PeachParts Mercedes-Benz Forum - Idle speed

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Idle speed - '87 W201 (602 NA)

Short version: loaned out my 190D that ran like a swiss watch. Upon getting it back it idles too slow (will barely stay running). I doubt that the borrower did anything under the hood other than change the oil; he had it for just over a year and put a bit over 11K on the odo. I just removed the intake rail assembly to install new glow plugs and all of the throttle linkages appear tight with no sign that they have been messed with. The one thing the borrower did do was put in a new radio, which I am assuming is why the tach no longer works. My guess is that he cut the wire to the tach and that the idle issue and the dead tachometer are related.

Any thoughts would be greatly appreciated!

Thanks,
Jim

Quote:

Originally Posted by jaoneill (Post 3315412)

Lessons learned here.

1) Never loan your car, even to good friend/relative. Not for a day, not for a year. I loaned my car to my brother-in-law and he totaled it. Not his fault but the other guy is un-insured.
2) Never assume he/she messed up the car. You will lose your friendship. It is an old car so anything could happen.

Putting a radio in should not affect the tach if one knows what one is doing. It is so simple. The tach is partly controled by the Kilma. It may be a faulty kilma relay.

Quote:

Originally Posted by ah-kay (Post 3315439)

Lessons learned here.
2) Never assume he/she messed up the car. You will lose your friendship. It is an old car so anything could happen.

Putting a radio in should not affect the tach if one knows what one is doing. It is so simple. The tach is partly controled by the Kilma. It may be a faulty kilma relay.

Thanks for the response; I understand what you're saying (old car, etc) and haven't even mentioned the problem to the borrower.

What I am trying to do here is not place blame but minimize the diagnostic time. I haven't looked to see where it originates but there is a cut wire hanging from under the dash. Everything appears to work except the tach so my question is: could/would that screw up a circuit that would affect the IP (idle speed)?

Jim

Your overvoltage protection relay is fersheetzen. It's located behind the battery, you have to remove the plastic covering below the windshield. It looks like attached photo. It's a surge protector. The circuits it protects include the tach circuit, ABS, and the EDS (which controls idle speed).

The solder joints inside have likely cracked. If you tap on the OVP relay and the idle speed jumps then you've found your problem. Take it apart and resolder the connections.

-J

Quote:

Originally Posted by compu_85 (Post 3315666)

The solder joints inside have likely cracked. If you tap on the OVP relay and the idle speed jumps then you've found your problem. Take it apart and resolder the connections.

-J

Nice try, but there's really no effective way to test the OVP, much less repair it. If the solder joints are bad, or even if the fuse is blown, then the zener has a high likelihood of being permanently damaged. It's also a not the easiest part to disassemble. If the symptoms point to the OVP, the only course of action is to replace it.

I opened and resoldered mine without issue :confused:

-J

Thank you gentlemen; next stop "Pelican Parts" to have an OVP heading this way…..

Update - thought it might be prudent to pull the relay before ordering a new one. Found that it contains a 10 amp fuse (that was blown). 5 minutes later we are back in the race; idle, tach working just fine :-)