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MG MGA - Smoking starter switch!

My MGA has a newly rebuilt MGB 3-main engine in it, plus a new wiring and a new starter switch. I am trying so far unsuccessfully --to start it for the first time. The engine turns over very sluggishly when I pull the starter cable. Also, the starter switch itself is getting hot. I saw smoke coming from it and immediately shut everything down.

The switch is wired as shown on the wiring diagram: hot cable from the battery to one side, and a cable to the starter on the other. When I by-pass the starter switch by hooking the two cables together, the starter turns the engine vigorously.

The battery shows 12.5 amp output at rest and about 10.5 when cranking.

Can anyone suggest a solution to my problem?

Thanks in advance for your help! This site has been a real life-saver for me.

Mike
M.E. Whalen

That would be volts, not amps?
Looks like a bad new switch.
Art Pearse

Some new starter switches have been pretty bad, and I suspect they are still being sold. The contacts are okay, but the internal insulators are garbage, so the moving contact plate can short to ground on the pull shaft. The ones with white nylon insulator are highly suspect. See here: http://mgaguru.com/mgtech/faulty/ft004.htm
Barney Gaylord

Mike

Barney has summed it up very well. I had poor starting in much the same way as you describe. This is what I found inside my switch (picture).

I have since fitted a pre-engaged hi torque starter. This gets round the poor starter switch quality quite well in that the battery cable and starter cable share the same terminal post on the starter switch. The other post has a very small cable that connects to the starter solenoid. When the starter cable is pulled far less current crosses through the contact disc.

Steve


Steve Gyles

I believe that the nylon-faced switches are sold on Ebay...
I have used them without a problem, but maybe it has to do with quality control, where some switches get out that have obviously (according to your article),
been poorly assembled with mis-matching parts....
I recently replaced one of the NOS style switches, with another NOS style, and this thread makes me glad that I did not buy one of the nylon-faced switches, this time.
The switch that I replaced may or may not have been defective, but the car failed to turn over on a couple of different occasions, and I felt it would be safer (and cheaper), to replace it, than to get stuck on the road...
Edward
Edward Wesson 60MGA

Thanks for all of the helpful comments!

Art, you're right, it is volts, not amps.

Barney, I also noticed some sparks jumping from the switch shaft to the steel housing that the pull wire is in. Sounds like what you say: the contact plate shorting to ground on the shaft.

The defective switch seems to be keeping the engine from cranking vigorously, as it cranks fast when I by-pass the switch. It seems that the hotter the switch gets, the slower the engine turns over. This makes more sense in light of your explanation.

I'll get a new switch. Does anybody have a recommendation for a good source for a switch that isn't nylon-faced? My problem switch has the nylon face.

Mike
M.E. Whalen

You could pretty much follow Steve but use the starter solenoid from an early "B." Moss #546-020.
David Werblow

One of the studs on the stater switch may have 2 wires on it, sometimes the smaller wire has been installed first with the battery cable on top. When the nut is tightened down the small wire can touch the case when it flexes during the pull to start process.
Does not appear to be the problem here but worth a look.
Don
D Hanna

The last one that I bought ,(recently), that looked like the original type, came from Moss....It does NOT have the nylon face, but the has two separate "rubber" insulators.
So far so good.
Edward
Edward Wesson 60MGA

I have given up on the original-type switches, I have been told that the ones that are available today may not be designed to carry the full 12 volts regularly. In any case, they have not lasted long. I am using modern switches on both of my MGA's, the dashboard button only sends a message to turn the switch on.
As is probably obvious, I am not an "expert". Luckily the large heater trunking hides the "mod". Perhaps you can do what I did and see an electrical expert.
Barry Bahnisch

This thread was discussed between 17/10/2013 and 19/10/2013

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