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Message: <blockquote data-quote="PerryC" data-source="post: 1251400" data-attributes="member: 33726">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:<ul> <li data-xf-list-type="ul">    </li> <li data-xf-list-type="ul">Engine runs fine with ignition on.     </li> <li data-xf-list-type="ul">Turning on headlights kills all electrics (ignition, lights, accessories).     </li> <li data-xf-list-type="ul">Turning headlights back off restores ignition power.</li> </ul>This strongly points to a main power feed or ground issue rather than the headlights themselves. Here are the most likely causes:<hr /><h3>1. Bad Ground Connection</h3><ul> <li data-xf-list-type="ul">    </li> <li data-xf-list-type="ul">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.     </li> <li data-xf-list-type="ul">Result: turning on the headlights “starves” the ignition circuit, and everything dies.</li> </ul>Check:<ul> <li data-xf-list-type="ul">Battery ground strap to chassis.     (I started here)     </li> <li data-xf-list-type="ul">Engine ground strap (from engine/transmission to body).    (I did this)     </li> <li data-xf-list-type="ul">Clean and tighten both ends.  (I took a break here)</li> </ul><hr /><h3>2. Faulty Power Feed or Connection at theSolenoid</h3><ul> <li data-xf-list-type="ul">On MGs, the main battery feed often runs to the starter solenoid terminal, which then distributes power to the rest of the car.     </li> <li data-xf-list-type="ul">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)</li> </ul>Check:   <ul> <li data-xf-list-type="ul">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)     </li> <li data-xf-list-type="ul">Look for overheating, corrosion, or a loose ring connector</li> </ul><hr /><h3>3. Headlight Switch Short or Miswiring   </h3> <ul> <li data-xf-list-type="ul">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.     </li> <li data-xf-list-type="ul">This would also explain why nothing lights up when    the headlights are switched on.</li> </ul>Check:  <ul> <li data-xf-list-type="ul">Inspect the headlight    switch wiring for melted insulation or misconnected wires.     </li> <li data-xf-list-type="ul">Trace the brown (always    hot) and red (headlight feed) circuits.</li> </ul><hr /><h3>4. Weak Battery / High Resistance</h3><ul> <li data-xf-list-type="ul">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.     </li> <li data-xf-list-type="ul">But usually you’d still see dim lights, not a total blackout, so this is less likely than a bad ground or main    feed.</li> </ul><hr /><img class="smilie smilie--emoji" loading="lazy" alt="✅" title="Check mark button    :white_check_mark:" src="https://cdn.jsdelivr.net/joypixels/assets/8.0/png/unicode/64/2705.png" data-shortname=":white_check_mark:" /> 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.</blockquote>

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