1982 W123 aux fan wiring diagnosis - Page 2 - PeachParts Mercedes-Benz Forum

vwnate1 · #16 07-05-2022, 08:26 PM

The W123 AC fan shouldn't draw over 8 amperes even on initial surge .

Be *VERY* cautious of using higher rated fuses when dealing with an old fan that has dry bushings and so draws more current .

The old fans can be taken apart and cleaned & lubricated .

They're not too $pendy new, I'm using a URO brand fan on one of my W123's, I left the finger guard off and it runs quieter and blows more CFM's to boot, helps a lot in Death Valley etc. where I like to go .

If anyone sticks their finger in there and gets cut, that's their fault .

I don't know the entire current path but the temperature sensor right near the thermostat housing is what kicks the fan relay proper .

Engine hot, A/C on HIGH try grounding this single wire, the fan should click on .

Sadly I'm not allowed to post any attachments else I'd take and share a picture of this temp switch .

tangofox007 · #17 07-06-2022, 11:35 AM

Quote:

Originally Posted by vwnate1

I don't know the entire current path but the temperature sensor right near the thermostat housing is what kicks the fan relay proper .

Engine hot, A/C on HIGH try grounding this single wire, the fan should click on .

That is absolutely incorrect.
The aux fan on a W123 operates solely on the basis of refrigerant temperature. Coolant temp plays no role whatsoever.
The switch on the thermostat housing is associated with the cabin blower, not the aux fan.

Quote:

Originally Posted by vwnate1

Sadly I'm not allowed to post any attachments...

That might be a good thing.

BillGrissom · #18 07-07-2022, 07:57 PM

Before rigging in your own relay, test the wiring at the relay base. In my cars, it is under the black plastic cover atop the L inner fender. In my 1984, there are 2 relays, the other for the AC clutch. In my 1985, just the fan relay since the AC clutch is actuated by the Klima box. Jumper from 30 to 87 in the base, which should actuate the fan. You can find the numbers on the relay (mirror image). Someone here said 87a, but that is the normally-open contact and not usually wired (sometimes no pin on the relay either). If the fan runs, try swapping in a different relay. You can also verify you are getting 12 V across the coil terminals (85 and 86) to actuate the relay. Should be true with key on and temperature switch leads shorted together (earlier photo).

vwnate1 · #19 07-09-2022, 12:08 PM

Sorry, I grounded this temp. sensor to test and the A/C fan came on.....

BillGrissom · #20 07-09-2022, 04:43 PM

Quote:

Originally Posted by vwnate1

Sorry, I grounded this temp. sensor to test and the A/C fan came on.....

To clarify, in the photo of post 6, there is a green and brown wire on the car side. Brown is usually ground in M-B, so assume true here. The switch connects the green wire to brown (gnd) when the temperature is not too hot. That is wired to coil- of the relay. Coil+ comes from the push-button climate box, 12 V when it wants the AC clutch actuated. The temp switch is a "disable", same as how the neutral-safety switch is wired to disable the starter relay (when not in P or N) at least in U.S. cars.

vwnate1 · #21 07-09-2022, 05:14 PM

THANX Bill ! .

I could have walked out to the car and popped the hood but it's hot here .

In any case, standard DIN color spec. for switched ground is brown with a stripe .

My poor memory says often a white stripe / trace but I've noticed M-B and VW (most of my training) occasionally use slightly differing color code on wires .

In any case, my 1982 240D's fan blower will stay on even after the A/C is shut off if the day is hot enough ~ this is why I disconnected that wire : the engine's coolant temp. was 110* Centigrade so I shut off the AC before parking the car to cool it off a bit, I was surprised to hear the noisy fan running and decided to check it out .

I have no beef with the fan running solo, it has never done so unless I run the A/C on a well over 100* Fahrenheit day .

In industrial / fleet use auxiliary fans are properly wired direct so they'll often continue to run a while after shut down, this helps with heat soaking .

My old 1949 Chevy truck I added a pusher fan to help in summer and often after I'd shut it down, locked it up and was walking away, the aux. fan would kick on as the engine heat soaked the radiator and folks would yell at me "hey ! you left the engine running !" when it wasn't, it was doing the exact correct thing .

tangofox007 · #22 07-10-2022, 10:46 AM

Quote:

Originally Posted by BillGrissom

To clarify, in the photo of post 6, there is a green and brown wire on the car side. Brown is usually ground in M-B, so assume true here. The switch connects the green wire to brown (gnd) when the temperature is not too hot.

The refrigerant temp switch is open when cold, closed when hot. When closed, it grounds the coil in the aux fan relay.

Quote:

Originally Posted by BillGrissom

Coil+ comes from the push-button climate box, 12 V when it wants the AC clutch actuated.

The refrigerant temp switch has nothing to do with the AC clutch.

Quote:

Originally Posted by BillGrissom

The temp switch is a "disable", same as how the neutral-safety switch is wired to disable the starter relay (when not in P or N) at least in U.S. cars.

That is incorrect.

tangofox007 · #23 07-12-2022, 09:29 AM

Quote:

Originally Posted by marcomurabia

The pressure switch will prevent the AC compressor from engaging or the fan from coming on if low on freon. This may also explain why your AC does not work well in hot weather.

The fact that the AC operates eliminates the pressure switch as a problem. The pressure switch has no direct electrical connection the the aux fan.

BillGrissom · #24 07-18-2022, 08:01 AM

tangofox007 is correct. I should have said the temp switch on the AC filter closes when too hot to enable the rad fan, by shorting coil- to gnd. I recall coil+ is powered when the key switch is in "run" (check diagram).

Forget my mention of the different AC clutch relay I confused.

resago2000 · #25 07-21-2022, 09:41 AM

The pressure switch switches +12V in series with the compressor
The temp switch switches ground, which triggers the aux fan relay. just FYI

vwnate1 · #26 07-21-2022, 11:18 AM

Is the relay hot even with the key off ? .

resago2000 · #27 07-21-2022, 11:24 AM

nope. just with key on.
if you jump the temp sensor, the fan will start when you turn the key.

I have a relay tapped from the ac/pressure wire that connects ground for the fan relay(brown wires) so the fan runs when the compressor is running or the temp sensor trips.

vwnate1 · #28 07-21-2022, 11:37 PM

Thank you .

It was slightly above 100 degrees F in the shade here the other day and I noticed the gauge slowly creeping up, in time it reached 115 degrees C so I turned the (manual) heater knobs full on, that cooled it off quickly .

When it cooled I checked the coolant, it's still clean and full so I guess time to replace the radiator .

In general I'm a big fan of electric fans being wired live so they come on after the engine shuts down and begins to heat soak .

There's no upside to heat soaking .