Alternator not charging/ignition light not on

[GeoffK]

Charles. Does this test confirm the integrity of the alternator to warning light connection and the warning light to ignition switch or just alternator to light.

[Vic Butler]

It appears the wiring is in a mess. Check whether the alternator is charging first.

[Charles Farran]

Geoff,

With the ignition switch in the "on position" , not cranking, + power should then flow from the terminal on the back of the ignition switch to the "IND" socket within the plug that attaches to the alternator - by connecting a temporary crocodile clip or probe to the plug, with the other end to the block or chassis as the "-" /earth return you should then create a circuit and the bulb should come on. If it doesn't then you must have a fault somewhere in the wiring between the switch / bulb /alternator plug. (assuming the correct terminal on the back of the switch is used - easily checked if required).

If the bulb lights up , then you have confirmed the switch to alternator plug is OK , which then indicates that it is a problem within the alternator itself.

Take care not to connect or touch the other contact sockets within the alternator plug with your temporary crocodile clip lead as they are permanently "+ " live & you could create a short (if the other end of the crocodile lead is connected / touching the block /chassis.

If the bulb doesn't light up with the plug off the alternator, you can check whether the terminal on the back of the switch goes live when you have the switch in the "on" position by either a using a test meter with one probe on that terminal & the other probe to a suitable earthing point with the meter set to voltage to see if you have a 12V PD. (Alternatively use a bulb test probe that will perform the same function).

If the terminal does go live with the switch in the "on" position then the problem is then either between the back of the ignition switch & the back of the bulb socket/ (one of terminals) or between the other bulb socket terminal & the alternator plug.


Hope that helps.

Cheers

Charles

[GeoffK]

Progress so far. New alternator ready for collection tomorrow. Good price from Halfords. Also bought my own multimeter.

‘Static’ battery voltage with battery fully charged 12.67 volts
‘Dynamic battery voltage engine running at 2000 rpm 12.43 volts
This seems to confirm that the alternator is not charging as there is a voltage drop and nowhere near the 13.5 to 14.0 volts suggested.
Voltage at brown/yellow terminal varying wildly between 0.8 and 6 volts also suggesting faulty alternator.
There is continuity between brown/yellow terminal on alternator and ignition warning light and the bulb is still okay but not enough volts from alternator to light the bulb.

Will fit new alternator, recheck all the above readings before looking at the ignition switch. Switch still works but no warning light. Have a clear idea about all the ignition switch connections except for position 3 which holds the 3 white cables, one of which should illuminate the ignition light.

[Charles Farran]

Geoff,
Not sure if you have seen / studied a wiring diagram so have attached a copy of a 1973 -4 version by Haynes which is freely available as part of the auto- wire set. (I am aware that there were many changes to the wiring looms over the years & year of manufacture is not necessarily the same as year marketed / car registered but i think the attached should be helpful with regards to the area covering the ignition switch & ignition warning light).
Cheers,
Charles

MGB 1973 to 1974 UK Haynes version.pdf

(346.61 KiB) Downloaded 466 times

[Bumpa]

12.43 volts at 2000 rpm means a duff alternator for sure. See how useful a cheap multimeter is? I can't understand how people run older cars without one.

[Charles Farran]

Geoff,

You appear to have checked the continuity between the bulb terminal & the brown /yellow terminal on the old alternator, but have you checked (without the ignition on ) whether there is continuity between the brown / yellow (Ind) terminal on the alternator & ground? If there isn't then the warning light cannot light up at all as the wiring circuit for the bulb is effectively broken & the alternator is faulty as i previously suggested.( I have assumed you have checked from the ignition switch to the bulb terminal is OK).
Check you have done all the tests in my post of 6.43 yesterday !

Cheers
Charles

[Vic Butler]

The ignition light is not lit by the alternator but by the act of turning on the ignition and the power comes from the battery. The current from the bulb excites the alternator into charging and that's why LED bulbs should not be used as ignition warning lights as insufficient current is produced unless a resistor is in the circuit. When the alternator starts generating and the voltage both sides of the bulb is 12v plus then the light goes out. The circuit to the alternator has to be complete for the bulb to light.
Be very careful not to run the new alternator open circuit or it's curtains for it.
Before testing it you should really investigate why the ignition light doesn't come on when the switch is turned on. I presume things like wipers and direction indicators work with the switch turned on.

[Vic Butler]

On the ignition switch there's a terminal with 3 white wires on it. The ignition warning light has to be one of them.
I have only a1971 wiring diagram in a Haynes manual and as far as I can see from it the wire to the starter relay is white and red and is on terminal 3.
The wiring colour codes won't have changed in 1973 and it's unlikely the ignition switch circuit won't have either.

[Charles Farran]

Vic & Geoff,

I posted up the 1973 wiring diagram as an attachment to my post of 4.59pm yesterday. If you click on the link ending ".pdf " you will see the wiring on the ignition switch. It also shows why, with the ignition switch in the "on" position (not cranking & before the engine is started & alternator not rotating ), the warning light should come on as the wiring circuit for the light comes from the ignition switch to the bulb & then to the "Ind" contact of the alternator where it then earths through the stationery alternator which is attached to the block / chassis. (If there is an internal fault to the alternator as i have previously referred to (& experienced myself , then that fault might manifest itself as breaking that contact to earth whilst not rotating/active).

Cheers
Charles