Would be grateful if anyone could confirm that my alternator has failed, please?
The car is a Series 2, 1977.
Switched the motor off and removed the keys and the ignition warning light came on. Re-inserted the keys and turned the switch to “ign” position and the light went out. Started the motor and the light stayed off.
Tested the voltage at the battery with the engine running and got a reading of 12.7V. Reading of 12.5V with the engine off.
With the engine off and no keys in the ignition switch I disconnected the small D+ wire from the back of the alternator and the light goes out.
Seems to me the diode pack is faulty so the choice is to buy a new one outright and see if I can get the original one repaired and keep it as a spare. Thinking that the ones that are available now are not such good quality?
Seems to me that the warning light is earthing through the D+ terminal on the back of the alternator but that’s something I need to test in the morning.
with a running engine everything below 13.6 V would make me nervous and everythin beyond 14.5 V as well.
Did you see the 12.7 V at idle? What happens above 1,500 RPM?
From what you write it looks like the alternator isn’t charging the battery. You might either try to overhaul the original alternator - especially if it’s an original Lucas part - or get yourself a replacement part.
I think you’ve got a permanent 12V (coming from the battery connection to the alternator, via a shorted diode) on D+. With ignition on there is 12V from the ignition switch to one side of the alternator light and 12 volts from the alternator to the other. The light had no net voltage across it and is off. With ignition off, the ignition side of the light sees ground through other devices connected to the ignition–like the coil. The faulty alternator still supplies 12V on its side. So the light lights, albeit with current flowing opposite from normal. When you start the engine, nothing changes; the light remains on because the alternator is not supplying 12V to D+ as it should. You might get by by replacing the diode board (not the main diodes). IMHO. Good luck!
A secondary test of diodes is to disconnect a battery clamp and measure current flow between the post and the cable. With all consumers ‘off’ you should read some 20 to 35 mAmp drawn by the clock and radio standby power…
A failed diode allows high current drain, possibly several amps…
The alternator produces AC, the diodes deliver ‘chopped’ DC - but lack of alternator output reflected by charging voltages with engine running may have various causes…
The part I don’t understand is that last night, without the keys in the car, the ignition light was on. Then when the car was started the light went out.
However, this morning I have done a few checks.
The light is off when the keys aren’t in the car as it should be and when the the key is in the “ign” position the light comes on as is normal.
Removed the small wire from the D+ terminal and connected it to 12V and the warning light went out which, to me, means the warning circuit is working ok.
Ran the car and checked the output at the D+ terminal and got 0V. Checked the voltasge accross the battery terminals with all the headlights and heater fans on and got 11.5V. Got the same 11.5V with the engine switched off. So the alternator is faulty.
What confused me is the light staying on with no keys in the ignition. Can’t understand that part at all.
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That implies that the ignition provides a current passage through the lamp to the alternator with the key out, David. Or, more likely; current is flowing from battery through the alt and light to be grounded in the ign switch - typical diode fault…
The light will drain the battery - disconnect until rectified…
I removed the alternator and stripped it down and this is what I found.
One of the connections to the diode pack didn’t look very good. The brush to the centre of the slip ring was worn down to about 1/4" and had a circular impression in the centre of it.
I read the workshop manual on how to test the windings but I don’t have the equipment or indeed, the knowledge, to do the tests so I’ll take a chance on them being good.
What I am going to do is buy a repair kit and see if it fixes it. They are not expensive and include the bearings but I’m not sure if I will change them as the original ones seem fine.