Another High Idle Problem...as well as Tach and ABS Light - PeachParts Mercedes-Benz Forum

sublettd · #1 07-15-2006, 09:01 PM

First, I've read innumerable threads on high idle and there appears to be no clear solution to the problem. Few of the folks actually report back on what finally cleared the problem.

According to one of the threads, the MB manual says idle is affected by one of the following five things: oil temp sensor; oxygen sensor; coolant sensor; air flow pump; idle speed solenoid. Other posts suggest a vacuum leak or a probem with the micro-switch. I would also think the Idle Speed Control Unit is a player, since it feeds the control valve on the engine.

- Car is an '85 500SL Euro, not that the latter should actually matter
- Problem started several weeks ago with a surge of a couple hundred RPM, but has progresively gotten worse...up to surges of 1500-2000 RPM. Car runs and idles perfectly when the engine is cold. The surge begins when the engine gets hot...not before.
- Cleaned the idle speed control valve three or four weeks ago and that seemed to help briefly
- Took the car to my mechanic this week and the idle was oscillating beautifully up and down from about 700-1700 RPM as I pulled into the shop. So, he knew what the problem was.
- He checked the coolant sensor and there was no connection. So, he fixed that. Seemed to temporarily correct the problem, but after the engine warmed up the idle surge began again.
- Then he glommed in on the idle speed control valve...which I had told him was replaced a couple years ago (along with the control unit) and recently pulled, cleaned and reinstalled. Even though I had a good spare in the trunk that he was aware of he installed one from his bench stock...for a very reasonable $150.00...put two new hoses on it, adjusted the lean/rich mixture and thought he'd solved the problem. Said it held idle at about 700 RPM and never moved. So, I picked the car up from him.

After pulling in the driveway and putting the car in neutral the RPM surged to about 1200 RPM and settled at about 800RPM. When it stabilized, the tach just seemed to go haywire, jumping all over the place out of consonance with the engine.

Finally, the ABS light comes on now after driving the car for awhile and making a couple of stops. I've replaced the OVP with a couple spares and that particular problem still persists.

I called the mechanic when I got home and advised the problem is not fixed. Said no problem, bring it back and he'll figure it out, and won't charge me an arm and a leg to do so. I'm hoping it's something as simple as the oxygen sensor which wasn't checked. However, if someone has suggestions...

Also, I'm curious what might be causing the ABS light to come on since it doesn't seem to be OVP related and what might be causing the tach to fluctuate wildly. I know this is long, but I've tried to be as explicit as possible in relating the problems. Thanks for any help that might be provided.

mbdoc · #2 07-17-2006, 08:23 AM

Bad over-voltage realy OR fuse...no power to those computers.

sublettd · #3 07-17-2006, 09:10 AM

Doc,

I gave a cursory look to the fuses and didn't see any blown. They were all swapped out three or four months ago, but I'll give them a closer look.

Any chance one of the ice cube relays could be the culprit. I have one that routinely 'buzzes'. Trying to track that one down, but when I have the wife drive and I get in the passenger seat they are all as quiet as can be. Will keep trying to nail it down.

Thanks!

mbdoc · #4 07-17-2006, 09:18 AM

NOT a regular fuse but but located IN the over-voltage relay...red 10A ATC style fuse...not ceramic.

sublettd · #5 07-17-2006, 09:38 AM

Doc,

Checked the 10A fuse and it was good. I've put three different OVP (two used, plus the one that was in there that was bought new) in the thing and it's hard to believe that all would be bad.

Thanks!

david s poole · #6 07-17-2006, 09:49 AM

if this is a true euro 85 500sl then it should have black valve covers and ke injection instead of just k.[remove air cleaner and check for black govenor on rear of fuel distributor with two pin plug going to it] if this is the case your idle problem probably is caused by the flywheel pick up that feeds the ignition module down in the left front fender.lrt me know and i can help and also expain why. david poole european performance dallas texas.

sublettd · #7 07-17-2006, 10:13 AM

David,

My understanding is the 84 and 85 500SL was not manufactured in the US and all are imported from Europe. Am I wrong?

This particular car came from Houston and I have the paperwork that was submitted to DOT on the conversion. This car also has the European VIN.
Regardless, I just checked and mine has the silver valve covers instead of black. Didn't pull the air cleaner though.

FYI, I previously bought several parts off of another '85 500SL w/56,000 miles on it. Among those parts was an ignition control module. When this problem started many weeks ago I replaced the ignition control module with the one I bought. No change, so I assume the problem is not in the unit.

Were you pointing to the Ignition Control Module, or headed in a different direction? Thanks for your interest in helping me out!!!