Low air pressure light/buzzer inop on M923

[TexAndy]

I found a somewhat detailed wiring diagram for the entire truck in apendix H of the -24 manual but it's hard to follow on screen. Resolution is quite low. Volume 5 of the -23 (the 2012 updated version) has the same but with higher resolution.


Unfortunately... it doesn't show what's INSIDE the Failsafe Warning Control Module. Nor can I find the buzzer(s) listed anywhere on the diagram.

It does say the Air switches are normally open tho, which I think reinforces my theory of how the control module works.

I wish I could get the buzzer to sound again. I'm curious to see if the buzzer might actually be located in the FWCM.

 

[Suprman]

The buzzer is internal in the unit. Some of them have a small hole on the front the buzzer is behind the hole. The 2 different versions of the module I have seen have been sealed units they were epoxy filled.

 

[Suprman]

If the lights flash properly you could get a simple buzzer at radio shack and use 2 diodes it would buzz on and off with the lights flashing but it would be simple to add. You might need to calculate a dropping resistor depending on the buzzers operating voltage

 

[TexAndy]

I haven't been able to get either my low air light or parking brake light to flash even once.

I have gotten the buzzer to sound for the low air and for the brake, but not very reliabily. That is to say, when it's working I can turn the buzzer on and off at will... until it stops working.

The more that I think about it, the more I'm thinking it's the FWCM. What I really would like someone to do is stick their ear next to the FWCM and flip the parking brake switch and see if they can detect a "clicking" sound.

 

[Rifleman]

TEXANDY, i have the problem of my parking light working, my low air light working, but no fail safe warning horn. To this day i have tried replacing the fail safe warning horn unit and it still does not work. One thing i did find on my truck that you might want to check on yours for your air pressure light problems. My light bulb for the low air would not work right on my truck, sometimes come on, sometimes not, what i found was the bulb holder would not allow the bulb to be seated all the way into the holder. I removed the bulb socket, pulled some crap out from under the contact point plate that the bottom of the bulb uses to make contact, put it back together and no more problems with my air pressure light. Now all i need to do is get that dang low air pressure fail safe horn working but after weeks of work still no joy.

 

[TexAndy]

Unfortunately, I haven't been able to get either of the lights to work at all. Not even briefly.

Electronics people... would it make sense for them to have put some auto-resetting circuit breakers to protect those lights inside the FWCM?

I am so tempted to go messing with my FWCM, but I've only got the one module! Artisan, you want to make me a deal on one of your messed up modules? I'll dissasemble it as best I can and take pictures for everyone.

 

[TexAndy]

Well I was able to get the buzzer to come on... by tapping it's face on the FWCM.


Prior to that, I went out side to go run errands in town. Just for grins, I went and flipped the switches and lo and behold... the parking brake warning light came on. Steady flash. Cool.

45 minutes later, when I got back... it wouldn't. And I didn't change NUFFIN.

Tapping the speaker face did cause the buzzer to work again, for both parking brake and low air. I'm thinking that at least for the BUZZER problem, there's loose wires inside the FWCM.


Another interesting thing I found... the buzzer does not care if the grounding wire coming out of the cannon plug on the FWCM is grounded or not. I had both the FWCM and the grounding wire hanging loose in air, and the buzzer still worked.

 

[timntex]

Good information being shared! I appreciate you guys sharing all of this as I am having some similar issues with my failsafe alarm / lighting. As described in a prior post, I am experiencing the buzzer not always going off when parking brake is applied. Usually the light flashes though. Also, the hot engine light flashing when temp reaches 180'-190'. I am still troubleshooting.

 

[Suprman]

I don't have a hot engine light on any of my trucks that's a new one to me.

 

[TexAndy]

I don't have a hot engine light on any of my trucks that's a new one to me.

Really? Is there a light missing on your cluster above the steering column?

I thought all the 939/A1/A2s had it.

 

[Suprman]

I looked at the tm it would be the 4th light from the left.

 

[TexAndy]

Check to see if there's a wire coming from the cannon plug on the FWCM that doesn't go anywhere. Wire #35 goes to the hot engine warning light, according to the diagram.

 

[Suprman]

I have 4 a2 trucks none of them have the indicator on the dash. I will peek around the alarm box and see. I know it's a decent size bundle. I know on really hot days the temp gage hovers around the 200 mark with the fan cycling on and off.

 

[Rifleman]

Andy, you said something in one of your post from today that made me stop and think. You said that when you first came out, flipped the switch on things worked. Then when you tried a little while later nothing happened. So since you live where it's hot i have an idea. So in the morning when it's cool metal parts on your truck have contracted, than after the suns been beating down on them they get hot and expand. So i am now thinking you have a bad connection where the wires are just touching with maybe a little corrosion on them, so when the parts heat up in the morning sun the expansion could causes it to break the circuit, plus heat and corrosion also increase resistance and that to can cause a lack of connection. What you might want to do is look at the cannon plugs to make sure that everything is tight with no corrosion. You might also try twisting and turning the wires feeding into the cannon plugs to see if you can get the lights to turn on. I think you have one wire somewhere in the circuit that's just not making contact all the time. Since you have the pin out info in the TM for the feed circuit it should make it easy to just concentrate on the leads in each cannon plug or connector that completes the circuit for that light.

