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Old 05-29-2019, 07:21 PM   #1
NC2000
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2000 L-Series 3.0L Sedan
Default AC Fuse blown mystery

I have a purchased a 2000 LS2. The ac didnít work. I looked at the compressor to see if it as turning and it wasnít. I checked the 10 amp fuse under the hood in the power distribution box and it was blown. I put a 20 amp fuse in just to see if the compressor would turn on and the fuse blew immediately.
I used a wire to jump the power just see if the compressor would turn on. The compressor turned on and I pulled out the wire to make sure I didnít burn anything up.

Where should I start to diagnose my problem?
Any help would be appreciated.
Thank you in advance.

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Old 05-29-2019, 11:18 PM   #2
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2003 L-Series 3.0L Sedan
Default Re: AC Fuse blown mystery

Pull the ac diode in the fuse box. The electrical symbol for a diode is an arrow pointing to a line. Sometimes, this diode shorts and acts as a wire to ground 12v in the ac circuit. As soon as a fuse is replaced and ac turned on, the fuse blows immediately. Do not use a higher rated fuse as all this does is allow wires to carry more current that can stress wiring, terminals, relays and other things connected to this fused line. Burning up wiring up can result in damaging other wiring when wire bundles are tied together into a harness.

A diode electrically stops dc current flow in one direction. It's used to absorb current generated when the ac is turned off; the clutch coil magnetic field generates a current going in the opposite direction to the ac relay contacts. As the contacts open, an arc burns the contacts a little at a time until the contacts wear out. The ac relay connects battery voltage to the coil. The reverse current is absorbed by this diode to prevent contact wear. When it shorts out, it acts as a wire and blows the ac fuse as soon as ac is turned on. Remove the diode and either measure it for resistance with a multimeter or replace the fuse with the same rating and try turning on ac again. If the diode is faulty and removed, ac should run again. I do not know if there are any other parts that blows ac fuses as quickly as a faulty ac diode (shorted). Diodes measure resistance in one direction and when the probes are reversed, resistance is higher. A diode can fail as either an open like a blown fuse or measure as a plain wire with zero resistance in no matter which way the probes are placed on the pins.
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File Type: jpg Diode, ac.jpg (10.4 KB, 3 views)

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Old 05-30-2019, 12:54 PM   #3
NC2000
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Default Re: AC Fuse blown mystery

Thank you for your help.

I took out the diode and it looked like it was misshaped (like it got hot). I replaced the 10 amp breaker for the ac and turned it on. The clutch wasn't turning. The out side pulley was turning but the interior of the ac compressor wasn't.
Are the ac diodes universal or is there a certain amp i should be looking for. My local auto parts stores do not carry them and i have to order online. I think Amazon has a pack of 6 but it'll take a couple of weeks to get here.

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Old 05-30-2019, 01:07 PM   #4
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Default Re: AC Fuse blown mystery

?...? You may have more than one problem now. Your initial post mentioned wiring a jumper to power the compressor clutch coil but now it won't power up after removal of the diode. This seems odd as verifying the clutch coil powered up pointed to a damage diode (shorted and blowing fuses). Since diode removal, a fuse hasn't blown but the clutch coil isn't powering up. Try replacing the ac relay with a similar one in the fuse box (wiper, horn, etc). The ac relay contacts may have burned and pitted, preventing contacts from making electrical connections.

Try wiring power again, direct to the compressor and see if the clutch engages. If the clutch doesn't engage with a metallic click then the clutch coil may have burned out - a thermal fuse protects against excessive heat and current draw if a short occurs in the coil or clutch slips and creates excessive heat. A multimeter can measure for 2-3 ohms resistance in the coil windings. Salvage yards are full of ac diodes, any of them with similar terminals will work as replacements.

If this ac system wasn't over filled with refrigerant or mystery sealer, the compressor can run without a diode since its used to protect the relay from electrical arcing creating pitted contacts. Try determining if the compressor is faulty before buying an expensive diode. An initial search shows ac diodes as $37 items(!?). A plain silicon diode rated for 5 amps+ may work but remember that polarity is important - placed incorrectly with the arrow pointing the wrong way and another fuse will blow instantly when ac is turned on. Salvage yards are full of ac diodes. Any one of them with similar terminals will work as a replacement.

Last edited by fdryer; 05-30-2019 at 01:16 PM..

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Old 05-30-2019, 01:34 PM   #5
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Default Re: AC Fuse blown mystery

Which way should the arrow point? Should it point towards the front or rear of the car?

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Old 05-30-2019, 01:39 PM   #6
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Default Re: AC Fuse blown mystery

Look inside the fuse cover. It should have labeling for fuses, relays and diode position. I'm not near a service manual to see what drawings show but leaving this diode removed is fine and doesn't affect ac operation. Troubleshoot why you were able to power the clutch before and now unable to. The suggestions given can help determine if the clutch coil is blown.

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Old 05-30-2019, 05:58 PM   #7
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Default Re: AC Fuse blown mystery

I put a new 10 amp fuse in and the compressor didnít turn on. I replaced the diode with one from a different vehicle. The compressor started I used a refill kit I purchased at Walmart because I figured I probably have a leak somewhere. The air has been blowing cold and has been off and on for the last couple of hours.
Now I can use a black light to track down the leak. Hopefully it wonít be too bad.

Thank you fdryer for all your help.

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