[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 

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 

Jim Adney, jadney at vwtype3.org
Madison, Wisconsin, USA

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