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Air Con/Heating Fuse D6 Keeps Blowing


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1999 C2 996 USA.

When I turn the blower fan up past half way the fuse D6 (air conditioning/heating) blows like clockwork. The blower fan stops, but everything else continues to work.

I've got a steady supply of replacement 10 amp fuses to pop in (is it supposed to be a 7.5 amp?), but I don't have a steady supply of patience for problems like this when it is 100 F outside.

Before I start tearing things apart, reading codes and so forth to diagnose an electrical problem or perhaps the failing or clogged blower fan causing excess resistince I searched through the forum and found a couple suggestions like swapping the relay for the blower.

"block 10 the fan, we switch it for the suspect relay in block 18 the blower. (they wre the same types of relays) Like magic the blower fan comes to life!"

Thanks in advance if you've dealt with this before!

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Does blower relay work if you place it in block 10? Fuses blow because the circuit you are protecting is drawing more current then the fuse is rated for. Either you have a bad relay, blower motor, fan resistor, or a short in the wiring.

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Thanks, I found out the problem.

I referenced the work shop manual to find out what the proper amperage was and remembered that I was troubleshooting an electrical issue the other day and I mixed up where two fuses went.

I accidentially placed fuse D6 (heater/air con) which is supposed to be 30amp into E7 (air con panel) which is supposed to be 7.5amp (and it actually had a 10 amp in there previously when I bought the car... interesting now that makes me want to go throuh the rest of the fuses and make sure I have the correct amperage at each position).

Put the correct size fuse in each position and it works perfect again.

Thanks!

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  • 7 years later...

I now on the second hottest day of the year am having the same issue. Replace the fuse and the second one piped instantly. I'll pull the relay and check the fan for a short but does anyone have a diagram of what the circuit looks like? Would be very helpful in troubleshooting. Is this fuse only for the fan or does it support other circuits?

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  • Admin
18 minutes ago, talkenrain said:

I now on the second hottest day of the year am having the same issue. Replace the fuse and the second one piped instantly. I'll pull the relay and check the fan for a short but does anyone have a diagram of what the circuit looks like? Would be very helpful in troubleshooting. Is this fuse only for the fan or does it support other circuits?

 

Which fuse?

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If you're talking about D6 (30A), it supplies power (via a relay) to the compressor (via the refrigerant pressure safety switch) and the a/c blower under the dash and nothing else.

Edited by Ahsai
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Ok, thanks. The 30A fuse blew while the air compressor was working. So my new project is to discover if it's the fan motor or the compressor thats shorted. My guess is the fan because I've turned off the compressor from the control unit then replace the fuse but it blew immediately. I'll unplug the compressor and try again to confirm. If you can point me to a wiring diagram, that would be very helpful.

 

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So I removed my fresh air blower. Bench checked it, it checks out fine. I'm going to check the Actuating heat relay and Air conditioning compressor relay tomorrow but I think my problem is somewhere around the 3-level cooling water switch. I don't think it's the air conditioning compressor or relay because I've switched it off and new fuses continues to blow. I tried a new fuse with the blower fan removed from the circuit as well (still blew) So i'm pretty sure I'm tracking down a hard short to ground. I'd be surprised if it's the control unit but I fortunately have a spare to try if needed. 

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Sounds like a great product. I'll try to find one locally. Thanks... although, now that I'm thinking about it I guess I could just use a small 12v bulb across the fuse holder contacts and get the same results.

Edited by talkenrain
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4 minutes ago, talkenrain said:

Sounds like a great product. I'll try to find one locally. Thanks... although, now that I'm thinking about it I guess I could just use a small 12v bulb across the fuse holder contacts and get the same results.

Absolutely. The smart fuse is just more convenient.

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My problem can be marked Solved

I'm not sure if the problem was self inflected since i was just recently in the same area (other side) but looking at the pictures of the relay/DME area of the rear deck on the relay side the chamber that houses all the wires had a few stray wires that where not in the chamber and where squished against the large aluminum panel of the relay circuits. I have a heavy subwoofer that sets on top of this area so I think the weight combined with the heat (over 100 degrees here lately) along with engine/subwoofer vibrations wore away the insulation and shorted against the aluminum panel (ground).

 

The wire being shorted was from the "high/low pressure 3 level cooling water switch" (pin 1). This sources 12v to the air conditioner compressor relay (pin 3, violet/yellow) whenever the ignition is on and cooling water switch is in it's normal closed position. This can all be found in the area of L49 of the wiring diagrams.

 

I insulated the wire, made sure they are all in the wire harness, then remounted the relay panel. Short removed, air conditioner and heater fan are both working properly again.

IMG_9771.jpg

IMG_9772.jpg

IMG_9773.jpg

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  • 1 year later...

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