cooling fan ON all the time

[Exekiel69]

I have a problem with this 87 300TD where the front cooling fan will not go off unless I turn it to the off position or pull the key out. I found which relay powers it and after pulling it the fan goes off then I replaced it with another relay and it turns back ON. There is so many wires on that fuse/relay box I didn't want to start pulling it unless I knew what to look for.
What could be causing this? Is the sensor shorted? Where is the sensor?
This isn't My vehicle I'm just helping a friend and feel bad to send Him to a mech shop if I can help it. Thanks in advance.

 

[Exekiel69]

Someone?

 

[The Warden]

I'm not sure it's the same on the W124 chassis...however, on the W123, the sensor's in one of the freon lines near the a/c receiver/dryer. On the W123, it's supposed to turn the fan on if the freon gets too hot, but I think on the W124 they switched to a setup that measures pressure instead of temperature.

On cars where the engine-driven fan clutch is worn out, I've jumped the two wires at the sensor to get the fan to come on constantly...so, if this has been doing this the entire time he's had the car, it's possible that a previous owner did this. Otherwise, I would look at the sensor...maybe unplug the wires and see if the fan comes off, then jump the wires and see if the fan comes on.

The receiver/dryer on the W123 is directly behind the passenger's side headlight assembly; I'm not sure if the W124 placed it in the same spot or not.

Hope this helps some...good luck!

 

[Exekiel69]

Thank You, I looked and yes there is what looks like a pressure switch next to the accumulator but it is all on the drivers side. It started doing this a few months ago, don't know why. I still have to help him and recharge the ac system but this time with r134a so I got a set of O-rings and a pressure relief valve, I will be pulling it all apart then pull a good vacuum and if it holds put some refrigerant in it, any idea which one is the high pressure port on a Mercedes? He is a good buddy and helps Me some times so I want to do the same if I can.

Thanks.

 

[The Warden]

Thank You, I looked and yes there is what looks like a pressure switch next to the accumulator but it is all on the drivers side. It started doing this a few months ago, don't know why. I still have to help him and recharge the ac system but this time with r134a so I got a set of O-rings and a pressure relief valve, I will be pulling it all apart then pull a good vacuum and if it holds put some refrigerant in it, any idea which one is the high pressure port on a Mercedes? He is a good buddy and helps Me some times so I want to do the same if I can.

Thanks.

How many switches were there? I ask because there should be a compressor cycle switch in the vicinity also...AFAIK those two switches should be all you'll find, although the W124 could be different.

As to which port is which...I could tell you on a W123, but I don't have a clue on the W124 chassis ...all I can say is that the high pressure port should be between the compressor and the evaporator, and the low pressure port should be on the cold side somewhere...either near the accumulator or on the line going from the evaporator and accumulator back to the compressor.

Oh, one other thing...I just remembered that, at some point, M-B also started using an engine temperature switch to activate the fan. This changeover may have started when they switched from the W123 to the W124, but I can't say for certain. Unfortunately, I don't have a wiring diagram with me...

 

[Exekiel69]

Thank You, I will check what You wrote. At first I thought the fan was switch by a engine temp sensor but it makes more sense it is triggered by the ac inside the cab and He already got a new panel inside from another car which fixed a lot of issues but may be failing at the AC. I will get a diagram of the Mercedes ac system hopefully with some indications of the ports.

I was just reading some information online, should the suction port be the lower one and high pressure one higher?