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Old 07-29-2002, 04:25 PM
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Too hot! Auxiliary fans not cutting in soon enough

Can any of you technical experts help me with a problem of overheating (E280/M104/24V with A/C), only apparent during the recent hot weather?

Iíve read somewhere that the auxiliary cooling fans work independently. One starting at 100 degrees and the other (i.e. both) at 115 degrees.

Whilst idling in traffic in very hot conditions, A/C on, the engine temp goes really hot Ė right up to nearly red then (finally) both auxiliary fans cut in together and the temp soon drops to normalÖ.only to repeat this every five minutes or so. Itís all getting a bit too hot for my liking and I donít remember it doing this last year. I think maybe the 100 degree switch / relay (or whatever) isnít working.

On the front of the engine, next to the thermostat housing there are two sensors. The front one with one wire I know works the temperature gauge. The other one has a square plug with four wires. When I unplug this, both fans come on after a slight delay.

Does this sensor monitor both 100 and 115 temperatures? If so, am I right to assume it need replacing, or is a relay at fault?

Is there any way I can test the sensor before buying a new one? I donít want to burn anything out shorting out the wires!

Anyone help?

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Old 07-29-2002, 06:02 PM
Join Date: Mar 1999
Location: Tucker, Ga USA
Posts: 12,153
Both fans are designed to come on first with A/C compressor engagement, then at 107C. If the fans don't come on with the A/C then the relay OR fuse may be bad. The relay is behind the fuses so you will have to un-screw the black plastic cover to acess the relays. ALSO the low speed goes thru a ballast resistor & the WIRE can melt or the resistor can fail.
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Old 07-30-2002, 06:48 PM
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Thanks for that, I'll spend some time on it this weekend. At least I know where to start looking.
If I understand you correctly, both fans are supposed to run slowly, all the time with the A/C's operating, running at full speed only when the engine reaches this high temp.
I'll try to find the relay, but A/C over here is a relatively rare option and the manual I have doesn't show any of the A/C electrics.
Englands weather being predictably unpredictable, it's turned cooler now, and the problem gone away - for now!!!!
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Old 07-30-2002, 10:11 PM
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Join Date: Jun 2001
Location: Omaha, NE
Posts: 962
Just to clarify, and your model MAY be different, but on US models the low-speed fans do not actually come on exactly when the compressor does, but instead when the AC refrigerant pressure exceeds a certain level. In cool weather, they may not come on at all; in warm weather, they will probably come on after the AC has been on a few minutes. There is a pressure switch in the receiver-drier which you can jumper and if your relay, fuse and ballast resistor are OK, the fans should come on low speed.

For more info, do a search on "aux fans" and you'll get lots of info.

On the other hand, if your coolant temp is getting near the red zone, I'd suspect there may be something else at play (partially occluded radiator, bad thermostat,etc.)
'93 400E
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Old 08-02-2002, 04:55 AM
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Check this web page:

This guy's got a pretty neat product that allows the aux fans to kick in at a lower temperature. Basically, it's a series resistor. But, he sells it in a neat/compact assembly. Looks like a quality product from what I can tell.

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Old 08-02-2002, 07:48 AM
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Join Date: Aug 2001
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actually the resistor is in parallel and works by tricking the ECT into believing that the engine is hotter than it really is. here's the original DIY thread:

how to change aux fan cut in point

it's also quite possible that your radiator has lost it's efficiency over the years. has it been cleaned of debris and flushed?
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Old 08-03-2002, 07:28 PM
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Thanks for the replys.

I found the two fan relays, where suggested. The green one in the middle right hand slot has a 30A fuse and I believe this runs the fan at full speed when the temp sensor on the thermostat housing tell it to.
The other, behind it, is blue and had a 15A fuse which had blown. I carefully opened up both relays and they looked as good as new inside.

I replaced the blown fuse(15A) fans started working turning slowly after the A/C had been on for a minute or so.
However, they stopped working the next day and I found the fuse had again blown.

