unexplainable acceleration - PeachParts Mercedes-Benz Forum

oso · #1 06-02-2010, 03:29 PM

Weird thing started to happen from time to time: my car starts to accelerate even though I am not even touching the gas pedal! Yes, I know.... This is for real and last night I just started the car, still in "P", and the engine continued to rev up and up until I shut it off with the ignition switch. I am going to take a look at the linkeage and spray some Blaster but would like to hear from anyone that may have had similar thing to happen. Something ain't right....

4thesporty · #2 06-02-2010, 03:31 PM

toyota?

tangofox007 · #3 06-02-2010, 03:43 PM

Quote:

Originally Posted by 4thesporty

toyota?

Place your bet on a '87 300TD.

compu_85 · #4 06-02-2010, 04:29 PM

Check your oil level.

-Jason

TX76513 · #5 06-02-2010, 04:37 PM

Had a cruise control go bezerk on me once.

oso · #6 06-02-2010, 05:02 PM

Quote:

Originally Posted by TX76513

Had a cruise control go bezerk on me once.

Come to think about it- last year I "accidentaly" mutilated the stem of the cruise control at the time I did the re-install of the dashboard (would have been much too smart for me to check the cause of the resistance....). I hardly ever use it so I did not even try to see if it worked. I will check on it today. Thank for the hint.

bustedbenz · #7 06-02-2010, 05:33 PM

Not likely to be the cruise control if you were in P. The cruise has an interlock that doesn't even allow the system to function unless you are above a certain ground speed. 15-20 mph maybe.

If this is a 602/603, then it probably has the ELR device to electronically control the idle speed. This could be flaking out. If it's a 616/617 then there's nothing but mechanical linkages.

oso · #8 06-02-2010, 05:35 PM

Quote:

Originally Posted by bustedbenz

Not likely to be the cruise control if you were in P. The cruise has an interlock that doesn't even allow the system to function unless you are above a certain ground speed. 15-20 mph maybe.

If this is a 602/603, then it probably has the ELR device to electronically control the idle speed. This could be flaking out. If it's a 616/617 then there's nothing but mechanical linkages.

OK than... It is a 603 so where do I look?

Alastair · #9 06-02-2010, 05:42 PM

Is this accompnied by large quantities of smoke!!??

bustedbenz · #10 06-02-2010, 05:43 PM

Up near the firewall on the drivers (left) side in the engine compartment, there's a black rotary dial. It has 7 numbers marked on it, 1 through 7. Unless it has been messed with, it should be set to "4".

You can't just turn this dial. You'll break it. The way to adjust it is to lift straight up on the knob, it comes up a fraction of an inch. Then you rotate it to the new number position, and then press it back down into its socket. When you press it down, the rpms will adjust. Each number represents an rpm change of about 20 rpms. Nothing major.

Just play with the thing. If it's working properly (set it to 1. listen to the engine rpm drop. Set it to 7. Listen to it rise to a fast idle. Set it to 4 and forget about it) then it's *probably* working. I don't really understand how it works, I just know what it's supposed to do under normal conditions. It'd be a good test, anyway.

bustedbenz · #11 06-02-2010, 05:45 PM

The dial is just aft of the fuse box if this helps you find it. At least it is on my 300SDL. I can't imagine where it is on a 300D but it's probably somewhere close up there.

Also, this device is only ever supposed to be able to raise it from about 500-700 or somewhere roughly in that range. It's unlikely that it would be causing such acceleration as you're describing, since that kind of acceleration is not within its range. It would have to be a *really* weird failure to cause it. However -- it's worth checking just to rule something out.

oso · #12 06-02-2010, 05:52 PM

Quote:

Originally Posted by Alastair

Is this accompnied by large quantities of smoke!!??

No. I almost wish it were the cruise control since I have to replace the stem/base anyway... And it is not that it wanted to idle higher- it wanted to bust out of the engine bay!

scottmcphee · #13 06-02-2010, 06:07 PM

My 87 300D will occasionally stumble up to a higher RPM at a stop light, like a cough. It's so brief and intermittent that I can even tell you how many RPM it goes but I guess a few hundred RPM for a quarter of a second. Kind of in the same league as your problem but maybe less severe.

I am thinking a faulty cruise control, because, also lately, the cruise will not hold a speed for long. It just releases, as if you touched the brakes.

The ELR glitch idea is interesting. I could disconnect the ELR plug at the IP and see if the "stumble up" ever happens a gain. Interestingly, ELR is only remaining function that is connected in the car still being driven by the engine management computer. If I disconnect ELR, and settle on the mechanical 550 RPM or whatever, I can yank the computer, and put it on the bench alongside all the emissions pods it used to monitor.

But I need to fix my cruise too. It's just not a high priority item. My first take to fix cruise will be to swap out the cruise amp for a spare.

scottmcphee · #14 06-02-2010, 06:09 PM

Quote:

Originally Posted by bustedbenz

Not likely to be the cruise control if you were in P. The cruise has an interlock that doesn't even allow the system to function unless you are above a certain ground speed. 15-20 mph maybe.

Such interlocks work only if the device is good. Start opening or shorting random points with in the cruise amp, and it's anybody's game.

Alastair · #15 06-02-2010, 06:12 PM

Hmm, No smoke, oil-level correct and not being used up, not just a very fast idle...

Can only think of the throttle-linkage to the pump, the cruise-control actuator or controller malfunction or the IP itself....

You can remove the linkage from the cruise actuator to rule out that part causing it....