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Discussion Starter #1
I would appreciate any help or suggestions that you can offer to resolve this problem. Idle RPM has recently increased from the normal 500 to about 750 with the transmission engaged, and to about 1000 to 1100 with the trans in neutral or park.
I've checked all the linkages, also for vacuum leaks, and the throttle body to ensure that the throttle is closing completely on the idle stop. I've also checked the resistances for the idle and full throttle stops on the throttle valve switch as well as the electrical connections to the warm-up enrichment compensator.
 

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Hi CDM1

Unfortunately, your 380sl has an electronically controlled idle .

When a high idle symptom occurs, this means that a failure between the idle air control valve, the idle speed contr relay, the OVP relay, and/or a fault in the wiring has caused the idle to default that high.

The first base of attack is to replace the over volt protection relay as this is a wear item. He lives next to the fuse box behind the vinyl panel. Two screws and he comes out.

If this doesn't fix it, I would look at the idle relay . He lives behind the glove box . These can be rebuilt by replacing the dead capacitors and reflowing solder jointa. Forum member Jamesdean will do this for a fee if you're like me and lack a steady hand.

Last but not least is the wiring and the idle air control valve. It might be worth spraying carb cleaner inside of the IACV . These get filled with gunk and cause a higher than normal draw which zaps the relay. Also a way to fix it.

Hope this helps point you in the right direction!
 

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The Over volt Protect relay is the one with the fuse on top (behind the right side kick panel just above the fuses). It is not in a very convenient location, in fact they couldn't have chosen a less convenient location. If you're lucky the only thing wrong with it might be the fuse (not likely) but it might be worth a try. Here's a picture of the OV relay.
Merc12VoltOverProtectRelay.jpg

Like Beast said, the Idle Control relay sits behind the glove box (if yours is a left-hand drive), I'm not sure how to get to it if yours is a right hand drive car. (keep in mind, right or left hand drive is from the drivers point of view sitting behind the wheel looking out the windshield). Here's a picture of the Idle Control Unit.(Sorry it's upside down, I didn't take the time to rotate it).
MercThrotleControlUnit.JPG

Here's a picture of the idle air control valve, it sits on the engine just aft of the thermostat housing. It has a two wire connector on the front.
MercIdleControlValve.JPG
 

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My RHD '82 500SL has the idle control unit under the glove box (left hand side of car). You may need to be able to get your body in contortions to get at it, or if you have small children, this is where they will earn their keep.
 

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Discussion Starter #6
Hi CDM1

Unfortunately, your 380sl has an electronically controlled idle .

When a high idle symptom occurs, this means that a failure between the idle air control valve, the idle speed contr relay, the OVP relay, and/or a fault in the wiring has caused the idle to default that high.

The first base of attack is to replace the over volt protection relay as this is a wear item. He lives next to the fuse box behind the vinyl panel. Two screws and he comes out.

If this doesn't fix it, I would look at the idle relay . He lives behind the glove box . These can be rebuilt by replacing the dead capacitors and reflowing solder jointa. Forum member Jamesdean will do this for a fee if you're like me and lack a steady hand.

Last but not least is the wiring and the idle air control valve. It might be worth spraying carb cleaner inside of the IACV . These get filled with gunk and cause a higher than normal draw which zaps the relay. Also a way to fix it.

Hope this helps point you in the right direction!
Thanks very much for your helpful description of the cause of the problem and for your approach to the solution. I got lucky when I removed the OVR and discovered that the 10A fuse was blown. I simply replaced the fuse and the problem disappeared. Even though the new fuse did not blow I removed the IACV and cleaned it as you suggested and also checked that it was operating correctly.

I was not looking forward to replacing the idle control relay which is a very expensive component ranging from $193 to $306 plus a $31 core charge.

Thanks again for your help
 

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Discussion Starter #7
The Over volt Protect relay is the one with the fuse on top (behind the right side kick panel just above the fuses). It is not in a very convenient location, in fact they couldn't have chosen a less convenient location. If you're lucky the only thing wrong with it might be the fuse (not likely) but it might be worth a try. Here's a picture of the OV relay.
View attachment 2604847

Like Beast said, the Idle Control relay sits behind the glove box (if yours is a left-hand drive), I'm not sure how to get to it if yours is a right hand drive car. (keep in mind, right or left hand drive is from the drivers point of view sitting behind the wheel looking out the windshield). Here's a picture of the Idle Control Unit.(Sorry it's upside down, I didn't take the time to rotate it).
View attachment 2604849

Here's a picture of the idle air control valve, it sits on the engine just aft of the thermostat housing. It has a two wire connector on the front.
View attachment 2604850
Thanks for your pictures of the various components. MY problem was caused by a blown fuse on the overvolt protection relay and was thankfully resolved by simply replacing the fuse.
 
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