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Discussion Starter #1
Hi, all. Nice work to BenzWorld on the UI refresh. Still getting used to it, but I love Dark Mode.

My 126’s auxiliary cooling fan stopped coming on. I noticed this the other day when coolant temp rose to about 100ºC and the AC wasn’t getting as cool. I’ve read there are several potential reasons for this failure, but I suspect one of two things first:

1. Check the connector: I can’t seem to find where this thing plugs in. Basic question, I know.
2. I suspect that if the connector’s not the problem, the ceramic resistor is. A month or two ago I smelled something burning from that area under the fuse box and determined that it was likely a leaf or something. In any event, the resistor may have been on its last legs.

My 126 is a 300SE (1989)

My questions, please:
1. Does anyone know where the power connector is for the aux fan?
2. Does anyone know how much work is involved in replacing the ceramic resistor—and what part number to order? AutohausAZ doesn’t even seem to carry it.

Thanks much.
 

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Hi there,

That 2 pole connector for the aux fan is located near the receiver/drier canister aft of the left headlamp atop the 'frame rail'.

You could test the resistance of the aux fan resistor by removing both aux fan relays K9 & K10 & take a reading across positions 1 in their respective sockets. If you get a resistance reading, this suggests the wiring & resistor are fine.

In addition, the low speed portion of aux fan operation won't work if the refrigerant level is too low. You can jump the refrigerant pressure switch to harness connectors with a blade fuse.

Oh yes, here is your resistor part number as well. A 000 158 25 45

Good luck man.

MBL
 

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Discussion Starter #4
Hi there,

That 2 pole connector for the aux fan is located near the receiver/drier canister aft of the left headlamp atop the 'frame rail'.

You could test the resistance of the aux fan resistor by removing both aux fan relays K9 & K10 & take a reading across positions 1 in their respective sockets. If you get a resistance reading, this suggests the wiring & resistor are fine.

In addition, the low speed portion of aux fan operation won't work if the refrigerant level is too low. You can jump the refrigerant pressure switch to harness connectors with a blade fuse.

Oh yes, here is your resistor part number as well. A 000 158 25 45

Good luck man.

MBL
Thanks for the info and photos, MLB. I’m going to check the connector on the receiver/dryer. If that solves the problem, then that’s the easiest fix. If not, then I’ll test K9 & K10. Total novice question: What do I use to test that?

If that works, then I’m going to check refrigerant.

Electrical work scares me because I’m basically a novice and don’t want to fry anything. Even the resistor scared me when I smelled smoke one day because I thought a wire was burning. Even after figuring out it was likely a leaf smoldering, for a week or more after blowing out all the leaves near that thing it still smelled. However, perhaps the resistor was on its last leg.

I don’t have small hands, so replacing it myself is going to be a drag. If you were to take a stab at how many mechanic hours are required to replace it, what would you estimate?

Again, thanks so much for your reply.
 

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Discussion Starter #5
Resistor replacement is fairly easy if you have tiny hands. Easiest if you remove the false firewall. That will also get you to clean out the junk there
Thanks, Stutz. Contrary to POTUS, my hands aren’t that small, so I might actually have to have this work done for me unless I can learn how to remove the fuse box or false firewall. Makes me nervous.
 

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Following this one. My aux fan on my SEC doesn't work automatically so I put a switch for it in the glove box. I eliminated some possible causes and decided it must be the resistor. Had thought it was really hard to change but maybe not after all?
 

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Discussion Starter #7
Following this one. My aux fan on my SEC doesn't work automatically so I put a switch for it in the glove box. I eliminated some possible causes and decided it must be the resistor. Had thought it was really hard to change but maybe not after all?
Like was said above: It’s not if you have small hands—or don’t mind removing the false firewall or the fuse box. Mine is clearly visible under the fuse box—provided you’ve used a compressed air to blow all the debris out of there (it’s uhhhmazing what you’ll get by doing that; be prepared to vacuum).

The part is only like $35. Cheaper if you go non-Benz. But before I delve into that I want to ensure the fan is working and the Freon is at spec. If I had to pay for Freon check vs. labor to replace the resistor, I’m betting the Freon check is cheaper.
 

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Thanks for the info and photos, MLB. I’m going to check the connector on the receiver/dryer. If that solves the problem, then that’s the easiest fix. If not, then I’ll test K9 & K10. Total novice question: What do I use to test that?

If that works, then I’m going to check refrigerant.

Electrical work scares me because I’m basically a novice and don’t want to fry anything. Even the resistor scared me when I smelled smoke one day because I thought a wire was burning. Even after figuring out it was likely a leaf smoldering, for a week or more after blowing out all the leaves near that thing it still smelled. However, perhaps the resistor was on its last leg.

I don’t have small hands, so replacing it myself is going to be a drag. If you were to take a stab at how many mechanic hours are required to replace it, what would you estimate?

