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Old 03-29-2005, 03:50 AM
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simmo300e simmo300e is offline
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Join Date: Jun 2004
Location: Bali, Indonesia
Posts: 200
300e running hot in traffic

Having some problems with my 103 engine overheating in traffic. previously the car would run at a constant 85 deg, and only rise to the second mark on the guage when really stuck in traffic.

However, since my fan clutch failed, the stationary temperature is all over the place. I've replaced the visco clutch, thermostat and radiator, because the neck had cracked. For a while, the car would run below normal temp, which was fine, but then it started to play up in traffic, sometimes going up to the 120 mark.

If I rev the engine to 2000 so the visco clutch kicks in, temp drops rapidly.

The aux fan works but the previous owner had jury rigged the sensor at the AC, replacing the pressure switch with what is apparently a 50 deg temp sensor, clipped to one of the Ac pipes. Don't ask me why. This is connected to the two wires which seem to power up the aux fan. If I jump the wires, the fan starts running at low speed.

OK, so I've done some searching and found out plenty about the aux fan circuit. I have connected up the AC pressure switch and found it works, or at least the aux fan starts up once the AC is running and has built up pressure. Its now connected in parallel with the temp sensor so both are working.

I have located the dropping resistor and ascertained that the fan is running at low speed when activated by the AC switch by jumping the connections. So now I know the fan runs at two speeds.

I switched off the AC and let the engine idle until it reached operating temperature, around 87 deg, but no life to the fan. I therefore guessed that the engine temp sensor was faulty and set off for a drive.

The engine still gets very hot when stationary or in traffic, and I have now removed the resistor so the fan only runs at high speed. Althoguh I haven't tested this fully, the aux fan does not seem to be powerful enough to bring the engine temp down to normal levels on its own.

I switched the AC off but the fan kept running, which makes me question whether the engine temp sensor is faulty. I tried to find it but couldn't see anything that matches the descriptions I've seen elsewhere in this forum. Around the middle of the head, there is a temperature sensor kind of plug, with just one post, which was not connected to anything. Can't find any reference to this.

At the back of the head, there is a 2 + 1 pin sensor, perhaps this is it? I tried disconnecting the plug with the engine running, expecting the fan to start. It didn't.

My questions are these:


If the engine temp sensor engages the aux fan at high speed, is this speed the same as jumping the resistor or should it run even faster?

Where is the engine temp sensor and what is it connected to?

Where is the K9 relay and how do I check it?

Should the aux fan continue to run after the engine is switched off, providing of course that engine is hot? I seem to recall this should be the case.

I understand the fan is activated by two circuits, are they entirely separate or do they meet up somewhere and power the fan on a single pair of wires?

Could it be that the fan is failing?


Thanks in advance for your help guys. I see this is a quite common problem and would appreciate some guidance on fixing it.
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1987 300e manual 250,000 km (sold)
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