WIMPUS Posted October 26, 2013 Share Posted October 26, 2013 Hi, On my friend his MG :X the ignition light is always burning .. :-/Last night we where driving to a friend and when he stops at the lights, then the ignition light burns not so bright .. When i drives of again it burns brighter .. & if he get the lights and radio on then it's burning fully .. If he's driving and he doesn't give throttle then the lights goes dimmer also .. Is this the alternator that is failing ? Quote Link to comment Share on other sites More sharing options...
Rallyspit Posted October 26, 2013 Share Posted October 26, 2013 Hi Wim,The igition light is fed on one side from the 12v live ignition circuit and the other side "excites" the alternator to get it charging. Once the alternator has started charging, there's 12 volts on both sides of the light, and it goes out. It does sound like the alternator is failing. Is his battery constantly going flat? Alternatively has someone been playing with the wiring to the ignition light, perhaps connecting it to earth? Quote Link to comment Share on other sites More sharing options...
WIMPUS Posted October 26, 2013 Author Share Posted October 26, 2013 2154 wrote:Hi Wim,The igition light is fed on one side from the 12v live ignition circuit and the other side "excites" the alternator to get it charging. Once the alternator has started charging, there's 12 volts on both sides of the light, and it goes out. It does sound like the alternator is failing. Is his battery constantly going flat? Alternatively has someone been playing with the wiring to the ignition light, perhaps connecting it to earth?Well i measured it once and when it was running i got 13,40volts on the battery.. (don't know how much it is with lights & radio on .. ) He can always (re)start it .. even after is sitting for a week.. A good week or 2 ago i was driving behind him after he's car came from mot .. and suddenly radio, lights & .. ignition where not working .. 3 sec later working again .. And i could hear a grinding noise then .. but i know now what it was ! The ignition lock is nor working good .. you can start the car and then the key sticks on, so the starter is still spinning it ! :-/Maybe that caused to fail everything at the time, because is was so pulling power from the battery.. But next week i'm going to get the prop shaft of his car and then i will look again what i measure at the battery (with lights on & co ).Somebody was thinking to put another battery in it for test, if it then goes out, then it maybe is the battery :-/But i still are thinking that it's going to be the alternator .. Quote Link to comment Share on other sites More sharing options...
Rallyspit Posted October 26, 2013 Share Posted October 26, 2013 Hi Wim, Definitely not the battery. You're almost certainly right about the alternator. It's not normally worth replacing the diodes, etc., a new/exchange one is cheap enough. If he uses a large amount of current, for example big stereo, or extra lights, it may be worth considering an upgrade to a higher power one. I seem to remember that the standard MGB alternator is only the same as our Triumphs, at 45 Amp output.Just a thought - you have checked that the fan belt is not too slack and slipping under load? Quote Link to comment Share on other sites More sharing options...
WIMPUS Posted October 26, 2013 Author Share Posted October 26, 2013 2154 wrote:Hi Wim, Definitely not the battery. You're almost certainly right about the alternator. It's not normally worth replacing the diodes, etc., a new/exchange one is cheap enough. If he uses a large amount of current, for example big stereo, or extra lights, it may be worth considering an upgrade to a higher power one. I seem to remember that the standard MGB alternator is only the same as our Triumphs, at 45 Amp output.Just a thought - you have checked that the fan belt is not too slack and slipping under load?fan belt is new from yesterday, done up tight so ... :-/It can only be the alternator i guess .. Quote Link to comment Share on other sites More sharing options...
junkuser Posted October 26, 2013 Share Posted October 26, 2013 Either measure the voltage across the lamp with the engine running at various speeds to detect which side is higher or measure the voltage to frame at each side of the light at various engine speeds.From your description it could well be that there is resistance in the ignition side of the circuit causing the voltage to be lower than that of the alternator side.Worth doing before expending money. Quote Link to comment Share on other sites More sharing options...
Dale Posted October 27, 2013 Share Posted October 27, 2013 3 Possibilities:1) Junk user is right2) Alternator - just buy one3) A build up of electromagnetic flux has overloaded the Flux-Capacitor ;)4?) See 1) Quote Link to comment Share on other sites More sharing options...
WIMPUS Posted October 31, 2013 Author Share Posted October 31, 2013 Well today got the battery out and took it home to charge it good .. After 6 hours the charger still didn't say it was full :-/So took the multimeter out .. measured 13,33volts .. then it was dropping and dropping to 12,30 volts .. after a while it was around 12,81 again ..Normal or not ? :-/ Don't know how old the battery is .. Quote Link to comment Share on other sites More sharing options...
Nick Jones Posted October 31, 2013 Share Posted October 31, 2013 12.6 - 12.8v is fairly typical for a charged battery. It should be left for 15 minutes or so after taking the charge off before measuring or you'll see the effect you describe.If the battery also turns the engine over nice and fast on the starter I would say it is ok.Nick Quote Link to comment Share on other sites More sharing options...
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