High Idle on '89 560 SL - Mercedes-Benz Forum
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#1 (permalink) Old 07-13-2009, 05:18 PM
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High Idle on '89 560 SL

Car won't settle down after idling or running. Let me back up and tell you what happened before the car started doing this. Battery was dead and had it jumped. Friendly passer by connected the wrong terminals. I put new battery in and seems like there must be a constant drain, wont hold a charge. Now it idles @1,500 rpm and won't kick down even after warming up.
Where do I start?
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#2 (permalink) Old 07-13-2009, 05:51 PM
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Originally Posted by refiguy View Post
Friendly passer by connected the wrong terminals.
Praying that your computer isn't fried is a good place to start.

Then I'd move on to the OVP relay, and hope that's what got damaged.

When I die, I want to go quietly in my sleep, like Grandpa.
Not screaming, like the people in his car.

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#3 (permalink) Old 07-13-2009, 08:09 PM
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Another possibility is blown short diodes in the alternator. Reverse polarity at the currents typical in primary automotive systems can be a Very Bad Thing(tm).

I think I'm going to have to finish writing the howto on auto electrical troubleshooting that's been kicking around in the back of my head.

Good Luck. May whatever got blown be cheap. For Mercedes levels of cheap, of course.

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#4 (permalink) Old 07-14-2009, 05:47 AM
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#5 (permalink) Old 07-14-2009, 07:32 AM
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Thank you for the input. Where do I find the OVP relay?
In addtion is there a way to test the Alt.?
I put a meter on the battery while the car is running and it appears that the Alt. is charging the battery (even though it won't hold a charge now on a brand new battery, yikes).
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#6 (permalink) Old 07-14-2009, 01:54 PM
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On my car when it did that, it turned out that the throttle position sensor went bad. It is mounted deep in the V of the engine on the butterfly shaft. It can easily be checked as the connection is at the top left front of the engine. There are three wires: common, full closed idle, and full open throttle. At idle, there should be continuity between the common and idle wires. At full throttle, there is continuity between the common and full throttle wires. My sensor went bad in the idle circuit and the engine would not idle down. Hopefully that is your problem as it was not all that expensive to fix, just a pain to get to.
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