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1981 mercedes 380sl
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33 Posts
Discussion Starter #1
Idles at 1500 when warm. Here is a list of things I've checked.
First i applied 12v to the idle control valve and It rattled. Is it supposed to click or rattle?
Then i tested 0 volts at the at the connector to the valve but 5 volts at the temp sensor in front of it. are the wires crossed?
I looked under the glove box and tapped/unplugged the over voltage relay. no effect on idle.

bent/unplugged the little idle controller board no effect either.

Thinking it might be the relay but mine is pinkish with a different pin out and no fuse on top. part number 116 545 0034.


when i Google the part number it comes up with this.


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obviously It wont work so what is my next step / why is my OVP different?
 

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'80 450SLC Afro RHD Ikonengold
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2,900 Posts
^ ^ ^ ^ ^ ^ ^ ^ ^

This is interesting!

It is actually not a relay but just a powerfull 22V Zener diode (technically it is an avalanche diode) that blows the fuse when circuit voltage goes over 22V and a bit more.
It means that every overvoltage "relay" can be repaired cheap cheap.
 

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420 Posts
It sounds like the idle speed controller is not outputting the right voltage to the control valve. This is often caused by the oil temp sensor wiring failing to provide the ground data to the controller and it reads temp is not up to par. Suggest to warm up the engine to running temp, then disconnect the oil temp sensor wire and pierce the wire cover a few inches away from the connector and ground the wire to a good earth. If it is this wire is burnt due to the proximity to the exhaust manifold it should bring the idle revs down to about 800 rpm with a good ground. Once you have eliminated this possible cause you can test the controller, control valve, OVP, etc.
 

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1981 mercedes 380sl
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33 Posts
Discussion Starter #5
When i apply 12 volts to idle valve externally it has no effect on the idle just clicks back and fourth rapidly. is this normal?
unplugged the oil temp switch which also has no effect.
 

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1998 SL500, 1959 220S, 1970 280SL
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777 Posts
On a 1981, the idle controller gets a temperature signal from a switch on the back of the intake manifold, not the oil temp switch. the controller gets ground until like 100 degrees and that makes it raise the idle a couple of hundred RPM. That oil temp switch is for lambda control.

If you are getting 0 volts to the valve, then either the controller is dead or it is not getting power from the OVP/xener diode. Can you check for power to the controller, and might as well check for ground while you're at it.
 

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1984 MB 380SL, 08 Ford Escape Hybrid, 1999 GSX1300R Hayabusa, 1989 Honda NX250
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93 Posts
I had the same high idle problem. Replaced the overvoltage protector. Had to be shipped in from Germany but.........Problem solved!
Greetz Johnny
 

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1984 MB 380sl Lapis Blue
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2 Posts
Re-soldered broken joints in the Idle Control Module

Re-soldered fractured electrical joints in the Idle Control Module and my intermittent high idle problem is fixed! 1984 380 SL USA model.
 
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