My wife was driving her Mom's S320 to work this morning and it started to act as if the cruise control was engaged. She described it as if someone was helping push the acclerator. When she got to a stop, it would surge up and down in idle. She turned the car off and restarted and it shot to 4000 rpms and then went back to acting normally. It drove a few miles fine, and then started doing the same thing. It is now at work, and I looked enough to see it is not a mechanical binding in the linkages.

Trying to work the cruise control had no effect (as in turning it off). What should I look at when I get it home? Is the throttle control unit going bad? Will that throw a code??

Thanks in advance!!

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You almost certainly have fault codes. Check both the 38 pin with a flash code reader and the OBDII port under the steering wheel.

Did the ASR light come on? It should have!

Most likely a throttle actuator issue.

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Thanks for the responses!! I am not getting any lights or codes through the odbII system (other than a long standing egr flow code). Have not checked the underhood diagnostics yet.

I did drive it home and experienced the surging for myself, however. Pressing down the cruise into the off position did seem to change things for a split second, but then would start acting up again.

Changed the engine wiring harness a few months back which looked like complete crap. Wonder if the throttle actuator wiring is toast as described in this write up http://www.benzworld.org/forums/w124-e-ce-d-td-class/1448549-how-rewire-m104-throttle-actuator.html

Does this sound like a logical next step? Luckily, mother-in-law was using our car for other reasons and this happened while it was here.

__________________
‘84 300TD “Mountain Goat”
'62 300d Adenauer-350 Chevy - "Max"
'15 VW Passat TDI
'16 Dodge Ram 3500 - Cummins w/6-speed
'68 Mustang fastback
'55 T-bird
‘63 Studebaker Hawk

Sure, could be bad throttle actuator wiring. Do you have ASR?

Spot on! Thanks guys! Pulled out the throttle unit and cut into the wiring. Complete and utter failure of the insulation. Will probably get my computer engineer buddy to rewire it.

That does raise the question- is the wiring from the chassis to the actuator plug wired with the same stuff? If so, is there a good way to set fire to a car to make it appear to be an accident ......

__________________
‘84 300TD “Mountain Goat”
'62 300d Adenauer-350 Chevy - "Max"
'15 VW Passat TDI
'16 Dodge Ram 3500 - Cummins w/6-speed
'68 Mustang fastback
'55 T-bird
‘63 Studebaker Hawk

Beckmann Technologies (right up the road in Durham) sells rebuilt ones (and many other rebuilt MB components)

http://www.beckmanntechnologies.com/

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