A/C fuse blows only when car is moving...sometimes.

1998 MERCURY MARQUIS
58,000 MILES
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'98 GRND MRQUE
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I have had this car for about a year. The A/C was working fine up until about two months ago...just when it got hot here in Cleveland. At first the fuse popped after driving on the highway with the A/C on for a good twenty minutes or so. I replaced it and it worked for a couple of weeks. I replaced it again and then it started popping everytime I ran the A/C. I took it in to my local mechanic who I have been going to for over a decade and he checked the system with a VOM while the A/C was running and could not get the fuse to blow. I picked the car up and within a mile of leaving his shop it blew. If the car is parked, or just not moving, it will work but as soon as I begin to drive, even slowly and carefully through a parking lot...pop, the fuse blows. He said that he can not figure it out unless it blows in his shop which we can't seem to get it to do. Have you ever heard such a thing and are you able to give me some advise as to what to check out. I am mechanically incined enough to do some real damage...oil, brakes, tune-up & belts.

Thank-you,

Dylan
Sep 1, 2011 at 5:54 PM
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CARADIODOC
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Replace the blown fuse with a pair of spade terminals, then use a pair of small jumper wires to connect them to a 12 volt light bulb. A brake light bulb should work fine. The circuit will not work normally because the bulb will limit current to a value that's too low, but it will allow you to search for the short.

When the short is present and the circuit is powered up, the bulb will be full brightness, so don't allow it to contact the carpet or plastic door panels. When the short goes away, the bulb will dim. Since it's trying to pass enough current to run the high-current AC compressor, the bulb might be real bright when the short is gone too. That kind of defeats its purpose of being an indicator. If that happens, replace the low-current brake light bulb with a higher-current head light bulb. That will be much dimmer when the short is gone.

Now you can do things to see what makes the short appear. If it does when you shift into "drive", look for a wiring harness that is being tugged on when the engine rocks. If it also occurs when shifting into drive without the engine running, look for a wiring harness that's rubbing against the shift lever on the transmission. If you can get the bulb to stay bright, without the engine running would be even better, you can run under the hood and start disconnecting things one at a time, or moving wiring harnesses around to see what makes the short go away. That will help you narrow down the cause.
Sep 1, 2011 at 6:23 PM