Horn Wiring

[Mike Allfrey]

G'dday From A Very Sunny South,

I have noted some comment, I think in the SC rebuild story, about the wiring of the horn. I can only relate to personal experience - the following happened to me.

When I first started driving my Javelin, back in 1982, I found that at times, use of the horns caused the engine to die. Also dead were other ignition switched circuits, such as wipers and heater. This was very mystifying for quite a while and, worst of all, the previous owner had 'modified' the car's electrical system. Under the dash panel there was an added fuse box with those awful ceramic type fuses that VW liked so much.

Fiddling with that, used to bring the engine and other electrics back to life. No fuse had blown, so it was assumed there was a poor contact somewhere. All worked well, until next use of the horns.

My first discovery was that the horns had been wired via the ignition switch - a definite no no for a Lucas system! That was corrected and the feed to the horn relay was taken from the battery cable at the starter solenoid. This cured the engine cutting out when the horns were used.

However, one wet night in winter, the wipers, heater and electric radiator fan (then fitted) were in use. Then, with all of that, the brakes were applied and the engine died. It soon came to life after some investigation, but nothing had been found to be at fault.

I removed the dash and took out the ignition c/w lighting switch and set to with a multi meter. The fault was found to be in the ignition switch itself. As the key was moved slightly. there was an open circuit from the switch. What had initially happened was the horns drew too much current for the switch to handle, and, with all accessories switched on, the same situation occurred. A new switch assembly and re-wiring of the horns has completely sorted out that electrical fault.

Moving forward to 2007 and a Rover P6B 3500 of 1976, and electrical problems were evident right away. The fuse box showed signs of having been very hot (melted plastic body), I decided to wire all lights and the air conditioning system using relays. A sound idea, basically, and there is now a relay for every filament in the headlamps' and fog lamps' wiring system.

On the Rover, the battery is properly located in the boot, with a heavy cable running forward to a terminal post low down in the engine bay on the driver's side. Very convenient, thought I! All of my new relays were fed from that terminal post. Imagine my horror when I took the car for a drive and saw the ammeter go to full charge when main beam was switched on. As engine revs increased, the indicated charge rate came down.

Discussion with my local auto electrician revealed that I should have spliced a feed for the relays at the heavy brown cable as close to the alternator as possible. I did as suggested and the situation came back to normal. In addition, the fuse box stays cool and there is 13 volts at the headlamps.

This proved to me that high current drain components, such as horns, should be wired as per the original Lucas diagrams.

The most difficult part was actually finding the dodgy ignition switch!

Hope this story helps someone on a cold, wet and windy night,

Mike Allfrey.

[Forumadmin]

In recent discussions with the wiring loom manufacturer he suggested not using an ammeter with an alternator. The reasoning being that the ammeter does not tell when things are going wrong only when they have. He suggested a voltmeter instead.

The reasoning for the problem above is the ammeter is measuring the flow of current from the alternator to not only the battery but to all the accessories.

What is an even worse situation is where you effectively do the same thing with a regulator/dynamo setup and bypass the current sensing coil. On Jowetts the horns do this and thus will drain the battery without the regulator knowing about it. So those impatient people who use the horn a lot beware. If you also put a rad fan to the same place you quickly run out of battery. It took me one rally to figure this out!