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.

Click Here for Larger Image


Click Here for Larger Image

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.

NOTE:

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.

Click Here for Larger Image


Click Here for Larger Image

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!