Fire Loop - Interesting Find

Interesting problem today. The right engine loop 1 was showing as failed on EICAS. The system installed was a Systron-Donner.

We disconnected the pylon plug and checked the total loop resistance 1.5KΩ, this is a good reading.

Moving to the detection card in the E&E, we had the same reading. The problem did not follow a card swap.

We tried something new and dialed in 1.5KΩ into the decade box and hooked it up to the pylon plug..... aircraft side. The system tested normal.

So..... the problem is on the engine, but total resistance is reading good???

We then disconnected one lead from the lower aft loop and used the decade box to simulate the loop (4.5KΩ). The system was failed.

When we did the same with the upper aft, the system checked good.

I don't recall having a loop read good resistance, but cause the system to fail. We got somewhat physical on tapping the white disc connection point with a screw driver. Resistance jumped to 30MΩ and back to the 4.5KΩ. After that..... the system tested normal with the lead connected.

The loop was changed anyway as we must of had a 'bad internal connection point on one of the studs.

Systron-Donner explained.