Hi all

I have a 280sel 4.5 and the other day when in bumper to bumper traffic with 90 degree temps and I noticed that the car was running a little hotter than it ever had in the past. I got home and found out that the auxiliary fan was not switching on when I turned on the AC. I thought that it should go on whenever the ac is on even though it had not reached 212 degrees. I checked the fuse and it looked fine so I grounded the wire on the sensor and the fan would run.

After this I thought it must be the sensor so I replaced it. It still did not go on with the ac running. then I tried to get the engine to 212 degrees in the driveway with the ac on but it never got close so I do not know it it will work if it gets that hot.

I thought that if I could get it to run when grounded that ment the fuse and the relay were both good and most likely it was the sensor if that is not true please someone clue me in. If I cannot get it to run I might just put a switch in the car and run it manually.

Any ideas on what I should try next?

__________________
1972 280 SEL

I'm not sure if that holds true for the relay.

__________________

Current:
1971 Mercedes Benz 250 (Euro Spec)
1972 Mercedes Benz 250 (US Spec)
Past:
1972 Mercedes Benz 280

See the thread in this section "Auxilliary Cooling Fan is Always On"--the switch in the receiver/drier may be bad--good luck



Tim Kraakevik
kraakevik@voyager.net

can someone tell me where the dryer switch is and where get a new dryer switch to replace the old one

and which relay is the aux fan relay

__________________
1972 280 SEL

Ok I got a new relay and replaced the old one I think I swapped out the aux fan relay but I am not sure if I changed the right one the one I changed is the rectangular one of the three on the Passenger side firewall next to the four small fuse holders. the fan still doesn not go in with the AC so the dryer switch is the next thing to change
Please a little help. can someone please let me know if I swapped the correct relay and I have no idea what the dryer switch is and where it is located or where to buy a new one.

__________________
1972 280 SEL

A fast way to tell if the low, or high for that matter, pressure switch on your drying is working is to bypass it. There is really nothing to it and no harm will be done.

First understand that the voltage to the fan passes through the switch on its' way to the fan. There is only one wire and it should be 12v +.

Do the test in this order: Get a voltmeter and find the wiring leading to the pressure switch on the dryer. This is the easy part.

Pull the plug off the top of the switch, start your engine and turn on the A/C. One of the wires leading up to the pressure switch should read 12 v or so. Anywhere between 10.5 and 13.5 is possible, but it should be close to 12 v or above just a bit.

Do you have voltage to one of the wires? If yes then use a small piece of wire, like a paper clip, and jump from one slot to the next on the plug. The fan, if it is working, should come on, and you have just found that the pressure switch is not working.

Too complicated? Take your voltmeter, put it on the ohms setting, pull the plug off and turn on your A/C. If the plug is working you will get a flow of electricity from one pole to another and your meter should show this.

Now you have to decide if you want to just wire in a jumper wire and bypass the pressure switch or if you want to replace the switch which would mean your system would need a recharge.

It could also be that you don't have enough pressure in your system to bring the switch up to contact pressure, but if this were the case your air would not blow cold.

You can also unplug the fan and throw 12 v to it. The brown wire is the negative one. If the fan does not come on you have found your problem.

thanks idle


the fan works as it goes on when I ground the wire from the temp sensor


before I check out the dryer switch can you tell me If the relay I swapped is the one for the AUX fan (the rectangular one of the three on the Passenger side firewall )

and where is the dryer switch?

thanks for the help

__________________
1972 280 SEL

With the hood open, look to the left of the radiator.

__________________
It is a truism that almost any sect, cult, or religion will legislate its creed into law if it acquires the political power to do so. Robert A. Heinlein


09 Jetta TDI
1985 300D

this is a pic of the sensor I changed and the wire connected to it turns the fan on when it is grounded so the fan works and the sensor is new




this is the relay I chenged can anyone confirm that this is the relay for the AUX fan



Can someone point out from this pic what I am looking at and which is the dryer switch.



thanks again or the help

__________________
1972 280 SEL

To the left of your #2.... is there a screw in plug with two connectors on it? If there is, put those two wires together and see if the fan comes on (ignition on).
If not, I read that the relay is round, not square, and behind some sort of ECU. - I don't know this for sure, as my 6cyl doesn't have an ECU.

__________________
It is a truism that almost any sect, cult, or religion will legislate its creed into law if it acquires the political power to do so. Robert A. Heinlein


09 Jetta TDI
1985 300D

I wired mine so it comes on when the AC compressor is on. With r134a the fan is supposed to run any time the compressor is running.

__________________
Csaba
1972 280SEL 4.5, silver

geen:

A couple of misconceptions may be hampering you.
1) The aux fan is not wired to run constantly with the A/C on.
2) The switch in the receiver/dryer is not a pressure switch; it is a temperature switch.
3) Neither of the temperature switches (coolant or refrigerant) carries power to the fan; they simply cause the fan relay to close.

Here is what you do know at this point: the fan does work, the relay does work (because the fan will run), the wiring from the coolant temp switch is good (fan runs when wire is grounded).

To be determined: condition of wiring between refrigerant temp switch and relay. Remove both wires from refrig. temp switch, and bridge the two wires. Turn key to ON, fan should run.

In the event that all the wiring is good, you are left with these questions: is the coolant temp switch good (100C)?, and is the refrigerant temp switch good (62C)?

thank all of you guys for the help

Frank I installed a new temp sensor (coolant) so I have to think it is working I and as you say the relay should be fine if grounding the sensor got the fan to run. I will bypass the dryer switch this weekend and see if I get the fan to run but I thought it should always run when the AC is on so you are saying that that is not the case if so I might be chasing my tail as the car does not overheat an has never got to the line on the temp gauge (I think is 212 degrees) so maybe the fan has never been triggered.

__________________
1972 280 SEL

Indeed, the fan does not necessarily run as soon as you turn on the A/C. (just to confirm)

__________________
It is a truism that almost any sect, cult, or religion will legislate its creed into law if it acquires the political power to do so. Robert A. Heinlein


09 Jetta TDI
1985 300D

What I was thinking I would do is add a wire coming off the coolant temp sensor in to the cabin with a switch and ground the other end so I can turn the fan on with the switch when I am in traffic on hot days. Do you see any downside with that ?

__________________
1972 280 SEL