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Old 10-17-2012, 05:52 PM   #1 (permalink)
BenzWorld Junior Member
 
Date registered: Oct 2012
Vehicle: 1994 Mercedes C220
Location: Flemington, NJ
Posts: 23
Post 94 c220 rpm fluctuating PLEASE HELP!!

Hey everybody,
Lately my car has been acting up, and was wondering if you guys could help. The problem I'm having is when i give it steady gas the rpms bounce back and forth (mainly between 1500-2000rpm). This happens while driving and in park. Another thing i noticed is that the OBD2 scanner rarely ever connects. Can this mean that i have a bad ECU or is it something else? Besides this, the car idles perfect and has good power.
Thanks

Things i already replaced:
engine wiring harness
spark plugs
fuel filter
cleaned MAF sensor and throttle body
vacuum lines look ok, and seems to have no leaks

Last edited by greasemonkeyjoe; 10-18-2012 at 06:45 AM.
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Old 10-18-2012, 07:48 AM   #2 (permalink)
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Date registered: Dec 2011
Vehicle: 1996 C220 Auto-tragic
Location: SLC, UT
Posts: 43
Check or replace (cheap) your OVP relay. This powers the Diagnostic module (code reading problems) and the MAF.

Is your ABS light coming on?
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Old 10-18-2012, 10:27 AM   #3 (permalink)
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Date registered: Oct 2012
Vehicle: 1994 Mercedes C220
Location: Flemington, NJ
Posts: 23
No ABS light... SRS light is on though. How do i test the ovp relay?
thanks

Last edited by greasemonkeyjoe; 10-18-2012 at 10:33 AM.
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Old 10-18-2012, 12:27 PM   #4 (permalink)
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Date registered: Dec 2011
Vehicle: 1996 C220 Auto-tragic
Location: SLC, UT
Posts: 43
I looked for a way of testing the OVP relay but could not find any specifications for this part. I would just replace as very cheap. I think this relay also delays power up the Diagnostic module and MAF until car is running. Not 100% about this though. I took cover off mine and noted there are three relays and some electronic devices. I suspect this was the problem with mine, as OVP was immediately powering up the Diagnostic module which caused the firmware to crash. Then I could never get the ODB II readiness monitor to go to ready no matter how many drive cycles went thru. As a matter of fact my ODB II scanner read "unavailable" on the readiness monitors. After OVP replacement, disconnected battery, did a ECM reset/re-learn procedure and drove for about 200 miles. Finally go 4/6 readiness monitor to pass which passed emissions here in UT.

Do you have a ODB II reader?

Reason ask about ABS is this module is also powered by OVP.

Check you MAF is working, no 0101 or 0170 codes.

My SRS light has always come on for about 1-2 minutes after starting the engine but always (knock on wood) shuts off. Need STAR diagnostics to get codes, I think?
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Old 10-18-2012, 12:40 PM   #5 (permalink)
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Date registered: Oct 2012
Vehicle: 1994 Mercedes C220
Location: Flemington, NJ
Posts: 23
I have an actron obd2 scanner. I will try and see if i can buy a new ovp relay today. I tested the MAF by disconnecting it and seeing if the problem persists, and it does. The rpms still fluctuate so i suspect there isnt a problem with the MAF, but i may be wrong. Also when i disconnected it, the car went to stall.

Last edited by greasemonkeyjoe; 10-19-2012 at 06:58 AM.
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Old 10-18-2012, 03:08 PM   #6 (permalink)
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Date registered: Oct 2012
Vehicle: 1994 Mercedes C220
Location: Flemington, NJ
Posts: 23
I ordered a new ovp relay and will be getting it sometime next week. Do you think this will fix the rpm fluctuating, or just the obd scanner not reading? no matter what its worth a shot right?
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Old 10-19-2012, 08:08 AM   #7 (permalink)
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Date registered: Dec 2011
Vehicle: 1996 C220 Auto-tragic
Location: SLC, UT
Posts: 43
Try swapping out the OVP relay and see if you can get diagnostic mode working. That way you can check for OBDII codes.

You can build a LED blink code reader to access the blink codes directly from the ECM using the 38-pin diagnostic connector. This will allow you to get codes without the using the diagnostic module.

After you replaced the wiring harness did you do a ECM re-set/relean procedure? I found that I had to do this to get the throttle actuator working after I repaired my wiring harness. Also disconnected my battery to reset the diagnostic module. I see you replaced your wiring harness.
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Old 10-19-2012, 12:43 PM   #8 (permalink)
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Date registered: Oct 2012
Vehicle: 1994 Mercedes C220
Location: Flemington, NJ
Posts: 23
I didnt reset the ecm after i replaced the harness. How do you reset it?
Thanks for all your help

Last edited by greasemonkeyjoe; 10-19-2012 at 03:44 PM.
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Old 10-23-2012, 06:13 AM   #9 (permalink)
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Date registered: Oct 2012
Vehicle: 1994 Mercedes C220
Location: Flemington, NJ
Posts: 23
I got the new ovp relay yesterday and hooked it up, but didnt fix the problem. Any idea on what to try/look for next?
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Old 10-23-2012, 07:37 AM   #10 (permalink)
BenzWorld Junior Member
 
Date registered: Dec 2011
Vehicle: 1996 C220 Auto-tragic
Location: SLC, UT
Posts: 43
Finally got information. To reset the ECM follow the attached procedure. You have to make sure the OBDII codes are all cleared or the procedure will not work. I have also attached a schematic of the pulse code reader which you can use to clear codes or use your OBD II code reader to clear the codes. Once you have codes cleared follow the resetting and reactivating ECM memory procedure. This will reset the ECM and will re-learn the throttle actuator position.

You might want to disconnect the battery to reset the diag module.

Give that a try to see if Throttle actuator starts working. Make sure you don't have another vacuum leak.
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File Type: txt Resetting and Reactivating ECM memory.txt (1.0 KB, 90 views)
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