1995 Mercedes Benz 300d auxillary fans

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Suspect faulty cooling fan relay. Replace relay with known good relay to see if solves problem. If not, inspect engine coolant temperature sensor.






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I have already replaced the old relay with a new one and there was no change. Is the engine coolant temperature sensor blue and located on the top front of the engine where the radiator hose enters the block? If so,is there a way to check it without replacing it?

I have already replaced the old relay with a new one and there was no change. Is the engine coolant temperature sensor blue and located on the top front of the engine where the radiator hose enters the block? If so is there a way to check it before replacing it?

It is located at center, upper engine area, mounted in rear of intake manifold. Best way to check is to use a scanner that shows what the sensor is doing. It is much cheaper just to replace the sensor. First inspect wiring to the sensor to ensure it is not damaged.







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I looked on the underside of the intake manifold.
What I see is not the same as a picture of a coolant temperature sensor that I get on the internet. What I see are 2 electrical plugs plugged into a part mounted on the manaifold with vaccum lines attached. Is there a picture for me to look at?
If you can't provide a picture where can I look for one?

You may be looking at the MAP sensor...below is an example picture of what the sensor will look similar to

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On the block toward the rear on the left side I find a part that is blueish in color with a wire plugged in. Your picture doesn't match any of the parts picture that I have pulled up. Could we be confusing diesel and gas engine parts?

Your vehicle is gasoline correct? I have not searched for any diesel parts..

When I logged on I entered that this vehicle is a 1995 E300D meaning diesel. Please advise if you need additional information.

Sorry about that. I can not find a picture of the Diesel ECT sensor but I have dug up some wiring diagrams. First of all, I would like to bring to your attention that you have 2 relays for the auxilliary fan. Did you test both of these relays? Next, it appears the ECT sensor has a brown/yellow wire and a blue/yellow wire to it. Perhaps this will help you determine the correct sensor. Once you find the sensor, ohm it while the engine is at operating temperature. It should be reading 300 - 400 ohms. If it is higher or lower than sensor needs replaced.

I located the brown/yellow and blue/yellow wires and they checked 400. Both relays have been checked and they are good. Waiting for your advice.
Thanks.

Ok. Since these are both good you will now have to check the preresistor. It should be resisting currents to prevent the fans from running when not needed. If it is letting currents pass all the time then it has failed. A wiring diagram of the cooling fan circuitry is below to help you locate teh preresistor.

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I removed the resistor and tested with the ohm and current flows through. Does this mean it is faulty and needs replacing?

Yes, most likely it has failed. To be certain you may want to switch it with a known good one if available before purchasing the new part just to be certain it is not something in the wiring instead of the preresisotr itself.







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The resistor part was not availabe locally. I ordered part and it will be up to 5 business days before I receive it. I will advise at that time. Thanks.

I ordered,received, and installed the preresistor with much disappointment. I still have the problem with fan coming on when the ignition is switched on.

On April 10 I CORRESPONDED that I ordered the preresistor and that it would take about 5 days to receive. I received it on April 15 and installed it and with great disappointment it did not correct the problem. The auxillary fan still comes on whenever the ignition switch is turned on. I notified bluelighting and have not heard from him. I would appreciate some answer. Thanks.

Sorry about the delay. I had to get some info. from partner technicians. Had two tell me that the Aux Fan/AC compressor pressure switch could be grounding the circuit to activate the relay. One of the techs has a lot more exp. with Mercs and stated that they have a lot of issues with there wiring harnesses when they age.

I inspected the wires and didn't see any damaged wires. However I unplugged the pressure switch and the fans still come on.Does that indicate that the pressure switch is bad? Can you offer any more information as to where I should look? What now?

Hi,

If fans still work after unplugging the pressure switch, it is not at fault.

If the fan is working at low speed when ignition switch is turned ON, unplug the A/C push button control unit and see if it stops. If yes, the control unit is shorting to ground causing the fans to turn on.

Of the 2 auxillary fan relays, does unplugging any particular one stops the fans? If yes, which one, or do you need to unplug both before the fans stops working?

Removing the 30A relay stops the fan. Removing the 15A relay does not affect the fan. I have not located where to unplug the A/C push button control. Can you help me locate it?

Removing 30A fuse means the auxiliary fans are working at high speed and the circuit should be controlled by the A/C compressor pressure switch. Test the Brown/Blue wire from relay to A/C pressure switch for shorting to ground. Unplug both relay and pressure switch to test. Here are a description of the Push Button and auxiliary fans.