Melted A/C switch !?

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Thread: Melted A/C switch !?

  1. #1
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    Melted A/C switch !?

    My air conditioner stopped working and I thought I had narrowed it down to the A/C controls. The button was sticking when I pressed it and the AC light did not turn on. I removed the radio, trim, etc to get the HVAC controls module out. According to the wiring diagram, the AC switch is connected to pins 1 and 2 on the 8 pin connector of the HVAC controls. Checking with a multi-meter I can see there is no connection being made on the 8 pin connector between pins 1 and 2.

    I bought a used replacement off eBay. When I checked pin 1 and 2 of the replacement with a multi-meter, the switch works. It makes and breaks contact as expected.

    I hooked things up and tried it out. The AC light came on briefly but air coming out of the vents was not cold. Then the AC light stopped working. And in a bit, I noticed faint smoke rising from the HVAC controls. On the multi-meter I see that pin 1 dropped to ~2V instead of the 13 or so volts at pin 2. I shutoff the car and disconnected the controls. Now, the replacement AC button sticks too. I also noticed that on both of controls, the back of the AC button are is melted a tiny bit.

    What should I check next? According to the air conditioner troubleshooting guide and based on the wiring diagram my next suspect is relay 384.

    A possible relevant event is the car overheated before the air conditioner stopped working. That turned out to be a broken thermostat and I had the timing belt and water pump replaced while fixing the thermostat. That was during the winter so I did not try out the air conditioner until later. So it may or may not be related.

    Thoughts?

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  3. #2
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    The A/C switch ordinarily carries a very small current, just enough to activate two relays (384 and 214), but it's only protected by Fuse 25, which is 30 amps. It sounds like there's a short circuit to ground somewhere downstream of the switch and the switch melts (self-destructs) before Fuse 25 blows.

    Downstream of the switch are:
    • the ambient temperature switch
    • relay 214 (which activates the electric fan)
    • the A/C low pressure switch
    • relay 384

    Of these, I'd suspect the wiring to the A/C pressure switch, since it's out under the hood where things got unusually hot when your engine overheated.

    Check resistance between the gray wire in the harness at the A/C switch (pin 1 of the connector) and ground. I'd expect at least a few hundred ohms if all is well. In your car, I suspect just a few Ohms or zero, i.e. a dead short. If that's the case, you'll have to correct the root cause.

    (If you want to rule out the relays 214 and 384, you can just unplug them and measure the resistance again.)
    Iowegian likes this.

  4. #3
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    Looks like I'll need to decide which is easier to get to, the relays or the pressure switch and check that next. Relays may need removing the dash panel by the driver door and the pressure switch requires removing the headlight.

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  6. #4
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    First check the wiring to the pressure switch, particularly where it would have gotten hot during an engine overheat.

    It could be shorted to ground, or it could just be shorted to another wire in the same bundle. There's a ground wire (brown) that goes to the same pressure switch (for the high-pressure set of contacts). An unwanted connection between either of the low-pressure switch wires would cause your exact symptoms.

  7. #5
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    I measured pin 1 at the A/C switch harness to ground and it reads about 0.6 ohms. This meter reads 0.3 with the leads shorted. So there's a short.
    In an effort to isolate it, I disconnected the A/C pressure switch and removed relay 384. Both are relatively easy to get to. I removed the passenger side fog light grill and reached up to disconnect the A/C pressure switch and I removed 384 by reaching in behind the fuse panel.

    Pin 1 of the A/C harness to ground still reads 0.6 ohms. On the A/C pressure switch pin 2 to pin 4 (ground) is open (i.e. higher than 2000 ohms). Based on the wiring diagram pin 2 should lead back to the ambient temperature switch and relay 214. For sanity, I also checked pin 4 to ground and that is connected.

    I think this means there's no short in the cabling that goes to the pressure switch.

    Which relay 214 am I looking for to check? I have multiple diagrams and it shows as either position 1 on the 13 fold relay panel or position 5 on the 8 fold. On my 13 fold I see a relay marked 373 in position 1. I can't see the 8 fold from the fuse box access panel as far as I can tell.

    To get to the ambient temperature sensor, it looks like I'll need to remove the glove box. I could also try to look at connector T14k and T15k which is in the drivers side foot well and connects between the engine compartment and inside. That's connection between the ambient temperature sensor and relay 214 and the a/c pressure switch.

  8. #6
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    I got the lower A-pillar trim off and checked the connection at T15k. The trim panel is only held on be 1 Phillips head screw (not sure why it's not torx like all the others) but requires some maneuvering and force to get the tab next the hood release lever to come off.

    With T15k disconnected, I see 80 ohms between pin 1 and ground on the A/C controls connector. When it's connected that drops to 0.6 ohms. So it seems the stuff inside the car is OK. I don't need to go check the ambient temperature switch or relay 214.

    Since the problem seems to be between the connector to the pressure switch and T15k, I need to trace the path of that wiring. Are there any diagrams that show where I should look? It looks like T14k is the next stop in the chain. I have not found any diagrams showing where that is.

  9. #7
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    The relay position will depend on your car's year and engine and A/C (I presume you have manual controls, not Climatronic). It's in the Bentley manual, but it's a little hard to interpret.

    What is your year and engine?

    Do you have the Bentley manual, or some other schematic?

  10. #8
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    There is no connector location chart that I know of.

    There are two clues to the location of T14k:
    1) The ground is spliced "in front wiring harness" (Bentley schematic 57/4 rung 40, 151 in a circle)
    2) The outside air temperature sensor wires also pass through T14k (Bentley schematic 48/7 rungs 74 & 79)

    So, I think T14k may be on the left (driver's) side, near the power steering reservoir. My 2005 A4 has a 14-pin connector there, but it sometimes differs a little from a Passat (my Passat is, sadly, gone).

  11. #9
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    Thanks for the tips. It's a 2002 1.8T GLS. It's manual hvac controls. I don't have the car with me at the moment and I don't remember the engine code.

    I don't have Bentley manual. I've been looking at diagrams I found on the web.

    Next to the power steering fill cap is a bunch of wires including the one that goes to the A/C clutch. I saw that in a YouTube video. So maybe T14k is there.

    Any idea where the wires coming out of T15k pass thru to the engine compartment?
    Or is removing the headlight and attempting to trace the cable from the pressure switch end easier?

  12. #10
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    The two wires to the A/C refrigerant low pressure switch should be:
    > brown/blue, from the switch (via ambient temperature switch), in pin 14 of the 14-pin connector; black/red on the cabin side of that connector
    > black/blue, to the 384 relay, in pin 13 of the 14-pin connector; black/green on the cabin side of that connector

    I would unplug that connector and check resistances there, to body ground and to the brown (ground) wire in that bundle. That will isolate the fault to one side of the connector or another.

  13. #11
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    Quote Originally Posted by FrescoGreen01.5 View Post
    So, I think T14k may be on the left (driver's) side, near the power steering reservoir. My 2005 A4 has a 14-pin connector there, but it sometimes differs a little from a Passat (my Passat is, sadly, gone).
    I was able to get back to this this weekend. You are correct. T14k is next to the power steering reservoir.
    t14k.jpg

    After disconnecting that, I determined the problem is between the pressure switch and T14k. Following the cable, I found that it had been squished between the bumper mount and frame. This picture shows it after I loosened the bolts and pulled it out.
    pinched_wire.jpg

    I repaired the wire. Now I don't see the short at the AC controls connector anymore. I also tested things by jumpering pins 1 and 2 at the controls connector. The AC comes on and works as expected. Now I need to get another replacement control set to replace the melted switch again.
    FrescoGreen01.5 likes this.

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