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Discussion Starter #1
I suspect a bad relay, but I'm not 100% sure.
What I do know is, when I ground the temp sender lead, the aux fan kicks in, seemingly at high speed.
When the engine is running, with AC on, the aux fan does nothing. It should be running at a lower speed, right?

What else can I check before I fire the parts cannon at this? I know the relay is only maybe $20, but I want to check everything I can first.

Thanks in advance for any help here, Greg
 

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I suspect a bad relay, but I'm not 100% sure.
What I do know is, when I ground the temp sender lead, the aux fan kicks in, seemingly at high speed.
When the engine is running, with AC on, the aux fan does nothing. It should be running at a lower speed, right?

What else can I check before I fire the parts cannon at this? I know the relay is only maybe $20, but I want to check everything I can first.

Thanks in advance for any help here, Greg
I think the refrigerant temp sensor on top of the receiver dryer located near driver side of radiator turns on the auxiliary fan at 105C. Bypass that sensor by bridging the two wires going to this sensor that will check the circuit. If the fan comes on the circuit is good. I don’t know what the test specs for that sensor are but I’m sure someone will chime in. In this heat the auxiliary fan should be on most of the time. If the circuit is good then you probably need a new temp sensor at the ac receiver dryer.
 

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Discussion Starter #3
If I unplug that sensor on the dryer, while the engine, and AC are running, the clutch on the pump releases, plugging back in, the clutch engages. So I am assuming that pressure switch is ok.
 

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If I unplug that sensor on the dryer, while the engine, and AC are running, the clutch on the pump releases, plugging back in, the clutch engages. So I am assuming that pressure switch is ok.
The A/C dryer has 2 switches:

1.) A low-pressure switch (gray-colored switch housing) that opens the circuit if the refrigerant pressure is too low, thereby keeping the compressor's clutch disengaged.
2.) A temperature switch (red-colored switch housing) that closes the circuit if the refrigerant temp is too hot, thereby activating the auxiliary fan.

From your action in the previous post, it sounds like you disconnected the low-pressure switch. I think on your particular car, the low-pressure switch is the one towards the bottom of the drier and the temp switch is the higher one.
 

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I almost forgot to mention that there's also another temperature switch for the auxiliary fan (also called 2nd stage supplementary fan). It has a blue connector housing with 2 poles and is located to the left of the inlet connection towards the top of the coolant pump. If functioning correctly, it should activate the auxiliary fan at 105 C and off at 98 C. It also switches off the A/C compressor at 115 C as an emergency off, and on at 108 C.

As you know, keep in mind of the coolant pressure if you need to replace this switch. Also take caution where refrigerant will evacuate if you remove the temp switch on the A/C drier.
 

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Discussion Starter #7
Thanks for the diagram, looks like I need to check for voltage at the resistor, that will tell me if I have a bad relay, or the resistor has died, Why I did not think of that last night, I don't know, but looking at the diagram, it's obvious.

Thanks again, Greg.

UPDATE, I have no voltage to the resistor, I wanted to try swapping relays, but they are not labeled any where, I need to locate a relay map.
Seems odd there would be a fuse map, but not a relay map in the fusebox.
 

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Did you jump the switch on the receiver/dryer? Note that the fan is not supposed to be coming on unless the temp of the refrigerant coming out of the condenser exceeds 52C. Also, this switch is to increase the efficiency of the AC and not to help cool the engine.
 

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Discussion Starter #9
I did jump that switch, and no joy. I'm starting to get worried that something else is not happening to make the relay and fan run.

I need to located a full schematic of this part of the system (and then I need to learn how to read it)

Greg
 

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The schematic in Post 6 shows everything having to do with the aux fan. The relay on the right (High speed relay) is activated by coolant temperature and turns the fan on full. If I understand your post, you've jumped the coolant temp switch and the fan ran, so the whole right side with the possible exception of the switch, is fine.

The coil of the relay on the left (low speed relay) is powered from the same fuse as the one on the right, so that's OK. Fan power for both relays also comes from the same fuse, although it is a different one from the coils, so that must be OK too. When the temp switch on the dryer activates it turns the left relay on which powers the fan through the resistor. If you jump the switch it should energize the relay and you should get 12 volts relative to ground on both sides of the resistor and the fan should run at low speed.

If the high speed relay works, you can ID it by figuring out which one clicks when you jump the switch. The low speed one is probably right next to it, and should look exactly the same. If you unplug it, there should be 4 contacts. The socket will have 5; the one in the center is unused. 2 of the 4 should be powered when the key is on. Stand so 3 are vertical and one is horizontal, with the horizontal one on top. Jump from the top (horizontal) to the bottom (single vertical) and the fan should run. One of the remaining two, the two vertical ones in the center "row", should be hot, The other should ground when the refrigerant temp switch closes, and activate the relay.
 

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Discussion Starter #11
I located the high speed relay, verified by jumping the switch to make the fan run, and pulled relays until I found it.
Then jumped the switch on the drier, and even by swapping relays, could not make it run.
I need to start tracing wires, something has to be open somewhere.

By the way, who on Gods green earth would put a resistor under a master cylinder, and in such a position it's near impossible to get at? ( I hope I don't need to change that)

Greg
 

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There should be 6 relays in 2 columns of 3 each. The high speed fan is relay "B". It should be in the middle of the first column. The one below it is the low speed, relay "C". Refer to my previous post on how to test the relay socket for power, etc.
 

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I did jump that switch, and no joy. I'm starting to get worried that something else is not happening to make the relay and fan run.

I need to located a full schematic of this part of the system (and then I need to learn how to read it)

Greg
Greg,

Here are the relay locations. Please forgive the image, but the pdf is all washed out in that regard.

B relay is the middle one on the LEFT vertical column, while C relay is the lowest on that column.. Like this below..

A
B
C



MBL
 

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Discussion Starter #15
Mine is a little different than the illustration, and My high fan relay is not where it should be, it's in the "E" position. I have not yet traced the functions of the other relays, maybe this weekend.
Could this be from a PO modification, or is this a non 85 500SEL box? I'm a little confused here.
20190722_211534.jpg
 

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Discussion Starter #16
After a little research, it appears the relay sockets can be moved when the fusebox is disassembled. Why would somebody do this? I don't know. But someone, somewhere, felt the need to open this up, scramble things a little, and put it back together incorrectly, and I now feel compelled to undo this madness and get things working as they should.
I'll report back after the weekend with what I find inside the fusebox.

Greg.
 

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Do you have 12V at the dryer switch when the ac is on? should be 0 when of and 12V when on.

disconnect both wires and apply 12V and see if fan turn on low.
 
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