Can horns be damaged by insufficient current?

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Scott Gardner - 07 Sep 2005 05:03 GMT

When I bought my 1972 250C last week, the horns didn't work. The
previous owner had replaced the stock horns with a set of Hella units,
and he said that they had worked after he installed them. When I took
the horns off and jumpered them directly to the battery, neither one
would sound.

I replaced the Hellas with a pair of Fiamm horns, and they worked for
about a day. Today, I honked the horns and they both cut out. Same
problem as the others - I disconnected the horns and wired them
directly to the battery to test them, and neither one worked.

The horn circuit is relatively simple. +12V from the hot side of the
horn fuse is applied to terminal "A" of each horn, and terminal "B" of
each horn is connected to a wire that is grounded through the steering
wheel horn switch. The car, as well as all four horns it's killed,
are twelve-volt, so it's not like I accidentally put in 6-volt horns
or anything like that.

I'm thinking that the Fiamm and Hella horns may be drawing more
current than the stock horns, and that they should have been wired
with a relay connected directly to the battery, so that the stock horn
wiring only has to activate the relay, and then the relay would
provide power to the horns directly from the battery.

The only thing that puzzles me is that if the new horns are drawing
too much current, I would have expected the horn fuse to blow, rather
than the horns themselves dying.

Has anyone ever had a similar experience with aftermarket horns?

Thanks,

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Scott Gardner

"After things go from bad to worse, the cycle will repeat itself."

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Hernando Correa - 07 Sep 2005 17:50 GMT

> When I bought my 1972 250C last week, the horns didn't work. The
> previous owner had replaced the stock horns with a set of Hella units,
[quoted text clipped - 27 lines]
>
> Thanks,

After you removed the "burn out" horns, did you check the voltage at the
connectors terminals while actuating the steering wheel horn switch? It
should be a healthy +12V

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Scott Gardner - 08 Sep 2005 04:44 GMT

>After you removed the "burn out" horns, did you check the voltage at the
>connectors terminals while actuating the steering wheel horn switch? It
>should be a healthy +12V

With the horns removed, the voltage across the terminals was about
+12V. It wasn't exactly battery voltage, so there's a small loss in
there somewhere, but not much.

I think the problem's solved now, though. I put in two new horns, but
rather than use the original horn wiring to activate them, I wired
them directly to the battery (via a fuse and a relay) and directly to
a clean chassis ground. The original horn connectors now activate the
relay, which completes the circuit from the battery to the positive
horn terminals.

After I was done, I went to a deserted parking lot and tested the horn
for about ten minutes. Now that they're getting full battery voltage,
they're loud as hell, and the horn ring acutated them perfectly every
time I pressed it. The horn switch will probably last longer too,
since now it only has to pass enough current to activate the relay,
instead of carrying all of the current for both horns.

The previous owner probably should have added a relay when he replaced
the stock horns with the Hellas in the first place, but since he
didn't, I should have done it the first time I replaced the Hellas
with the new Fiamms. Serves me right for just re-using the existing
wiring instead of figuring out the best solution.

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Scott Gardner

"Tis an ill wind that blows no minds."

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Martin Joseph - 08 Sep 2005 07:49 GMT

> The previous owner probably should have added a relay when he replaced
> the stock horns with the Hellas in the first place, but since he
> didn't, I should have done it the first time I replaced the Hellas
> with the new Fiamms. Serves me right for just re-using the existing
> wiring instead of figuring out the best solution.

Wasn't there a relay to begin with and perhaps that was your whole issue?

Marty

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Scott Gardner - 08 Sep 2005 20:01 GMT

>> The previous owner probably should have added a relay when he replaced
>> the stock horns with the Hellas in the first place, but since he
[quoted text clipped - 5 lines]
>
>Marty

No, before I tore into the wiring, I checked with some other Benz
owners, as well as examining the wiring diagram for the car, and
there's no horn relay.

The stock wiring simply has a wire from the horn fuse to the positive
terminals of the horns, and a second wire from the negative terminals
of the horns that connects to ground via the horn switch in the
steering wheel. The factory horns must not have drawn much current,
because the factory wiring passes all of the horn current through the
horn switch.

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Scott Gardner

"Never get your heart set on a peanut butter and jelly sandwich until you're sure there's jelly in the house." (Linus Van Pelt)

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Hernando Correa - 09 Sep 2005 01:29 GMT

>>After you removed the "burn out" horns, did you check the voltage at the
>>connectors terminals while actuating the steering wheel horn switch? It
[quoted text clipped - 23 lines]
> with the new Fiamms. Serves me right for just re-using the existing
> wiring instead of figuring out the best solution.

Your adding the relay makes perfect sense since the current required to
activate the relay is very small compared to the current needed to blow
the horns. The side effect of your fix is that the steering wheel switch
will outlast the life of the horns! These, however, will burn out if
you were to test them CONTINUOUSLY for several minutes.

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