Numerous years and many-many lost brain cells, I again had to troubleshoot a engine fire warning system. Same as above with "B" loop being inop. We had a hard fault (good thing) on the overhead.
We gained access to the controllers and shot each loop system. Loop "A" was 1.4K ohms thru pins D and R (as you count the pins...... "A" is not marked but it aligns with the main key-way and there is no I,O,or Q). Loop "B" read 6.8K ohms. Both loops on the opposite engine read 1.4K also.
Out on the engine we wanted to break the loops in half. This "was not" convenient to do. The easiest break point was the lower turbine element. Breaking into the system here was a 3 to1 split with the gearbox loop being the 1 and the remaining 3 being all the turbine elements. We disconnected one wire from the "lower" lower turbine loop connector. Located engine right about 5 o'clock. This isolated the two sides. It is also important to note that the plug was "off" the controller in the E/E to keep it out of the system.
That's B.B.B.B.....Bob
We shot both directions. The hard loop connections and up, read 1.8K (all three turbine elements), the harness side read open. We then went down to the gearbox "aft" loop connection and disconnected one wire also. The loop itself read 6.8K. The harness was open on one of the wires.
This is all a little tricky because the actual aircraft layout dos not match the schematic. From the pylon plug down you hit TB3. (Engine right at 3 o'clock)
That harness then splits. One segment to the gearbox, one to the lower turbine. In actuality, two sets of wires go to the gearbox and then one set back-tracks up the harness, through the "Y" and then on to the lower turbine element.
On the upper connector of the lower turbine, the next two elements split off and go upward. The schematic depicts a harness between each loop, but only one harness is used. The loops are still all in parallel, but it can be confusing when trying to compare the engine to the print.
The 6.8K that we read from the E/E was "just" the gearbox loop. With the harness open to the lower turbine. It and the two loops above were not in the circuit. As stated above, when the other three elements were shot, they read 1.8K as a group. The aircraft had a significant history of "B" loop problems. I shook and tapped the remaining sections. Nothing jumped out and grabbed me as being screwed up.
This problem almost exactly mirrored the one above. Both had a open harness segment leg.....