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MG Midget and Sprite Technical - Starter motor running continuously

Currently restoring a late 1500 that has dual braking circuit and brake warning light (removed with the console). Finished the welding and went restart it for the first time in a couple of months and when I turned the key the starter ran but wouldnt stop, I had to rush to pull the battery terminal off. When connected again all is fine until key turned again and it repeats. When the white and red starter lead is pulled off the back of the keyswitch it still runs but stops when same wire is pulled off the starter solenoid.
I'm strongly suspecting the diode that feeds the brake warning light circuit but cant find it. Any ideas?
Alan McKewan

Hmmm... Thinking out loud here...

When operating correctly:

- The brown wire to the key barrel is +12v.
- When you turn the key, this connects the brown to white/red, supplying +12v to the solenoid.
- White/red activates the solenoid, connecting brown at solenoid to starter motor.
- When the key is released, white/red and brown connection at the key is broken, deactivating the solenoid, stopping power to the starter.

But... You disconnect the white/red at the key and the solenoid does not disengage. So...

1. Is it getting stuck?
2. Is it getting power from somewhere else?

If it was 1, a problem within the solenoid itself, why would disconnecting the white/red at the solenoid stop the starter spinning? Dunno.

So no. 2 is perhaps more likely. But why?

Alan, why do you suspect this diode (don't have access to a late model wiring diagram at the moment)?

Sure your solenoid and ignition barrel are wired correctly?

Cheers,
Malcolm

Edit: Is the engine running in all of this? Or does it fail to start?
M Le Chevalier

I remember something similar with MGBs. Do you also have a seat belt warning light? Possible diode there, too.

A good wiring diagram should help.
Dave O'Neill 2

Found a wiring diagram.

Dunno what all these bits are that are circled, but problem must be in there!

I'm out! :-)

Malcolm

Edit: Link to diagram as attachment is a bit small.

http://www.1978mgmidget.com/78-80_Calif_schem_color2.gif

M Le Chevalier

Alan,
I suspect you're right :-)
Sorry, can't help with location but you can prove this is the culprit by starting/stopping the engine with the hand brake in the off position or pull the fuse on the Green circuit which could be a little less dangerous ;-)
If the fault is still present you would then be looking at the seatbelt warning circuit (for which you pull the fuse in the purple cct).

Best of....
MGmike
M McAndrew

If you dissconect the w/r at the solenoid you can check to see if it is live all the time ,you could also disconnect the lead to the starter and fit a lamp to it and test curcuit without starter turning some of our american member may be able to tell you where the diodes are located.
mark 1500 on the road Preston Lancs

This might help explain what we think is going on.
http://www.mgb-stuff.org.uk/bf.htm#7
(thanks again to Paul Hunt)

Best of...
MGmike
M McAndrew

this may help

http://www.mgexp.com/phorum/read.php?3,2290474
mark 1500 on the road Preston Lancs

Thanks Mike, everyday is a school day! An awful lot of idiot proofing going on. It's as though people were incapable of driving pre 1978.

Malcolm.

M Le Chevalier

Sounds like the diode has gone open circuit in the brake warning lamp circuit.


Try letting the handbrake off and see if that stops it.

It's a small cylindrical thing, marked with a diode symbol. Fitted up in the dash loom somewhere, male lucar one end female lucar the other end.



Just disconnect it.
SR Smith 1

I like where mark 1500 is going with this

Im guessing if the engine is running and you pull the r/w wire on the selinoid and it the starter stops then an issue with selinoid / dioades/ worn ignition key/barrel area

But if starter keeps then id think the problem would be in the starter its self like a bad abendix

Agian... thats just my thought,

You might try disconnecting the R/W wire at the selinoid and using a piece of jumper wire flash the selinoid to the battery cable directly and see what that does...remember to put the key in and turn to run if you want the car to start, or leave the key out if you just want the starter to turn over the engine without starting

Prop
Prop and the Blackhole Midget

I tried running a temporary wire from the switch straight to the solenoid and all is well.
Put the original wires back and problem is back.
I found 2 white / red wires connected to the spade on the keyswitch. So I thought only 1 must go to the solenoid and the problem must be coming from the other. EUREKA. I cut both and connected one at a time. One does nothing so must go to the hidden diode.
The other goes to the solenoid and when connected on its own makes everything ok.
So for now the brake circuit warning will be left off!
The light has been removed anyway for now as it was on the horrible centre radio console that hit my knee every time I got in.
I will have the dash off in a few weeks and might have a look for the elusive diode then!
But I still cant figure out why it only started to feed back to the solenoid only after the keyswitch was turned..
Alan McKewan

Because the brake warning circuit is live with the key on.


As the diode has failed its allowing current to backfeed via the handbrake warning circuit, and making the white/red live.
SR Smith 1

This thread was discussed between 14/10/2015 and 15/10/2015

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