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MG MGB Technical - 1972 MGB GT Starter Problem

I wonder if any members can point me in the right direction. The car has just started turning over very slowly. The battery voltage is being dragged below 10v. The 12 volt battery is fairly new, it is constantly on charge via a CTEK electronic charger. It is normally 13.8+ volts. The new fault started this morning, the engine turns over very slowly then fires up. I checked the leads to the solenoid and there are all tight. I rechecked the starter and it turned over the engine perfectly 5 times. Then on the sixth try the fault immediately returned and the motor 'laboured' to turn the engine over. I can't get access to the electrolyte as the battery is sealed - so I can't check the SG to determine the batteries true health. Is it easiest to remove and take to an auto electrician to check over the solenoid and motor. Does the starter motor relay suffer any problems - or it usually either comnpletely okay or not? Thanks.

Elliot Smith

Paul will be able to help pin this down for you but in the meantime a few thoughts -

starter relay seems to often be the problem but I'd have thought the problem would be more frequent if it was that but it's always worth checking and cleaning the connects then put in a very small dab of electrical contact lube to help prevent moisture or crud in future

you could take the battery in for a load test

also check all battery posts, clamps, main leads and earths are clean, secure and protected
Nigel Atkins

Are you measuring the voltage directly on the battery posts? Or on the connectors? Only the battery posts will give a true indication of battery condition, anywhere else includes the effects of connections. A 72 will originally have had twin 6v batteries, if that's still the case then you need to measure each one separately and add the results together, as doing both together includes the link cable and its connections.

Less than 10v indicates a partially discharged battery, or one that has lost cranking ability. However it will also drop if the load on the starter has suddenly become greater for any reason, such that it is turning more slowly than usual.

Unfortunately, anything that supplies less current and voltage to the starter will also cause it to crank more slowly, so you have to decide whether it is chicken or egg. The difference is that if there are bad connections anywhere in the cranking circuit you will tend to get higher than normal voltage at the battery posts, but lower at the starter.

So after doing the battery voltage tests, measure the voltage between the battery cable stud and the starter body as well. The difference between the battery voltage and the starter voltage, is how much you are losing in all the connections. It should be possible to get it to one volt in total or less. If you are getting more than that then you need to start breaking it down and testing individual sections of the circuit to see if one is causing it, in which case it is probably worth addressing it.
Paul Hunt

What do you mean "I rechecked the starter ?". What year is the car?
Do you mean "Relay" or solenoid? I assume pre-engaged starter. The relay only activates the solenoid which a) engages the pinion with the starter ring and closes the contacts to switch on the motor. If the problem is intermittent it sounds like these contacts are the problem and your motor is not getting it's fair whack of current It is possible to strip the motor and solenoid and clean things up.
Allan Reeling

Paul, firstly, the 6 volt batteries have been replaced by 1 x 12 volt. The voltage was measured on the battery posts. I'll take the battery off charge and do the voltage test at the battery posts and then at the bolt that holds the battery cable onto the solenoid, this afternoon. I'll let you know the results.
Nigel - I removed all 4 x spade connectors from the starter relay on the inside of the offside wing - and they are all clean - re-connected same with a smear of vaseline and no difference. I've removed the earth lead and am cleaning up the lead terminals and where it bolts onto the bodywork - it all looked pretty good - but task this will alleviate the possibility of a poor earth connection. The battery posts, clamps and leads are all on very good condition - I replaced with new 6 months ago.
Alan - when I said I rechecked the starter - I should have said, after checking the battery lead to the solenoid wwas sound and secure, I re-checked the starters' condition by turnning the engine over, it worked perfectly 5 times and started the slow turning over fault on the sixth try. The thread title gives the year 1972. When I said relay - I was referring the starter motor relay on the inner wing - when I say solenoid I mean the one attached to the starter motor.

Thanks to everyone - I'll get back with the volatge test results later today.

Elliot Smith


With the charger disconnected - the voltage at the battery posts was 13.05 vdc - measuring the voltage at the starter motor solenoid termninal I got 13.03 vdc. Tried again after 10 minutes and got a similar drop of 0.02 vdc. The earth lead terminals and bodywork bolt hole were thoroughtly cleaned and reconnected for this test.

Elliot Smith


To help try to try to eliminate the battery being the problem,I connected another 12 volt battery in parallel to the car battery and exactly the same occured. This time 8 perfect fast turns and quick starts with the starter motor, then 2 very slow attempts. This also dragged the voltage down to approx 10v (with both batts connected) It's although there is something amiss that has angular rotation - and depending on where the starter comes to rest, decides whether the starter motor immediately turns fast and the engine starts properly, or labours. I don't know it this helps or not...

Elliot Smith

Engine/body earth lead?

Battery/body earth lead?
SR Smith 1

Elliot, Apologies, didn't read the banner!! But my comment remains to suspect the solenoid motor contacts.
Allan Reeling


I've given this fault a lot more thought. Firstly, I would have thought the solenoid is either going to work or not. The fact is delivers the current for the first 'x' number of attempts and then doesn't - a think it is okay - what do you tink.

Secondly, my comment about being possibly something to do with angular rotation. I think it is more likely to be that the copper coils have either a crack or area of high restistance. The last few days have been very warm, 25+ the starter motror spins fast for the first attempts (when the coils are cold and warming up) and then turns slowly - as they have heated up and their resistance has increased. Again what's your thoughts.

SR Smith - I've had both the battery earth lead and the engine earth lead off, cleaned and re-installed - I'm now totally happy with that side.

Elliot Smith

Any chance this is a mechanical issue rather than electrical? Starter shaft binding, for example.

Patrick Callan

In my Army time , a 5 second check for solenoid problems (or emergency start) was to get a hefty screwdriver and simply short the solenoid terminals out.

Takes a bit of care making sure you have a firm contact and you need to ignore the big sparks, but it will kick the starter over and tell you straight away if the solenoid is causing problems.
R owen

If you have two batteries in parallel and both are dropping to 10v when it starts cranking slowly, then either both are suddenly losing the capacity to turn the starter, which is highly unlikely, or more likely the starter is suddenly finding it difficult to turn the engine. If you can turn the engine manually without it becoming much harder to turn when the starter is having difficulty, then it must be the starter itself partially seizing. Maybe the pinion or flywheel teeth are worn and partially jamming.

Shorting the solenoid studs out only works with the earlier inertia starter and the remote solenoid in general, the attached solenoids on pre-engaged starters usually have the solenoid to motor connection insulated.

If it were a problem with battery, solenoid or engine earth strap connections, or solenoid contacts, the battery voltage would get higher with the slower cranking, not lower, as the bad connections mean less current is being taken from the battery.
Paul Hunt

I agree with your dianosis - I've already been onto MG spares and ordered a replacement starter motor, but unfortunately they are on back order, so another week or so before I can change it out. Thanks for your input.

Elliot Smith

This thread was discussed between 18/06/2014 and 24/06/2014

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