923A2 Failure warning module won't shut up!

[dikwks]

Anybody run in to this? I've checked the three inputs, overheat, low air, and parking brake switches. All is well with them. It has power at terminal A and the ground is good. I checked for 24V power at terminal B, but can't seem to get it with the light switch in any position. From looking at the schematic, I still can't figure out what it is supposed to do anyway. The TMs aren't much help as they only cover two of the inputs. The audible alarm won't shut up and I've already tried a known good part. Has anybody ever had this problem before and what did you do? And what is the purpose of 24V at terminal B?

 

[Rifleman]

In your thread you said you used a schematic in the TM's to help you trouble shoot your problem. Did you also use all the step by step ladder logic trouble shooting guides, they cover all the systems that provide outputs signals to the warning horn, they can help in pin pointing the problem.

 

[Suprman]

I believe the sensor inputs silence when grounded so a faulty switch cannot cause a low air or parking brake engaged warning to not signal.

 

[dikwks]

@ Rifleman-Yes, I used the troubleshooting guides in the TMs. Trouble is, the only thing they cover is the low air switches and the parking brake inputs. I didn't see anything about the high engine temp switch there. I got the info about that from the schematic. In the schematic, only the box terminals are shown. Nothing WRT the alarm. It is a "black box".
@ Suprman- Yes the switches in the for the low air switch have to be grounded, the circuit for the parking brake has to be open.

There is a connection between the lighting system and the module, it should see 24V from the light switch. I thought it might be connected to the fact that the tail light lighting was damaged. Hoped I'd get lucky if I fixed it(Kind of makes sense that a failure in the lighting system might be something to make noise about). Didn't happen that way, but at least the lights are all working now. I'm going to test the light switch and see if I can figure out why there is no 24V input from it to the module tomorrow. If you guys see something I've missed, please chime in. Every one of the troubleshooting guides ends with "Tell your supervisor", mine just looks at me funny and says "You'll figure it out, baby."

 

[Artisan]

FWIW I had 3 FWCM's PLUS the OE unit and all four of them produced different undesirable results.
I bought a new FWCM from Alex at idahomotorpool and now all is well. This was after I replaced both
low air warning switches and cleaned there ground.

Additionally, the A2 is different than the nhc250 but the high temp is deleted on the nhc250's.
FWIW.