Left Wing Duct Leak

We had another problem similar to this one dealing with the left bleed air system. In the case of this particular aircraft, the overheat switches were part of a different warning system so we were dealing with just the loops.

As with either type of configuration, everything can be checked from the system test panel. Both ends of each loop.... (left, right, and body) converge at the plugs behind the panel.
  
     
  
The reading from end-to-end of the loop in question was around 9Ω, which is a good reading.
  
     
  
A problem was noted when a short to ground was checked. Just over 500Ω while a known good system was 2.4MΩ+.
  
     
     
  
500Ω does sound like a pretty high resistance, but we're dealing with a static aircraft with no wing fluctuations from flight.

Now comes the "fun" part. If both ends of the loop show a short to ground, how do you trace it down?
  
     
  
You try to find an accessible point halfway in the loop series and disconnect the junction. The short will now only be on one pin. That takes you either outboard or inboard. Ours was outboard. When the next junction was separated..... the short went away. Meaning, the loop between the first disconnect and the second was bad.

A bracket arm had broken free of its mount and had rubbed into the loop.
  
     

Gaining access to these connections can be quite difficult. The more "hands on deck" the better.....
  
  

Awful job, we ran into same issue intermittent duct heat only happened with engines running at high power very intermittent. We ended up having to re run new loops. Never found bad loop even after removed. We tried the megger test to off the airplane and still passed. After replacement never happened agian.

You didn't see "me" out there on the wing. Old folks like myself need the "young bucks" to do the lifting.....

Yes, it sucks. The first loop in the A/C bay is the only one reachable. All the rest require panel removal and a set of nibble hands to get over and behind the bleed duct itself.

Yeah was a terrible job, it was 120 outside and we replaced from pack bay to pylon.