Help with ignition light and alternator please

[Bumpa]

As my rebuild of the 1971 roadster progresses I have reached the point of connecting the batteries and trying circuits out. One thing I immediately noticed is that the ignition light doesn't come on when I set the key to the running position, just prior to operating the starter motor.

I have used the multimeter to check the white wire on the light's bulb holder and it has 12.4 volts power. Then I unplugged the alternator and checked the brown/yellow wire in the plug and the power reached there too, so is obviously passing through the bulb OK. But the terminal on the alternator where the brown/yellow plugs in is not connecting to earth (ground).

My understanding is that until the engine starts that terminal should be earthed through the field windings. Does this mean the alternator is faulty? If so, can I fix it, or would a replacement be the best option?

[Vic Butler]

With the engine running is the alternator charging? It should be about 14v across the battery terminals at about 2,500 rpm. The ignition light goes out when the voltage either side of the bulb is equal. Daft question maybe but is the ignition light bulb ok and you haven't fitted an LED one?

[Bumpa]

Thanks for the suggestions Vic - they aren't daft at all. However, the engine isn't yet fully assembled (no carbs, or radiator yet) so I can't run it. I am trying to overcome electrical gremlins first. The warning light bulb is new (not LED) as is the bulb holder, and I am using a new Autosparks harness. Having done some more research I will try disconnecting the alternator plug and earthing the brown/yellow wire. The ignition light should light with the key at the "ON" position.

I suspect that the alternator is the original unit from 1971, so rather than try to repair it I think I will go for an uprated 18ACR type. They aren't too expensive in comparison with what I have spent already on the car!!!

[Vic Butler]

Is the engine earthed to the body?

[Bumpa]

Yep, engine earth strap in place. As I suspected, when I earth the brown/yellow the ignition light comes on, so the fault is definitely in the alternator.

New ones aren't too expensive, so I'll go down that route.

[waltmgb]

Curiosity question from the response. What difference would an LED bulb make?

[Vic Butler]

An LED bulb in the ignition warning light doesn't draw enough current to "excite" the alternator into charging unless a resistor is included across the terminals. My Series 3 Land Rover has LED bulbs except the ignition light for that reason.

[Bumpa]

I agree Vic, but isn't it also to do with LED bulbs being polarity specific? With the engine off the bulb is lit from the battery, earthing through the alternator. Once the engine starts and the alternator starts charging, the polarity reverses because the alternator produces more volts than the battery does. An LED can't cope with the polarity reversing. Please correct me if I'm talking out the back of my head !!

[Charles Farran]

With the engine at rest with the ignition switch in the on position there should be a potential difference each side of the ignition warning light as the + (assuming a negative earthed car) comes via the ignition switch wire with the circuit being earthed through the alternator at rest. The alternator is excited into operation by the load of the bulb (non led) once the engine is fired up. The ignition bulb goes out once the potential difference across the bulb terminal reduces to nil. The bulb will come on when the power generated by the alternator is insufficient to meet the electrical demands - e.g. a slipping belt , too many electrical accessories on with possibly unreliable wiring / corrosion causing excessive resistance etc.& a PD is created.
Many with traditional indicator set ups & original flasher units will have noticed the rate of flashing slowing down when they have headlights on , heater blower on , windscreen wipers on etc. when slowing down & waiting to make a turn at a junction . (The original flasher units work on a thermal break (that is sensitive to voltage drop) - all cured by going to a modern style flasher whether it be one for incandescent bulbs or one for LEDs.
The PD can also be created if the battery is defective & won't either retain or recover to normal operating voltage.
Cheers,
Charles

[GeoffK]

Hi I had a similar problem and if you look down the topics you will find one dated 2nd Feb 2023 with 49 responses!
The upshot of it all was that the circuit was not earthing through the alternator. New Lucas alternator approximately £35 fixed the problem straight away plus no more flat batteries. Cheers Geoff