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simmo300e 03-29-2005 03:50 AM

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.

cc260E 03-29-2005 08:05 AM

Water pump, check whether you have a leak of coolant or level decreasing.

aharley1 03-29-2005 08:30 AM

i didn't catch if you replaced the fan clutch... mine (560SL) was doing the same thing. I replaced the fan clutch & that fixed it. Simple test for the fan clutch...let the car warm up. At idle, stick a used radiator hose (or something similar...rolled up towel, etc) into the fan BE CAREFUL. If the fan stops easily, the clutch is bad.

jayb79 03-29-2005 09:27 AM

Check out post #8, it may help.

simmo300e 03-30-2005 03:20 AM

thanks for the suggestions.

yes, I did replace the fan clutch, and yes my mechanic did the tea towel in the blades test beforehand to check it was bad :)

Checked yesterday and found the engine temp sensor is kicking the fan in at high speed when the temp gets to 107 or thereabouts, so not worrying about that any more.

Having checked all the components, seems the fan control is acting as it should, which leaves only the cooling process.

A failing fan I find unlikely seeing as it actually runs. Can't think why an electric fan would slow down instead of stop if it was failing.

The other suspect is the AC condensor, core or whatever its called that sits in front of the radiator. its either running too hot or its not letting air through at all, which seems more likely. Plan to try cleaning it out this weekend.

haasman 03-30-2005 04:42 AM

Make sure you don't have an air pocket. There is a 14mm bolt on top of the head where you can bleed.

I have started using Water Wetter product and have seen a significant lowering of engine temps.

Keep us posted.


r_p_ryan 03-30-2005 05:49 PM

hmmm, bad brushes on an electric motor will cause it to slow down and draw more amps. I have two M103's and they run at the same temp based on measuring with a $95 IR thermometer, but they read about 10-15 different at the dash guage. I don't have much of a solution, but the IR thermo is fantastic at diagnosing cooling system problems. You can point it at the inlet and outlet at the radiator and determine how much cooling is happening, you can also determine if the thermostat is opening, if a cylinder is running hot, the temperature of the fan clutch, the temperature at the temp sensors, etc, etc.


jayb79 03-30-2005 09:33 PM

Dry bushings will also slow down the fan and make it draw more amps. A few drops of oil in the right place could do the trick.

1991300SEL 03-30-2005 09:45 PM


Originally Posted by haasman
Make sure you don't have an air pocket. There is a 14mm bolt on top of the head where you can bleed.

haasman is correct about the 14mm bleed plug on top of the 103 head. There are actually 2 - you remove the one closest to the front of the car, BUT............

Be REAL careful taking it out. This plug strips REALLY easy and if that happens......

simmo300e 03-30-2005 10:09 PM

my mechanic came over last night and he thinks its an air pocket, so we'e going to bleed the system tonight.

as for the single post sensor (see my earlier question), he says it is an 85 deg sensor and probably isn't functioning, and the second stage fan kicking in at 107 deg is governed by the 2+1 sensor at the back of the head. we're also going to change that tonightt and see if we can get it working.

haasman 03-31-2005 01:45 AM

PS regarding the

There is a 14mm bolt on top of the head where you can bleed.
be sure to not do it when the engine is hot. A real pain to get the bolt in when hot!


simmo300e 04-01-2005 05:35 AM

Air pocket. Took more than two litres of water once I removed the 85deg temperature sensor, which was not functioning and has been replaced.

Problem solved.

Thnaks for all the tips and suggestions. I recommend removing the temp sensor to bleed the head, easier to get at than the bleed screw and not so fiddly to replace.

pentoman 04-02-2005 09:30 AM


Originally Posted by aharley1
i didn't catch if you replaced the fan clutch... mine (560SL) was doing the same thing. I replaced the fan clutch & that fixed it. Simple test for the fan clutch...let the car warm up. At idle, stick a used radiator hose (or something similar...rolled up towel, etc) into the fan BE CAREFUL. If the fan stops easily, the clutch is bad.

I don't know about the M103 and the V8 (111? 112?) but on a 102 four cylinder, at idle you can stop the blades with your hand, the fan basically doesn't spin at all - so long as the engine temp stays below ~105 degC (pretty much all the time in England) at which point the fan switch cuts in - then the fan should spin and I wouldn't try to stop it!

Is the 103's fan really supposed to be engaged all of the time?? Doesn't that sap power and make it sound like a vacuum cleaner? There must be no fan switch then?


simmo300e 04-07-2005 07:10 PM

Ah, that's because the 102 engine has an electromagnetic fan clutch, which engages at 105deg, I guess.

The fan blades on my 102 have been spot welded to the rotor, so the fan is on all the time. Not sure if this was done by the PO because the solenoid was busted or because we live in the tropics, but it seems to work fine.

Bit noisy though, so might get round to changing it in June when I have to drive the 230 around (300e going in for paint)

On the 103 engine, the visco clutch runs the fan at low speed up to 2000 rpm, at which point it kicks in to high gear and really starts moving some air.

I think the extra two cylinders are there to compensate for the power loss :)

BillyRx 10-31-2016 08:13 AM

Hi I'm a newbie. Where is this bolt located?

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