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MG MGB Technical - Electrical problem

bought a 68 roadster 3 weeks ago , the first weekend all was OK.Went to start it following week, turned the key and nothing !( sometimes clicking can be heard). Checked both 6v batteries at leads and read 6.2 & 6.3v respectivley and 12.5 across both. With engine running (jump start fine) voltage increases to 14.4v. After taking the car for a reasonable run 45-60 mins and turning ignition off she is still dead. Fan belt is tensioned ok and does not appear to slip at higher revs. The ignition light does not come on when key turned and on last run this afternoon asI had engine ticking over I went to switch on the lights and she cut out ??
The car is fitted with a Lucas alternator and I believe is negative earth.


I'd check the wiring from the solenoid to the starter. Sounds like the low ampage side is OK as you can hear the clicking of the solenoid but the heavy ampage power is not making it to the starter. That would be the easy bit first to see if the starter is getting power. After that I'd suspect the starter.
Roy Soper

If you have an alternator, it's definitely negative earth.

Reading 12.5 volt across the batteries with a meter doesn't mean a thing, as there is no load. What's the voltage when you are trying to start? It sounds to me that you have some poor connections somewhere along the line, especially as it starts with jump leads. Incidentally, where did you connect the jump leads on the MG? Directly onto the battery terminals?

What sort of battery terminals have you got? If they are the fully enclosed sort with a screw through the top, you need to change them for the open ring type clamped up tight with a bolt.

You need to consider that when making a cold start, the starter is drawing in excess of 300 amps from the batteries - pretty close to a dead short. If there is a less than perfect connection anywhere, the current simply won't pass and nothing works. By the way, 14.4 volts from the alternator is bang-on dead right.

Come back with more information.

Mike Howlett

Thanks for responses,

I will check voltage when trying to start, what reading should I see ?

When jump starting I connected +ve lead to +ve terminal of driver side battery and -ve to -ve terminal of passenger side battery.

The terminals are a combination, which could indicate historical problems ??

I've read that changing to a single 12v battery is worthwhile, is this correct ??

When cranking a cold engine the voltage might go as low as 8 volts. There is nothing wrong with the twin batteries so long as the connections are all sound. I really feel that this is most likely to be your problem. When you jump started the car it started, which shows that from the battery leads forward, everything was working. Therefore the terminals on the batteries must be the most suspect items. By the way, you connected the jump leads up correctly.

Mike Howlett

Nigel - the clicking is a classic low power signal. As Mike and Roy have pointed out - the battery is the likely cause but before you pull them run a single jump lead between the big nut securing the chassis rail to the front cross member and the other end to an unpainted cylinder head nut on the engine - 30% chance she will start. If that fails - clean up the earth strap connection to the car in the battery box. Failing that new batteries.

The earthing strap battery to chassis and chassis to the gearbox give problems and you would be well advised to have a second somewhere you can keep an eye on it - in the engine bay pergaps.

Single 12V batteries = more quality options/value for money. (get one guaranteed for the duration of your ownership).



If there was no warning light (did it come on when the jump batteries were connected?) then either it is faulty anyway (which can cause charging problems) either the batteries are very bad or one of the battery connectors is completely open-circuit or very nearly so.

If it started with the jump leads on the connectors at the batteries that only leaves the batteries themselves and the connections between the connectors and the battery posts. Put the voltmeter probes directly on the battery posts, not the connectors, each battery in turn and see what voltage you get when someone tries to crank. If the voltage drops to zero it is the battery. If not then it is the connections. If one battery is dud replace both.

If it is the connections and you have the cup-shaped connectors then I'd recommend changing them to the later clamp-type, they are much more reliable.
Paul Hunt

This thread was discussed between 26/10/2004 and 27/10/2004

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