MG Your Midget Electrical System Dies When You Turn on the Headlights

The car started and ran fine until I: Turned on the headlights, stepped on the brake, used a turn signal, or ran the heater fan. I asked Chat GPT what could cause it, and here's its reply (which was absolutely right!):


That’s a very classic electrical fault, especially in older British cars like your 1976 MG Midget. Based on your description:

• Engine runs fine with ignition on.
  
• Turning on headlights kills all electrics (ignition, lights, accessories).
  
• Turning headlights back off restores ignition power.

This strongly points to a main power feed or ground issue rather than the headlights themselves. Here are the most likely causes:

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1. Bad Ground Connection​

• The battery’s negative cable should be bolted solidly to the body and/or engine block. If this connection is corroded, loose,or damaged, the system can’t carry the extra current draw when the headlights are switched on.
  
• Result: turning on the headlights “starves” the ignition circuit, and everything dies.

Check:

• Battery ground strap to chassis. (I started here)
  
• Engine ground strap (from engine/transmission to body). (I did this)
  
• Clean and tighten both ends. (I took a break here)

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2. Faulty Power Feed or Connection at theSolenoid​

• On MGs, the main battery feed often runs to the starter solenoid terminal, which then distributes power to the rest of the car.
  
• If the connection there is loose or corroded, the extra load of the headlights can “break” the circuit and cut off all power. (This checked out okay)

Check:

• Big brown wires at the solenoid and fuse box. (JACKPOT! The brown wire was wedged down against the chassis just ahead of the firewall. I will list the rest if your isn't fixed yet)
  
• Look for overheating, corrosion, or a loose ring connector

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3. Headlight Switch Short or Miswiring​

• Less likely, but if the headlight switch or its wiring is shorting to ground instead of routing power, flipping it on could effectively “kill” the power bus.
  
• This would also explain why nothing lights up when the headlights are switched on.

Check:

• Inspect the headlight switch wiring for melted insulation or misconnected wires.
  
• Trace the brown (always hot) and red (headlight feed) circuits.

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4. Weak Battery / High Resistance​

• If the battery is weak or a cable has internal corrosion, turning on the lights may drop voltage so low that the ignition coil cuts out.
  
• But usually you’d still see dim lights, not a total blackout, so this is less likely than a bad ground or main feed.

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Most Probable Cause:
A bad groundstrap (battery to chassis or engine) or a bad mainpower connection at the solenoid/fuse box.

Best of luck to you in your quest. By the way, be sure your battery is fully charged.