Vacuum Master Switch (LF Door) - PeachParts Mercedes-Benz Forum

BrierS · #1 09-09-2009, 08:03 AM

1984 300D T

Is the vacuum master switch/valve in the driver's door supposed to have leakby from the unit itself when all lines are connected (or 2 capped with the Miti-Vac on the third)?

After having read what seemed to be 100+ postings related to these door lock and inability to shut the engine off, I still did not find the answer to the above. I suspect mine is defunct yet I hate to replace ($16+/-) it if it still works correctly.

It leaks from both ends with a few exceptions...moving the plunger slowly in or out I can find a couple of spots where it will hold vacuum. However, installed it leaks like a sucking sieve, and of course the shutoff switch will not function properly (engine remains running). If I remove the key from the shutoff switch, place it in the driver's door and actuate the lock, the engine will slowly stop. That's what lead me to the master switch/valve.

If if missed an explicit posting, please let me know.
Best,
Steve

nhdoc · #2 09-09-2009, 08:13 AM

The valve does bleed down the vacuum from the circuit which is "shutoff" - in other words if you lock the car, vacuum is applied to the locking actuators and bled from the unlock side. If it did not then the diaphragms would be fighting each other. That's also why you hear the "whoosh" noise from the valve when you lock and unlock the car.

When I test a vacuum lock system the first thing I do is check each circuit to make sure it holds vacuum. Just attach your mity-vac to the lock side and pump it up until the doors lock and watch the gauge. It should hold perfectly. If it leaks down then you know you have a leak somewhere on that side. If it doesn't then do the unlock side. If both sides hold vacuum without the valve in place then hook the mity-vac to the valve in place of the vacuum source and do the test again with the lock and unlock lines attached to the valve. If it leaks down now you know the valve is faulty. The older, metal ones can be rebuilt with new o-rings but the plastic ones usually can't.

BrierS · #3 09-09-2009, 08:30 AM

Thank you. On my way outside to recheck what I suspect (you also listed) as the culprit. Bad valve since all prior tests showed the actuators were working.
Steve

tangofox007 · #4 09-09-2009, 10:11 AM

Quote:

Originally Posted by BrierS

Bad valve since all prior tests showed the actuators were working.

A leaking actuator can still work well. You need to determine if each side of each actuator holds vacuum.

nhdoc · #5 09-09-2009, 12:49 PM

Yes, tango is correct. You can have leaking actuators that will work, the key test is to isolate each circuit and do a "leakdown" on them to know if they hold vacuum or not. If they leak enough then it can cause the engine shutoff valve to stop working, especially if it is leaky too.