[T3] On that 73 T-3 horn
Jim Adney
jadney at vwtype3.org
Mon Nov 13 12:54:38 PST 2017
On 13 Nov 2017 at 12:27, William J wrote:
> I wanted to add I drive the car a bit ago and the horn works with the
> engine running which makes no sense .
>
> The battery puts out 45 AH was is fully charged. Even with the charger
> connected and on just key on the horn didn't work .
I think that if you check, you'll find that your system voltage is lower with the
key simply ON than with the engine running. While generators put out little
or nothing at idle, once you've started the engine, it's either running at a fast
idle, or it's been sped up a bit and pushed the battery to a higher voltage.
After the speed has fallen to the regular idle, the higher voltage drops only
slowly, so your system voltage will still be higher than with the key simply
ON.
With the key simply ON, you've got a load on the battery, because you're
supplying the ignition and FI systems, and there are voltage drops in the
chain from the battery to the horn.
The short answer is that the horn is a electromagnetic device that requires a
certain amount of current to work properly. It's likely that a new horn would
still work at the lower voltage, but your old horn is worn and needs the extra
bit to work.
The fact that your dash lights dim when you press the horn button tells us
that the horn circuit is working, but there is not enough current thru the horn
coil to pull the internal contact open. This is probably due to a combination
of resistive voltage drops and lower system voltage. Both cause reduced
current.
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Jim Adney, jadney at vwtype3.org
Madison, Wisconsin, USA
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