Ignition light

[Tony Lilly]

Hi All - newbie tucked away in Cornwall.
1979 GT - Few weeks back battery was dead. Not sure why so charged it and all was well. Week later it was dead again? Charged and all good until few days back and dead again. Noticed the ignition light does not come on - ever - although can't be certain whether it was coming on before the problems started. So I have:
- When fully charged I get between 6 and 10 starts then it dies.
- Fully charged and everything works but as the battery dwindles the indicator flashers slow down or don't flash at all.
- Recon alternator was fitted about 8 months ago.
- In the engine room I've noticed (near the coil) a green 'male' connector going nowhere and a green/brown male and female not connected ! I haven't connected them.
- Car does max 50 miles a week but no run over 10 miles. Battery doesn't seem to deteriorate between use (thinking of a trickle drain somewhere) I've cleaned up the battery connections and earth connection by battery.
My thoughts/questions:
a. Duff battery? (although it has an indicator on the top which shows green (good) but it shows green when its flat as well)
b. Duff Alternator?
c. If the bulb has gone does that affect the charging circuit? (read that somewhere)
d. I doubt the short runs help.

Before I hand over to my garage any more tips would be helpful. Thanks

[Peter Cresswell]

Hi Tony,
This sort of problem can be quite difficult to get to the bottom of.
I see you have already found your way to another thread:
http://mgbforum.mgb-register.org/viewto ... ?f=4&t=372
Where you will have found info about batteries, but the fitting of a single 12v battery in one of the trays doesn't (I think) apply to your car as it will have a single 12v battery.
Have you checked the fan belt is tight? If not do so. The standard alternator is an 18ACR which outputs 45amps, equivalent to over 1BHP so the skinny belt that was fitted to MGBs has to transmit a lot of torque to drive the alternator.
I feel you have something draining the battery (interior light perhaps, or radio just turned down and not switched off completely), so it is a case of looking at those things. Try to do this at night to see if a any light is coming out cracks around the car. Modern batteries do fall off a cliff when they fail so it might be the battery.
The indicators flashing slowly is because the battery voltage is dropping too low.
Looking at the wiring diagram for the late MGB it seems to me that a green wire is the switched (by the ignition switch) power feed via the fuse box. Does the heater fan work? This is fed by a Green/Brown wire. it doesn't help though that the late MGB also had wires coloured Light Green/Brown!
I hope these thought help you (at least) a little.
If all else fails you might have to get the services of an auto electrician.

[Vic Butler]

Evening Tony.
If the ignition light does not come on when the ignition is switched on then the alternator cannot charge as the voltage through the bulb "excites" the alternator into charging.
Easy to check whether the bulb is blown but if it's ok then check the small wire on the alternator hasn't come off. This small wire goes to the bulb.
. Vic Butler

[Tony Lilly]

Thank you. I'll check fan belt.
I did try and get to the bulb holder from behind but couldn't. Can I just ease it out the front?

Heater fan? agh! It seems to blow fine on drivers side but not passengers side. Should I connect the male/female green/brown?

This clump of wires is just to left of coil and I mentioned the lonely green wire but I did notice that some of the double connectors (black rubber thingy's) had 2 wires one end and only one the other but that's probably normal for a lot of cars.

The alternator was actually new not recon and by chance I bumped into a chap in an early 70s Roadster in my village and mentioned the snags. He said to check the wires on back of alternator as some garages forget to connect the required wire for the lamp? My alternator has 3 wires attached which are brown / yellow and blue. Should there be more?

Thanks again - I'll report back idc. Luckily my garage man knows about MGB's and I also have a local guy who restores them as a business so if all else fails ....

[Vic Butler]

Morning Tony.
Brown and yellow wires on the alternator are the main output ones and the blue one is the one that goes to the ignition light. 3 wires are correct.
. Vic Butler

[Vic Butler]

Morning again Tony.
According to the wiring diagram the 2 thick wires from the alternator are brown and the thin wire going to the ignition light is brown and yellow. If that one is blue then it would appear that the wiring has been messed about with.
However check the ignition light bulb first.
. Vic Butler

[Tony Lilly]

Vic Butler wrote:
> Morning again Tony.
> According to the wiring diagram the 2 thick wires from the alternator are
> brown and the thin wire going to the ignition light is brown and yellow. If
> that one is blue then it would appear that the wiring has been messed about
> with.
> However check the ignition light bulb first.
> . Vic Butler
Thankyou. Sorry my colour coding was the plugs not the wires. The Alt has 2 x brown and 1 x brown/yellow. All seem secure and fan belt is tight.
What a job with the bulb!! Took the cover off - easy. The bulb holder (+bulb) had dropped back out of the hole and after much fiddling with home made wire hooks got the bulb to emerge from the front. Taped it protruding out.
On the way to work the bulb (still not glowing with ignition on) started to glow dimly at about 2500 revs.
I think I had missed this before because the bulb had slipped back out of the holder. Winged past Tim Kelly MG and he said the Alt seemed correctly wired. I think I may try and drag the wires and holder out from the back to check connections, check all other wires etc and then if no joy off to my garage.
One thing the car has got is a basic theft device which is a switch on passenger well bulkhead which when switched just stops the car running. How does that work?
Thanks again

[Vic Butler]

Evening Tony
It's likely the anti theft switch is wired to the low tension side of the ignition. It will be wired to one of the two wires on the coil.
The way the bulb is behaving is symptomatic of an alternator fault. I'll look it up and contact you later with what the fault in is likely to be.
. Vic

[Vic Butler]

Evening again Tony.
It appears that the alternator has a short circuit in the live side main output diode. As the alternator is only 8 months old it should be still under warranty so best to contact the supplier. (I keep a spare alternator for both my 77 BGT and Land Rover, they use the same alternator type.)
It would be a good idea to check the alternator plug contacts for any corrosion. I use electrical switch cleaner on dirty contacts. It works very well and is obtainable from electrical outlets.
Hope this helps. Vic

[Tony Lilly]

Thanks Vic.
I'm going to ''clean up'' as you suggested and then go back to the garage who fitted the alternator. I've used them for many years and really good so don't envisage any snags with replacement. I guess there is little point dragging the wires and holder out because if its glowing then the wires must be connected! It's not my daily runner so no panic - I'll post the result once its complete. Thanks again.