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1989 300SE 240k miles
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3,538 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
1989 300SE 240k miles
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3,538 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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Registered
1989 300SE 240k miles
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3,538 Posts
Discussion Starter · #4 ·
So, I’ve been tracking this down. I replaced my ICV with a new EBAY knockoff. I figured I’d notice if it changed anything. It didn’t.

But it’s in place now. So I’m leaving it for now.

I bought a used idle control relay. The one in the (US) passenger’s footwell, along the outside. I tried multiple configurations of my original and replacement ICR with each of my 3 OVP relays.

In every case, at operating temperature, I have higher idle speed with the used (new to me) ICR. With that ICR, I have a rise and fall of idle. In D, it’s between 700 - 500 RPM. The MB OVP I bought in the last 2 years has the least rise and fall.

One thing that happens with my new to me ICR is my SRS light comes on. It does this with all 3 of my OVP relays. My original ICR does not have the SRS light come on.

I decided to leave the new to me ICR with my 2 year old MB OVP installed. I then adjusted my EHA and idle duty cycle. The EHA was now reading in the 60’s. I reset it to 40 - 45. The idle was set to this.

In this condition, the SRS light stayed off for 3/4 of the drive. Once the SRS light came on, though, my idle was perfect the rest of the drive.

I know the alternator brushes can affect the SRS. And I did replace those. So I’m going to install the old brushes tomorrow and see if that’s it.

If it’s not, I’m thinking of replacing the OVP. That even though it’s testing perfectly, the SRS light and rise and fall are known to be tied to these.

Anyone have any thoughts?
Bill
 

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1989 300SE 240k miles
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3,538 Posts
Discussion Starter · #6 ·
As I understand it, I have the idle control relay in the footwell. And it then goes to the closest thing the car has to an ECU. This ECU is located in rear of the engine bay, between the battery and the OVP.
 

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1989 300SE 240k miles
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3,538 Posts
Discussion Starter · #8 ·
New to me ICR. Part numbers match the original. As I understand it, these are interchangeable. As opposed to the electronics in newer MB’s like the r129’s.
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1989 300SE 240k miles
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3,538 Posts
Discussion Starter · #9 ·
So… I drove to my parents’ house. The car stalled when stopped at a red light (with the SRS light on), in D. The car’s idle dropped and almost stalled when driving around turns. But when I got to my folks’, it idled perfect (in both P and D)…. Ugh.
 
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