Isuzu pickup - the silent horn

piddlerdad3August 30, 2007

1994 Isuzu pickup - 2wd, 4 cyl 2.3L with carb. My horn quit working, so I got a meter out, and measured around 18 vdc at the horn fuse (both sides). hmmmmm. I stared at my battery and saw some corrosion, then cleaned it up. hooked battery cables back, and filled cells with a bit more distilled water (aahhh, this Texas summer!). Had to drive the truck on errand, then came back after 10 miles. Checked the horn fuse again. this time reading 12.5 vdc. hmmmm. So, now i look into the horn circuit. My son sits in the driver seat and pushes the steering pad center to activate the horn switch. He can hear no relay clicking, but I am reading 12 vdc on the hot wire (marked 12 volts on the terminal clip on horn casing) when he is not switching, then when he pushes the switch, i'm reading 0 volts. So, the horn's bad, and the relay is just very quiet - i theorize. Using amazing dexterity and superior intelligence, I remove the horn, and take it apart. I see nothing wrong visibly. This is an aftermarket horn, not the original. I see 2 coils with a resonator bar going across. everything seems okay. i take a few resistance measurements to check connectivity. put the horn back together, and hook it up to a shop supply i have (not sure why i didn't just use the battery in the truck)?? anyhow, i adjust the supply to around 12-15 volts and limit current at first. i hear a weak click / beep. As i increase the current, I hear a bit louder - what seems to be the horn trying hard to make the expected sound, while it is out of its normal circuit. ok. i "fixed" the horn.

Next day, I put the horn on truck and this is the kind that apparently needs to be isolated away from the truck chassis (electrical Ground). horn is located on driver's side in front of radiator. so, it is back in place and green wire from harness (coming from fuse box under the hood) is hooked to the GND stud and is secured with a nut. Blue/white wire is hooked back onto the 12 volt terminal clip on horn. i take out my meter and am reading 12 vdc at both GND and the hot terminal. hmmmm. I push the switch (steering wheel pad), and nothing happens. not a weak peep or nothing. I am thinking at this point, that the relay is shorted somehow between GND and horn hot terminal wiring.

I remove the horn, and take reading on the wiring. I am now getting -12vdc on the GROUND WIRE, and 0 volts on the blue/white hot wire. yep. I measure my battery, and it is reading normal ... +12vdc on positive terminal post, and 0 volts on negative terminal post. I am thankful, then hook up the horn to my jumper cables. Black cable from GND stud on horn to negative battery post. Red cable to 12 volt terminal clip on horn. then i touch the red cable's other end to the + post on the battery. Nothing. ok, horn is dead, i guess.

I go back to the wiring harness, and accidentally touch the GROUND WIRE to the truck chassis and pop my horn fuse. ok, that's logical since my GROUND wire is now hot. I leave the fuse popped, and search for the horn relay. I have yet to find it.

Would appreciate anyone's help on location of horn relay (assuming that is what i should be looking for), and any advice on how i could proceed from here. I hear something laughing underneath my hood ....

Many thanks, dennis

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Check for corrosion where the fromt wire harness plugs into the body wiring. Typically, this connector is located near the front of the vehicle and under something like the air intake box. This connector may not be of the weather-proof variety, but instead, an ordianry multi-pin connector. I've had horn and fog lamp circuits go bad at this location.

    Bookmark   August 31, 2007 at 5:14AM
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Thanks. I haven't had much time to look into this yet, but I did look for a brief time today. Still cannot locate a plug for the front wiring harness, and have still not located any horn relay or any other component that might have failed in this circuit.

I got a hold of a parts / service dept at an Isuzu dealership and they told me that there does NOT appear to be a horn relay in the circuit. I'll look more later, but cannot locate anything else that has a common connection with this horn circuit.

thanks, dennis

    Bookmark   September 2, 2007 at 12:19AM
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