General MG 76 MG Midget Electrical Anomaly

[PerryC]

My Midget starts, but if I turn on the headlamps or step on the brake, the entire electrical system goes dead. I turn off the headlamps, and it starts right up again. I think it is due to a short somewhere in the lighting wiring. Am I right? If so, where's the best place to start looking for the short?

 

[YakkoWarner]

You could have a short, and/or grounding issues. If it is a short you don't want to risk a fire or burning up part of the harness.

Do the headlights and brake lights actually work when you turn them on? Or are they also dead. I would start at the battery - you need a current limiter to avoid burning down the whole car if there is a dead short somewhere. An easy way to make one is use an old headlight - wire it in series with the positive cable on the battery so all current has to flow through the headlight. That way if there is a dead short, the worst that will happen is the headlight comes on full brightness. I used this technique to initally test the wiring on my MGB because I didn't know the condition of the harnesses when I got the body, even an absolute dead short can't burn the wires with that headlight bulb in the circuit. The headlight bulb is acting sort of like a fuse but instead of blowing and stopping all current, its limiting how much current can flow. There will be voltage drop across it, so testing voltages downstream from it will not be accurate.

Once you have that rigged, disconnect the coil wires and fuel pump (you know those work because the car starts and runs, taking them out of the equation helps eliminate variables). Disconnect the alternator for now, because it will draw a small amount of current for the exciter coils. Then turn on the ignition at the key (you won't get the alternator warning light because its disconnected at the moment) - there should effectively be no current flowing and the headlight should remain dark. If it comes on immediately, you know something is drawing excessive current.

Now press the brakes - the power flow to the brake lights should not be enough to cause more than a slight glow in the headlight. If it comes on full brightness, then you know there is a short. If the headlight remains dark but you get no brake lights at all, then its looking more like a grounding issue than a short.

Do the same thing with running lights - turn on just the parking lights (the current limiting headlight bulb will prevent the main headlights from coming on much). Again there should be maybe a dim glow from the current limiting bulb but the smaller running lights should come on. If they do, press the brake. The limiting bulb should might get a little brighter, but not much. If either nothing comes on, or all the lights go out when you press the brake, that really points to grounding issues.

 

[PerryC]

You could have a short, and/or grounding issues. If it is a short you don't want to risk a fire or burning up part of the harness.

Do the headlights and brake lights actually work when you turn them on? Or are they also dead. I would start at the battery - you need a current limiter to avoid burning down the whole car if there is a dead short somewhere. An easy way to make one is use an old headlight - wire it in series with the positive cable on the battery so all current has to flow through the headlight. That way if there is a dead short, the worst that will happen is the headlight comes on full brightness. I used this technique to initally test the wiring on my MGB because I didn't know the condition of the harnesses when I got the body, even an absolute dead short can't burn the wires with that headlight bulb in the circuit. The headlight bulb is acting sort of like a fuse but instead of blowing and stopping all current, its limiting how much current can flow. There will be voltage drop across it, so testing voltages downstream from it will not be accurate.

Once you have that rigged, disconnect the coil wires and fuel pump (you know those work because the car starts and runs, taking them out of the equation helps eliminate variables). Disconnect the alternator for now, because it will draw a small amount of current for the exciter coils. Then turn on the ignition at the key (you won't get the alternator warning light because its disconnected at the moment) - there should effectively be no current flowing and the headlight should remain dark. If it comes on immediately, you know something is drawing excessive current.

Now press the brakes - the power flow to the brake lights should not be enough to cause more than a slight glow in the headlight. If it comes on full brightness, then you know there is a short. If the headlight remains dark but you get no brake lights at all, then its looking more like a grounding issue than a short.

Do the same thing with running lights - turn on just the parking lights (the current limiting headlight bulb will prevent the main headlights from coming on much). Again there should be maybe a dim glow from the current limiting bulb but the smaller running lights should come on. If they do, press the brake. The limiting bulb should might get a little brighter, but not much. If either nothing comes on, or all the lights go out when you press the brake, that really points to grounding issues.

Thank you very much for replying . I'll take your advice and start the hunt.