Strange One

Captain's pitot heat fail light on after engine start.

Please reference https://rotate.aero/forum/767-ice30/242-ata-30-31-pitot-heat for an understanding of the 767's pitot heat system.

The system has two heat modes, low heat after engine start and high heat in air mode.

This particular problem (failed Capt's Pitot Heat) showed up after engine start.

Pitot heat test from the P61 panel was normal.

I took out the Current Sense relay (K241) and shot the probe coil from "Pin C1". 38 Ohms..... seems normal?? Pulled the "A Phase" breaker..... still had continuity..... not normal.

What the hell is going on here?

90% of pitot heat failures are heater coils burning open. This one tested from the cockpit test switch, so the coil is still good.

Well kinda..... In test mode, you're simulating "air" mode or high heat. "A" and "B" phases are connected through the heater coil in air mode, instead of "A" phase to ground in "low heat" mode.

We pulled the probe and shot it with VOM. We found out one side of the coil wire was shorted to ground and the other completely open.

That would explain our problem. In low mode, "A" phase was going nowhere (open leg on coil), In high heat mode, "B" phase was coming in from the opposite side, going through the coil to ground. It was enough current to turn off the fail light. The pitot probe would of been heated in the air, but only in low heat mode.

I have never seen a probe fail in such a manner.

Red represents short to ground. Green, the open pin.

I wish I could show the whole circuit, but Boeing has been watching the site for plagiarism.