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MG Midget and Sprite Technical - Wiring Issue

I am currently restoring a 1959 mk1 sprite which has been a pile of bits since 1979. Whilst sorting out the leaking clutch pipe, the pipe accidentally shorted onto one or more terminals on the voltage regulator. Since then nothing happens when turning the ignition switch, I believe the brown wire with blue streak should be the live feed to the ignition switch but when I measured this with a multi-meter there is only 3.5 volts getting to the switch and this is the same at the voltage regulator. It's a new wiring line. Has anyone got any suggestions?

Before I did this the rear lights, dash lights and fuel pump were working fine.

Thanks
HS Stone

Have you checked the fuses? There are two, one for the horn and one for the rest of the car.
Martin Washington

Hi,


It's not a fuse, any brown wires are unfused battery supply.


Check the voltage at the other brown wires, ie at the starter solenoid, fusebox etc
SR Smith 1

Thanks for your advice.

I have checked again and I have 12v at the two brown wires attached to the voltage regulator and the brown wire at the fuse box. All the fuses are intact and its a new wiring loom so there are no inline fuses. There doesn't appear to be a solenoid on the starter and the starter motor will turn over fine but that is on a separate switch.

Any other suggestions?
HS Stone

The 12v on the brown wires you have measured is straight from the battery. The brown/blue wire to the ignition switch comes out of the regulator and therefore is affected by whatever is going on in the regulator. I wonder therefore whether the regulator has been damaged in the short.
Graeme W

There is a wire wound shunt resistor under the base of the reg box, I have seen these burn out and odd things happen around the circuit because of it

lift it off the bulkhead and check for obvious signs of burnt resistor, possibly even a gap in the wire coil

But

before anything else check the battery voltage

A heavy short can damage the cells and drop the available lecktrickery to next-to-nuffink
Bill sdgpM

Yes I agree, the regulator needs checking. If its a reproduction one, they can be unreliable.

I would also check the lighting/ignition switch. The dash and main lights are on a different circuit to the regulator and should be unaffected by any regulator woes!

As you know the car would have had a mechanical fuel pump originally so not certain where the electric one you have is wired in.
Bob Beaumont

Many thanks for your suggestions. As recommended I will take the regulator off and give it once over for signs of distress. I haven't drop tested the battery but currently there is over 12.5v in it and the starter motor turns over fine. I don't think it was shorted for any length of time but I guess that doesn't matter! It is an original regulator. I also have a couple more but they have spade connectors - I assume I could use one of these if the other one looks faulty or would it be best to get an exact replacement?

I will check the lighting as suggested but from memory the lights are connected to one of the terminals on the ignition switch as it is the type that has an outer bakerlight knob to switch this. I have replaced the pump with an electric pump and that is currently connected to be live when the ignition is switched on. I realise this needs to be altered as the pump runs whenever the ignition switch is on which wouldn't be great in an accident. I am not sure where the power for the dash lights comes from.


Anyway, as suggested first port of call is the regulator.
HS Stone

I think common practice back in the good old days for road cars at least was for the electric fuel pump to run with the ignition.
I suspect that high output pumps associated with fuel injection, together with greater safety awareness created a need for inertia switches in the fuel pump power circuit. These can be purchased and fitted easily enough.
But that is just one of the many areas in our cars which have had redesigns in modern counterparts to improve safety. Where do you draw the line? Typically, no seat belts, no roll-over protection, no crumple zones, no collapsible steering column, no air bags, no side swipe protection, less effective braking, single circuit brakes etc. - probably safer on a motor bike! But the inertia switch reduces one risk.
Graeme W

Ok, let's get back to basics here.


the control box/regulator is purely acting as a junction point in the circuit you a problem with.



Ditch the meter and get a proper 12v test light for testing, a meter will show voltage, but not current.



Firstly check for good voltage at the 2 brown wires at terminal A on the regulator.



Then check for good voltage at terminal A1, the brown/blue.


If this test shows low or no, current it maybe just a loose/dirty screw at A1. Assuming the car has the original type of screw fitting control box.


If this is all ok and the lights work, the 12v supply to the ignition/lighting switch is ok.


Next is key to "on", does the ignition come on and do you get a good 12v on the white wires at the fusebox?


If not, the ignition switch has failed.
SR Smith 1

Thank you all for your help.

I took the regulator off last night and found one of the contacts underneath had melted. So I replaced with a spare one I had and everything is working - hurrah!
HS Stone

Good to hear you're sorted out.
SR Smith 1

This thread was discussed between 12/10/2015 and 14/10/2015

MG Midget and Sprite Technical index

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