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MG MGB Technical - Coil Overheating (revisited)

Thanks everyone for your thoughts on my recent engine cutout problems.

Unfortunately/fortunately the problem has not happened since so I haven't been able to diagnose the fault completely.

However, today I found time to go over some of the wiring, cleaning up contacts etc.

When I was looking at the connections to the starter solenoid I noticed that the brown and white wire had badly cracked insulation, meaning that there was a lot of copper showing. This in and of itself might not have been much of a problem but the wire does run alongside a short stretch of copper hydrulic line and the day on which the engine cutout occurred was really quite wet. I have wrapped this section of wire quite thickly in electrical tape in case this was creating a short.

Other than that I didn't find any badly corroded terminals. I think I might replace the coil anyway as it has not been changed once in the time I have had the car.

One quick question, now it occurs to me - on the starter motor there were two spade terminals, one long which exits straight out and one shorter which exits at more of an angle. When I put a resistance meter across the two with the battery reconnected they didn't appear to be twinned. I have connected the brown and white to the shorter spade terminal, where I am pretty sure it came from. All the other connetors attach to the copper binding post with ring type connectors and everything seems to be running fine.

Does anyone know what the two spade connectors are for?

Thanks for any help,

Alec
Alec

Alec. The later model cars used a starter having two blade terminals. One, the lower of the two as the starter is seen when mounted to the engine, is attached to the line coming from the stater relay and causes the starter to turn over when the ignition switch is in the start position.

The second terminal provides 12V power to the coil when the starter is being operated and is providing no power when the starter is not operating.

This form of starter was introduced about 1975 when they went from a 12V coil input to using a resistance wire in the circuit which cut the input voltage down to about 6-8 volts when the ignition switch is in the "run" position. Thus, two wires going to the coil--the standard wire, with internal resistance, for use when the engine is running, and a second wire, off the starter, providing full 12V input for easier starting.

Both starters will, physically, interchange and the last time I had to replace a starter only the later model version was available as a rebuilt unit. So, it sounds like you are running the later model starter on an earlier model car.

Les
Les Bengtson

Ah, thanks Les - yes, that's exactly what's going on - it's a 72 car (with 71 wiring according to the Haynes manual) and a new starter was fitted a few years ago by a local mechanic. Thanks for the info.

Alec.
Alec

Alec. No problem. My 68 got a later model starter when I needed to replace it--all that was available.

Les
Les Bengtson

Unlikely to have been a short on the wire you mentioned, that isn't powered when the car is running or switched off, only when cranking.

Unusual to have a late-model starter on an earlier car, it is usually the other way round and people are faced with what to do about the coil boost circuit.

Note that while the starters are interchangeable on 68 and later chrome bumper cars and rubber bumper i.e. both pre-engaged, the earlier cars with inertia starters use a different number of pinion teeth.
Paul Hunt 2

Many thanks guys (as ever).

Alec
Alec

This thread was discussed between 22/07/2007 and 25/07/2007

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