Cruise control not holding speed - related to door locks vacuum leak??? - PeachParts Mercedes-Benz Forum

rgnprof · #1 04-10-2009, 12:28 PM

I have isolated a leak in my '78 300D central locking system - the 3 way master vacuum valve in the driver's door is leaking. Correspondingly, I have noticed for the past several months (about the same amount of time I have been having problems with the door locks) that the cruise control will not maintain the set speed (new cruise amp installed within the last year, maybe 18 months??). Most noticeable whenever the cruise has to make adjustments - while going up or down a hill. It doesn't just shut off - it just slows down - and depressing the accelerator pedal usually gets it back to the set speed.

Could these problems be related? Bad cruise amp, or vacuum supply problem?

ryan

#2 04-10-2009, 06:46 PM

I would also like the answer to this cruise control issue, mine wont maintain a speed more accurate than + / - 10 mph. 83 240d auto.
Sometimes it will just keep revving, if I let it, it will climb from 50 mph, to 75, on its own. Is this a bad sensor in the speedo? Or a bad amp?

2.5Turbo · #3 04-10-2009, 07:52 PM

Quote:

Originally Posted by 83240D

I would also like the answer to this cruise control issue, mine wont maintain a speed more accurate than + / - 10 mph. 83 240d auto.
Sometimes it will just keep revving, if I let it, it will climb from 50 mph, to 75, on its own. Is this a bad sensor in the speedo? Or a bad amp?

I had the same problem on my '90 300D. This happened on a new remanufactured amp; as it turns out it was bad and I had it replaced under warranty. Dropped in the new one and it's dead-on accurate.

On the OP's problem, I think the 116 SD's have a vacuum actuator for the cruise. Since it's all off the same vacuum system, I would think a leak somewhere else would cause problems. Teeing a vacuum gauge (visible while driving) into the actuator line should ease your troubleshooting. I've never worked on one of these vacuum cruise systems, therefore this may be a total load of BS, so YMMV....

rgnprof · #4 04-10-2009, 08:02 PM

The '78 300D is a W123 with a vacuum controlled actuator. That vacuum canister holds vacuum just fine. I think I will try to hook up a gauge to see what is happening while driving...

I just am not sure if these vacuum leaks always other vacuum controlled accessories...

rgnprof · #5 04-15-2009, 03:31 PM

OK, I just replaced the master vacuum switch in the driver's door and so far so good.

Can someone confirm - or disconfirm - my thinking on my cruise issue. The cruise and door locks are hooked up to the same 3-way check valve in the engine compartment - but the yellow hose that runs back to the vacuum switch in the driver's door has another check valve installed inside the cab, up under the dash. I pulled it out and checked it today and it works fine - so...I've concluded that my cruise problem is unrelated to my doors leaking. This second check valve has to be preventing any leaking from the locks from affecting the cruise. Right?

ryan