300SDL temp sensor question

The Mercedes diesel engines around this vintage all have the same wiring for the aux fan. These engines include OM601 (2.2, never turbo), OM602 (2.5, turbo or non) and OM603 (3.0, turbo). I am not familiar with earlier or later diesel.

There are two parallel circuits driving the aux fan. You can distinguish which circuit is running by listening to the fan noise.

1. A/C. When A/C is on, the pressure will close a switch (I forget which one, what color, though I can check ETM if you want) near the dryer/accumulator. This switch turns on a relay which turns on the aux fan in HIGH speed (pretty noisy). This circuit has nothing to do with the engine temp. The A/C pressure can be built-up within seconds after turning on A/C.
2. Engine temperature. The temp sensor will close around 105 or 110 degree, not 85-90. This sensor also turns on a relay which turns on the aux fan via a resistor (white, ceramic, about finger length) near the dryer. The fan is running on LOW speed in this case (due to the resistor). I have seen car bypassing the resistor. If so, the fan will run on high too. So make sure the resistor is still there and working.

So there is no "magic", unlike modern cars where most actions are driven by computer and there is almost nothing for a DIYer to check. I feel that your "mechanic" did not really know what he was doing. When you know how the system works, all debugging can be done by a multi-meter and a jumper wire.

I cannot explain all the symptoms you saw, but most seem to be normal (as of now). I would suggest you to watch it closely. As long as the A/C circuit is OK, if you don't feel comfortable seeing the engine temp, just turn on A/C to cool it down. On mine, turning on A/C keeps the engine temp really stable, not low but staying around 90.

Good luck.

wc98 wrote:

My first thought was where is he living that the aux fan would be coming on at all this time of year? As WT pointed out, there are 2 things that will run the aux fan. First is if the coolant temp is high enough. Not sure of the exact temp, but it is quite high. I have a 1980 300SD and it only goes on in very warm summer weather when idling. Typically it would come on after driving at highway speed, then parking and leaving the engine running.

The other mode is with AC on. Don't know how later models work, but on my car, that too is driven by temp. The refrigerant temp at the receiver/dryer has to be above a certain temp for that to activate the aux fan as well. On mine, again it has to be a hot day for this to activate. With the AC on, the fan will not run on moderate Spring day, say 75 outside temp.

I am in Atlanta GA. It isn't that living here is all that hot, it is just that a few months ago i found out the fan was not coming on because i was stuck in morning traffic because of an accident and i noticed the car was running abouty 110C but i knew the owners manual said sometimes it moght come close to 120 in heavy conditions, so i didn't worry. Soon after, the radiator popped. When i had the car taken to the shop, that is when he replaced the temperature switch. Since then, any time i gets close to 100C i get nervous. Normal driving it is running at 80 or even a little under. After getting off the highway and onto surface streets (on 75-80 F days and all sunny) after sitting through 3 or 4 traffic lights i see the temp up from 80C to 100 or a hair over. That is when i usually turn the climate control up to HOT and fan on high to get the water temp down because i was not aware at what temp the fan should kick in.

My suspicion would be you have a partially blocked radiator or bad thermostat. My 80 300SD maintains the temp just a hair above 80 in winter and maybe 85 in summer on 80 deg days and that's without the aux fan running. In the type of driving you're talking about, ie highway, then going through 3 or four lights, etc, on a 75-80 deg day, the aux fan shouldn't even have to come on. If it's an 85 or 90 degree day and I'm stuck in bumper to bumper traffic, barely moving for several miles, then it comes on.

These 3.0L sixes have aluminum cylinder heads on cast iron blocks. The two metals expand and contract at different rates, OK so long as the motor isn't overheated - then the head gasket is put at risk.

So the key is to over maintain the cooling system, be sure the fans run appropriately and don't charge long hills in hot weather with the A/C on full. You're already aware of the engine's temperature so the task is to fix the fans and perhaps flow test the radiator at a radiator shop, or just replace it if there's any doubt.

Nice car.

If you found 2 pins shorted together, do you really know that the closed mech actually changed the 212 switch? They are very commonly found to be defective and should be changed every so often just because of the protection that they offer. And there are about 3 different kinds with different actuating temps.

I was wondering why Mercedes had different switches for the same car. In my parts book i fould the grey 3 prong i have and a blue 2 prong. On-line i found a green 3 prong as well. Why is the not one standard switch for this car?

On Thu, 03 Apr 2008 18:39:47 -0700, "-->> T.G. Lambach

Not paranoid, cautious.

I suggest that you read #3 at

Thanks for great info... I too am having trouble with this issue too.

Now that Marshall Booth is gone (as of Nov 07 which I just learned today) Dave is THE man on W124 and OM603 engines.

I'm troubleshooting a cooling fan system right now. I've determined that the fan works by using jumpers to the fan contacts. I just need to get juice to it though. At 110C it still doesn't come on, and the AC system on this car isn't operational. I took the belt off since I never use AC. I just open the sunroof.

So I have a few questions. Where is the aforementioned temp switch? It is mentioned that the pressure switch is near the dryer, and the white ceramic resistor is also there. This raises two questions for me. Where is the actual temperature sensor for the fan? and where is the dryer? I know where the compressor is. But where is the dryer? Is a dryer the evaporator coil? Wouldn't that be inside the dash?

I know that the system shouldn't be at 110 this time of year in 80 degree weather. I am addressing the cause of that(air in the system, and a plugged air vent line between the radiator and the expansion tank.) but I would like to have this system operational for the dead of summer when I will likely need it to keep my engine cool for stop and go driving on 95 or 100 degree days.

Thank you for any help, Bill

Bill

I assume you are talking about the 84 300D you have mentioned in other post. The OM60x diesel that I am familiar with uses one serpentine belt for all accessories. If you can take one belt off, the engine must be earlier model. I guess it is OM617. Unfortunately, I am not familiar with OM617, nor W123 body.

If the design is similar to OM601, the temp switch is around the coolant thermostat/outlet at the front of engine on passenger (right) side. The switch has one pin on top. The switch connects to ground when the preset temp is reached. You can try to jump that pin to ground to see if aux fan comes on. If no, trace back to the relay (driver side, near fender). Try to switch to another (good) relay. You can also jump the sockets if you know which one goes where. I personally would not prefer to jump at the socket because the fan uses a big current. Improper jump may melt the contact or hurt yourself.

If you have the official manual (especially the ETM, Electric Troubleshoot Manual), tracing can be easily figured out. All component locations are identified in ETM too.

The dryer is a cylinder about the size of a soda can, probably behind driver side headlamp, half buried.

If you can find either switch (temp or AC) and make sure the circuit (relay and wire) works, I have seen people putting a toggle switch on top of the switch pins (or you can do so at the relay) so they can manually turn on the aux fan. Do not use toggle switch to control the fan DIRECTLY unless it (and the wires too) can handle the big current.

snipped-for-privacy@gmail.com wrote: