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MG MGB Technical - Elec -> Horns/Fan Motors 78B

Hello,

Am working through some electrical problems on my 78B.

Symtoms:
1) Horn does not work (when pushed squeakes for 1/8 second and then nothing).
2) Melted wires from Starter solenoid relay - to ground (black).
3) Temp guage was replaced by PO. Burnt wires behind existing guage shows previous "wiring fix..."

Of course, all above symtoms may be related or not at all...

*Note Engine may have been replaced with a 74 model year engine. The battery has been disconnected.

I enlarged a (later model) wiring diagram and hung it on the wall. I divided the diagram into grids and am systematically working through each grid.

Horn- There are two wires coming from each horn (Purple and Purple with Black tracer). From looking at the 'later model wiring diagrams for US models', it only shows one wire (Purple with Black tracer) with the horns being grounded.

The purple and purple with black tracer wires both seem to go to the Horn push. When I disconnect the purple wire (at the push)and hook up a meter, OHMS read from BOTH wires coming from each horn. In other words, no matter which wire I touch (at each horn) with the meter, OHMS are present.

Also, no matter which fuse I touch at the fuse box, it too registers ohms (at different levels)

The earlier wiring diagrams (73-74) do show two wires coming from each horn.

Should each wire register OHMS?

Is there a better way to trace/track each wiring circuit to ensure it is hooked up properly?

Also, I noticed on my Electric fan motors, the left motor has 4 wires coming out of it and the right just two. The extra two wires are yellow and red and are soldered by the PO. Do you think that was just an after market fan and the extra two wires are for an option 'manual switch' for the fan motors?

Thanks in advance.

Todd C
Todd C

Hi Todd.

I believe that the purple wires should carry a 12v feed to the horns, and that the purple / black wires should go to the horn push, which (when pushed)connects them to ground.

You should get low ohms from the purple wires to battery positive, and low ohms from purple / black to chassis when you push the horn button.

Several of the diagrams in the 'official' workshop manual show this arrangement.

The melted black wire to the starter relay may have occurred if the relay was incorrectly wired in the past, I can't think how else this would happen.

I would unwrap parts of the loom and check for other melted wires.

HTH.. Don
Don

Late model cars had the polarity to the horns reversed - the horn push gets 12v on the purple and puts it out on the purple/black, the horns have a local ground through the physical mountings and this is what I would have expected a 78 to have, unless it is 78 registered and an earlier model year. Or maybe the PO rewired it with an earlier loom.

Forget ohms. If yours has both wires at the horns then measure the voltages on them with the horn push operated. significantly less than 12v on the purple shows there are bad connections back though that wire and the purple fuse. Significantly higher than 0v on the purple/black indicates the same but back through that wire, the horn push and column ground. Telescopic columns and some wheels do have bad grounding problems, I've had to put a relay on mine.

In ya car with the two wsets of wires at the horns the purple at the lights and horn switch is for the headlamp flasher, not the horn.

A late model car I rewired also had burnt out black wires at that point, and elsewhere in the loom, hence the rewire.

The extra two wires sound more like they went to a third fan, an override switch would only need one connection to a switched 12v supply. But if the PO didn't know what he was doing anything could have been done.
Paul Hunt

This thread was discussed between 18/04/2004 and 19/04/2004

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