ATA 26-11, Systron-Donner Fire Detection

Most engine fire detection systems come in two forms,  Systron-Donner or Kidde .

CF6-80A and CF6-80C2 that use Systron fire detection systems have three equal resistance loop sensors. Each loop measures 4.5K Ohms. They are wired in a parallel circuit.

In a functional system 1.5K Ohms should be read at the fire detection card pins 19 & 20.
  
     
  
If 2.25K Ohms is read..... you've lost one loop. If 4.5K Ohms is read..... you've lost two.
  
     
  
The "only" way to verify an open loop is to disconnect one (or both) wires from the loop terminals and measure resistance at the terminals (if the wires are connected..... you're reading the other loops and possibly the card).

A loop is either good or bad. There is no halfway. 4.5K Ohms or open.

If a bad loop has been isolated, throwing a decade box across the terminals should at least give you a valid test from the cockpit.
  
     
  
Or..... just throw a 4.5K Ohm resistor across them.
  
     
 

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.

Mark did you try to test the loop with a megger once removed? I remember it being a way to find intermittent loop failures but blurb in the manual that the loop must be clean prior to using the megger.

I have used a Megger on engine loops, but I'm not a fan of using one.

A Systron loop is pressure filled, so a Megger shouldn't hurt it. The Kidde???? I would stay away from a Megger.

With this problem, after we used the decade box for substitution, the loop read 30MΩ (that was just from removing the one terminal lug).

We smacked the white doughnut pretty hard and the 4.5KΩ returned. It didn't fail after the beating.

I use a regular VOM on most engines loops. A LCR Meter can also be used.

A LCR Meter should "always" be used on body and wing overheat loops.