I have a 1985 300D and I'm having problems with the vaccum system. The engine doesn't shut off when the key is turned off, the door locks don't work properly and the the dash light that indicats a brake problem is illuminated intermittently, presumably due to the vacuum asstist to the brakes. I did some initial trouble shooting a couple of weeks ago and found the vacuum modulator wasn't holding vacuum. I replaced the modulator but it didn't fix the problem. At least the engine will shut off now although it idles for about ten seconds after the key is turned off. Dash light still illuminates intermittently and the door locks don't work. Is there a good source for trouble shooting the vaccum system, with diagrams, step by step testing etc.? Does anyone have a suggestion on how to proceed?

There should be two brake indicator lights. Neither has anythig to do with vacuum.

Can you describe the brake light that is illuminating?

It's the light that the owner manual shows as the brake pad wear indicator. I forgot to mention in my original post that I put new pads and rotors front and rear about 10 months ago. so I can't imagine that the pads are worn out already.

Actually I rechecked which light is illumitnated and it's the brake warning light, not the brake pad indicator.

As Tango says, neither light knows anything about the vacuum system. You may have a brake problem in addition to the vacuum leak.

There are lots of threads in this forum on vacuum system diagnosis. Check the DIY section and the various wiki articles under "Resources" at the top of this page. Buy yourself a MightyVac, which is a hand-operated vacuum pump/gauge. It allows you to test the vacuum system and see where the leaks are.

The basic rules are (1) on an old car like yours, all of the rubber connectors are suspect; consider replacing them en masse, (2) break the system into smaller pieces and test one section at a time (a MightyVac kit will come with various caps and plugs that are handy).

BTW, I advise buying a MightyVac that is made of metal (sturdier than the plastic versions) and spend a few more bucks to get one that can do pressure as well as vacuum. The one I bought is a model 04050. Just my opinion, others may disagree and YMMV as always.

Jeremy

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That's an indication of low brake fluid.

Are your door locks hooked up at the vac line under hood near the firewall? There is a yellow check valve, if it's plugged in, unplug it and put a stopper in the open line.

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83 SD

84 CD

Okay folks, things are looking up (I think). I borrowed a portable vacuum pump from a mechanic friend and by-passed the vacuum pump on the car. When connected to the portable pump everything functioned as it should, engine shut off immediately after turning the key off, door locks worked, life is good. The brake light on the dash even went out. The system had about 22 Hg under the portable pump. So..........looks like the vacuum pump on the car is the guilty party, but...........when I reconnected the vacuum pump on the car back to the system it's providing about 20Hg and the system continues to function as it should.

My theory on this mystery is that when I had my oil changed several weeks ago the guys at the shop disconnected some of the vacuum lines to remove the oil filter and some vaccum in the system was lost. Combine that with a vaccum modulator that was bleeding off vacuum and the system just didn't have enough vacuum to function properly. Connecting it to the portable vacuum pump seemed to be the key.

That's not a very sound theory. Any lost "vacuum" should have been restored within seconds of the vacuum pump coming on line.

It's a valid theory if the shop disconnected the line for the engine stop. It's also possible that they disconnected a line to the cabin for the door locks. However, the use of the portable vacuum pump won't magically reconnect those lines. If the lines are open with the engine driven pump, the same symptoms will be observed with the portable vacuum pump.