 

[TexAndy]

I looked again. It's on the diagram for the 939 and 939A1 trucks... but the 939a2 has it's own diagram.

For the 939 and 939A1s, wire 35 goes from Pine E on the FWCM to the Engine hot warning light. On the 939A2s, pin E is unused and no hot engine warning light is apparent.

I guess they got rid of the Engine Hot Warning light for the A2.

 

[TexAndy]

Andy, you said something in one of your post from today that made me stop and think. You said that when you first came out, flipped the switch on things worked. Then when you tried a little while later nothing happened. So since you live where it's hot i have an idea. So in the morning when it's cool metal parts on your truck have contracted, than after the suns been beating down on them they get hot and expand. So i am now thinking you have a bad connection where the wires are just touching with maybe a little corrosion on them, so when the parts heat up in the morning sun the expansion could causes it to break the circuit, plus heat and corrosion also increase resistance and that to can cause a lack of connection. What you might want to do is look at the cannon plugs to make sure that everything is tight with no corrosion. You might also try twisting and turning the wires feeding into the cannon plugs to see if you can get the lights to turn on. I think you have one wire somewhere in the circuit that's just not making contact all the time. Since you have the pin out info in the TM for the feed circuit it should make it easy to just concentrate on the leads in each cannon plug or connector that completes the circuit for that light.

I think you may be onto something... but the problem is, I think, whatever connections have poor contact are probably inside the FWCM itself. I'll try cleaning the pins and sockets of that cannon plug tho to see if that helps at all. I did unplug it and plug it back in earlier, but that didn't seem to do anything.

Failing a good cleaning of the plug and socket pin connections, I think the next step is to try to dig into the FWCM itself.

 

[Rifleman]

Just to add to the Hot Engine light thread, i have a 1983 M925 and it has a spot for that light on the dash, but there is a painted green cover over that spot that looks just like the red lens cover's but with no bulb underneath it. My truck went through Red River in May of 2009 so maybe removing that light was part of the update they did on these truck? So how about we ask someone who works at Red River to chime in and let us know if deleting that light was part of the Red River "overhaul" process.

 

[Rifleman]

Andy,i have one more idea for you to check out. I think that you may have a cold solder joint in your cannon plug, to fix it you just need to remove the leads and solder them again. With the TM's you will know what wires are the one's that complete the light circuit and those are the ones i would remove and resolder.

 

[TexAndy]

I don't think I'm goinng to be able to dissasemble the cannon plug without damaging something. It doesn't appear to be made to take apart.

What I'm thinking about doing now is cutting the two wires that go from the FWCM to the Parking brake light and the low air light. Then splice a wire into wire #578 from the air pressure switches. That new wire will go to the coil of a Normally Closed relay through which power will be given from one of the batts to the low air warning light. The new wire will be the input logic that tells the relay contacts to open (shutting off the light) when both air tanks are pressurized.

Similar setup for the parking brake warning light. Splice off of wire #584 to a new relay controlling the power to the parking brake light.

I even found a circuit that I could add to either or both to make them flash:

http://www.hobby-circuits.com/circuits/led-and-light/light-flasher/819/led-or-lamp-flasher-circuit


Right now my situation is:

Buzzer works properly. Both for low air or for parking brake engaged.

Parking brake light sometimes flashes. Usually not. Low air light does nothing.

I think the easiest way to fix this issue is just going to be to make what amounts to a new failsafe warning control module, just for these two lights.

Honestly, I think I could even make a new FWCM from scratch. It seems pretty simple. The engine temp circuit would just be the opposite of those other two lights. It's input on the diagram, "Engine Temp Switch" is listed as Normally Closed. So your relay in the FWCM would be normally open. Which makes sense, since it's operation is the opposite of the low air and parking brake warning (your normal condition is engine hot light OFF).

Two NC light circuits, one NO light circuit, and a NC buzzer circuit. Buzzer circuit relay would have two parallel inputs to it's coil; it would be active normally and only turn off if both parking brake were off AND air tanks greater than 60 psi. That's all you'd need.

Before I order a bunch of components, I'm going to see if I can nail down that it's a problem with the FWCM, tho. I'm thinking if it works the way I'm thinking it does, I should be able to read continuity from ground to both of the light socket and pin pairs I'm having trouble with, right? Lack of continuity would mean the circuit is not closed inside whatever switch is inside the FWCM, right? If the FWCM is working properly, I should see continuity from Pin J to ground, Socket J to ground, and Pin G to ground and socket G to ground, correct?

Again, I am not an electronics person so this is mostly groping around. I'm sure a real electronics person would have had this all fixed by now. Any help would be much appreciated...

 

[ryan77]

I have the same issue with my 923a1! I have not had time to start messing with it! My low air buzzer doesnt work and the light comes on once in a while! The parking brake buzzer comes on but the light doesn't come on always ??Im curious to see what you issue turns out to be!