AeroElectric Connection

Bob's Shop Notes:
Probing the Current in an ATN Fuse

The A/C in my best road car has been shutting down intermitently. Seems the fuse that protects the compressor clutch power was experiencing an overload. This is a very simple circuit. The fuse supplies power to contacts in a clutch relay. Power through an energized relay is rounted to the compressor clutch. At first, the fuse seemed to be opening on engine start-up. Replacing the fuse would get me hours to perhaps a day of normal a/c operation.

Question: Am I looking for a hard fault (wire rubbed to ground) or a clutch with shorted turns that draws too much current? I needed to discover the over-current conditions without getting into the car's wire bundles.

Went to my supply of K and S Engineering brass shapes and found some 3/16" square square brass tube that would make a good fit to a standard bananna plug. I then sanded the sides of a pair of banana plugs so that they could be bonded together (E6000) with 0.320" centers (spacing of blades on the ATN fuse).

With a little work at the 1" belt sander, I fabricated these shapes from the square tube.

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The 'blades' make a snug installation onto the bananna plugs. A visual check compares the spacing with an ATN fuse (A similar process can be sued to fabricate a probe for ATC fuse holders).

The tool is finished by adding lead wires with bananna plugs to mate with a handy multimeter.


It would be good if the test lead wires are be fitted with an
inline fuse holder to protect both the fixture and ship's wiring
should the problem prove to be a hard fault.

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Out in the driveway, I can now replace the tormented fuse with the test fixture. Here we see that clutch current (normally 3-4A) is now more like 10.5A suggesting that the fault is not intermiitent but continuous . . . most probably caused by shorted turns in the compressor clutch. This is confirmed by the fact that replacing the nomral 7.5A fuse with a 10A device would yield un-inerupted a/c perforamance. It also explains why replacing the 7.5A fuse would produce some duration of normal a/c operation . . . the shorted coil was going to warm up at elevated current draw. The draw goes down as the copper warms up.

With this data I can now go on the computer to get a new compressor on order . . . sure glad I don't have to trace wires/bundles looking for damage!