deandec
Northern CA
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Hopefully there is an auto AC expert out there. 
The 20A AC clutch fuse blows when the system calls for air conditioning.
I have tested the wire run to the AC Clutch and measured 12.5v.
I have Ohm tested the AC Clutch Coil at 3.5 Ohms.
Inserting a 30A fuse causes the AC clutch to engage and air to cool. But...the fuse and the Clutch wire get very hot. So have not done this for any length of time.
The Result:
12.5v to the Clutch coil which it needs.
Clutch coil tests as if it is not shorted.
What else could be the problem that causes the fuse to blow when the call for AC occurs? And, how to test for that problem?
Dean
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smkettner
Southern California
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Any chance the coil shorts when engaged?
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Chris Bryant
DeLand, Florida, USA
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smkettner wrote:Any chance the coil shorts when engaged?
That would be my guess.
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j-d
Sunny Florida USA
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You could unplug the coil, call for Compressor, then check the fuse and for 12VDC at the plug if the fuse didn't blow.
Possibility: Many auto A/C systems incorporate a Diode in the clutch circuit. When the coil Energizes, it's only a Load. But when it DE-Energizes, the collapsing magnetic field acts as a little generator and sends a voltage spike called "Back EMF" OUT of its windings. They do this to keep that spike from damaging or interfering with the operation of other vehicle systems. If that little diode went Shorted, AND it was a hard enough short to just not burn out and go OPEN, then that Short could blow the Fuse.
Diode doesn't look like much, usually buried under tape right at the plug end of the compressor clutch wiring harness.
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Ozlander
Rose Hill, Kansas
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j-d wrote:If that little diode went Shorted, AND it was a hard enough short to just not burn out and go OPEN, then that Short could blow the Fuse.
Me thinks that with a 30 amp fuse you would blow that diode to pieces if it was bad.
And I don't understand how the diode being OPEN could blow the fuse.
Ozlander
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j-d
Sunny Florida USA
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Long shot, I agree.
What I tried to say was if the diode failed shorted +++and+++ it was robust enough to resist 20-30A and still stay shorted, then the fuse would blow. I'd agree that the diode if shorted would probably just blow out to an open, as if the diode were not there at all. This whole logic presumes the diode is ACROSS the Clutch Circuit...
Failed open or missing diode there, chances are it wouldn't ever be missed. Maybe a POP in AM Radio...
FYI - Some circuits with Relays use a Diode to quiet down the Back EMF from those contactor coils also.
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DryCamper11
Hartford
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If the A/C coil was tested while pulled out or disconnected, there could be a high resistance short after the fuse - a crimped wire might do it. It sure sounds like something is shorting or nearly shorting when the compressor is running and the engine is vibrating, but isn't showing up when you just stick a meter on the coil to check resistance. I'd look the wiring over very closely after the fuse all the way to the coil.
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MrWizard
Traveling
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diode being open
means that the when the clutch is turned off
the collapsing magnetic field, sends power back into the electrical system
its like monetarily connecting a DC generator to the fuse
the power spike blows the fuse
this may or may not ne the problem,but should be checked
the fact that it fails to pull in and blows the 20 amp and will work with the 30 making the wires hot
indicates a shorting clutch, some of the windings are burnt
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MEXICOWANDERER
las peñas, michoacan, mexico
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Shorts in an inductor a (coil of wire) can be reactive. Impossible to deal with using a resistance measurement. A twenty amp system requirement and a 3 ? measurement don't jibe. You can spend hours beating your head against the wall or just replace or repair the part with the magnetic coil, and go have a cold one to celebrate how smart you were.
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DryCamper11
Hartford
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There are some designs that have a low resistance to pull in the solenoid with lots of power, then a switch operates to lower the current and decrease heating and power used. That switch might be partially shorted out internally.
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