A friend has a 1988 420SEL and apparently when the driver door lock is actuated by key or manual lock the air pump stays running for about 30 seconds or so causing the locks to stay shut and it's extremely difficult to pull open the driver side door until the pump shuts off due to fighting the vacuum. This problem is also duplicated when the door is locked from the passenger side and or by key on the passenger side.

Is there a specific switch that times the vacuum pump in the system?

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1974 240D 3.0L 4(M) - 250K -OM617.910 + 300D FW + Rajay turbo power!
1977 6.9 3(A) - 84k

if there is a leak, either pressure or vacuum at any of the elements, the pump will run excessively, there may be a leaking actuator somewhere

That is a possibility. Each lock holds down firmly if that in fact makes a difference.

So the solution is to check for leaks at each of the doors, trunk and gas cap?

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1974 240D 3.0L 4(M) - 250K -OM617.910 + 300D FW + Rajay turbo power!
1977 6.9 3(A) - 84k

yes,each should hold pressure and vacuum

I've heard of wear in the internal switches causing the pump to run longer than it should.

Sixto

Pump is located next to the spare tire, and if it has a leak in the locking system the pump is timed. The pump will run for app. 45 sec. Next to the passenger seat under the matt is a single yellow vacuum line, see where it branches off and test with vacuum and pressure with a vacuum gauge, find the one thats leaking nd replace, or you can cap it off if you would rather have someone who wont break the door pannel getting it off do it. The left side is on the drivers side same place.

the pump has pressure and vaccum valves inside, it turns off then the threshold vacuum or pressure is reached, it that value is not reached the pump will run on. if this isn't fixed soon the pump will burn up. you can verify this on a woking system by disconnecting the line and connecting a guage, the pump if working properly will turn off almost immediately. if you do not have a gauge, use your finger while actuating the lock at the trunk, or at either front door with an assistant

So I should:

1. get into the trunk and test the vacuum to see if it shuts off immediately

if it shuts off then:

test each of the vacuum elements in the doors?

Can they be tested with one of those hand pump vacuum testers or will more draw be needed to register? I presume it will either hold or leak since there is only one vac line for pressure or vacuum?

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1974 240D 3.0L 4(M) - 250K -OM617.910 + 300D FW + Rajay turbo power!
1977 6.9 3(A) - 84k

It takes a bit of pumping to actuate the pods from the trunk but you can do it with a hand pump.

Sixto

Will using the hand pump be a worthwhile test at the main trunk connection? (presumably to see if there is a leak in the system at all?)

or should I skip that and test each vacuum element individually?

__________________
1974 240D 3.0L 4(M) - 250K -OM617.910 + 300D FW + Rajay turbo power!
1977 6.9 3(A) - 84k

Within the trunk you can test the trunk lock and fuel filler door actuators. If the door lock actuators are fine at the main junction then there's no point in checking individual actuators. Seems worthwhile to me.

Sixto

i have a hand pump that makes pressure and vacuum, i cannot recall where i bought it from, but that is what i use at work when testing this sytem. usually if the pods are good, it goes pretty quick either way, so i would unlock all doors and pull vacuum with a hand pump, if they all lock , the pump is likely your culprit. i have a few extra pumps, if you are interested in trying out one, i will install it in my 500 and verify it is good,. and can send it in the mail

The MityVac I have has a rubber vent cap just in front of the gauge. Pull the vent cap to expose the port through which air is expelled when pulling a vacuum. I've used it to test a turbo boost gauge. IIRC it's good to 12-14 psi.

Sixto