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MG MGB Technical - positive earth B electric woes

Hi all,

Having a bit of a time here. I've a 67 B GT that has been running fine for a while but recently the starter started to play up.

Fine I thought, time for a high torque upgrade. Checked with MGOC etc on how to wire and basically was told to piggy back the white connect from the existing solenoid to the starter solenoid (on the starter) and just connect up the big battery line from the old starter onto the new.

On turning the key the starter jumps into life. Tick v.g. Problem, let go of key and the starter remains powered. Only way out is to disconnect the white from the old solenoid. This cuts it out.

So Q. 1 Any ideas what I've missed ?

Q 2. It also turned out that the clearance for the new starter was slightly out. On the last fault find with the ammeter etc the starter remain both powered and engaged. Whilst I shut it down pdq (trust me I do mean pdq) it looks as though somethings distinctly unhappy. On turning the key now the charge light does not come up, no sound of pump ticking.

Fuses look o.k, I've tried a replacement solenoid (old style) and voltage control box. No obvious external marks on the start switch, starter, voltage box etc.

so .. what is the most likely thing I've fried ?

Many thanks in advance for any suggestions. Feel free to remind me how daft I've been (after telling me what I need to do to fix it :-) )

Pete
P Clapham

The white lead is hot all the time when switch is not in the off position. I believe you should have a white with a red tracer to operate the solenoid.
Sandy
Sandy Sanders

Sorry yes, the lead from ignition switch to the solenoid (old) is white with red trace. This is then piggy backed to another lead to the solenoid on the high torque starter.

Pete
P Clapham

What lead is the white/red piggy-backed to? At a minimum it should be connected exactly the same as before, i.e. on its own to the solenoid operate terminal.

However it sounds to me like you may have the white/red connected to *two* spades on the solenoid, which is definitely not correct. If the solenoid has two spades it is designed for rubber bumper cars with ballasted ignition, the extra contact supplies full 12v to the coil during cranking to aid starting. If you connect the two spades together, then yes the solenoid will lock in until you disconnect the wire, not even switching off the ignition will stop it. If the MGOC told you to do that, it is very serious error.

The other potential (no pun intended) problem is that 67 and earlier +ve earth cars had a remote solenoid and an inertia starter. You have a pre-engaged starter with attached solenoid, and the solenoid on those takes more current as it has to push the pinion into contact with the flywheel as well as close the contacts to pass 12v on to the starter motor. MGBs were like that for just one year, then a starter relay was provided. This greatly reduces the electrical load on the ignition switch contacts and wiring. I'd advise fitting a relay like the later cars, or the ignition switch may be the next thing to fail.

Indeed, it may already have failed, as it sounds like you have no power to the white now. Check the brown at the ignition switch to see if it still shows battery voltage when you turn the key first to ignition. If it does, then check the white. If there is no 12v there then the ignition switch has failed.

If there is no 12v on the brown, or it drops right down when you turn the ignition on, then you may have been lucky and it is a connector elsewhere that has failed under the extra current. The brown to the ignition switch comes direct from the B terminal on the dynamo control box, and it is the brown on the other B terminal that picks up 12v from the battery cable on the solenoid, so that may have failed as well, testing with a voltmeter on the spades and connectors on the control box should tell you which. But if the lights and horn still work, with the ignition on, then it can only be the wire between the control box and switch at fault, or the switch.

If there is 12v on the white then something else has been disturbed or failed, but as there are three separate whites from the ignition switch and one feeds the ignition warning light and another feeds the pump, then I rather suspect it is either the ignition switch that has failed or or there is a problem in the brown as above.
Paul Hunt2

Thanks for the heads up and the suggestions Paul,

I'll check out the wiring and voltage outputs tonight and let you know how I get on.

As for how to wire the new starter...

At present:

The white/red comes from the ignition switch to the old solenoid (still in place) and this is currently (pun certainly unintended :-) ) spliced to a white red to the starter solenoid (which is a part of the new starter unit).

If this is incorrect a Noddy does electrics diagram would be a good thing to have :-). If you can help here, much appreciated.

Thanks again for your assistance

Pete
P Clapham

Paul has good suggestions. You can also use your original starter solenoid as a relay to trigger your solenoid on the starter. That way is easier to wire- just take the cable from the starter side and move it over with the battery cable side, then attach it to the terminal on the new starter that would have the battery cable. Then run a wire from the starter terminal on the original solenoid to the starter terminal on the new solenoid on the new pre-engaged starter.
Matt
M. H. Dabney

Pete - that sounds correct as long as the new white/red is only connected to one spade, and not two if there are two. If there is only one, or the white/red is only connected to one if there are two, then it is still possible that the additional current of the new starter has caused the ignition switch contacts to weld together to give you the continuous cranking.

Matt makes a good suggestion, I'm going to reference that in an update to my web site. It all depends on what you took out when fitting the new starter, you may have to refit it to some extent. I'm attaching a diagram of how this would be wired, original on top, modified below.


Paul Hunt2

Many thanks for the assist here, and the wiring diagram. I'll give this a go over the w/end and (hopefully) trace the generic electric fault.

Once again the MG community has been most helpful. Thanks again :-)

Pete
P Clapham

For coloured wiring diagrams, go here and select the right page for your year of car.
http://www.advanceautowire.com/mgb.pdf
Mike Howlett

This thread was discussed between 30/04/2008 and 02/05/2008

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