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MG Car Repair/1980 MGB Horn and Headlight problem

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Question
QUESTION: Horn stops working when headlights are turned on. Turn off headlights and horn works fine. When I hit the horn with the headlights on, they dim and when horn is released they go back to normal. What's going on?

         Ken

ANSWER: Hi Ken,

Do this, Put a 12v test light on the #8 fuse (with the purple wires on it) on the fuse box. Confirm that the test light does light up bright. Now have someone operate the horn while you watch the light. The light should stay lit. Now have them turn on the head lights and check that the test light is still lit. Now with the head lights on have them push the horn while you watch the test light. The test light should stay lit all the time. If not your problem is the power to the fuse box or that fuse.

Is it still lit or not? It should stay lit. If so go to the horns and put the test light on the purple wire on the horns and have them push the horn again (headlights still on) If so put the test light on the purple w/black tracer wire side of the horn (test light should light up) and have them push the horn again. The test light should go out when they push the horn.

If the test light stayed on when the horn was pushed but possibly went dim then you need to go to the steering column plug connector and use the test light to probe the purple w/black tracer wire on the harness side of the plug. (on a MG with a horn push in the steering wheel that wire may be a separate wire but if you have the horn push on the end of the turn signal switch it will be in the same plug as all the wires as the light dimmer and turn signals.)

The steering column switch for the high/low beam and the horn wire are close to each other in the steering column so this is a likely place one can affect the other.

Let me know,
Howard



---------- FOLLOW-UP ----------

QUESTION: Howard:

    Followed your instructions and when I tested the fuse it did light at first. But when I went to operate the horn all it did was make this very low sound. When I started the car the horn worked normally. Also when the car was turned off the test light did dim when the horn was pushed even though all the horns did was make a very low burp sound. Does this give you any clue as to what the problem may be?  Thanks.

         Ken

ANSWER: Symptoms can sometimes lead you in a direction but most of the time they just tell you something is wrong and that you need to test.

In a case where something is trying to operate but is weak. An electric motor runs slowly or a horn just clicks or makes a slight sound is an indication of a poor connection which causes a high resistance to current flow.

You already proved this with the dim test light. Then test for high resistance in either the positive side of a circuit (power side) or the ground side of a circuit. This is easy to test on a horn circuit by supplying the ground circuit yourself and not depend on the car.

Go to the horns and have someone test the horn push and when you get the low burp sound as you call it. have a test lead with one end connected to a good ground like any clean bolt on the car. Then touch the terminal on the horn that has the purple w/black tracer wire. (BE CAREFUL NOT TO TOUCH THE PURPLE WIRE ON THE HORN WITH YOUR GROUND TEST LEAD)

If the horns blow normally then the problem is in the ground circuit of the horn circuit. If however, it does not make the horn blow normally then the problem is in the power side of the horn circuit. To confirm this, run a long test lead from a power wire on the fuse box that is a large brown wire to the terminal on the horn that has the PURPLE wire on it and then have someone try the horn button. (BE CAREFUL NOT TO TOUCH THIS POWER TEST LEAD TO THE SIDE OF THE HORN THAT HAS THE PURPLE W/BLACK TRACER WIRE ON IT) If the horn works normally then you have proved that the problem is in the power circuit of the horn circuit and you need to test further to narrow the problem down to the real problem and the most likely place to start is the fuse and fuse holder. Here you can use the test light to spot the problem. Connect the clip to a good ground and with the test light probe touch the terminals of the fuse box that has the several purple wires on it and it should light the test light brightly. Hold it there and have someone operate the horn and if the light dims move the test light to the other end of that fuse to the terminals of that fuse. If the light lights up brightly then the problem is most likely the fuse holders. To confirm this connect a clip ended test lead from one side of the fuse to the other side of the fuse thus eliminating the fuse with your test lead and try the horn.

High resistance in any circuit drops the available current and voltage so you can see with the tests you are bypassing parts of the circuit to isolate the problem into a smaller and smaller area. This type of test works on any circuit. Some circuits are complicated when they have several different power sources or several grounds like a wiper motor circuit. But a horn circuit is simple circuit with a power supplied to a fuse and from that fuse to the horns and then back to the horn button and on to a ground.

All electric items operate by current traveling in a circle (circuit). Power (battery +) to load (item that operates) to ground of the (battery -).

let me know what you find.
Howard



---------- FOLLOW-UP ----------

QUESTION: Howard:

    So sorry it took me so long to get back to you but after chasing circuits and life happening it took me forever to discover that it appeared the fuse block was the problem. Replaced the fuse block and everything seems to be ok now except for one thing. The gas gage is functioning with the key on or off. I can't seem to remember if it always did that or if it just started when I changed the fuse block. It seems to function ok but is always working no matter if the key is on or off. Is that correct or is something wrong?

Answer
For the fuel gauge to operate all the time it is required to have power on the Green wire going to the voltage stabilizer which powers two gauges at 10 volts. The green wire is normally powered by 12 volts from the fuse panel. So it sounds like you have plugged the wrong wires in the wrong positions on the fuse panel and have power on the green wires with the key off.

Follow the wiring diagram to see which wires are in the wrong position. Or it is possible that the wrong fuse box was installed and had some pins on it connected together. To check that take the old fuse box and check connections between each set of pins and fuses.

Most of the time when a fuse box fails it is corrosion on the clamps that hold the fuse. Many times you can correct a fuse box by sanding the fuse contacts and press them together so as to clamp down on the fuse harder.

Howard

MG Car Repair

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Howard M. Fitzcharles III

Expertise

MG from 1956 (USA versions only) up and Engine theory.

Experience

Dealership line mechanic on MG, Triumph, Jaguar for 15 years, Instructor in commercial mechanics school 2 yr. Product information manager for piston and valve manufacture, Instructor & hotline answer man for import car parts importer 15 yrs.

Organizations
Associate member SAE EAA member

Publications
Import Car magazine

Education/Credentials
ASE Master Auto with L-1 certification up to 2000

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