Low air pressure light/buzzer inop on M923

[TexAndy]

Low air pressure warning light won't come on. Buzzer won't sound. Unfortunately, 9-2320-272-24-1 only talks about what to do if the buzzer and light won't turn off.

I've checked the connections at the two air pressure switches on the primary and secondary tanks and they are hooked up. Disconnected the leads on both and reconnected them. And I've tried replacing bulbs.

I'm guessing that for super safety purposes, the buzzer and light are normally active and that the air switches are NC, so that the light and buzzer only turn off when pressure is applied and opens the circuit at either switch. At least, that's the way the troubleshooting problem in the TM I mentioned made it sound.


If that's the case, then my problem is an open circuit... broken wire or bad ground somewhere?

Where are the wires for this circuit routed?



Also, my "Parking brake engaged" light and buzzer aren't working, either. The buzzer was working for a short time, tho. Interestingly, the bulb for that light is an LED.


Any help would be much appreciated.

 

[212sparky]

Do you have voltage to one side of the pressure switch? Did you jump the switch out to see if it would complete the circuit?

 

[TexAndy]

I did briefly try to skip one of the switches but I was getting overheated and eaten up by mosquitoes so I didn't give it a fair chance.

Didn't take my DMM out with me, but I will do that tomorrow evening. You're talking about putting one lead of the DMM on the truck frame and then testing both switch leads, correct?

 

[two.fiveM35]

I know this is no help, but my M923a1 has a buzzer that is out but the light does work. The truck was rebuilt in 2010 and has not worked since I've owned it. Could the Failsafe alarm be the problem?

From Erik's website:
Failsafe alarm monitor (warning control module) for all of the M939 series trucks. P/N 11669142. NSN6350010892987. Controls various warning lights on the dash and sounds its built in alarm when a problem occurs.

Connector wiring:
A, from engine ignition circuit
B, blackout override input
C, not used
D, to engine temp switch
E, to engine over temp light
F, to air pressure switches
G, to low air warning light
H, ground
I, to parking brake switch
J, to parking brake warning light

 

[juanprado]

It sounds like the fail safe warning module failed or the ground wire(s). Real common issue with the m939. It is on the left inner cab fender by the fresh air door.

 

[TexAndy]

Had a minute, so I went out and tried something:

Disconnected the lead from air switch one that comes from the cab and disconnected the lead from air switch 2 that goes to the frame. Situation, circuit is open and the air switches are completely disconnected.

Went around to the cab, flipped the batt switch and the start switch to "run". Low air (I think) buzzer immediately came on, but the light did not. Went back around to the air switches and touched the two disconnected leads together (shorting past both switches) closing the circuit, and the buzzer turned off. Used the DMM to test voltage between the two leads and read 14.2V

Walked back around to the cab to look at the warning light, but it was still off. While I was fiddling around, the buzzer turned off by itself. Walked back around to check the leads but they weren't touching eachother or the frame. Test voltage again, still read 14.2. Touched the leads together again, no change. Let them go, walked back around to the cab... and the buzzer came on again. Touching the leads together and taking them apart still turned the buzzer on and off at will.


So my guess is... that the light and the buzzer are two separate issues. And my pressure switches are both stuck in the closed position. Which I theorize could happen with lack of use.


Juanprado: Does the failsafe warning module control all the warning lights on the cluster above the steering wheel? Because on mine, the only two lights that don't work are the parking brake light and the low air warning light.

Are there fuses for these lights? I noticed two fuses under my dash. One 3 amp-rated fuse that I'm thinking protects the ABS warning light. And one 15 amp-rated fused that I'm not sure what it controls.


Still not sure of anything.

 

[juanprado]

The other fuse is also for the ABS.

The fail safe warning control module only operates the parking brake light and the low air pressure on the cluster. (2 on far left)

The newest 23 series tm manuals from Sept 2012 have great trouble shooting flow charts not available here yet but can be downloaded @
http://www.jatonkam35s.com/jatonkaM939seriesTMdownloadpage.htm

 

[TexAndy]

Thanks, but those appear to be the same manuals from 1998 I've been reading.

Looking again did give me a good reference for testing the switches themselves, tho. In the "activated" position (system at normal pressure), the resistance across the switch should be 300 ohms or less.

Granted, the problem where this figure is mentioned is for the air buzzer not shutting off. If it reads greater than 300 ohms, I'm assuming that means the switch is either stuck in it's normally open state, or one or the other leads is broken inside it.

 

[TexAndy]

With the tanks pressurized, each switch measures about 0.7 ohms resistance across the contacts. With the tanks drained, open or infinite. Which seems reasonable to me...

Buzzer worked briefly while I was testing this time then inexplicably turned off again.

I'm at a loss.

 

[juanprado]

check under the Sept 2012 heading. The best trouble flow charts are in the 23 series 5 part not the 24 series 4 part tm.

Some have reported the grounding at the frame by the air tanks or at the switch itself in the cab to be troublesome or even the grounds to the frame and starter under the hood. Artisian had a thread on his extensive troubleshooting.

