why did my coil melt?

[Norfolkandchance]

Mk1 Golf gti with electronic ignition.

Car wouldn't turn over after months of not being used. After much effort with various chargers, boosters and jumps starting I replaced the starter motor.

Engine span really fast but would not fire. I left it for a while and put some extra fuel in even though it had some and it started straight away. Too quickly to have sucked the fuel through, I would have thought.

Take it for test drive. All good for 20 minutes then cuts out instantly. Coil is so hot it has melted the plastic cover and dropped molten plastic all over the heater pipes.

What could have caused it? Do I need to check anything before fitting new coil?

Thanks

[Norfolkandchance]

Parked up was running perfectly. Only change was starter motor which had an earth wire attached to one of the mounting bolts. Coil could well be original. Certainty looks old. Ht leads and plugs all fairly new.

Thanks

[Norfolkandchance]

Doofus said:

Bad earth

What would you do?

Thanks

[Norfolkandchance]

oakdale said:

Coil should be fed through a ballast resistor when running but fed by direct battery 12v from a terminal on the starter motor during cranking.

Check that you haven't connected the 12v cranking feed for the coil to a terminal on the starter motor that is constant 12v.

Very plausible. Will check tom. Assume engine neededent be running to look for feed as long as the ignition is on.
Thanks

[Norfolkandchance]

OP here: Update:

A bit of ferreting around suggests that the ignition system on my car may be a bit of a bitsa with some Mk2 bits here and there.

I fitted a spare coil that came with the car and car started and runs great. Though I haven't let it run for any length of time.

The coil has two sets of terminals. One set has a green and a red wire attached, the other two black wires.

Other than the big wire from the battery, the starter has one wire coming in to the solenoid. If you take this wire off then the starter won't work!

I've got three options as I see it:

1) Assume the changed starter motor didn't lead the coil to fail. Start engine and see if it lasts a long time without melting the coil. Worst case scenario need to buy another coil and am no wiser.

2) Take lots of readings, ask lots of advice try and solve problem. But its a bitsa and advice tends to be contradictory.

3) Pay mobile auto electrician to come out and check it.

I'm minded to go for option 1.

[Norfolkandchance]

Thanks to everyone who has taken the time to reply, by the way. Especially the detailed and researched ones.

[Norfolkandchance]

Thanks for the advice everybody - very helpful. I have a spare and the car seems to be running very well so it seems to have been a coincidence.

Cheers

Chris