Car is a 1996 S320. Only the RH fan will come on regardless of temp, but turns off ok when the car is off. I have replaced the two temp sensors on the t-stat housing. I looked for the relay that controls the aux fans behind the fuse box. There are 7 relays - I removed each one, one at a time and the fan never turned off. Pulling the fuse does. I am thinking there must be a relay I am not finding for the aux fans, maybe a change in 1996?
Vehicle: Current 1992 - 500SEL, W124 - 1989 230E (sold)
Posts: 1,363
did you consider fact that it could be the actual fan, as there are only two relays for fans one for stage I and another for stage II but they should run both fans at the same time.....
they are behind the fuse box(you need to remove cover and unscrew 4 bolts inside fuse box and remove whole top and they are in a back)
there should be some 1 silver cube, 1 pink cube (i think stage I) and one larger black one (stage II)
It's not the fan, I checked for voltage to it and there was none. All of the relays are basically the same size - cubes. I don't have a larger one that I can see. I removed each relay and the fan still came on, so none of those can be the right one. I am thinking this car might have the 6-stage fans - maybe a solid state controller. Anyone know?
More about the mystery:
The AC diagnostics says the coolant temp is 105. It always says that, even first thing in the am. When I pull the connector off of the sensor, it STILL says 105. The sensor was replaced a month ago and before then I got the same readings.
I did learn that one fan is bad. A direct 12v to it does nothing and metering the fan itself, I get an open.
I am able to turn the remaining fan on hi using the AC diagnostic control so I know the controller is ok. But the AC thinks the temp is always 105. What is even stranger is that the engine management knows better - the trans has its usual delayed upshift when cold and the engine start perfectly when cold.
Vehicle: Current 1992 - 500SEL, W124 - 1989 230E (sold)
Posts: 1,363
what is the resistance on the new sensor when cold? (just pull the plug and multimeter it....) also which sensor we talking about the coolant one with two prongs?
I'll check the resistance this weekend - the car never sits long enough to cool down during the week. The sensor is a four-prong. I metered opposing pins and on one set got 482 ohms and the other about 1100 after it had sat for a couple of hours. The other sensor on the t-stat housing is a single prong and it goes to the temp gauge. I am thinking that the four-prong sensor is actually two and the signals go to two different places. One may be for engine management and that must work, because I get an instant cold start with high idle and the customary delayed upshift when cold. But it is a new sensor and the old one did the same thing with the aux fans, so I doubt it is the sensor(s).
I am wondering now if someone fitted some kind of resistor in the wiring to make the aux fans come on all the time to cover up an overheating issue. But you'd think they would put it right at the sensor, and there is nothing there. Still, why would it always say 105 at the climate control?
Replaced the LH aux fan today. PIA. Most of the little plastic clips broke while removing them and it took some disassembly to get the fan to come out. Took maybe 2.5 hours, could do it in an hour now.
Of course, the reason the fan quit is because it wore out -the fans are on all the time, summer and winter because they think the coolant temp is 105C all the freakin' time. I have no idea why. I followed the wires as far as I could and there is no obvious place anyone could have inserted a resistor. Maybe in the CC unit itself? The sensor is ok, yet the reading is always the same. Quite a mystery. I'd love to solve this before I wear out any more fans.
Replaced the LH aux fan today. PIA. Most of the little plastic clips broke while removing them and it took some disassembly to get the fan to come out. Took maybe 2.5 hours, could do it in an hour now.
Of course, the reason the fan quit is because it wore out -the fans are on all the time, summer and winter because they think the coolant temp is 105C all the freakin' time. I have no idea why. I followed the wires as far as I could and there is no obvious place anyone could have inserted a resistor. Maybe in the CC unit itself? The sensor is ok, yet the reading is always the same. Quite a mystery. I'd love to solve this before I wear out any more fans.
Is this the set up you have?
Oh, and the four prong one IS two sensors in one pack.
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