M38 Horn Amps

RonD2 · Post by **RonD2** » Tue Aug 15, 2017 10:53 am

My M38 is converted to 12 volts and has an early military Sparton horn that sounds good. The previous owner had it alone on an un-fused circuit. I'm going to put a fuse in the circuit and need to determine what fuse size would best do the job. I've searched and researched and can't find anything about how many amps it draws, 12v or 24v.

I thought I'd ask for pointers before I have to cut in an ammeter and measure it. Does anybody know how many amps this horn or any similar horn might draw on either 12v or 24v for a fuse size?

Thanks!

4x4M38 · Post by **4x4M38** » Tue Aug 15, 2017 11:07 am

Hey Ron,
Without your horn data it's hard to know for sure.
I think Bretto is using a 15 amp fuse on his 12 volt
converted jeep.

What horn is it?

One other thing you might consider is also adding a relay
in the circuit.

Just my two cents.

RonD2 · Post by **RonD2** » Tue Aug 15, 2017 11:21 am

Hey Brian. That's what I'm looking for --- the horn's data. No joy in my research. It's a "round-back" Sparton horn, the same as shown in the M38 TM by the looks of it. It's missing it's little data plate and has no other marking that I can see so there's no way to tell for sure. It looks original and un-molested to me. 15 amps seems a little heavy just for a horn and nothing else in the circuit? A relay set-up is a possibility, but am trying to keep things as simple as possible. And wouldn't a relay take 2 fuses? One on the primary side and one on the secondary side?

Everything else is correct for the M38 (horn rod through the steering gear box, switch screwed into the bottom of the steering gear box).

RonD2 · Post by **RonD2** » Tue Aug 15, 2017 12:09 pm

I did come across some info that horns on early 6V jeeps drew 12-15 amps. If that's true, then a 12v jeep should draw about half that, or 6 to 7-1/2 amps.

4x4M38 · Post by **4x4M38** » Tue Aug 15, 2017 2:15 pm

Hi Ron,
Is your horn waterproof with Douglas or
Packard connectors? I don't think the 6 volt horns
were waterproof, hence no Douglas or
Packards.

I'm wondering if the M606 if that's the right model
had a 12 volt horn. Isn't that the later
12 volt vehicle? If so, is the horn waterproof?

Just seems improbable that a 24 volt horn would
work on 12 volts.

I believe the battery box breaker for the
horn on the M38 is 15 amps.

Bretto ran a heavy wire through a relay and fuse
to the horn, and ran a smaller gauge wire for the
button to the relay. That way only the main amp
draw is from the battery to the horn.

Take care,

RonD2 · Post by **RonD2** » Tue Aug 15, 2017 4:03 pm

The horn has Douglas connectors. It's identical in appearance to the horn in the M38 TM and ORD9. I have no reason to think it's not a 24v horn, but it has no data plate or markings to prove it. I guess it could be anything.

I'll cut in an ammeter and measure it so I can size a fuse.

RonD2 · Post by **RonD2** » Wed Aug 16, 2017 8:33 pm

I manually tested the horn <again> with multi-meter test leads direct to the battery (12V) -- no switch. Honked loud and clear. Then did the same with an ammeter in the circuit. Measured 4.64 amps --- but the horn wouldn't sound this time. Took the meter out and tested the horn again like the first time --- loud and clear. Now I'm looking for the manual to my Fluke 77 to see what the deal is with the ammeter. More to follow.

Post by **wesk** » Wed Aug 16, 2017 10:54 pm

Did you check the amp rate in series or parallel?

RonD2 · Post by **RonD2** » Thu Aug 17, 2017 7:38 am

Series.

I'm pretty sure I had one of the meter leads in the wrong port on the meter and that's the issue. Still looking for the manual. I ought to know better by now to read the manual first.

Especially since I don't use the ammeter side of it very often. The good news is I didn't smoke the meter.

Post by **wesk** » Thu Aug 17, 2017 9:46 am

Which fluke 77 do you have? There are a dozen versions. You could just go to the Fluke web site and download a PDF copy of the correct 77 model manual.

RonD2 · Post by **RonD2** » Thu Aug 17, 2017 5:55 pm

Thanks Wes. I've got a Series II. I was just about to take your suggestion when I found the original manual in the last place I looked.

Yes, I had a lead in the wrong port on the meter in the first try.

Hooked the meter up correctly and tried the test again (no meter first), horn sounds loud and clear. Put the meter in the circuit, horn sounds loud and clear and I read 2.15 amps.

I'm thinking an AGC 2.5 amp fuse will do the trick.

RonD2 · Post by **RonD2** » Thu Aug 17, 2017 9:12 pm

This measurement was without the M38 horn switch in the steering gear box and the 14 gauge wire in the circuit so I'll probably take another measurement once I get it all installed and hooked up. May have to bump the fuse size a tad. I'm glad to see it's not a lot of amps anyway. I was concerned about the unknown amp rating on the repro switch I bought.

Brinken · Post by **Brinken** » Fri Aug 18, 2017 4:50 am

Hi Ron, Sweden here
There is always a "Rush-in current" at the very begining of the operation, which means that the first part of a second the electric consumer will take about 4 time the steady current. Therefore I would advice you to go for an ordinary auto fuse of 8 amps. This will still protect your circuit properly.
//Jan Olof