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Auxiliary Fan(s) Not functioning

2947 Views 64 Replies 5 Participants Last post by  Kevin G.
This if my first post so bear with me. I've reviewed the threads on this issue and am still quite confused. First some background

The Car:
W124 1993 300E
235,000 Miles
Well maintained, though not perfectly
Burns a little oil, radiator probably needs replacing, recent inspection shows most everything in working order.

I have the desire to keep this car going as long as possible, so I want to identify what needs replacing and take care of it. I am familiar with these models, as I have had 5 previous to this. I still don't know every bit and piece but I know enough to be dangerous.

The Issue:

I recently had a son, and this is our only car in Washington DC. It has been hotter than the blazes of hell recently and so Air Conditioning has been a must. I never much bothered with it before as I just used window A/C as it were. But since having the kid, it needs to work. It never really blew cold, but I could do a weird hack. If I shut down the car, when I restarted it, the AC would blow cold, for a bout 10 mins or less, then go hot again.

I checked the pressure in the AC system and it was way too high. I got it back down to legitimate levels and it would blow cold air for longer for about a day. I have noticed that the car would get quite hot quite fast, and when it would get hot, the Aux fan would kick in, the AC would blow cold and the temp would go down.

After a drive today with the same intermittent AC and hot as hell outside temp, I decided to check out the whole system. I am pretty sure that when the AC is turned on the aux fan is supposed to turn on and cool the engine, and perhaps also help it blow colder. When I turned on the AC the aux fan didn't go at all.

Testing the Aux Fan

Here is the test I did.

1) With the key ON I pulled the wire on the temp sensor. The first time I did this, the passenger side fan started right up, but the driver's side didn't, as it did when the motor got hot.
2) With the key ON and the Plug OFF the temp sensor, I pulled the plug for the fans to check the connections, the left fan shut off, the connections were clear and tight, and it started up when I put it back together.
3) Put the plug back ON the temp sensor and turned the key OFF.

This seemed to confirm the aux fan would turn on, but perhaps the drivers side needed to be replaced. I wanted to use a voltimeter to check the power at the connections.

1) With the key ON I pulled the plug off the Temp sensor - fan did not engage
2) Checked the volts at the fan plugs, not juice
3) Bridged the temp sensor plug, no volts at fan connector

This indicated that the fans had just stopped working, perhaps the relay?

1) Open the relay box, pull the Relays (A,B I believe), clean the connectors, use compressed air to blow out the plugs, and put them back in,
2) Key ON, temp sensor plug OFF - no fans, no volts at the connector
3) When I pull the temp sensor plug i cant hear a pop in the relay box coming from the D relay. Remove the D relay and no pop.


I am confused. This all seems to suggest a relay, right? Can I bypass the relay to see if the fans turn?

Looking for suggestions.

Update: Connected the connector to the fans directly to the battery and they didn't turn on. Pulled the plug on the temp sensor with key ON and checked the female connector side, showing 12v. This seems to indicate that it is the Aux fans that are bad.
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The one on the bottom of your picture is the connector for the aux fans. The one on the top is the pressure sensor. Right?
The one on the bottom of your picture is the connector for the aux fans. The one on the top is the pressure sensor. Right?
You'll find out when you unplug one of them. Nothing catastrophic will happen if you unplug them. Jumper the fan sensor plugs(on body side) to each other and see if that turns on the fans. I use a small piece a reg wire.
I have disconnected the fan connector plug and the fans shut off. I've plugged one of the plugs on the pressure sensors and clutch goes.
I don't think so - Check this out.

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That sensor plug you disconnected is for you fans since the a/c clutch was still engaged. Disconnect both connectors for that sensor and touch them together, that bypasses the sensor. At that point the fans should kick on.If your refrigerant is to low the fans won't kick on. Your refrigerant seems low, pressure should be at 30-50.
Also that port is your low side, not high side. High side pressure should be around 185 to 225, depending on ambient temp.
I'll try that tomorrow after I install the new wiper assembly.
I'll try that tomorrow after I install the new wiper assembly.
If when you bypass that sensor the fans kick on that could mean
A. A/c pressure to low, fill with refrigerant some more until your at around 40psi.
B. You're pressure is good and they still dont kick on, sensor is bad. You'll have to evacuate the system, replace the sensor and recharge. I would replace both sensors while you're at it.
I also bought a new aux fan relay and it looks like this. Which relay does this go in, B or C - According to this there are two?
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Yes two relays, one for low speed and one for high speed but dont quote me,
For the K9kl relays, otherwise a K10 and K9
I put power from the battery to the rusty part of the resistor and the fans fired up. I suspect that is the root cause. Going to try to replace soonish. Still, added some freon and it seems to be much colder.
Do you have voltage at the resistor input side? Pull the temp sensor and check for voltage at that resistor when the fans are running, If you have voltage at it then the resistor isnt your problem. It's the control side. Did you bypass the fan sensor??
Yeah I have volts at the input side of the resistor.
Yeah I have volts at the input side of the resistor.
Voltage to input of resistor but nothing on the output side then your resistor is toast.
Well, the bolt is so rusty, I want to replace the bolt and see.
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Well, the bolt is so rusty, I want to replace the bolt and see.
Cool, clean up those contacts
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