From reading other posts, I am getting the feeling that the cruise operates off of the vacuum just like the locks. I am new to Mercedes but have experience working on other cars. My locks only work now if the car has been running for at least 20 min. Cruise will engage but not hold speed. Any "first" places to check to remedy these issues in my 1980 300TD? Thanks in advance...

Check the check valve and vacuum lines that are in between the vacuum pump and the firewall. The vacuum lines and/or the valve might be brittle and cracked, therefore not holding enough vacuum for the central locking system to operate.

hlamar,

The leak could be anywhere, but the two most common, and easiest places to check are; the four way connector next to the oil filter. It is in the way when changing the oil filter and commonly gets jarred loose, and the tee from the big line that goes to the brake booster, this is the main vac source.

If you had a serious leak, you're engine wouldn't shut off, so you need to run down the accessories you've already to discovered to be nonoperational. If you don't find something simple, get a vacuum schematic and a vacuum test pump from the Auto Parts store.

Good luck,

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Larry Bible
'84 Euro 240D, 523K miles
'88 300E 5 Speed
'81 300D Daughter's Car
Over 800,000 miles in
Mercedes automobiles

Thanks for your reply. Unfortunately, you said, "if you had a serious leak, you're engine wouldn't shut off". Well, on two occasions now, the engine would not shut off (happens very randomly - shuts off fine 99% of the time). Does that suggest another place to start my troubleshooting?

Take Larry's advice on checking the lines that pass over the oil filter housing. This happened to me when I was changing my oil once. A line came loose and I could not find where it connected to. After I changed the oil and drove the car, the engine shut off. However, a few hours later I went to a party and I couldn't turn the car off without pressing the manual stop lever. Also, the locks would not work. The next morning, I looked again to find where the mystery line came from, and found that it goes to the injection pump. I plugged it back in and the vacuum system was back to normal. I think the color of the line was blue with a brown stripe, or vice versa.

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'82 300D Turbo 215k miles

If the above suggestions haven't solved the problem the next places to look are the door locks.

With the engine running, test each door lock, if any of the locks are easy to move up and down, then it is likely that the vacuum control for that door has a leak and needs to be replaced. This test on the driver's door may not work.
repeat above test, soon after stopping the engine

Michael