1990 W124 300E Checkengine light at low RPM - PeachParts Mercedes-Benz Forum

buddyligon · #1 03-21-2019, 10:38 PM

I have been getting a check engine light at low (idle) rpm. When checked the codes are for the #14 pin the following:
2: Fuel pump relay (circuit 87) not functioning
3: TN/TD signal (RPM) interrupted
4: Output for oxygen sensor heater control defective
5: Output for air injection pump control defective
6: Output for kickdown switch control defective
12: Output for A/C compressor control defective
13: Excessive A/C compressor clutch slippage
When I check the wiring diagram most of these go through the Engine Systems control unit. Could it be this unit or actually all of these things? How to check?
The check engine light goes away upon acceleration.

Sugar Bear · #2 03-21-2019, 11:02 PM

What is the idle RPM?
Battery good or great?
What is the charge voltage at idle?
I'm asking/thinking the idle RPM is too low causing low voltage, there is a charge output problem (regulator) or a bad OVP relay.
Remove, clean and reinstall the grounds at and near the battery.
Good luck!!!

buddyligon · #3 03-22-2019, 12:40 AM

Thank you.
Idles at 500-700 with trans engaged and 700-800 in neutral
Voltage in neutral has been 13.7 to 13.9 at the battery haven't checked with trans engaged.
Battery seems solid less than year old Mercedes battery. Starts easily etc.
Ill checked the terminals for the corrosion and give it a good clean.
I was thinking at the low idle maybe the alternator was dropping to low for all the systems to working properly but I thought that was more on newer cars. I did take unscrew the grounds in the engine bay and clean each of them for good contact to the body point. I know on my other Mercs that grounds have a lot to do with having things work properly especially the ones behind the instrument cluster and in one case the antenna aerial ground for the speedo (other model)

buddyligon · #4 03-22-2019, 12:42 AM

Oh and did check the OVP works great.