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MG MGA - Very odd electrical problem

Sorry to start another thread but something very odd just happened. Please dont think me mad!
I thought I would test my new sender unit, off the car. So I disconnected the terminal on the old unit and connected it to a long piece of cable running to the new unit sitting in the car. I also connected an "earth cable" to the sender unit.
I turned the ignition on, and lifted the float to see the fuel needle move from full to empty. All good there.

I promise I havent been drinking, but when I then switched the ignition off there seemed to be a very tiny surge forward of the car (it was in gear), as if the starter motor tried to turn - only very gently and only for a split second.
I repeated and the same happened.
I disconnected the new unit and put everything back as it was, switched ignition on and off, and nothing other than a slight click as I turn off from the engine bay, that seems to come from the distributor - I have electronics, not points.
I have absolutely no idea what gremlins have got inside. My first thought was something in the control box (still on a dynamo) but that wouldn't overide the manual starter switch of course. And nothing odd looking on the cable between the starter switch and the starter ??????
Graham V

Graham,
I believe your first thoughts are almost correct in that something in the control box is wrong, however I think it is your dynamo that is trying to turn the engine & not the starter.
I believe your cut out relay within the control box is latched or latching in incorrectly when switching on the ignition. I don’t think this has anything to do with your new sender unit but is just a coincidence.
Suggest that during further investigation that you loosen or remove the fan belt to stop the possibility of a high current burning out the dynamo or control box.
Richard
R A Evans

Hi Richard
Thanks for your post. Very interesting observation. That certainly gives me something helpful to work on. Never touched a control box before so will get in to my manuals and Barneys site to read up! I do enjoy learning about the car!
I realised it couldn't have anything to do with the sender but thought I should present the entire scenario.
Just returned a 100 mile round trip to MGCC in Abingdon without incident. They made us very welcome.
Graham V

Graham
Another thought for you. I too have electronic ignition (pertronix/Aldon) and have noticed on occasion when turning ignition off or on that the click you referred to in the electronics in the distributor, presumably it switching in some way, sometimes occurs in conjunction with the very softest of backfire noise from one of the cylinders. It is never more than the slightest sound but definitely from the engine and I have always assumed that the electronic ignition has fired a random spark into a cylinder and burnt off a bit of residual vapour in there. Presumably it would be enough if in gear to nudge the car. I have only experienced the phenomenon in neutral so can't comment on your experience in gear.
Bruce.
B Mayo

Thanks Bruce
Interesting - Thats another one for me to think about.
I cant remember for sure but dont think I had run the engine for at least 3 hours before I tested the sender unit, so am I right in thinking its unlikely for there to be any vapour floating around to ignite?
Graham
Graham V

It is most unlikely for the generator to produce sufficient torque to cause the car to surge forward regardless of how slight that might be. It is more likely that when you turned the ignition off, it triggered a spark from the coil and the rotor in the distributor was on the wire to a plug in a cylinder that was on TDC for an ignition stroke. What little fuel air mixture was in the cylinder was enough to produce a bit of a burn to cause the slight surge. Cheers - Dave
DW DuBois

Graham
I think a whiff of fuel vapour could linger quite a while. My experience of this was just a slight "cough" and not a big deal but definitely a bit of engine movement.
Bruce.
B Mayo

Thanks for that. I will keep an eye on it and report back if I discover anything more.
Graham
Graham V

My A does that all the time....electronic ignition. (pertronix brand)

The pertronix fires when you turn the key to ignition on and off and occasionally my engine kicks as if its fired one of the cylinders off. Not enough to start the engine but just bumps the crank forward abit.

Hope that helps:-)
S

Thanks. Yes that does help. I had originally thought I was going mad, but from what I now understand, I am not. And if it ain't broken, I don't need to fix it!
Thanks
Graham V

This thread was discussed between 19/04/2016 and 22/04/2016

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