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MG TD TF 1500 - electrical trouble

I've inherited my dad's 1950 MG TD, he's 93 and purchased the MG in 1969. I had the carbs rebuilt by Joe Curto (an expert with SU), installed a new fuel pump, new battery, had the gas tank lined and a new ignition switch. I thought that would do it! I turned the key and nothing. I felt the wires and they were really hot and there was electric smoke coming from the control box. Anyone have any ideas?
jrk knight

Disconnect the battery asap! Did you put the battery in backwards? The TD would have been positive earth originally but owners often convert to negative earth (like modern cars) to suit modern add-ons like GPS, rev limiters, radios etc. When you turned on the ignition, did the ammeter show charge or discharge?

I am not an auto electrician but If you have the wrong polarity, you might need to check a few things before just turning it around. Generator polarity might be one.

Bob Schapel
R L Schapel

Check the wiring that you touched when replacing the ignition switch.

P G Gilvarry

Bob i did disconnect the battery! However I did not notice whether the ammeter was showing charge or discharge. if it showed discharge would that tell you something?

jrk knight

If wiring was hot and smoke coming out it might have been showing a discharge, may even have been full scale deflection.

P G Gilvarry

Many things could have happened. Why was the switch being replaced? TDs have many circuits that are not fused, and a loose wire can short. If you are not pretty good on 12V/auto wiring, it may be a good idea to find someone locally that can help you check the harness and everything out. A wiring harness fire could end up burning the whole car. George
George Butz III

George the ignition switch was replaced because the old one was past it's prime and we had to fiddle with it to get it to work the last time the car ran, which we think was 12-15 years ago. So I thought it best to replace it with a new one.

My dad replaced the harness, so I'm not sure that's the problem.
jrk knight

Disconnect the connections to the battery and put a battery charger with a circuit breaker in place of the battery. That will allow you to check all the wiring without frying your harness. Until you find the short circuit, the worst that will happen is that the circuit breaker will trip.
Then methodically double check all the wiring against the circuit diagram. When you have found and fixed the problem, the circuit breaker will stop tripping and things should start to work. The only things that may not work on the battery charger are the starter and horns.
There's a short circuit there somewhere.
Lew Palmer

jrk, since the smoke didn't show up until you turned the ignition on, you have a limited area to start searching in. There are only a few circuits that are energized by turning on the ignition key. Virtually all of them are involved with the white wires attached to the A3 terminal of the fuse holder, or the green wires attached to the A4 terminal(s) of the fuse holder. One of these white wires is attached to the fuel pump. Remove it. One is attached to the coil. Remove it. Is there a chance that the new fuel pump is a solid-state one that may be polarized? Remove the cover from the voltage regulator and see if you can tell where the smoke was coming from. Any chance that the regulator is not an OEM, i.e., mechanical, type?
Do any lamps light up when you turn on the key? Bud
Bud Krueger

A word of warning on the ignition switch. Moss had a batch of faulty ignition switches a while back that were responsible for causing wiring fires. I believe that they withdrew them once they became aware of the problem. My strong suggestion would be to have the switch checked out particularly if it came from Moss. Cheers
Peter TD 5801
P Hehir

I was going to mention the replacement switches as well but Peter beat me to it. Also if the wiring harness is the old original lacquered cloth wiring now is the time to replace it especially since you’ve experienced some sort of overheating and smoke event. The insulation has become brittle with age and any kind of manipulation opens it up. Allowing adjacent wires to come in contact with one another.

Bill Chasser
W A Chasser

The ammeter should show a small discharge when ignition is turned on because the coil and petrol pump are drawing current. If it showed charge, (without the engine running) it is still drawing current but indicates that the battery has the wrong polarity (unless the car has been modified to negative earth without switching the ammeter connections). I agree with Pete ... if smoking, it was almost certainly full scale deflection one way or other. Bob
R L Schapel

Pull the 3-4 fuse, that will then only power the white wires on the side that is live with the ignition on.

Is the IGN or GEN warning light coming on? That is the major item that is powered with the IGN on and involves the regulator

I would carefully check the IGN switch wiring that you worked with.

P G Gilvarry

For trouble shooting suspect wire harness or correct routing of same, suggest using a secondary power source such has slot car toy transformer, output 12-14 volts, but low amps, in place of battery. can check for continuity and voltage without frying components. DC power supply also available at electronic supply stores.
R W Hinton

This thread was discussed between 11/05/2018 and 13/05/2018

MG TD TF 1500 index

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