Aux Fan Relay Location - Help!
 

I have a 1994 C280 with 90K miles. I was stuck in a long traffic jam on the highway on the weekend and watched at my coolant temp went up to 115 degrees before the fans kicked in. The fans cooled the car back down to about 105 before shutting off. I've done my searching and from what I've read, this indicates there is a problem with the low-speed fan, which should have come on by itself at around 105 degrees. I would like to check the fuses on the relay that control fan operation, but I just don't know where the relay is. I've checked my Haynes manual, but as it doesn't include the C280 version, I've had no luck.

I know that the answer is going to be right infront of my face, and I'll feel like a big idiot when I find it, but could someone tell me where to find the relay so I can check the fuse?

Just to assist in the diagnosis, I understand that having low refrigerant pressure in the a/c system can sometimes cause the fan to not engage. I don't have any refrigerant at all, as my compressor leaks and I have to replace it later this month.

Thanks!

Jonathan

The low fan is ac system activated , so don't look for a low fan with a bad ac system
The high fan is coolant temp activated at 105C , so you have high fan, but it seems to be cutting in a little late .. that is the blue 2 wire sensor on the thermo housing area.
The realys are behind the fuse box, under a lid that has to be unscrewed .. but you know fan works , so it is not a fuse problem.
I suspect sensor is a little out of spec.

If you a/c system is kaput and you have a lot of heat related driving to do, you can put a simple switch in the cabin that jumpers the high pressure sw at the reciever/drier and that will allow you to control the low fan circuit manually when you need them to get out of a jam. seeing the ac is not using them ,you might as well get something out of them , even if just for emergency back-up.

Thanks for your reply, Arthur. I appreciate it.

Jonathan

You are welcome and Thank you for the explicid posting, makes it easy..

Follow-up Post
 

Just to keep all the information on record here, I did open up the lid behind the engine-compartment fusebox to check the 15A fuse in the blue relay. The fuse was blown. The 30A green fuse was fine. I replaced the fuse and I shall see what happens. If that doesn't fix my problem I'll change the sensor that Arthur mentioned, and report back on that. The sensor is only a $50 (Canadian) part, but I'll go with the .50 fuse first.

Thanks again.

Jonathan

Problem Solved
 

Just to close off this thread, it was in fact the fuse that was causing the problem. I took the car out for a nice hard run, then parked it with the motor idling. I watched as the needle crept up towards 100C, and just as the needle crossed the 100C line the fans kicked in and ran until the temperature was down to about 95 or so.

Jonathan

The blue relay is the a/c aux fan relay and it is a 15A fuse . This relay is LOW fan, not high. Your green relay is the high fan and you did not have a blown fuse .. You started earlier that the fan did come on high, so that is indication that you did not have a bad fuse on high fan.
Your low fan fuse replacement may have you thinking that you solved the problem , but you have simply replaced an earlier blown fuse for the a/c low fan.
Blue relay has nothing to do with coolant temp sensed high fan. The only way it would is if someone had swapped the blu/grn relays . but then you woiuld have never had a high fan.
Watch the temps and see if the sensor is erratic..

http://www.peachparts.com/shopforum/diesel-discussion/125502-95-e300d-high-engine-temp-only-ac.html?highlight=blue+relay

Arthur,

I'll definitely keep an eye on the temperature that the high speed fans come on to see if it is consistent or not. If there is anything erratic about it, then I'll change the coolant temp sensor right away.

Thanks again for all your advice through this. I appreciate the time you've put in helping me.

Jonathan

J
Very Good

just wanted make you aware of the relay/fuse difference , as it is a common confusing point on these systems .
..and you will see the relays swapped sometimes , adding more confusion..
[that usually happens when someone suspects a bad reay and does a swap for testing purposes]
The basic thing one wants to remember is:
HIGH FAN is for engine temp [ and triggered by the ECT sensor on the engine]
LOW FAN is for a/c system [ and triggered by the high pressure sw on the a/c drier]

Each has it's own relay and fuse , and each is an entirely seperate system.
If one has no high fan, he does not go looking into the low fan circuit and vica/versa..
Another tip for any readers of this post:
The fuse on the blue relay that always seems to blow has been upgraded by Benz to a 25Amp, as thay found the 15 was not sufficient , specially when they came out with the dual fan set-up... So, if one has no LOW fan , one of the first palces to look is at this 15A fuse , as it is the most common electrical fault on NO low Fan complaint. The other would be a pressure problem with low refrigerant charge being the culprit [ not reaching adequate HIGH side pressure to trip low fan].
A simple jumper of the high P/SW. is a good diagnostic trick here as with the jumper, if the fan does not come on , the fuse is suspect, but if the fan then starts , low refrig. is suspect.... [ or a remotely possible bad high pressure sw. ]