I'm working on a '94 E320 that has a strange aux fan issue.

Its an earlier car with the separate gauge sender and blue two pin fan sender. The aux fans come on at high speed as soon as the ignition is turned on.

The fans cut off when I remove fuse 7 or the single pole sender connection.

The car has a recently replaced engine wiring harness.

I replaced both the high speed and low speed relays and the blue two pin sender. The fans still run on high speed regardless.

Also the temp gauge does not work.

Thoughts? Bad ECU?

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David Hendy
'97 Renntech E60RS
'97 GMC 2500 Sub' Diesel
'95 E300D
'88 300TE
'88 250TD
'84 L/Rover 109

Probably a short somewhere. Trace the wires starting from the blue plug.

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1990 190E 3.0L

Check the blue plug on the temp sensor. If someone shorted it, then the fan will come on full speed when you switch on the ignition and the A/C compressor will not engage.

Remove the plug and check the resistance across the pins.

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1986 300E (3/2002) Rear ended and totaled (10/2009)
2000 Xterra (5/2000)
1992 400E (11/2009) -Sold
1986 300E (12/2009) - Sold
2004 E500 Wagon (6/2013) - Sold

Is this an updated wiring harness or an old style "ECO JUNK" replacement?

Seems to me there is short in the wiring.
The temp gauge does not work either, something is pulling the signals to ground.
If I only could remember how this is supposed to work with the fans.
I think, I remember from the electrical diagram, that they don't go through the Computer.
They purely get switched by relay (I believe) that gets triggered by the engine coolant sender unit and / or by the AC pressure switch.
Also, I believe there is no HI or LO speed with them. There is only an ON or OFF. Unless the 94 had a single fan with two speeds.
If the Engine temperature is below 100C (I believe) then no AUX fan should run, without AC on, that is.
If the AC is on, the AC pressure switch is switching on one (SUB fan, again my memory is short) while driving and the temperature is going beyond 100-110C the other fan kicks in too (AC ON).
If they come on as soon as the ignition is on, then there is a short circuit, I bleieve there was something like a failsafe.
My best bet, your replacement harness is faulty.

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Mostly, I don't know notin, I just know where to look.
I am looking back, to over 30 years in Electronics Design.
Electrons don't care if they move in a car, computer or relay!

95 W124 E320 M104.992 - Because, I love to repair, naaaah!
Over 221,000 Miles
Cheers,
Norbert