 

[TexAndy]

Thanks, sounds like artisan had the same issue as myself.

http://www.steelsoldiers.com/showth...ule-Plug-Repair-Help&highlight=low+air+buzzer

I'm going to start by cleaning and re-seating every ground I can find.

 

[Artisan]

I have attempted to make this silly low air warning light come on till I am blue in the face. I replaced a-new both low air warning switches at the primary and secondary tank. I pulled the ground for them and cleaned to bare metal and re-affixed. The bulb is good. I have tried 2 other used FWCM's to no avail, actually all 3 had different results. (makes me think I have 3 bad FWCM's ) I pulled ALL GROUNDS throughout the truck and glass bead blasted them and I added a couple grounds as well, cab to frame and bed to frame. *** . I no can get the silly thing to come on if I run the truck to full air then pump them down, BUTSOMEHOWEVER, when I start the truck w/ no air pressure it DOES NOT LIGHT UP! If I shut off the top battery switch and turn it back on sometimes it starts to work. I continue to have issues alnough I can make it work usually. My last resort is to throw parts at it , that being a brand new FWCW and I do NOT want to do that. I SO WISH I had a known working FWCM to play with...***My e brake light works flawlessly, I did find ANOTHER ground for it tat I will pull and clean soon as well. SUB'd In and GOOD LUCK!

 

[Suprman]

I do know the switch on the handbrake gives a ground when the brake is off. I presume its this way so if the switch failed the warning would just stay on instead of not coming on at all.

 

[Artisan]

Unbelievable. I pulled the E Brake ground wire in the cab to clean it and when done I tried to make the low air light come on as I had it working and NOTHING, PLUS the E Brake light was now off as well! I grabbed 2 other FWCM's I had and did some swaps and the E brake light came on w/ one of them. I think I have THREE defective FWCM's. I just found one brand new in box online and ordered it up.

 

[TexAndy]

Have you taken any of your FWCM's apart?

I'm wondering how user serviceable they are. The -24 does describe a little bit of multi-meter diagnostic involving the module.

eta: Here's how the system COULD work, and it could explain why some things are working when others are not. All speculation, but:


There could be a relay inside the FWCM for each light and buzzer separately. It would be better to diagram this, but my MSpaint skills are garbage.

Basically, positive lead goes from the positive terminal of the battery through the CONTACT side of the relay to the light, ground wire goes from the light to the frame or negative terminal of the battery. That relay is Normally Closed, so unless it recieves input to the contrary, that light will always be on.

The input goes: Positive wire from the battery, throw the two series-wired air pressure switches, THEN through the coil side of that same relay and finally to ground.

The BUZZER could then be on it's own relay-controlled circuit, except the coild for that relay is also controlled by the same air-pressure switches.

This could explain why the buzzer can work when the light does not or vice versa. And it might also explain why some of the lights/buzzers are working with one particular module but not another. Certain of the relays could have bad coils or connections on the contact circuit.

Again, pure speculation. I wonder if in 1984 to 1989 they were using solid state or traditional clicky clacky relays.

 

[Artisan]

All the following grounds on my truck were taken apart and all crud and paint removed to bare metal and all
Put back together w/ dielectric grease, start washers and tight.
E-Brake ground wire in cab.
FWCM ground in cab at FWCM device.
Hood to Frame.
Frame to Compressor.
Engine to Body.
Starter to Engine.
I think it was Alt to Frame , a smaller wire in that area.
Additionally I added a #4 ground wire from body to frame and from bed to frame. Any sparky knowledgeable guys have any input on what I may have missed ?

 

[TexAndy]

Since you have some FWCMs already taken off the truck, do you think you could open one up and take a few pictures of the inside for us?

 

[Artisan]

There was talk in the past about melting one open. If I remember right member NXF6 or something REAL close to that and a pilot member were discussing it...I might just grab a torch and see what happens...

 

[juanprado]

There is at least 3 different styles of them from one being cast with cooling fins and 2 different ones I have seen that are plastic bodies. Seems to be a very common trouble with many M939.

One of the things that bothers me on this part is that the early trucks that originally had a temp light also went through the module/buzzer. I am wandering what happened to those wires or are they terminated in the wiring harness? Can this be part of the problem?

 

[TexAndy]

I took my FWCM off, sanded down the copper grounding terminal and the sidewall of the cab and reinstalled it. And I was immediately able to get the low air buzzer to sound... but not the light.

So I taped together the leads that normally go to the air pressure switches to shut off the low air buzzer so I could hear if the parking brake buzzer worked.

Taped them together, the low air buzzer turned off, then walked back to the cab and engaged the parking brake. Buzzer came on for about 3 seconds then stopped. Went to untape the low air circuit leads and the low air buzzer did NOT come back on like it should have.

Going to take the FWCM ground off and clean it even more thoroughly and see if that fixes it again.

For what it's worth, my style of FWCM appears to be cast aluminum with cooling fins with RTV? or some like substance filling the back.


If cleaning grounds doesn't work, I'm considering just building my own failsafe control module from scratch. I think I've got the logic down for how I can make it work. And anyways, new components and a project box would be cheaper than a milsurp FCWM I think.