Conductive Sealant??

Intermittent "Duct Overheat" EICAS and overhead amber light.

The bleed air ducting uses two types of sensors on a 767. Overheat loops along with overheat switches. The loops checked good. Upon meggering the switches to ground all read open except one. Granted the resistance was around 870 Meg Ohms, but it was all we had to go with.

These switches only have two contacts. One goes to the warning system and the other is connected directly to ground.

We found one overheat switch with 1422B1/2 or 870 type sealant covering the connection points. All the others had RTV (white in the image).

We dug the darker sealant off the back of the switch and our megger reading showed open.

This was good enough reason to sign off the write-up, but the verdict is still out if this was the actual problem.
  
     
 

It was a "big negatory" on this being the problem. It was written-up within two days.