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Auxiliary Fan(s) Not functioning

2967 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 AC was blowing cold before, but I think mostly when the Aux fan would kick in. I bought two new fans, as I suspect that is the core issue, but don't really have the tools or the space to do the repair myself. Lucky I have a classic Mercedes mechanic that can take care of it for me.
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Turns out it was, replaced the fans today and they worked like, well, brand new. AC still not cold though. Just checked the pressure and it read...0? Not sure what is up, but it looks like the compressor clutch isn't engaging. Will bridge the sensor and see.
So I narrowed down that it was the fans, bought new ones and installed them. Tested them by taking the temp sensor plug off and the fired right up. However, that hasn't fixed the AC issue, here they are:

Re-tested the AC high pressure again with the AC on and it read 30. I don't have a way to test the low pressure, since I don't have a pressure gauge that fits that one as it is a different size from the high pressure. The clutch is engaging on the AC compressor and the air is blowing sort of cold, but not ice cold as it had before. The Aux fans do not come on slowly when the AC is on,so I don't know how much that would affect the coldness of the air. I pulled the plug on the fans and turned the AC on to check current to the fans when the AC is on and get nothing.
I also did some looking and need to check the ceramic ballast resistor, but I don't know where that is! Any ideas?
You sure you dont mean the low side? I've never seen a high side only pressure gauge. The single gauges are low side from what I've seen. If low side is 30, what's the ambient temperature at? You can buy a set of gauges at harbor freight or amazon for cheap.
I am referring to the side near the top of the engine, not the one near the driver-side headlight. It has a smaller diameter. ambient air temperature is around 90 degrees F. Car is rock solid at 80C.

That fan resistor is on the driver side mounted on the frame rail on the engine bay or next to the headlamp
I'll take a look and post a picture of what it looks like. If I take a voltmeter out there, what can I test?
No, I will take a picture of my relays as well and show you. In the relay one is green, one is silver, and two are yellow. The yellow ones are the ones use for the aux fans. I assume those are working, since the fans turn on when I pull the temp sensor. Is that mistaken?
So I just went out and checked things.

Here are the relays:



I checked the AC pressure again and it read higher than before - near 40


Then I checked the resistor and got volts on the better looking bolt in the picture below. Got nothing on the one closer to the firewall that looks all rusty. Bad resistor?


The AC is still not blowing cold.The Aux fans kicked on when it got over 80C on the low setting and they kick on high when I pull the temp plug. So I am not sure what is up.
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One more thing, did you try and bypass the pressure sensor for the fans on the ac drier
I didn't think that would be necessary given that the clutch engages, which suggests that the pressure is good, no? All that bypassing the pressure sensor would do is engage the clutch.
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?
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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I'll try that tomorrow after I install the new wiper assembly.
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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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.
Yeah I have volts at the input side of the resistor.
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