Master door lock vacuum switch test
 

I have a vacuum leak (on the lock circuit) that I think I have traced to the master door lock vacuum switch in the driver's door. No leaks downstream connector along floor at driver's door. Leaked upstream so I removed driver's door panel and disconnected the lock circuit hard line (red stripe) thinking I had a crack in the hard line between door and body (rubber tube is shot) but it held vacuum. When I tested it reconnected to master switch I could hear air escaping and assume that means switch is leaking and needs replacing.

Then I read in other threads where it was that the switch vented and hearing a hissing noise is normal. So now I am confused.

Is there a different procedure for checking the switch?

Thanks

Yep, plug your Mityvac into one port and check the switch response, then do the others.

test results
 

Thanks for the suggestion.
So I have now tested each of the three inlets to the driver's door unit. I blocked the other two inlets off when testing.

Test results:
Lock line inlet with plunger in = no vacuum
Lock line inlet with plunger out = holds vacuum

Vacuum source with plunger in = holds vacuum
Vacuum source with plunger out = holds vacuum

Unlock line with plunger in = holds vacuum
Unlock line with plunger out = no vacuum

From this test it seems to me that the unit is working correctly (but of course my logic may be flawed). If so, then where's the leak?

Here is a good document on Vacuum door locks.

Be aware that you have a check valve under the dash "if" someone has attempted to pull the yellow lines further out from the firewall they will disconnect it.

Also my euro had a cracked in the main supply line to the master switch. My 78 had a rubber coupler in the door jam to the master switch...on the 78 when the door was open it held vacuum...closed it would not...the line was coming undone from the coupler.

https://www.ifixit.com/Guide/Mercedes+W123+Vacuum+Lock+System+Diagnosis+Technique/20201

Thanks
 

Thanks cooljay I will check that check valve -- I was messing around under there recently so I may have done something to the connection.

Hmm... a major help would be telling us the year and model you're working with. If it's a W126 series, you may be looking in all the wrong places vs. a W123 series or W124 etc....

Solved
 

Problem solved loose fitting 4-way connector at check valve before firewall. Car is a 83 300TD, sorry I thought that automatically came up when I posted.

I borrowed back my FSM and found test procedure for the master switch:
block off locking and unlocking line connections and then connect tester to middle, suction line. If it holds vacuum unit is ok -- which mine did.

Next I tested suction line from after check valve to driver's door (line plugged) -- held vacuum so the hard to get at check valve under the dash was OK. I reconnected lines to master switch and was able to actuate locks by applying vacuum with tester and moving the plunger on switch in and out on both sides of the check valve.

So I figured problem must be before check valve. Tested connection at brake booster line -- plenty of vacuum. Tested for vacuum at the 4-way connector -- very weak vacuum. Since I didn't have a spare I was able to get vacuum back through check valve and get locks to work.

Now, I need to order some connectors. I noticed that the car in the ifixit tutorial that cooljay mentioned has two yellow check-valves before the firewall. I only have one with two outlets but am wondering if there should be another one for the ignition vacuum lines (brown and green on my car)?

Thanks for your help everyone.

Jim

The car in the guide is an early w123 and has two check valves one that works the locks and one that works the climate control. The cruise control also runs off one. I cannot be positive if later w123's also came with one for the climate control. I would see if someone else chimes in.