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MG MGB Technical - Major short circuit

I just drove the 70 B from New Mexico down the Baja California peninsula, a rough trip for anything that isn't a 4 x 4.

On the return, I hit minor flash flooding in a small coastal town that had suffered rains. As I drove through about 40 cm of standing water, a terrible chatter came from the engine compartment. It stopped and then started a few minutes later, and the car quickly went dark and died. Turning the key would not disengage the ignition (what was left of it).

It turned out that the starter solenoid/relay had engaged on its own with the car at speed. The current drain of the starter working against the engine in 3rd gear brought the car to a stop, unfortunately in about 20 cm of mud on the highway, headlights on, etc.

I lifted the bonnet and found all of the mechanical cables glowing red hot (especially heater valve cable). I later found that the engine ground strap was bolted to the "engine" side of the rubber engine mount, not the chassis side so the starter, alternator and coil had been grounding through the various cables that disintegrated red hot like the heating elements in a toaster. Fortunately, I was able to disconnect the batteries before the car burned down.

I replaced the starter with a spare that I was carrying, corrected the ground strap problem and the car started and ran perfectly, albeit without choke, speedometer or heater control.

The question is, why did the starter solenoid engage on its own after hours of running? It happened when submerged. I suspect that months of starting and charging through the mechanical cables didn't do the solenoid any favors as current at cranking was undoubtedly high and voltage reduced. Maybe the solenoid coil was shorted and rapid cooling upon contact with water engaged it? Dunno but it was pretty scary, especially in the desolation of Baja California.

Glenn Mallory

Glenn,

Sounds like a nice trip other than the flash flood part. I can take a few guesses on the delayed reaction in the starter. One is simply that the water and muck made a connection later that was not made at the time you got out of the water. Also, it may be that the starter relay (if your model has one) caused the problem.

Finally, at times when I had to drive through deep water, I removed the cooling fan or removed the fan belt before driving into the water. When the fan hits the water at speed it can bend the blades and bring them into contact with things, such as the radiator. So, maybe the initial noise you heard was the fan and not the starter?

Charley
C R Huff

A 1970 should have a starter relay, but that may not be relevant. 40cm of standing water is a lot, and driving through it will create quite some force on components in the engine bay especially the lower half. This could have pushed wiring i.e. the solenoid operate wire onto the solenoid stud hard enough to break through the insulation.

Given the current the solenoid takes I can't see water on it own providing enough of a circuit to operate it. If the car went dead when you were diving, but restarted on the same battery later on, then I suspect other solenoid connections had also been dislodged. The engine will run on power from the battery or power from the alternator, but they both involve the solenoid stud.

The fact that the engine earth strap was correctly positioned was probably not relevant. Far from increasing current the resulting bad connection would reduce current, as well as voltage to the motor. As well as make starting more difficult from the low voltage point of view it would also reduce HT spark as the distributor was probably several volts above earth where it should be when cranking. Those symptoms, and the heating up of accelerator, choke and heater cables would have been happening all the time.

If the starter didn't resume cranking when you eventually reconnected the batteries, then whatever had cause the solenoid to energise had detached itself. You you need to have a good look round at the solenoid wiring.

Incidentally, this is a good advert for a battery cut-off switch positioned where you can reach it quickly in an emergency, the heel-board is usual.
paulh4

Just a comment based on my own experience.

I had the starter engage on a 1970 MGB, whilst driving on a major highway at probably 60 or 70 MPH. By the time I had gotten to the shoulder of the road and disconnected my batteries (which is why I have always used a cut-off switch since) the starter had been turned into a generator, and was completely fried.

Replaced the starter, and could never discover the source of the problem. I drove that car for another 80,000 miles, and never again had the problem.

It's one of those itches that never got scratched.
JR Jim) Ross

This thread was discussed between 26/12/2016 and 20/01/2017

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