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Discussion Starter · #1 ·
I need some help figuring what is going on with my 2002 R170 A/C, and also some help figuring out what kind of jury rigged temp sensor I have inherited.

Today I had to go to MB independent shop to pick up my keys they had ordered. When I bought the car a month or so ago, it only came with one remote key, and the remote part did not work. This shop ordered a replacement remote key and one non-remote key. So since I was going to be there, I asked them to schedule an A/C diagnosis for me. AC has not blown cold since I bought the car.

Upon initial testing they determined it had zero freon in it, so they loaded it up and set out to test it. This is when things went south.

  • [*]Coolant temps began to rise, and fan failed to kick in.
    [*]compressor began to labor and actually began to smoke
    [*]they shut down the car​

Okay, that did not go well.

When they began looking around for WTF went wrong they noticed that a prior owner had wired a non-factory temp sensor in. It is one of the generic ones that you push through the fins of the radiator. You can kinda see it here, nestled behind the coolant tank. The gold looking item is the base of the sensor, there is a metal probe that is pushed through the radiator fins.

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Coming from this add-on is a yellow wire that leads to the nearby Aux Fan Control module.

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Here it is leading into the control module.

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Some of those other wires, like the pink and gray ones, do not look factory either.

So, it seems that something in the original aux fan circuit failed and someone did this workaround instead of fixing it correctly. So, where is the temp sensor that triggers the aux fan? How do I test it?

They want me to schedule another appointment to let them diagnose it, but the idea of paying shop rates for hunting and seeking electrical gremlins makes my wallet cringe.

As for the A/C, I'm assuming a smoking A/C compressor is likely an indicator that I need a new A/C compressor. That one I can maybe figure out!:smile:

Good news is that my new keys work great. :|
 

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Discussion Starter · #3 ·
New info, new symptoms, and a video

So, I continued to look around my engine bay trying to figure out what this mystery wiring is all about, and I realize there is a second wire coming from the "replacement" temp sensor. It snakes across the front of the radiator and is spliced into a wire near the battery. No idea wat that is for.

Also, now I have a new noise. In the too long video below (sorry) I discover that the buzzing is coming from the round silver component near the aux fan module on the driver's side. What is this thing? It's partially visible at the right edge of the third photo in my original post. It was buzzing and hot to the touch.

So, here is a video. Sorry for the length.

 

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Discussion Starter · #5 ·
So obviously the brake vacuum pump is a separate issue, but no one has any info on where the aux fan circuit is supposed to be getting a temperature reading? I'm assuming that where ever that temperature sensor is, that must be what the aftermarket sensor is replacing. If I can find/identify the original sensor, maybe I can restore normal operation, and not the half-assed bypass system I have now.'

Anyone?
 

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Discussion Starter · #6 ·
Okay, trying one more time for help on this topic, with some new developments.

  1. Previous owner had completely jury-rigged the fan circuit. The aux fan control module you see in the pic above has the wrong part number on it, so I removed it to replace it with a brand new CORRECT one. Imagine my surprise to discover it (the old one) was completely gutted.
  2. Someone has basically jumpered the connection in the plug itself and just had the aux fan module there for appearances. They were using a non-original 12V relay wired to the sensor, which had been added to the radiator, to trigger the fan at 80 degrees C.
  3. I removed all the non-factory wiring and put everything back to stock.
  4. Installed new correct fan module
  5. checked all fuses
  6. fan rotates freely by hand, and if jumped with 12V direct

When it is all put back together, I'm still having issues:

  1. Fan does not come on, I let the engine temp get up to about 100 C. Gauge still works, no problem.
  2. Coolant low light now on, was not before.

The wires coming into the aux fan module are (reading rear to front of car)
  • M (rear most big terminal) reads over 12V at all times, engine on or off
  • 31 (2nd big terminal) no voltage
  • 30 (3rd big terminal) no voltage
  • M- (big terminal closest to front of car) no voltage
  • PWM (small terminal closest to rear of car) no voltage
  • 15 (small terminal in middle top of plug) over 12V when car is running only

Short of getting a shop to run the diagnostics, is there anything left for me to check or try? I'm broke as heck right now from a much bigger than expected tax bill last week. :surprise:
 

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Discussion Starter · #7 · (Edited)
Bumping in the hope of getting some suggestions, and I'll add this new bit of info.

Some threads I have found on various forums pointed me towards the infamous K40 as a possible contributor to fan issues. So, in the interest of ruling out something, I followed the excellent DIY on Pelican's website to remove my K40.

Two things:
1) The solder joints "look" good. I don't see any solder cracks and I don't see any areas where it looks like the board has overheated.
2) BUT, it does look like someone has been here before me. The entire K40 relay assembly is obviously from a used part yard. On the black "box" the K40 is housed in, there are paint pen markings like parts yards use to mark their parts. I double checked the part number (A 170 545 02 05) and it does appear to be the correct K40 for my car.

Does this new info give anyone an idea where I should look next?
 

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this guy needs help

Hey come on you techies, this poor guy needs help, surely with 266 views someone must be able to point him in the right direction.

Good luck and trusting to someone offering you some assistance soon. R)
 
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