My rudimentary knowledge of electricity made me query why, if the two fans drew 30A in one operating case and it is the start-up surge that blows the fuse, why only 15A on the A/C relay I think the previous owner may have fitted the wrong fuse.

The handbook and manual are no help here. It would have been very useful if MB (or whoever) had printed the fuse rating on the relay, along with all the other numbers and makers codes.

Anyway, I've upped the fuse to 30A and it has worked okay since.

Next payday, I'm investing in new MB antifreeze, and will thoroughly flush out the system first. Anyone have knowledge of "Febi" antifreeze which my local independent tells me is to MB spec - but less tha half the price!

Fingers crossed that the uprated fuse fixes the problem!!!
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Old 08-03-2002, 10:26 PM
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Join Date: Aug 1999
Location: Florida / N.H.
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The relays and fuses are a little confusing.
The a/c relay is actually the low fan, but takes the 30A fuse . The reason for this is that the low fan uses a resistor in series to drop the voltage to obtain the lower speed, but this also makes for a higher current demand.
The High fan is is straight 12v with activation from the Blue relay
and there has been a change on the 15 A to 25A.
Also , be careful of relay location as they are interchangeable...
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Old 08-04-2002, 05:45 PM
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Thanks Arthur, that is exactly what I found.

I've replaced the blown 15A fuse on the blue (rearmost) relay with a 30A fuse and the fans now run properly - a minute or so after the A/C is switched on.

The Green relay - high speed/high temp - has a functioning 15A fuse which I will replace tomorrow with a 25A as you suggest. I don't want that fan blowing it's fuse in "emergency, super hot" situations. I suppose if everything is working correctly, this mode of fan operation will never be needed.

Whilst I was in the independent dealers buying a new pack of fuses, I splashed out and treated my E280 to some new repeater indicator lenses. Over here, our cars have to have repeater turn indicators towards the rear of the front wings (sorry...fenders?)about six inches in front of the door. These are orange, rectangular, about 2.5 x 1 inch high and are the only orange lenses on the car. I think this really cheapens the look of the car and my new white lenses (with "invisible" orange bulbs) look 100% better.

Anyway, thanks again from a very wet (and unseasonably) humid London!
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Old 08-04-2002, 07:00 PM
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Join Date: Aug 1999
Location: Florida / N.H.
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The Green relay - high speed/high temp - has a functioning 15A fuse which I will replace tomorrow with a 25A as you suggest.>>

I don't have wiring diag. for UK spec cars , but all US version 104 engs relay layouts show the blue relay to go in slot "C" [foward]
This is relay K9/high speed -eng temp activated.
The green relay is the low speed [a/c high pressure activated] and it's posistion is slot "B" [ rearward]. This is relay K10.

If the wiring is the same as US versions , I believe someone has switched the relay locations and that is why you have had fuse failures...

If you find this to be the case , then you want the posistions as stated and want a 30A in the green relay , in the rear slot,,,, and the blue relay in the foward slot with a 25 A fuse....
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Old 08-05-2002, 03:22 PM
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Arthur, you're a star.

As you suggested, the relays were the wrong way round!!!

The numbers and colours are as you say, and - stupidly - these relays fit in each others sockets.

The fans now run slowly, intermittently, from about 90 degrees and I'm hoping not to have to experience the "fast" fans at 100 degrees plus! However, I know they work as when I remove the plug from the sensor on the thermostat housing, they speed up after about one seconds delay.

Anyway, it's all sorted now, with the correct fuses.

All I need now is for the rain to stop and the sun to return in order to test it!!!

Can you fix that?

Thanks again. This is the first ever no-cost repair to my Mercedes Benz!!!
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Old 08-05-2002, 04:33 PM
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Join Date: Aug 1999
Location: Florida / N.H.
Posts: 8,784

Once you see how the 2 systems work, it's pretty simple...

I think the confusion comes from the fact that the same fans/motors are used by two different independent systems/circuits with there own sensors/relays/fuses

Last edited by Arthur Dalton; 08-05-2002 at 05:34 PM.
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