Again, thanks so much for your reply.
Hey there,

In order to save steps perhaps, I'd jump the wiring connector that hooks to the red refrigerant pressure switch located on the receiver/drier. You can't miss it with the white/translucent plastic insulators. Just insert a blade fuse between the now unplugged connectors and start the motor up.

In doing this test, you provide ground supply to Aux Fan Preresistor Relay K10, allowing current to flow from the relay > through the resistor R15 and down to the auxiliary fan motor M4. At this point, that fan should start right up & shut off when the ignition is shut off.

If NO fan, it would follow that either a bad K10 relay or R15 resistor would not allow the current flow.

If the fan DOES work, it has been my experience that the refrigerant level in the system very well be too low as the red pressure switch is open circuit at 15 BAR. This was my experience exactly one hot HOT day in Atlanta traffic.

Hope this smooths the water some for you.

MBL
Screen Shot 2019-08-29 at 4.55.34 PM.png
Screen Shot 2019-08-29 at 4.59.03 PM.png
 

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Discussion Starter #9
Hey there,

In order to save steps perhaps, I'd jump the wiring connector that hooks to the red refrigerant pressure switch located on the receiver/drier. You can't miss it with the white/translucent plastic insulators. Just insert a blade fuse between the now unplugged connectors and start the motor up.

In doing this test, you provide ground supply to Aux Fan Preresistor Relay K10, allowing current to flow from the relay > through the resistor R15 and down to the auxiliary fan motor M4. At this point, that fan should start right up & shut off when the ignition is shut off.

If NO fan, it would follow that either a bad K10 relay or R15 resistor would not allow the current flow.

If the fan DOES work, it has been my experience that the refrigerant level in the system very well be too low as the red pressure switch is open circuit at 15 BAR. This was my experience exactly one hot HOT day in Atlanta traffic.

Hope this smooths the water some for you.

MBL
MBL, thanks much for the advice. Will do.

I used to have a blade fuse—just need to find it. Any particular amp rating required?
 

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Discussion Starter #11
So using a 15-amp blade fuse I jumped the connectors that come off the wires attached to the receiver/dryer’s red connector and fired up the engine. No auxiliary fan.

However, I need to confirm I jumped the correct ends—the two connectors that pull off from the receiver/dryer end expose their female connectors—not the male ones? If that’s right, then yeah: The fan didn’t come on.

And if that’s true, then it’s now one or more of the following:
  • Bad fan motor
  • Bad K10 relay
  • Bad R15 resistor
  • Loose connection somewhere
I still think the resistor is the prime suspect since a month or so ago I had that burn issue with it (bad electrical smell coming from under the fuse box and crisped leaves)—which kind of sucks because it’s the hardest to replace. My hands are way too big to get at it, and doing it with extension screwdrivers, etc. sounds like nothing but frustration.

Question 1: How do I test just the fan motor itself, i.e., free of any fanfare (pun intended) that controls it?

Question 2: Where is the K10 relay and how does it come off? Maybe I should replace that.

Question 3: If I were to take the car to my mechanic, what kind of labor (hours) would you say is reasonable for the resistor replacement?

Thanks so much. I really appreciate the help here! (y)
 

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So using a 15-amp blade fuse I jumped the connectors that come off the wires attached to the receiver/dryer’s red connector and fired up the engine. No auxiliary fan.

However, I need to confirm I jumped the correct ends—the two connectors that pull off from the receiver/dryer end expose their female connectors—not the male ones? If that’s right, then yeah: The fan didn’t come on.

And if that’s true, then it’s now one or more of the following:
  • Bad fan motor
  • Bad K10 relay
  • Bad R15 resistor
  • Loose connection somewhere
I still think the resistor is the prime suspect since a month or so ago I had that burn issue with it (bad electrical smell coming from under the fuse box and crisped leaves)—which kind of sucks because it’s the hardest to replace. My hands are way too big to get at it, and doing it with extension screwdrivers, etc. sounds like nothing but frustration.

Question 1: How do I test just the fan motor itself, i.e., free of any fanfare (pun intended) that controls it?

Question 2: Where is the K10 relay and how does it come off? Maybe I should replace that.

Question 3: If I were to take the car to my mechanic, what kind of labor (hours) would you say is reasonable for the resistor replacement?

Thanks so much. I really appreciate the help here! (y)

Hello again..

I believe I was unclear in my recent reply..Apologies..

"However, I need to confirm I jumped the correct ends—the two connectors that pull off from the receiver/dryer end expose their female connectors—not the male ones? " One does not jump the red pressure switch wires, rather jump the corresponding car wiring harness connectors instead. In testing, turn the ignition on that fan should run at the low speed. On my '87, they are female.. easy.

Here are some pics of K10 location inside the fusebox. from the MBCDs.
Screen Shot 2019-09-09 at 3.38.25 PM.png



Screen Shot 2019-09-09 at 3.39.23 PM.png
 

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Discussion Starter #13
Hello again..

