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1989 300SE 240k miles
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3,360 Posts
Discussion Starter · #1 ·
Hello All,

This regards my 1989 300SE with 242k miles (m103, I-6). I’m working on my idle. A month + back, my idle dropped. It went from 750 rpm in P down to 600 rpm. And in D it dropped from 600 to 500 rpm. Also, it now sporadically spikes upward, and downward. Sometimes it’s once. Sometimes it spikes and drops continuously for 30 seconds. Three or four times when in P, the swing was so large the engine stalled.

I hoped it was a result of mis-calibration of my air fuel ratio and misalignment of my air flow plate height. Or an undiscovered vacuum leak. However, the system is now reset and recalibrated. While doing so I checked the rubber boot under the air mixture box; it’s soft and supple. There are no cracks. And I replaced all the vacuum lines and connectors in the engine bay and from the intake manifold.

Previously I cleaned the ICV in an ultrasonic cleaner. It had no effect. Additionally, I’ve been tracing a “tick” that I’m convinced comes from the unit. In fact, one time I disconnected the ICV and the “tick” started immediately upon reconnection.

I have used 2 MB OVP relays, both with new fuses. One of which I bought new in the last year.

This morning I did some electrical tests. And my results were confusing.

Engine at operating temperature for all. The volts are dc. And I placed the multimeter leads one in each plug in the harness.
***Key at position 2. Wire harness to ICV read 11.4v ~Should be 12v. Is this close enough?
***Engine running. Wire harness to ICV read 12.9v ~ Should be 4-5v. I understand a lower volt equals a higher rpm. So could this be my cause???
***After this, I tested continuity in ohms, which I understand I should have done first. My multimeter read 0.000. It read OL prior to connection, but switched to 0.000. when I touched one probe to each prong. ~ Should be 4ohms (I believe. I forgot the number while typing this. But it’s definitely not zero).

By this testing I believe I’m showing 2 problems. 1) No continuity on the ICV. Which I understand means the unit is bad. And 2) 12v going to the unit when the engine is running and at operating temperature. Which is bad and means control module or OVP.

Unplugging the ICV does not change the idle. Unplugging the EHA and Throttle Position Sensor both result in a higher idle.

I was going to remove the adjustment screw on the ICV and bump up my rpm, but now I don’t believe that will fix the problem.

Any ideas? Or input? Suggestions on how to proceed? And did I make any mistakes? …If you show me exactly what to do with a multimeter, I can do it. But otherwise I’m doing what I think is appropriate.

Thanks,
Bill
 

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Registered
1990 420 SEC
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919 Posts
ICV of 6.cyl M103? engine differs from 8-cyl engines. I believe it makes engine idle a bit high when cable is disconnected. Then with some current it closes to bring idle down to lowest and after that with more current it opens to increase idle speed. If someone knows better correct me. It would be worth checking service manual on this.
 

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Registered
1989 300SE 240k miles
Joined
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3,360 Posts
Discussion Starter · #3 ·
ICV of 6.cyl M103? engine differs from 8-cyl engines. I believe it makes engine idle a bit high when cable is disconnected. Then with some current it closes to bring idle down to lowest and after that with more current it opens to increase idle speed. If someone knows better correct me. It would be worth checking service manual on this.
Ahhhhh. If this is accurate, my OVP and Idle Control Module may be functioning properly. And the lack of continuity would indicate a bad ICV.

Also, this makes sense as there is inconsistency in individual m103 responses to disconnecting the ICV. Some have the idle increasing, some report decreasing.
 
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