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Q: My '77 MGB Roadster has been going quite well until yesterday. I was out to the shops, about 2 miles from home. After shopping, got back in; one click-grunt, then nothing. Ignition light was on initially after trying to restart, but went out, doesn't come back on.
The clock stopped for a while, but is going now. Interior light is on (when door open), but dim and "slow to warm up". Whenever I try to turn anything on (side lights; ignition etc.) nothing happens and the clock stops and the interior light goes out till I stop doing it.
The fuses are intact, and all the connections I can find are making contacts. Of course, first thought was: "battery" (interior short maybe). However, I tried to jump start, but "nothing". Most recently (today) I disconnected the battery (to escape from a possibly shorted cell) and applied a completely separate new battery via jumper cables from another car.
Strangely enough, the SAME symptoms occurred: no cranking; turn sidelights (or anything else) on and the clock stops and the interior light goes off, for a slow return when I stop doing it.
I've studied the wiring diagram, and can't come up with a solution. The battery reads 12 volts (although of course I can't be sure of available crank amps; but recall the "replace battery" trick already tried).
The only other symptoms I've observed recently (within last month): one set of opposite corner sidelights went out till I cleaned off the right connector on the fuse box; slow cranking last week caused me to charge up the battery, which worked and had me driving around all day Friday (before "dark Saturday").
All indications are of a short to ground somewhere, but probably not just on the high-current starter circuit, since the sidelights are separately fused? (No "fizzing" sounds on applying the new external battery either)
Anyway, hating to be stumped, thought I'd call on your expertise and advice.
Looking forward to defeating this particular gremlin!
A: The side lamp issue may be related, but having just found a short in my own car that did the same thing front and back in separate harnesses, it might not be.
Your car was factory equipped with a single 12 volt battery. All MGBs were 12 volt systems, though early cars through about 74 used (2) 6 volt batteries in series. Very early cars had positive ground. Many changed these cars to single 12 volt batteries, but they are not the same as you use on yours, the battery boxes being smaller for the 6 volt units.
What is common to all is the exceptionally long positive battery cable that runs beneath the car along the passenger side for almost its full length to the starter. Subject to deterioration from oil leakage, road grime and weather, after over 30 years, it and its connections to the battery (which you already checked) and starter solenoid would be my firsts suspects. There are several clips that hold it to the chassis and they are supposed to have rubber isolators in them. If those are gone, the cable insulation may be rubbed off. The insulation is also known to crack, allowing the cable to corrode unseen or ground to the chassis.
The ground (negative post) cable or strip is short and goes right to the chassis near the battery. You need to make sure that is clean, the bolt to the chassis and connection point is not corroded and the connection good as well.
Start there before tearing up the harness. If that solves it, we'll address the lights in another note.