Fanbelt is gone warning light


The other day Shawn and I were knocking around ideas re. how the driver might be warned that he’s lost his fan belt and therefore the water pump is no longer turning., I thought that a coolant pressure light would warn the driver, but an experiment proved that to be wrong. The pressure of hot coolant is enough to keep pressure switches from warning the driver that the water pump is no longer turning.

I came up with another idea and I tested it tonight. It’s easy and it works.

Why this matters. If you go off into gravel, rocks spray into your engine bay. With a little bad luck one will pop into the gap between a pulley and the belt and that can knock the belt off. Since the driver gets no warning other then the dim little OEM Alternator light on the dash, it’s easy to make the mistake of keeping the engine on. But the engine is already hot as hell, so with coolant not flowing you have only seconds to turn the engine off before something bad happens.

The above scenario happened to Shawn the other weekend which prompted this discussion.

Another way for this to happen is if the brace that adjusts the alternator position breaks. That allows the alternator to rotate inboard loosening the belt, which then escapes. I’ve seen this happen 2x, one of those was to Fred Switzer when he was at MidO, 12hrs from home, in '09.

Cliffnotes. Wire in a dash light between 12V KeyOn and the exciter stud on the alternator.

Background. 2 wires go to the alternator. The big one supplies power to the battery and the small one, lets call it the exciter, goes to the dash alternator light and then to the battery.

The alternator consists of a rotor that has a bunch of wire wound around it. When the alternator is stopped, the wound wire is a path to ground. So if you connect that to a light and then to KeyOn 12V, the light will illum when you turn the key. Think about that. If the key is on and the alternator is stopped, the light goes on.

When the alternator is turning, it’s charging the battery. This also means that the wound wire, and therefore the exciter wire and your KeyOn 12V are now at the same voltage. So now your light doesn’t illum because both sides of it are connected to 12V.

It’s actually a little more than 12V, but that doesn’t matter.

So put a big light on your dash. Wire one side of it to KeyOn 12V, and the other end to the alternator exciter 5mm stud on the back of the alternator. Just spin off the 8mm nut, crimp a connector on the end of your wire that will fit over the stud, and spin the nut back on. Done.

As a bonus, if the alternator light in your cluster fails, this light on your dash will keep your alternator working. That exciter wire has to get KeyOn 12V for the alternator to charge.


In Lemons we wrapped the front of the engine bay in mesh to keep the rocks out.