DougE280 in UK

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?

I’ve 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 – right up to nearly red then (finally) both auxiliary fans cut in together and the temp soon drops to normal….only to repeat this every five minutes or so. It’s all getting a bit too hot for my liking and I don’t remember it doing this last year. I think maybe the 100 degree switch / relay (or whatever) isn’t 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 don’t want to burn anything out shorting out the wires!

Anyone help?

Thanks

Ron in SC

I've been told that if there is not enough refrigerant in the system thtat the fans will not come on when they should

RPM

You more than likely have a blown fuse on the relay for the low speed for the aux fans.Both fans run at the same time and are turned on by the pressure switch on the a.c. receiver/drier for low speed and by the temp switch on the water pump for high speed. Both speeds are controlled by seperate relays located in the back portion of the fuse box [towards the firewall]. It is necessary to remove the cover over the rear portion of the fuse box to access these relays.The cover is held on with six screws.You will find the relays have a fuse on top of the relay. If you pull the relays out one at a time you can check the fuses.

DougE280 in UK

Thanks chaps - particularly RPM

That's exactly what I found. The green relay has a 30A fuse which was okay - this is for the high temp/high speed mode, I believe.

The blue relay behind it had a blown 15A fuse.

I replaced the fuse and ....bingo...it all started working properly.

This new 15A fuse blew within an hour and I have replaced it with a 30A and it all now seems okay.

My rudimentary knowledge of electrics makes me think that these two fans must draw the same amount of current whatever their mode/speed of operation - as maximum load/surge occurs at their start-up.

It would have been useful if MB or whoever printed, amongst all the other writing on the top of the relays, the fuse rating.

Anyway thanks again and "fingers crossed"!!!

Arthur Dalton

The green relay is actually the low fan [ a/c pressure activated].
The reason the fuse is larger for low fan is cuz there is a resistor in the circuit to limit the power to the fans. This makes for a larger draw.
The high fan fuse has been up-graded to a 25 A by Benz.
Remember, the a/c pressure controls low fan and if it does not come on as often as it used to, that is an indication of slight low freon level..

Joseph_H

For those of you who complain of overheating problems with their Benz, I have been bothered by my E420 running around 100C and higher during stop-and-go traffic and I had been researching this same issue and found quite an interesting site by a gentleman who seems to have a simple, cheap, but yet a practical solution. He calls it Cool Harness (CH) and he sells it for $45 and takes 5 minutes to install. This solution seems practical from engineering standpoint and no risks involved. Rather than going into more details, just check this site out on the following link: (Also he offers many other solutions. Advises etc…)

http://pages.prodigy.net/jforgione/MB_S500.html

I just ordered this product and will update everyone with results.

Good Luck!