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MG MGB Technical - Check the things that don't break!

So the Jubilee decided not to start on Monday lunchtime, my initial (uneducated) guess was battery or starter motor.

So returning to work in the midget to get my battery pack and separate charger I then try to start the car with the different booster packs on. Still no joy!

So there I am contemplating removing the starter and swapping it for the one that is on the parts car that is at work and I switch off the battery charger and see that it goes from a healthy green light to a quite poor yellow light (on the charger). Ah, so it's a duff battery after all.

This lunchtime I buy a new battery and get home after work to fit it, turn the key and no difference at all - flippin' heck I think (or words similar!), so I call my mate who's an Auto-Electrician.

"Sounds like a bad connection to me, have you checked the cable?"

So I look at the cable, no wear evident, the terminals are tight, all looks okay. But I ask him to make me a new cable.

So after collecting it from him, I fit it and the Jubilee fires first time.....

Moral of the story, is to check the things that don't break or wear out,

The cause of the problem was found to be the battery to solenoid cable, it suddenly decided that it did not want to conduct enough amps, watts or whatever it is to the starter solenoid....

I'm left wondering how can that be?
A I McGee

Corrosion can build up under the insullation and cause high resistance in the cable. Note, high resistance is a relative thing and in a battery cable 1 ohm of resistance is enough to drop all 12 volts when the starter is engaged and is drawing 40 or 50 amps (40 amps times 1 ohm of resistance is a 40 volt drop, several times the voltage of the battery). This is a suptal problem because it is totally unseen. Cheers - Dave
David DuBois

AI. I have had the same problem, with three different cars, over the last five years. Hence, you are noting something that all of us have a potential to have a problem with and more of a problem as time goes by.

I found what Dave noted--build up of corrosion inside the cable's insulation.

One quick check, if you have a volt meter avaliable is to check cranking voltage at the battery (should be about 12.5V), then at the starter solenoid. (I use a battery charge spring clamp on a long wire with a small alligator clip on the other end which allows me to test the cranking voltage at the starter by myself.) All three of the bad cables showed about 5V at the starter when they quit working.

Les Bengtson

Hi all.

I have recenly 'nailed' an odd problem on my hatchback 'shopping trolley'. It has always (for the 3 years I have owned it, anyway) had a slow cranking speed, though it would crank happily for several minutes and the battery voltage seemed ok (with the headlamps for a load) so I assumed that the battery was probably ok and the problem was elsewhere (all connections were clean and tight).
Since it always started (eventually) I didn't worry too much.

When I replaced the battery recently (the original had been ruined by being totally discharged, but that's another story) the cranking speed rose dramatically.. I can only assume that there had always been a slightly high resistance somewhere in the old battery, something I have never met before.

Back at the totally discharged battery... an intermittant interior light switch had been left in the 'always on' position, but this wasn't noticed as the light didn't happen to be on at a time that it would be seen.



If you have battery cables with molded on battery terminals make sure to tug on the cable at this joint. I have seen several where the cable corroded inside the terminal. Looked perfect but wasn't.
John H

Had this happen to me as well. No electrical power anywhere, except at the battery. Ran a jumper cable from the battery to the starter and everything worked. When I replaced the cable and looked at it, it looked perfect. I am glad I am not crazy, and this has happened to others.



I think the calc is wrong - the voltage drop needs to be worked out over the whole circuit - so you need to look at the relative resistances of the cable, the solenoid contacts, the starter motor itself and the return path through the chassis to the battery as well as the battery internal resistance.

You can't have a voltage drop that is greater than the battery voltage!

However, the intention is right - a relatively small resistance will seriously restrict the CURRENT flowing in the circuit and since the current taken by the starter is in the order of 250 amps when turning the engine ( 475 amps locked and 70 amps free running) then a bad connection or cable will mean the starter won't start the engine!
Chris at Octarine Services

Had my roadster do something similar while undergoing an MOT at Halfords, and the mechanics there were totally stumped. All I had to do was remove the three cables from the solenoid stud, clean them up with emery, and away we went. The Workshop Manual talks about a maximum of 0.25v volt-drop with just the headlights, but you should be able to get that when *cranking*. Anything higher than 0.5v between battery post and solenoid (12v to stud and ground to body) is worth investigating, I have seen as much as 3v lost on just one leg. Because these are bad connections we are talking about you can't go by resistance, this will vary with heat, current and vibration. Volt-drop under load is the only way.
Paul Hunt 2

This thread was discussed between 05/04/2006 and 07/04/2006

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