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Air horn woes
- Thread starter tbar123
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I think this might be your problem. It is in the ground wire off the air horns close to the heater box. Might just look like a big bubble in the wire.
74M35A2
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It is needed because when the horn solenoid releases/closes, it generates a big spike that would otherwise go through the system, much like an ignition coil (they discharge their high voltage when power is removed and the mag field collapses). If your horn switch tests OK, then it is probably this diode. That was the case of mine staying on once pushed, at 3am, on a weekday, in an expensive neighborhood (not mine though).
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If you do a search which I suck at..... There is a thread on here with the part number. I ordered off epay, I think maybe $3 for 10.
It has been a few years but if you do not find it, I could check my epay email confirmations on my home computer for the diode number.
It has been a few years but if you do not find it, I could check my epay email confirmations on my home computer for the diode number.
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http://www.steelsoldiers.com/showth...g-button-wont-stop-until-turn-shut-down-truck post #3
It's my opinion that you can simply eliminate the diode. Others may disagree...
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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 no voltage when measured to +24V with the horn button not pressed, and should read +24V when measured to +24V with the horn button pressed.
-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 no voltage when measured to +24V with the horn button not pressed, and should read +24V when measured to +24V with the horn button pressed.
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 dissipate energy from the solenoid coil when the horn is deactivated. Testing without the diode shouldn't hurt anything however the solenoid life expectancy may be reduced if it is permanantly removed.
74M35A2
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Disagree. Going without it may cause voltage spikes into your system, and could fail electronics which are not protected against such, such as the ABS module (proven to fail) or the voltage regulator in the alternator. They added it for a reason. You'd have to put a scope on it to confirm, but a diode is a less than dollar part. Just replace it, close the hood, and not have to worry about it. It's removal should not affect solenoid life, but instead everything that is electrically connected to it.
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.
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Here is what I did over the past few hours, Not being a patient man I went out before the responses! Probably a bad idea. I disconnected the fusible link what it looks like to me. I then checked the ohms and of course my meter is going hay wire. So i made a jumper cable and the horn at 60 psi was still blaring. So i then took the steering column out put the old horn switch in the new steering column and no change.Every time psi gets to 60 the horns go off. I disconnect the wire at the horn switch they stop. While I did all of this the horn button was not connected.I'm at wits end!
74M35A2
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None of the M939/A1/A2 were originally equipped with ABS. They all should have been MWO's to have ABS added, unless liquidated before the MWO went active.
http://www.roscommonequipmentcenter.com/MWO 9-2320-272-35-1.pdf
Battery may and should absorb any spikes, but an oscilloscope would confirm.
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If you find you need a new horn button to fix the problem, I have a NOS button and NOS button kit that includes everything in the column if you need it. You have a couple small things I'd trade even for.
I also sent you an email.
It doesn't matter what I do the horn will not stop blowing. I had the diode jumped and was expecting that to be it. Once the horn started to blow, I removed the jumper wire which I thought the cercuit would've been cut at that point, but No it just kept blowing. I'll add picks in the morning. I do thank you all for the help.
Cape Coastie
CWO4 ENG/MSS, USCG, RET.
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Hey tbar, sorry you are having all the trouble with something that you think should be simple. Been there done that, very frustrating. Still sounds to me like you have a solenoid valve stuck open. If the valve is stuck open they will sound as soon as the circuit gets air pressure. I know you have tried severel of them. When I took mine apart there was a small spring and a button, which is actually the valve. The spring was collapsed. Stretched it out and reassembled it. Cleaned the horns internally and they worked, not the best sound but they work. The reason they come on at 60 psi is the PPV valve in the air circuit doesn't allow air to them until that pressure is reached also supplying the wipers. Keeps an air supply for the brakes and eliminates the non necessary circuits. Just make sure everything is assembled correctly and you will figure it out. Good luck and keep us updated.
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.
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