Why would my aux fans come on at startup with a cold engine?

For the morning commute, with night time temperatures having been in the low 60'sF, the start is normal. The car will then sit for about 8 hours with ambient air temperatures in the mid 70'sF. With the second start of the day, the aux fans will be on with the start and cycle on and off for about a minute. The temperature gauge is at 40-50C and rising to operating temps. Then everything is normal for the rest of the day. Starts with the car warm are normal.

Any ideas?

Thanks,

glenmore
1991 300CE

Mine does kind of the same thing. I'm interested in the answer.

I noticed you're from Los Angeles, maybe there's an earthquake coming:p

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5 speed '91 190E 2.6 320,000 mi. (new car, fast, smooth as silk six, couldn't find any more Peugeots)
5 speed '85 Peugeot 505 2.5l Turbo Diesel 266,000 mi. (old car, fast for a diesel, had 2 others)
5 speed '01 Jetta V6 (new wifes car, pretty quick)
5 speed '85 Peugeot 505 2.2l Turbo Gas 197,000 mi. (wifes car, faster, sadly gone just short of 200k )
5 speed '83 Yamaha 750 Maxim 14,000 mi. (fastest)
0 speed 4' x 8' 1800 lb Harbor Freight utility trailer (only as fast as what's pulling it)

Yeap ... I have the same thing going on too in my 92 500E. My MB Technician said it could be High/Low Pressure issues in the AC system ( ... $$$$). I hope not. It could also be bad sensors (AC system).

I'll report any new info as I get it ...

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1992 500E (Daily Driver)
2004 Porsche RUF 955 Dakara 550

If it is happening with the climate control off, it could be the temp. sensor in the head going bad. This is a brass colored sensor with green plastic. It will have one "2" wire connector and one "1" wire connector. Will be just past about half way back on the head new the air cleaner. When you start the engine and the fan comes on, pull the "2" wire connector off and see if the fan stops. If it does, there is a real good chance the sensor is failing. Seems to me the sensor price was bearable, but was a pain to install since you had to drain half the coolant to avoid leakage, and then had to remove the valve cover in order to get a wrench on the sensor. See if the a/c clutch will engage when the fan is on. If not, then check the sensor.

Thanks Bob ... I will try this checkout soon ...

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1992 500E (Daily Driver)
2004 Porsche RUF 955 Dakara 550

With your year car you may want to look at the Blue 2-pole temp switch as the possible culprit. Relays can stick occasionally too.

Are the fans operating at high or low speed? If stuck on high the blue switch might be the solution.

Mr C:

Who's car are you refering to?

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1992 500E (Daily Driver)
2004 Porsche RUF 955 Dakara 550

Bobk, MrC,

Thanks, that sounds like the culprit.

The fan will go on at high speed immediately with starting the car. No AC on.

I see the part at Fastlane, AC Temperature sensor, brass with blue plastic, about $30.

Thanks,

glenmore
1991 300CE

Your 400E may have the same problem. Check the voltage across the blue switch, if I remeber correctly at 1.75 volts the fans will come on and at 1.54 the fans will go off.


I played with the 400-E circuit a lot becuase most owners are not comfortable with the operating temperature being above 100-C. I have a really great modification to keep it below 90-C all day long, let me know if your interested.