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MG MGB Technical - Electrical help needed

I'm still attempting to get my newly acquired '72 GT on the road. Now that I've replaced the fuel pump so it's getting gas, and the water pump so it doesn't leak coolant, it's time to tackle the electical problems.

1. No turn signal, hazard flasher, horn.

2. Do have windshield (windscreen) wipers.

3. When I turn on the headlights - the tach falls to zero.

Any help would be greatly appreciated as always.

Thanks,
Dave
Dave Shively

These may sound like minor things to check, but I have learned that there are no minor issues when it comes to Lucas wiring. I have just finished my 73 MGB and have gone completely through all the electrical system. First, check ALL your fuses. Even though they may look good, check them with an ohm meter. Also check their connections and be sure they are not corroded. Then I would consider replacing the hazard warning switch. These switches control a lot of circuits. After a lot of time and effort, I found that this was one of my big problems. Also, check and clean the wiring connections going from the blinker stalk on the steering column to the main wiring harness. If these are corroded or not making the proper connections it would cause everything you described, including the horn as this is the where the horn connects through to the main electrical system. This is at least where I would start. One thing I did, which seem to really help, was to take a wire brush on a Dremel and I cleaned every electrical connection. This way I knew I was making positive contact. When I put my dash back in my car a month ago, after rebuilding all the gages and electrical system, everything worked the first time. No surprises. I know I was very luck, but the electrical systems on these cars are very tricky and require a lot of patience. Also, do not depend on one wiring diagram, get several and compare them and you will see small, but important differences between them. I use three wiring diagrams for my car and they all are different.
Robert Browning

On that era the washers, wipers and heater fan work off the accessories position of the ignition switch via their own in-line fuse (under the fusebox), not the ignition position as you might think.

The turn signals and hazard flasher both go via the hazard switch, so that could be a common failure point, check for 12v with the ignition on on both green wires at the switch. One is 12v in and the other sends it on to the turn flasher. The hazard flasher also has its own in-line fuse tucked away somewhere (behind the console, I think). Of course both flasher units could be duff, don't get them mixed up, and make sure you get a proper turn flasher that either stops flashing ot goes noticeably faster with one bulb out.

The horns work off the purple circuit together with the interior lights and headlight flasher. If these don't work either check the bottom fuse. If only the horns don't work then try grounding the purple/black at the column and the horns themselves to see if it is the column ground or switch or connections to the horns. With a ground on the purple black at the horns make sure you still have 12v on the purple there also, if not there are bad connections back to the fusebox.

If you mean the tach drops to zero when you turn the lighting switch to its *first* position i.e. parking lights then check there is a black ground wire under one of the fixing screws for the tach.
Paul Hunt

This thread was discussed between 02/06/2003 and 04/06/2003

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