1907 W108 280sel electric fan - PeachParts Mercedes-Benz Forum

dirkbalter · #1 09-27-2018, 06:26 PM

Hello. I am new to this forum. The electrical (auxiliary) fan on my 1970 280 SEL does not come on as the temperature rises in stop and go traffic or standing in gear. The car has AC and power steering.
My question is: What triggers the fan to come on? How can I check if whatever initiates it, is working properly?
(I unplugged the fan connection in the harness and verified that the fan itself is working.)
Your help is appreciated.

Tomguy · #2 09-28-2018, 08:17 AM

The fan comes on any time the A/C is on, and also any time it rises above 212F / 100C. If your fan isn't on when your A/C is on, check the fuses (on the firewall under the hood). And then check the wiring to the fan to ensure it's getting 12V.

Idle · #3 09-28-2018, 12:19 PM

I have often found that the failure of this is related to a relay. Check the relays you find on the insides of the front fenders and see if one has stopped working.

There is also a high pressure switch that comes off of your a/c dryer on some cars. This senses high (which really, in this case, means normal) pressure in your ac system. In other words: It senses when your ac system is running.

This switch can fail and then no signal is sent to your fan that the ac is running.

It is possible to run a wire right from the hot side of the wiring for this switch to the fan but then the fan will run all the time. I did this when I lived in Texas where it was hot outside almost every day. I didn't mind the fan running as much as I did the constant overheating due to the normal 95 degree days.

dirkbalter · #4 09-28-2018, 04:31 PM

Thank you to both of you. I will work on it this weekend and report back.

Ziad · #5 09-30-2018, 04:40 PM

Ground the auxiliary fan switch, fan should come on..end of story.

Ziad · #6 09-30-2018, 04:42 PM

I have the switch if you need one.

dirkbalter · #7 10-02-2018, 01:57 PM

Thanks again for the help. I finally got hold of a wiring diagram for the car. as you mentioned, two thermostatic switches are integrated to trigger the fan. One of them is supposed to turn on the fan at 100 degrees C and is located on top of the thermostat housing. The other one is labeled thermostatic switch for dehydrator (option ac). As suggested, I grounded the wire at the switch but the fan doesn’t turn on. If I ground the wire at the relay plug, the fan turns on, no problem. Concluding, I believe I have a wiring issue between the switches and the relay plug. (Anyone know where the thermostatic switch for dehydrator (option ac) is located and what it looks like, didn’t find it yet).
I am also tempted to just add a manual trigger switch hidden somewhere to turn the fan on manually.

dirkbalter · #8 10-02-2018, 01:57 PM

Thanks again for the help. I finally got hold of a wiring diagram for the car. Two thermostatic switches are integrated to trigger the fan. One of them is supposed to turn on the fan at 100 degrees C and is located on top of the thermostat housing. The other one is labeled thermostatic switch for dehydrator (option ac). As suggested, I grounded the wire at the switch but the fan doesn’t turn on. If I ground the wire at the relay plug, the fan turns on, no problem. Concluding, I believe I have a wiring issue between the switches and the relay plug. (Anyone know where the thermostatic switch for dehydrator (option ac) is located and what it looks like, didn’t find it yet).
I am also tempted to just add a manual trigger switch hidden somewhere to turn the fan on manually.

Ziad · #9 10-03-2018, 02:33 PM

It's on the side of the drier

Ziad · #10 10-03-2018, 02:42 PM

Ps. You need to ground the wire on the switch.
Remove off the sensor, and ground ( the connector wire )
I use a baby screwdriver inserted and then ground on the valve cover.

dirkbalter · #11 10-03-2018, 06:12 PM

@ Ziad, that’s exactly what I did. I used a piece of bare wire. I also measured continuity between the wire end at the thermostat and the connection point at the relay and don’t get any. That's why I think I have a broken wire or something. I will check at the drier side for the other switch. Thanks for the help.

Idle · #12 10-03-2018, 09:24 PM

Quote:

Originally Posted by Ziad

It's on the side of the drier

Don't remove the sensors but take a look at the part numbers. I have seen dryers with two of these, a high pressure (which turns on the fan) and a low pressure (which cuts out the system to prevent damage to your compressor).

But usually you just see the high pressure switch. It senses higher pressure in the system, which means it is on and running, and is not so much a sensor as a switch. It just allows current to flow from one pin to the other.

You can easily check the high pressure switch. Just unplug it and, with the engine off, test if for continuity. There should not be any change in the meter.

Now start the engine and turn on the AC. This time you should have continuity across he poles. That is, if the switch is working.

And if the switch is working move on to the next testing of the circuit. If the switch is working there is nothing more to be done with it. By the way, if this switch needs to be replaced you will have to bleed down the system to remove it. This means a refill of the system. For this reason when I find one that has failed I just wire the fan to the hot side of the switch. Then the fan comes on whenever the engine is running.

But this was in Texas, where it was always hot weather and the fan was always running anyway.

Arthur Dalton · #13 10-04-2018, 02:50 AM

<<<>>>

Aux fan switch on that model is drier temp sensed . not pressure sensed.

So, it can be changed w/o letting any refrig out.

Do not test this sw by grounding a terminal..the test is to jumper the switch
Reason is b/c if you are grounding the ground side of the sw. the fan will still not run...you must jumper the switch. [ or make sure you ground the hot side]

The fan does not run until the drier temp is increased by high side refrigerant heat @ drier.