I replaced all of the fuses in my 87 SDL today, even though they all looked fine.

When I turned the key to the 2nd position the aux fan came on high. This didn't happen with the old fuse, so I'm guessing that was bad, but I'm also thinking that the aux fan shouldn't run constantly with the key in 2nd position.

I juggled the icecube relays by the fuse box and the fan continued to run with each of the relays I plugged in.

After searching the forum for a bit, I came upon the possibility that the temp sensor might be causing the aux fan to run.

I found this:



and when I unplugged the wire with two plugs, the fan stopped.

Is the sensor my problem, or am I barking up the wrong tree/misunderstanding the issue?

Any insight is much appreciated.

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87 300SDL

Measure the resistance of the 2 prongs with a multimeter. It is a normally open thermo-switch. It should be open circuit at room temperature, close at 105C ( grey color ). You may have a bad wiring within the 2 prong plugs. It is too much of a coincidence that the thermo-switch broke while you were changing the fuse. One of the wire contact is chassis, the other goes to a relay inside the fuse box.

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Not MBZ nor A/C trained professional but a die-hard DIY and green engineer. Use the info at your own peril. Picked up 2 Infractions because of disagreements. NOW reversed.

W124 Keyless remote, PM for details. http://www.peachparts.com/shopforum/mercedes-used-parts-sale-wanted/334620-fs-w124-chasis-keyless-remote-%2450-shipped.html

1 X 2006 CDI
1 x 87 300SDL
1 x 87 300D
1 x 87 300TDT wagon
1 x 83 300D
1 x 84 190D ( 5 sp ) - All R134 converted + keyless entry.

I don't think the thermo switch broke while I was changing the fuse, this car was asleep for a long time, I have no idea how long it's been running with no aux fan. All I know is that after replacing the fuse the fan came on for the first time...and stayed on.

I haven't even started the car b/c the fuel tank is empty atm (replacing tank screen), so there shouldn't be any reason for that thermo sensor to start the fan since the engine hasn't been run in days.

Am I missing something?

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87 300SDL

This is what you wrote. Am I missing something?? Just change out the switch and be done.

__________________
Not MBZ nor A/C trained professional but a die-hard DIY and green engineer. Use the info at your own peril. Picked up 2 Infractions because of disagreements. NOW reversed.

W124 Keyless remote, PM for details. http://www.peachparts.com/shopforum/mercedes-used-parts-sale-wanted/334620-fs-w124-chasis-keyless-remote-%2450-shipped.html

1 X 2006 CDI
1 x 87 300SDL
1 x 87 300D
1 x 87 300TDT wagon
1 x 83 300D
1 x 84 190D ( 5 sp ) - All R134 converted + keyless entry.

No, you're not missing anything, I just wanted to get confirmation that a failed switch could cause the aux fan to run constantly before I replace it.

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87 300SDL

It should have a specific resistance at ambient, and at 100. It does govern the fans. I have a similar issue in the 96, but its more intermittent ( yet at startup regardless of temps) AND the temp sensor checks ok. ???? . I simply disconnect the fans in the winter when I don't need the AC.

I actually had this same issue in my 75 300D. the car was parked in a garage for 15 years just being out about 2 months when I got it. I reversed some cooky work and noticed that when the A/C was on and or when the temp climbed slightly the AUX fan did not come on. On my model (and on most cars) the fan is controlled by the water temp sensor in the t-stat housing as well as the refrigerant temp sensor on the A/C dryer. I closed the circuit on the temp sensor and dryer and no fan. then I inspected the fan, I had power... disassembled the fan and cleaned it made sure the brushes were making solid contact. the last thing I did is replace the 8a fuse for a 16a as the diagram depicted and now the fan runs in key pos. #2 constantly from the first turn in the morning to the last revolution of the day... i have disconnected the two sensors and it still operates... everything looks undisturbed...

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MAKDUFF3

1975 300D, Feuerrot and Parchment interior

1971 220D (RIP)

I had a 603 aux fan switch stick on two occasions from wetting them when washing the engine.

Sixto
87 300D

My switch on my 300TD failed shorted, this caused the aux fan to run all the time.

If the fan stops when you pull the 2 pin plug, there's your problem.

You do not need to drain the coolant to change this, it's high enough that you might get a dribble but that's it.

The two pin plug is the fan relay coil and ground, the single pin is for the ACC compressor shut down. Threre are 2 switches in there, the stock switch is 105 c for the fan and 115 c for the ACC shut down. Both switches ground on temperature rise.

Some folks get a cooler rated switch to bring the fan on at a lower temperature. That is a good idea IMO.

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The OM 642/722.9 powered family
Still going strong
2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)

both my kids cars went to junkyard in 2023
2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles)
2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech