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Correct, however he says wasnt sure whether or not a jumper was in place which would cause the horns to sound. That is what I wanted him to confirm. Now that he has explained removing GND from the horn switch stops the horns, it's obvious there is contact through the switch when there shouldn't be.

If this is the case then the horn switch is providing GND to the solenoid when not being pressed. Figure out why this is happening or get a switch that doesn't do this.

Power ia always supplied to the solenoid from wire 26 as you mentioned. GND is supplied to the solenoid through wire 25 when the horn switch is pressed as you mentioned. Wire 25A is tied to GND, has a reversed polarity diode, and connects to wire 25. The reverse polarity diode should not...

Lol. Good idea, best of luck.

A jumper across the diode will cause the horn to sound. I would remove any jumper and disconnect 25A at it's ground point. Then see if that stops the horn from sounding.

Did you have the GND disconnected without the jumper wire in place?

Remove wire 25A from where it connects to GND. Does that stop the horn from sounding?

That is so disturbing lol.

what he said!

Good for you, your machine is ready should it be needed, as it should be. I hope all is well with him.

I'm sure there are folks here who could tell you how many hours labor is fair. It's simple mathmatics from there.

Exactly what I am thinking at this point. Here is how to tell if it's not an electrical problem...start up, build air, horns are blowing, turn ignition to off and turn off battery. If they are still blowing then they are getting air through the solenoid valve when they shouldn't be.

If the fusable link you mention is in wire#25A which leads to GND jumping it will cause the horn to sound. This is really a diode and if removing the jumper and diode causes the horn to stop, then the diode is bad.

Not sure the charged horn solenoid would discharge enough amperage to spike the entire electrical system including the battries damaging other components, however I'm far from an expert. A continuity check across the diode to see if it's shorted may be a safer way to test it.

Bypassing it will cause the horn to sound as you would be applying GND to the solenoid which is what the horn switch does. Removing the diode is the way to troubleshoot it. If the horn deactivates with the diode removed then the diode is bad(shorted). As was mentioned the diodes purpose is to...

There are two wires connected to the horn solenoid. -Wire #26 should read +24V when measured to GND at all times (battery & ignition ON). If it does not then check the circuit breaker next to the air governor on the firewall for open circuit. If open then replace it. -Wire #25 should read...

The voltage needle staying in the yellow is usually indicative of no charging output from the alternator. I would check the wiring connections at the battery and alternator. Be carefull not to ground out any wrenches while doing this.

Yes the muffler will fit, it's a common part on all of the M939 models. Manuals are here... http://www.steelsoldiers.com/showthread.php?121487-TMs-for-M939-Series-Trucks

The purpose of the solenoid diodes is to dissipate stored energy in the solenoid coil when it is deactivated so a voltage spike doesnt occur. It's a circuit protection thing. Where do the other ends of the green wires go?

Thanks for posting, they are easier to find for me then waldo. This should help rivercreek get to the bottom of his no gage, no blower problem.