I believe I was unclear in my recent reply..Apologies..

"However, I need to confirm I jumped the correct ends—the two connectors that pull off from the receiver/dryer end expose their female connectors—not the male ones? " One does not jump the red pressure switch wires, rather jump the corresponding car wiring harness connectors instead. In testing, turn the ignition on that fan should run at the low speed. On my '87, they are female.. easy.

Here are some pics of K10 location inside the fusebox. from the MBCDs. View attachment 2603620


View attachment 2603622

OK. I’m having a totally dense moment. So if in the pic I supplied below, those are NOT the connectors to jump, then I’m having trouble picturing the wiring harness connectors.

I do see the fuses, however. I assume I can just get those from the auto parts store, or do they have to be ordered from Daimler Bends-Your-Wallet?

Sorry: Total car electrical novice here. :rolleyes:

Thanks, MBL.
2603649
 

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OK. I’m having a totally dense moment. So if in the pic I supplied below, those are NOT the connectors to jump, then I’m having trouble picturing the wiring harness connectors.

I do see the fuses, however. I assume I can just get those from the auto parts store, or do they have to be ordered from Daimler Bends-Your-Wallet?

Sorry: Total car electrical novice here. :rolleyes:

Thanks, MBL.
View attachment 2603649
You are on the way! :)

Those are the ones to stick a fuse into & start the motor..Your local garage/gas station/repair shop/car enthusiast will have one of those fuses Or maybe your modern car will have one as a spare in the fusebox too? Our Volvos use this sort of fuse for example..Good luck

MBL
 

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Discussion Starter #15
You are on the way! :)

Those are the ones to stick a fuse into & start the motor..Your local garage/gas station/repair shop/car enthusiast will have one of those fuses Or maybe your modern car will have one as a spare in the fusebox too? Our Volvos use this sort of fuse for example..Good luck

MBL
Thank you. Those are in fact the two that I bridged with a 15A blade fuse last week. Fired up the engine, but the fan didn’t go on. Which means, to your earlier email, that it’s the relay(s) or the resistor.

But first I wanted to rule out the fan motor itself. I was trying to figure out how to test the fan itself, i.e., absent any controls. If it works, then I’ll then suspect either the relay(s) or the resistor.

I still think it’s the resistor...which is a drag because I can’t replace it myself.

Do you know how to test the fan motor itself?
 

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Does your electrical fan spin freely by hand? I only ask because when my SEC I eventually realised it didn't turn. I eased it off and lubricated it and it came back to life perfectly after being almost rock solid. Albeit now controlled by a switch in the glovebox.
 

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Discussion Starter #17
Does your electrical fan spin freely by hand? I only ask because when my SEC I eventually realised it didn't turn. I eased it off and lubricated it and it came back to life perfectly after being almost rock solid. Albeit now controlled by a switch in the glovebox.
Hi, Ian. Yes, it does. I can spin it with my finger.
 

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Following this one. My aux fan on my SEC doesn't work automatically so I put a switch for it in the glove box. I eliminated some possible causes and decided it must be the resistor. Had thought it was really hard to change but maybe not after all?
I too want to install a manual switch to turn on fan when it's very hot outside, should lower temperature by 10 degrees. I currently clip on alligator clips to connector and ground to turn it on. Can you post instructions how to install this switch and where to purchase, thanks.
 

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Thank you. Those are in fact the two that I bridged with a 15A blade fuse last week. Fired up the engine, but the fan didn’t go on. Which means, to your earlier email, that it’s the relay(s) or the resistor.

But first I wanted to rule out the fan motor itself. I was trying to figure out how to test the fan itself, i.e., absent any controls. If it works, then I’ll then suspect either the relay(s) or the resistor.

I still think it’s the resistor...which is a drag because I can’t replace it myself.

Do you know how to test the fan motor itself?
Yes you can test the motor by applying 12V to the BK/WT connector [female], one of the ones you just jumped X64/6 connection with the brown [GND] connection made...

Or you can jump the switch pins of the Aux Fan Relay K9. No key needed there.

Good luck man!

M.
 

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Discussion Starter #20
Thanks again to all who provided guidance. Turns out it was in fact the resistor—the simplest of fixes except for getting at the darned thing. It had completely fried—and that’s what I smelled that day a couple of months ago even I smelled something electrical burning and was worried I’d started a fire.

One couldn’t tell how bad it was by looking straight down at it, but when my mechanic got it out, it was badly damaged.

The upside: (1) Cheaper than a new aux fan, and (2) because I was told that resistor gets hot enough to smolder leaves, I now have a nice air compressor that I used to blow out all the crap that had accumulated down there over 30 years.

Thanks again, guys. As always, the help here is invaluable and very educational.
2605176
 
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