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Old 04-21-2003, 11:43 PM
Registered User
Join Date: Aug 2000
Posts: 15
560SL High Idle and Aux Cooling Fan Problems

Hi guys. It's been a couple years since I've been online and now need more advice. A couple years ago I acquired the subject 1988 SL which had been stored five years, and put it back on the road. Had lots of input from many of you, especially Larry Bible, on what to do to get it back on the road. I now have just over 22,000 miles on it and am having a continuing problem with high idle (1100rpm in Park, engine hot) and with a slight overheating problem in stop and go traffic while sitting at a stop light. Temp gage goes up to about 10 o'clock position. Stays at 9 o'clock position while driving, regardless of speed or OA temp. I'm convinced the front aux fan is not running, even though I had this repaired a couple years ago at the local dealer with a new switch of some kind and several O rings (don't have the RO handy at this time). Now the dealer tells me I need lots of AC components replaced due to a leak in the compressor which has caused the freon level to drop, which, in turn causes the aux fan to not operate due to pressure or temp. Cost of repairs is $1900.00. I can see no evidence of freon /oil leaking. No spray line across the hood which was the tell tale evidence of AC leaks in older cars, no drips on the garage floor, no oil on the engine. The AC works fine, air from outlets is cold, interior is comfortable and seemingly quarts of water drain out on the garage floor after parking. (this is humid Florida, you know).

The dealer- recommended AC repairs are one thing, but after an abortive attempt by the same dealer to correct the high idle problem, I have little or no confidence in their ability to honestly diagnose or repair a problem.

Here's the high idle problem: Diagnosis: the idle speed control module was bad and needed to be replaced. Cost of part, $280, cost of labor $287, plus this and that for a $600 bill. I gave them the go ahead. After repairs, I picked up the car, engine already warm, drove to the first traffic light (not far in Tampa) put it in neutral and the tach went to 1100rpm. Took it back to the dealer, got a new diagnosis of some sort of idle control switch/valve that was bad and required the intake manifold to be removed and many bits and pieces to be replaced in the process to the tune of $2100.00. Enough was enough. I told the dealer to reinstall my original control module ( a black, cigarette-box sized unit with several silver-colored prongs sticking out of one end) which they carefully left on the center console when I picked the car up, and refund my $600, note my dissatifaction, and I would not bother them again. After some dialogue, they did so.

Interestingly, records indicate the former owner also had the idle speed contol module replaced several years, but few miles, ago.

So, I need some expert advice on the probable causes for the high idle, the aux fan/high engine temp problem, and most of all, a recommendation for an honest, experienced, dependable MB tech in the Tampa area.

Thanks in advance. Harcar
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Old 04-22-2003, 07:32 AM
it leaks, its german
Join Date: Mar 2002
Location: raleigh nc
Posts: 1,111
hi idle. ISC motor bad, ISC module bad or bad connection. If I remember right, the OVP on that will do it as well. Check the tps for idle switch over, duty cycle exsisting for the isc motor/valve, ovp for blown fuse and sticking linkage.

overheat. t-stat, stopped up radiator, fan clutch, aux fan switch in the intake, relays, fuses or fan motor.

I dont know how far Continential Imports in Gainsville is but Steve is a regular contributor to the forum and seems to be able to fix cars. (a valuable assest these days)

Project Smoker, '87 603 powered wagon
Hauler, 96 CTD can you say torque?
Toy 73 Cougar xr7 convertible
Acme Automotive Inc.
Raleigh NC
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Old 04-22-2003, 07:48 AM
Posts: n/a
That's exactly what I was thinking. Are you near Gainesville. Steve is a scientist troubleshooter. Many of the dealers only replace parts until they have accidentally replaced the one that solves the problem.

Can a vacuum leak not cause a high idle, not only the solenoid etc.?

Sorry I can't be more helpful on this.

Good luck,
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Old 04-22-2003, 03:48 PM
Registered User
Join Date: Jun 2000
Location: Saugus, CA USA
Posts: 1,974
Run away from the dealer

The AC fan switch and the hot engine fan switchs are in parallel. Either one will turn the fan on regardless of what the other one is or is not doing.

The high idle and no fan may be related. The fan is off and the idle is high when the car thinks the engine is cold. Maybe they use the same sensor, or coolant isn't getting to that spot.
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Old 05-23-2003, 01:41 PM
Registered User
Join Date: Aug 2000
Posts: 15
560sl idle and cooling problems

Just wanted to let the forum members know I have successfully resolved the problems stated in this thread. Or, rather, I have found a great MB mechanic in the Tampa area. Final cost was $570, vs the $4000 quoted by the local dealer. A couple new switches were required and while at it, the garage converted the A/C to R134, saving me the replacement of freon at $80/lb. The garage that did the work, on time, under budget is "Frenchy's" on Lois Ave in Tampa. Located almost behind the Mercedes dealer. Frenchy himself is the owner/technician, and knows his business and is honest. A remarkable combination in my judgement. Frenchy (yes, he is) works only on MB, BMW, and Volvos.

Best part of the experience is that I am now convinced to keep this little silver beauty (1988 with 22500 miles) knowing I can get repairs done at a fair price.
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