Scary Stuck Throttle 1993 W124 300D 2.5 Turbo - PeachParts Mercedes-Benz Forum

Dinoman · #1 03-24-2017, 09:56 PM

Symptoms .. When cruise control is engaged, (for example set at 55), and then a hill is encountered, the throttle accelerates (as it should), but, doesn't stop accelerating (as it shouldn't). So, the speed quickly picks up to 70-80-90 etc. and I'm standing on the brakes and trying to steer off the road. I come to a stop and pop the car into Park and the RPM's are now off the Tach (over 6000).
I shut off the ignition and pop the hood up. I find that the throttle cable is stuck
squeezed on a square piece of plastic washer type of fitting. I 'think' the fitting is broken from off from another plastic rounded fitting on the cable itself? (see photo's .. of where I 'think' the two plastic pieces should fit together). Can anyone check their own W124 300D 2.5 Turbo or any good Mercedes reference to see if that is the cause (broken plastic do-hickey) or if something else needs to be fixed for this not to occur. It's scary as hell, when it suddenly happens. I now avoid using the cruise control, but, I really need to get this fixed, as I depend on the cruise control on long trips. P1-Throttle at rest. P2-Full Power. P3-Cable stuck on Square washer. P4-where I think the plastic parts should be connecting? Thank you. Richard

vstech · #2 03-24-2017, 10:09 PM

I doubt it is in the cable.
More likely, there is a part stuck in the linkage for the cruise actuator.

ROLLGUY · #3 03-24-2017, 10:10 PM

DISCONNECT THE CRUISE CONTROL ROD NOW!!!!! I had the exact same thing happen to me on a '91 300D 2.5. The cruise control servo is the problem. I replied to another thread about a fast idle condition, and I suspected the servo was the problem there as well. It took many posts and suggestions to look elsewhere before the guy finding out it was the servo all along. Just disconnect the control rod first. It is easy to do, and it will most likely save you from getting in an accident!

HarvAMG · #4 03-24-2017, 11:50 PM

Yeah, like RollGuy said, take off the cruise control rod while you get this sorted.

Also why wouldn't you put it in neutral if the car was running away at 90mph? Bouncing it off the redline is far better than flying into something at that speed.

97 SL320 · #5 03-25-2017, 07:02 AM

Turning the key back one position will shut off the engine, practice this while stationary so you are aware of position. There are interlocks that prevent the steering from locking unless the shifter is in park, as long as the system is working properly.

And, panic leads to bad results.

gsxr · #6 03-25-2017, 12:50 PM

Triple-check that all the throttle linkages are moving smoothly - there are a lot of lubrication points, both the ball/sockets and also the cam roller/slot, IIRC. Any hint of stickiness and you need to fix that first. It is possible the cruise actuator (or control module under the dash) is faulty but those will be more difficult to diagnose without swapping them out.

BTW, it's unlikely the engine RPM's actually went over 6000 in Park, unless your IP governor is incorrectly adjusted, or you modified the high idle (max RPM) setting. The factory spec is 5150rpm ±150 or ±250 depending on if you are reading the old printed specs or the newer WIS specs. If you saw over 6000 on the tach, I'd suspect the tach is not accurate... would need to use an optical tachometer on the balancer (or other external tach) to verify.

Dinoman · #7 03-26-2017, 08:52 PM

Thank you all for your quick replies. The reason that the speed got away from me, is that this happens going uphill, and the RPM's start getting away gradually at first and then full throttle. Then when I notice it, I mash on the brakes, to stop the cruise control. Since the accelerator cable is stuck at this point, I have to ride the brakes while turning off the road. I didn't think about turning the key off one position. I guess I could have dropped the car into Neutral, but, I didn't know what was going on. I was worried that the diesel engine was in some sort of run-away situation. I was glad it shut off when I put it into park and turned the ignition off. It is scary being out of control and not knowing what's going on. I had to ride the brakes for a little while, till a good spot came up to turn off. I ordered a new throttle cable guide as it looks like my current one is broken. I'm going to replace it and then see if I can manually get the cable caught as I can now. I'm hunting around town to see if there's another 300D Mercedes around that I can take a look at the engine for a moment. I'm wondering if the small bit of plastic coating the cable at the end is missing some length, as this seems to be where it's catching on the guide. I know a few of you have mentioned that the servo actuator might be the cause, but, when I pull up the hood after this happens, the cable is caught on the guide. I'm wondering if there is a missing spring or clamp that prevents this from happening. Thanks again for all of your suggestions and I'll keep you posted on what I find out.

Maxbumpo · #8 03-27-2017, 05:02 AM

The pictures look normal to me, but if you can detect that the cable is getting caught on the guide, then I agree that you need to fix that.

Dinoman · #9 03-30-2017, 05:06 PM

Status update .. I bought a new plastic "Guide Clip - Accelerator Cable" .. part number 22-301-00-93-M22 and replaced it (simple 15 minute fix).
It now moves as it should, staying 'connected' with the other plastic part (some sort of rounded banjo type fitting on the actual accelerator cable).
This should stop the accelerator cable from sticking to the guide clip as the actual cable won't be sliding through the guide clip. I'll give a road test later. It's just starting to rain, and it should be rainy for the next day or so. I don't want to 'test' a stuck accelerator in the rain. I"m very confident that this should fix the problem, but, I'm one of those trust but verify types, so I'm not going to try it, until it's dry out and I can find a 'safe' place to test it out. Attached is a photo showing the two plastic parts staying together avoiding the cable getting caught on the plastic guide clip.

Dinoman · #10 04-20-2017, 11:00 PM

after the plastic throttle guide was replaced, all works fine now. It's been holding cruise without any stuck accelerator now for almost a month. I'm relieved both from a safety standpoint and a cost standpoint (fix only cost me $3) Thanks for all of your help. Richard