt contamination.
How to Check Each Component of the Locking System:
(1) Crawl under and start from the 3-balled vacuum reservoir. With the engine running pull off the line and listen for the tank to hiss loudly. If it doesn’t the check valve is right there by the tank, pull it off, blast it clean with spray lube like WD-40. The Valve is all plastic and will revive if dirty, which often happens. Once done try the tank again.
(2) If no luck, trace back to engine and find the leak, likely a bit of rubber hose. The tank filling point is a steel tube, they may rust shut, have a go at that. Once satisfied that the tank is getting air,
(3) go to the dash and pull the black pipe off the switch and listen for hiss. If nothing, then that line is broken.
If good, (4) then reconnect and pull off the blue or white pipe and play with the switch to get vacuum at each line. Valve body is plastic and may have cracked, epoxy/replace if needed.
(5) Final step is to crawl under now and pull line off the servo by the trans and listen for hiss at each line, if good, continue, if not broken pipe(s)
(6) Remove two nuts from servo bracket to allow the servo box to float free. Keeping it STRAIGHT! get some one to work the dash valve. Box should move in/out when active. If not dead servo, replace (can be split open to check diaphragm, maybe goop-able)
(7) If servo is good, but diff lock wouldn’t move then remove the servo and its bracket. You need to drive out a 3mm drift pin from the actuator shaft first DON’T bend the servo rod!!!!! Slip a small nail into the 1/2″ diam actuator shaft on the diff and go to it, penetrating oil, bristle brush/emery cloth etc. and get it moving again.. This is the most likely failure point, gummed up from lack of use, so you may wish to start here BTW.
(8) Once the shaft is free about 1/6th of a turn of a tire will let the lock engage, about 1″ travel inwards of the shaft. If it is really freed up properly the tiny return spring inside the diff will push the lock out to ‘off’ by itself. Very light force to move shaft is all. At same time, see if the dash light came on when shaft pushed in, if not unplug the connector and bridge the terminals. After that check it as needed, maybe a dead LED. There is an O-ring around an aluminium sleeve (that is around the shaft). Sleeve will move out when you tug to expose ring, replace that via dealer part, cheap.
Once fixed cycle the lock about once a week regardless of need, it likes to be used. HTH, bye, Tim
Another Take on Repairing the Servo Unit
From Don McCallum e-mail to the Vanagon List
1. Remove the servo unit. This involves first of all knocking out a 3mm roll pin which locks the servo shaft into the diff lock shaft (you may have a rubber boot which you will have to pull back to do this). Then remove the servo unit from its bracket (2 x M5 screws). Care to pull off
straight or you can snap the end of the servo shaft.
2. Check the operation of the servo by operating the dash control. The easiest way to check the servo for leaks is to take it off and suck on the two vacuum pipes!
3. Check the mechanical operation of the diff lock by jacking up both wheels on one side – try to push in the diff lock shaft while turning the wheels. It should go in within about 1/2 turn of the wheel – you will know it is locked because the wheels will no longer turn!
4. Remove the servo mounting bracket, one circlip and 2 x M6 bolts. If the shaft is seized, remove the indicator switch (it just screws out). You will see that the diff lock shaft runs in a steel bush and the plunger on the indicator switch bears on the shaft through a hole in this bush. Removing this switch allows the complete bush/shaft assembly to move in and out and you should be able to get the diff to lock by pushing it in.
5. Free the shaft in the bush by gripping the bush with a pair of vice-grips and turning it backwards and forwards about 1/4 turn while squirting releasing fluid into the indicator switch hole. Push and pull on the shaft while doing this (use an allen key though the 3mm hole in the shaft to get a grip on it). Once it starts to move, things will get easy quickly!
6. Chances are you will damage a 19.7mm x 1.7mm O-ring near the end of the bush on its outer diameter. This can be replaced by pulling out the bush until you can see the groove and slipping the new O-ring into place.
7. Apply lots of grease and re-assemble. Don’t forget to line up the hole in the bush so the indicator switch plunger fits back in. If you can find a rubber boot to put over the shaft, so much the better. Its also a good idea to operate the lock on a regular basis to keep it free. Best of luck – you are lucky, the front one is even harder!
Don McCallum (1992 1.6 Turbo Diesel Syncro, North Wales, UK)