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Is this A6 wired correctly?

Robert57 on Sun November 30, 2008 5:18 PM User is offline

Year: 1972
Make: Chevy
Model: K-10
Engine Size: 350

I got to looking at the double terminal going to my stock a/c compressor and noticed there is a jumper wire from one terminal to the other. Seems to be factory. Why? Seems like it would result in a short. There is continuity from one side to the other.

mhamilton on Sun November 30, 2008 7:41 PM User is offlineView users profile

That's a diode. They're usually seen on computer controlled vehicles to prevent (I believe) any "back voltage" from going to the computer. A diode only conducts (short circuit) in one direction, and looks like an open circuit in the other direction. It will be wired so that with normal clutch voltage polarity, it is reverse biased (open circuit).

It's odd that you would have it on a 1972 vehicle, but it won't hurt anything. It looks like those connectors were done at a later time--the stock GM connector is a single plug. But it's hard to tell from the pic if those terminals are the OE type that go in the GM connector.

GM Tech on Sun November 30, 2008 7:47 PM User is offline

If that was "just a wire" then you would be correct- except that the little molded ceramic piece inline is the diode- which allows current to flow in only one direction- in this case the reverse direction of coil activation-- it is there to disipate the reverse current spike encountered when the coil is de-energized- so that none of that voltage spike gets back into any of your other electronics and causes other electrical problems. Your compressor coil does not care which way the terminals are plugged in (polarity is not important) just be sure not to unhook the diode- so that it is always wired in the proper direction- if the diode were to be reversed- then you would experience the short circuit you spoke of...

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The number one A/C diagnostic tool there is- is to know how much refrigerant is in the system- this can only be done by recovering and weighing the refrigerant!!
Just a thought.... 65% of A/C failures in my 3200 car diagnostic database (GM vehicles) are due to loss of refrigerant due to a leak......

bohica2xo on Sun November 30, 2008 7:52 PM User is offline

Check the diode both directions with your meter - when diodes fail they often short internally. It should conduct in only one direction, so if you get the same reading bvoth ways, it is bad.

B.

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"Among the many misdeeds of the British rule in India, history will look upon the act of depriving a whole nation of arms, as the blackest."
~ Mahatma Gandhi, Gandhi, An Autobiography, M. K. Gandhi, page 446.

Matt L on Sun November 30, 2008 10:00 PM User is offline

Diodes are used with all electronic solenoid drivers, but are also often used with mechanical contacts to minimize arcing upon opening. I've seen them on plenty of 70's vehicles.

The degrading field in the clutch coil generates current, and it has to go somewhere.

Dougflas on Mon December 01, 2008 4:59 AM User is offline

They were used in the 70's to prevent back EMF from reaching the alternator doides was an explaination I rec'd many moons ago at a Delco training class. Now they are used to protect computers and electronics as mentioned above.

GM Tech on Mon December 01, 2008 9:19 AM User is offline

Actually they are still in all A/C automotive clutch coil electrical systems-still today- usually buried in the electrical conduit- or sometime in the underhood fuse block- or often times in the compressor coil itself-if the coil has "+" and "-" molded into it- you can bet there is a diode in the coil molding- but they are always there- just not nearly as obvious or as "ratty" looking as they were in the '70s....

-------------------------
The number one A/C diagnostic tool there is- is to know how much refrigerant is in the system- this can only be done by recovering and weighing the refrigerant!!
Just a thought.... 65% of A/C failures in my 3200 car diagnostic database (GM vehicles) are due to loss of refrigerant due to a leak......

mk378 on Mon December 01, 2008 10:02 AM User is offline

GM loves the buried diodes. Also GM dealers do not sell diodes.

If the diode is shorted, cut it out and solder in a new diode. You can pull one out of a dead TV, computer power supply, etc. they're pretty interchangeable. Look for the big ones near where the line cord comes in. The polarity is critical. The end with the painted band goes to the live wire. The other end goes to the ground wire.

Robert57 on Mon December 01, 2008 11:47 AM User is offline

Thanks guys very informative. What would happen if it was taken off altogether?

GM Tech on Mon December 01, 2008 12:09 PM User is offline

I've always contended that nothing would happen- as long as there is an a/c relay in the circuit- because it opens first- to shut off the circuit- and that the diode is there for safety reasons- in case of a stuck relay or the like- I have never in over 20 years seen a failure related to a non- existent diode-- every a/c test stand I have ever seen does not use diodes- and are usually fed by power supplies- with no problems- I'm not an electrical expert- so others may have better opinions- but I'd test the old one and put it in place if fine- just to be sure- Looks like you broke off the plastic connector that housed the terminals as well- would be good and look better if you had that connector over the terminals...

-------------------------
The number one A/C diagnostic tool there is- is to know how much refrigerant is in the system- this can only be done by recovering and weighing the refrigerant!!
Just a thought.... 65% of A/C failures in my 3200 car diagnostic database (GM vehicles) are due to loss of refrigerant due to a leak......

Robert57 on Mon December 01, 2008 7:05 PM User is offline

Quote
Looks like you broke off the plastic connector that housed the terminals as well- would be good and look better if you had that connector over the terminals...

I just took off the double connector to detail the wires a bit..Always a mystery to be solved on these things.

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