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Old 07-01-2002, 03:42 PM
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Join Date: May 2002
Location: Snohomish, WA
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Question W140 Cooling fan Won't Shut off?

93 500SEL. Runs great, temp is fine, oil is fine, yadda, yadda yadda. Sometimes when we park the car in the garage and turn the engine off, you hear the cooling fans stay on for a while. We go inside and think nothing of it.

But lately, as the summer has come, we go inside and come out like 4-5 hours later and they are still on. And a couple of times they've stayed on all night and the battery doesn't have enough juice to turn the car over in the morning. But it does seem ambient temp sensitive.

We live in the Seattle, WA area, so high temps are usually not an issue. But the last few weeks we did have a few 95+ days and on a few occasions, this type of problem arose. I've even taken the car out in the evening, after its been sitting with the fans on for several hours, for a quick spin around the block to make sure everything internally was cooler. When I park it and turn off the motor, the fans are still going and as stated earlier, stay on the entire night, causing a very low battery.

Any ideas as to what would be causing this?
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Old 07-01-2002, 04:08 PM
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Join Date: May 2001
Location: Florida
Posts: 1,105
this is a shot in the dark, but i'd look for a temp switch somewhere. I *think* that the fans will run in proportion to the coolant temp, so if the sensor thinks the coolant is still hot it will continue to run the fans. That'd be my best guess
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Old 07-01-2002, 04:09 PM
Join Date: Mar 2000
Location: Gainesville FL
Posts: 6,844
From looking at the diagram, it looks real strong to be a stuck K9 aux fan relay. Normal operation seems to be dependant on term 15 which is ignition hot. The source is term 30 which stays hot all the time. The only way for the fans to get current is through the K9 and it shouldn't work with the ignition off.
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician
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Old 07-01-2002, 04:26 PM
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Old 07-08-2002, 03:23 PM
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Join Date: Apr 2002
Location: westchester, ny
Posts: 747
w124 cooling fan comes on all the time

On my 1987 300e, the aux cooling fan comes on once the ignition key is inserted & moved to the start position, even if the engine is not started, and even if the air conditioning is off ( the "O" button is depressed on the controls). Is it likely the fuse or relay, & where is the relay. Thanks in advance for the advice.
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Old 07-08-2002, 09:25 PM
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'94 S500: only 793 sold!
Join Date: Mar 2001
Location: San Diego, CA
Posts: 1,929
Stevebfl's theory is correct but . .

. . maybe the ignition switch is bad allowing power to be on with the key of 'off' (very unlikely!).

I vote for a bad K9 relay.

The fans should never run with the key off. Do you think these are Hondas????
Regards . . . . JimF
'94 S500 Cpe

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Old 08-03-2002, 08:57 PM
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Join Date: May 2002
Location: Snohomish, WA
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We took it to the dealer to have them check it out. They couldn't find really anything wrong so they gave me the song and dance about how the coating on the wires deteriorate overtime on the early W-140 models and bascially said the replacing the engine wiring harness would fix the problem. $400 part and $600 labor to install. I told them I would think about it.

I have read on this forum about the wiring issues, but I don't think that this cooling fan problem that we are experiencing is caused by that. Wouldn't other things be effected also if that were the case?

Anyhow, I wanted to ask if stevebfl had a part # for the K9 aux fan relay? Is the K9 relay something I could replace myself?

Thanks again so much!
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