A Small Thing That Matters

Air conditioning work is a big issue in lots of Canadian shops this time of year, and whether or not you believe that global warming is a fact, only the most cash-strapped of Canadian motorists will balk at repairing the system. I recently experimented with a Ford FS-10 compressor, or more accurately, its clutch assembly. The FS-10 is ubiquitous, reasonably durable and not expensive as compressors go, but in this case, I elected to replace the clutch only as the unit was still strong, with a clean orifice tube and no sign of the dreaded “black death” in the system. The clutch was shot, as evidenced by the fragments of clutch facing all over the engine compartment. The service clutch is really the whole pulley assembly and costs almost as much as a low end reman compressor, so it’s not often replaced by itself. In my application, however, it was easy enough to slip off the belt, back out the 6mm centre bolt and remove the splined clutch disk to expose the snap ring. After the snap ring pliers released the ring with enough force to fire it across the shop (never to be seen again) the pulley released with a simple puller. The new assembly (and a new snap ring) went on even faster, but the result was still no cold air. In fact there was no clutch movement at all. The gap looked a little big, but I expected at least an attempt to engage when the coil was energized. The coil tested OK (just under 4 ohms of resistance) and there was power to the coil so I examined the gap again. It was too big, much too big and after some work with the standard shim kit, the unit performed perfectly. But why didn’t the clutch move at all with the big gap? A quick look at one of my old textbooks showed why. It turns out that the strength of a magnetic field in a simple system like our round coil electromagnet is proportional to the square of the distance between magnet and iron clutch plate.

 Magnetic Field Strength        X      1           

                                            (Distance from Magnet)²

X = ‘Is Proportional To”

The math isn’t too serious, but expressed as a ratio, the actual gap, 60 thousandths of an inch, was a lot bigger than the specified 35 thousandths. But is it that important?

The result of the math (the “inverse square law”) shows that the extra gap reduces the magnet’s strength by three times! It’s no wonder I didn’t see any plate movement. Now imagine if you could cheat the spec a little and get a 25 thousandths gap without dragging the clutch. The magnetic field strength would double! The inverse square law means that the maximum clearance spec is critical for pulling in the compressor, and as the clutch wears, removing a shim or two might be worth doing on an easy to service unit like the FS-10, where it’s one bolt and no belt removal required. It could be an added value service for a midlife vehicle in for an A/C system “tune up.” And if you’re working on an FS-10, don’t drop the shims. They don’t fly as far as the snap ring, but they’re just as impossible to find when you do.