M923a1 air horn problems

[Farmallmilitary]

Ok like i said before i hate electrical wiring. So today i was going to try to get the air horn functioning properly but i failed. So i first started by seeing if the air valve worked so i hooked up a 12v battery to it and it clicked like its supposed to. Next i checked to see if i have air to the horns and it does. Then i tested to see if i was getting 24v to the horn when the button was pressed. I c-clamped the button down turned the power on to the truck and used my volt meter to see if there was power to the horn and the was, only like .64v which is not near enough to make it work. So i go into the cab and pull the plug out of the box at the bottom of the steering column and hook a 12v battery to the lead to see if i hear the valve clicking. Nothing. Then i test the box to see if there was power when the button was depressed and STILL nothing. I took the box apart and found there was some rust. So i cleaned everything in there with CLR and made it spotless. Then i test it again this time inside of the box and there was 24v when the button was pushed. YEAH! At least i got power to the box now. So i hook it up and still nothing. Then i test the outside of the box in the plug and i have no juice. What! By that time i was to tired to mess with it anymore and gave up for today. I just want the horn to work! Anyone have a clue whats wrong? By the looks of it it almost seems like there is a wrong wire connected to the box on the steering column but everything else works. Could it be a fuse in the wire from the box to the horn if so where is it located? Also where is the ground for the horn found? Is it the ground right under the horn? I really dont want to pay someone to fix the little horn problem all i want is for it to work.

 

[Flyingvan911]

I don't have anything helpful to add but I want to thank you for giving me the idea to clamp down the horn button on my deuce. It also does not work.

 

[goldneagle]

The horn button sends the negative of ground to the horn. The horn already has the Positive from the battery. Also make sure the ground wire is going to the screw at the top of the cover plate on the bottom of the steering column. Also make sure the nut that holds the steering wheel on is rust free (clean). Also the spring and the metallic bottom of the horn button assembly have to be rust free (clean).

 

[Farmallmilitary]

Ok that helps i also tried putting the voltage meter with the positive on the wire right where it plugs into the valve and the negative on the body of the truck (ground). I got nothing from either wire so that probably tells me that the positive power isn't getting to the horn but the negative is. Where is the power source for the positive end of the horn located. Where is the other end of the wire connected is another way to put it. Thanks that helps alot as i was approaching it all wrong thinking the positive power was the one that runs through the horn button.

 

[Farmallmilitary]

Anyone have an electrical diagram of the m939 series trucks? I was unable to find one in the tm's or on the internet.I think that would help alot of my electrical questions related to the truck.

 

[juanprado]

the p2p software is awesome. there is a sticky in the top of the 5 ton forum.

Horn wiring

 

[Rkrug]

There is a diode up by the horn it is probably bad, i think it is on wire 25A if you can see the numbers.

 

[Rkrug]

looks like some one got a picture.

 

[Vintage iron]

I have found that the original horn button set up in the column is a PITA. They either don't work or they come on when you don't want it. I had bought all new parts for mine and just ended up putting a button on the dash. Works great! Good luck with yours.