My aux fan runs full speed all the time.

Can somebody tell me which temperature sensor is the one controlling the electric fan on my 87 420sel? Is this a normally closed or normally open switch?

Also, which switch (location) in the ac system turns the fan on if high head pressure is present? Is this NO or NC?

Finally, Relay D is for the aux fan resistor. Where is this resistor, and how does it work?

Thanks in advance.

__________________
Thank You!
Fred
2009 ML350
2004 SL600
2004 SL500
1996 SL600
2002 SLK32
2005 CLK320 cabrio
2003 ML350
1997 C280 Sport

A Dalton

__________________
A Dalton

I can't find the resistor. I went to the parts section and found a picture (great way to find the unknown parts - thanks!). It looks similar to resistor found on Chrysler ignitions many years ago - is it about the same size as that? Where else could it be located on the car? I followed the fan wire - it goes to the fuse box. I can't imagine putting a resistor in the fuse box - it would get too hot.

Again, thanks!

__________________
Thank You!
Fred
2009 ML350
2004 SL600
2004 SL500
1996 SL600
2002 SLK32
2005 CLK320 cabrio
2003 ML350
1997 C280 Sport

Certain years of the W126 had the resistor around the fusebox, but i think they moved it as it was a fire hazard as leaves built up.

__________________
'94 E320 with 272,000 miles. '92 500SEL 133,000 miles.
Mods to the '94: TomTom GPS in place of ash tray, Auto Dimming Mirror ,Memory seat, DEI 545T Auto lamps, DEI 530T Auto Up/Down Windows, DEI Remote Start, Motorola Bluetooth® Car Kit, Built in phone charger using factory phone wiring, RainTracker Rain sensing wiper, Keyless Entry, Ambient lighting of center storage compartment. Soon: factory heated seats, REST
Mercedes-Benz is my addiction of choice.
http://www.youtube.com/vanwiek

I tested the temperature sensor and it indicates a closed switch with the engine cold. I figured disconnecting the wire would simulate an open switch, but the fan still runs at full speed. So, I do need a new temperature switch because it is closed when it should be open.

The AC switch is open as it should be.

I can not find the resistor. It is not anywhere near the headlight or the inner fender. I even disassembled the fuse box to see if it was in there. Not there.

1) By disconnecting the temperature switch, wouldn't that be the same as having a NO switch? It is only a one wire switch.
2) Anybody know where this resistor is? What should its resistance be?
3) What else can cause my issue?

Thanks
Fred

__________________
Thank You!
Fred
2009 ML350
2004 SL600
2004 SL500
1996 SL600
2002 SLK32
2005 CLK320 cabrio
2003 ML350
1997 C280 Sport

I think they have a mistake in the parts listing.
That single wire sensor may be the temp gauge sensor.
Here is the aux fan High speed sensor
http://catalog.peachparts.com/RenderScriptTemplate.epc?_cmd=epccat_VehicleAAA&cookieID=2FG17RJ3A2FM17NS85&yearid=1987%40%401987&makeid=MB%40%40MB%40%40X&modelid=420%2DSEL%2D001%40%40420SEL&catid=G%40%40Cooling+System&mode=PA&subcatid=G5045@@Engine+Temp%2E+Sensor&source=www.peachparts.com&clientid=catalog.mercedesshop

Do a test :
Look at the temp gause with warm car and then unplug that single sensor and tell me what happens to gauge .......

then, unplug that 3 wire sensor and see if fans go off.
Is this single fan??
You may also have a stuch high fan relay..

__________________
A Dalton

Aha! That is the problem!

I failed to mention in this post that the car has sat essentially unused for two years. I thought I did a good job of preparing it. Anyway, turns out a related issue is solved as well. The AC clutch would not engage. I figured after two years, it probably leaked out from my 21 year old beast.

I disconnected the single wire, no change, fan blowing full, no A/C. I reconnected it and disconnected the 2-pin connector, the fan stops, and the ac engaged. And it blows cold too!

As for the other sensor, the temperature gage did not move from 40 in either case. But the temperature gage does work. So who knows hwat that sensor does.

Thank you Arthur Dalton!

Now to buy a switch and replace it.....

__________________
Thank You!
Fred
2009 ML350
2004 SL600
2004 SL500
1996 SL600
2002 SLK32
2005 CLK320 cabrio
2003 ML350
1997 C280 Sport

I went to AllParts Express right here at mercedesshop and paid $37.07 for the Behr switch delivered to my Wisc home, along with two windshield washer pump grommets that are leaking.

Great price

__________________
Thank You!
Fred
2009 ML350
2004 SL600
2004 SL500
1996 SL600
2002 SLK32
2005 CLK320 cabrio
2003 ML350
1997 C280 Sport

I believe that chassis also has an electric clutch on the main fan behind the radiator and that would be the sensor for the clutch engagement................

I will even give you a little tutorial on that 3 wire sensor/ sw.

It is the 105/115 sw
It closes one set of contacts at 105 C for the high fans to prevent overheat condition of coolant and it closes the other set at 115C to kill the a/c compressor in the event the coolant gets into that danger zone.
So, the diagnosis on yours is an internal short to ON for both contacts , giving you both complaints of Constant High Fan and NO A/C...simple as that.

__________________
A Dalton

And again thanks! I learn something everyday on this site! I don't know how anybody could afford a fleet of old Benzes without this place.

__________________
Thank You!
Fred
2009 ML350
2004 SL600
2004 SL500
1996 SL600
2002 SLK32
2005 CLK320 cabrio
2003 ML350
1997 C280 Sport

Now yet another question on these relays. With the car running, the seat relay and the window relay heat up, even though no seat or window actuation is occuring. I hear no motors running for either accessory.

What could cause this issue?

__________________
Thank You!
Fred
2009 ML350
2004 SL600
2004 SL500
1996 SL600
2002 SLK32
2005 CLK320 cabrio
2003 ML350
1997 C280 Sport