W124 92 300D Ignition Vacuum switch - PeachParts Mercedes-Benz Forum

pdrayton · #1 12-27-2008, 03:10 PM

My ignition vacuum switch over valve seems to be a problem. The supply and return line to the ignition switch are fine, so is the shutoff valve (tested with mityvac.

So i'm thinking it must be the ignition vacuum switch. But I'm a little concerned it could be something to do with the new ignition switch I put in. It works fine except that it doesn't spring back from START like it should. You have to manually move it back to RUN after the starter has started the engine.

Is there anyway that could be the problem? Another reason I'm doubtful is with the ignition off, the switch doesn't hold any vacuum. With the ignition in RUN, it holds vacuum. Does that sound like a broken vacuum switch? I'm not sure what it is supposed to do.

What section is the ignition in the FSM? I've looked and looked, didn't find it anywhere.

Thanks,

Paul

sixto · #2 12-27-2008, 05:11 PM

Sounds like you attached the vacuum switched backwards or otherwise incorrectly.

Does the key switch behave the same way when the vacuum switch isn't attached?

When you say the vacuum switch holds or doesn't hold vacuum, how do you have the vacuum gauge hooked up? The supply side should always hold a vacuum; at the switch in run or start (99% sure), at the actuator in off or accessory (100% sure). The actuator side will never hold a vacuum in the way you'd test; vented at the swtich in run or start (100% sure), vented at the non-running engine vacuum pump in off or accessory (50% sure, not sure how check valves come into play).

Sixto
87 300D

Sixto
87 300D

pdrayton · #3 12-27-2008, 08:00 PM

Yah, I tried connecting the vacuum lines in the opposite way, and it still did the same thing.

I've bypassed the ignition vacuum switch and attached a 3/2 valve I had from a W123 spare. I push it to let vacuum through to the shut-off valve. Possibly my most wonky-ass quick fix ever!

Without the vacuum attached the switch behaves the same - ie doesn't spring back.

Testing the vacuum on the switch was done by connecting the mityvac directly to the vacuum switch on the ignition switch. But then it could be something I don't know about.

To clarify. I'm thinking I must test the valve in OFF. If I put vacuum to the ignition vacuum valve in OFF, it should pass through to the IP shut off valve right? If it doesn't, and all the lines to and from the ignition switch are holding vacuum, then it must be the ignition vacuum switch. Right?

BTW, the boost actuated turbo is working great - peaks at 15 PSI and no worries!

Paul

The thing is the ignition switch was working fine for a bit and then quit. So I'm thinking it must be it

pdrayton · #4 12-29-2008, 08:21 AM

I tested the vacuum on my other 92 300D (the swimming car), and in OFF, it lets vacuum straight through to the shut off valve.

The car with the problem dumps the vacuum. So I've ordered the valve from Brumos in Jacksonville for $29. Doesn't seem to be available at aftermarket places.

The FSM says you don't have to remove the instrument cluster to get the ignition switch out, unlike the post at anyone replace vacuum switch-off valve on ignition switch?
says.

We'll see.

LUVMBDiesels · #5 12-29-2008, 09:58 AM

One thing to look out for (at least on the W126) is that the vacuum switch needs to be shimmed when it is attached to the key switch. If it is too tight to the key switch body it becomes very hard to turn the key and could prevent the key from going over to the 'start' position. I found that out when I replaced the switch on my 300SD and looked out for it when I did the 300SDL. Since your key is not springing back from the 'start' position I believe that youhave the vacuum switch on too tight. The vacuum switch is worked by a cam on the key switch. Maybe they way it is installed the vacuum switch is being kept open. It is only a reed valve

pdrayton · #6 01-06-2009, 07:46 AM

I pulled the ignition and replaced the vacuum pod and put it all back only to find everything was working backwards - OFF was on, and RUN was off.

Finally figured out the ignition switch itself was 180 degrees to the where it should have been in regard to where the spindle on the tumbler was. Not sure how it got there - this ignition is all messed up (the car was stolen and I think they put a hacksaw and gawd know's what else to the ignition).

Anyway, put it all back the right way, and it works 100% now. Even springs back from START to RUN. So it's all good in ignition land... Thanks for the tips guys.

More info on W124 ignition switch wiring here
W124 Ignition wiring diagram needed urgently!