MGB-GT 1977 MG BGT electrical

[tufsis]

Hi all, hope someone out there can help me. I have just got an MG, with a few problems, but this one is baffling me. I have fitted a new battery, and when I turn on the ignition, the warning light is very very dim, but gets slightly brighter the longer it stays on. If I then start the car, it really missfires. It has an alternator and is neg earth. Any help appreciated.
Thanks
Jon

 

[philman]

any more clues? try putting a meter on the battery while the engine is running and checking the voltage. then turn on all the accessories and check the voltage.

 

[gjoakes]

when I turn on the ignition, the warning light is very very dim, but gets slightly brighter the longer it stays on.

Jon,

Based on this my first guess would be dirty or loose battery terminals. If they check out then start working your way forward. Checking voltages with a meter at various points would be helpful if you can compare the no-load and under-load readings.

Good luck ...

Greg Oakes

 

[Kimberly]

Wiring diagrams are available at https://www.advanceautowire.com click on stock schematics

Make yourself a couple of enlarged copies, one for reference, and one to make notes on.
Start at the battery, verifying voltage output and making sure all the battery cable connections (four of them if you have one battery) are clean and secure. Check all the connections that are accessible under the bonnet, making sure they are clean and secure. Note which connectors you have inspected and cleaned on your diagram.

Check the belt tension
Measure the output of the alternator at idle and at 2000rpms. Verify you aren't having charging problems.
Provide the model year of your car. The ignition light will stay on if the alternator is not charging. The ignition light being dim and then getting brighter can be indicative of high resistant (corroded) connections. Clean or replace connectors and use dielectric grease to prevent future corrosion.

 

[tufsis]

Thank you all very much, I will be having a look tomorrow, but the info given is already very helpful
Thanks again
Jon

 

[tufsis]

Hi everyone,
Managed to get time to look at the MG over the weekend, and have sorted out a few other electrical problems, but this one is still hanging on. When the ignition is turned on, the charge light is very dim, but when it is started, the charge light becomes much brighter, and does not go out, but the engine runs fine. I feel as if it could be the alternator at fault, but how do I test this????

When I put a meter across the battery, it shows over 12v, and when the engine is running, it almost seems to count down slightly. Any feedback greatly appreciated.

Thanks

Jon

 

[RonMacPherson]

Please explain count down slightly.

Battery, fully charged voltage should be 12.6 volts, anything over 12.2 is acceptable.


when your engine is running with no accessories, lights, blowers, radio off. at 1500+ rpm you should be running 13.5 possibly up to 14.7 volts. If at idle the charging voltage drops below 12 volts, you either have an enormous draw(in which case your battery would be completely dead as it is being drained) or your charging system has failed.(i.e., alternator)