Help!! Strange alternator problem!

[matthew6974]

Hi all
I own a Marlin Berlinetta Kit-car (based on a Ford Cortina) which I'm nursing back to health. I'm a bit stumped however by a strange alternator problem so I'd be grateful of any suggestions. Unfortunately I'm not an expert :-)

I noticed two problems initially - that the alternator was not providing any charging current (voltage across battery terminals 12.6v) and that the ignition light fails to come on at ignition.

The alternator is the standard one for Ford Cortina/Sierra SOHC 2.0 Pinto engines. It has three connector pins going to a. the positive battery terminal, b. the ignition switch, c. the ignition light.

I have tested continuity for the ignition light connection (c) and tested the bulb. It's fine, there's no voltage being supplied at ignition as there should be.

I have tested the voltage for connections a&b and it's over 12v.

I get no voltage increase to 14v however when the engine is revving as I would expect.

I'm confused. My understanding is that the 'ignition light' really is representing a charge warning light - so when the ignition is on but the engine not running (ie battery producing more than the alternator) then the light should be lit - but with the engine running and the alternator producing power the light should go out. A lit bulb when the engine is running should indicate that the battery is producing more than the alternator and therefore there is a problem. Is my understanding correct?

It's a new alternator but I had the same problems with the old one. Any suggestions?? :-)

Thanks

Matt

Edited by matthew6974 on Monday 25th October 13:43

[thescamper]

not 100% sure on the wiring for the alternator, but I do know if the ignition light does not come on when you switch the ignition on the alternator will not charge.

i believe that the light goes out when the voltage on bot sides of the bulb equalises as the alternator starts to charge. I would suggest that you look towards the wiring that lights the ignition/charge lamp as a starter for 10.

[fake7]

"I have tested continuity for the ignition light connection (c) and tested the bulb. It's fine, there's no voltage being supplied at ignition as there should be."

"I have tested the voltage for connections a&b and it's over 12v."

i'm no expert either but i think you should be able to measure 12v at all 3 pins on the alternator connector (with it disconnected) and the ignition switch at the on position. If you are not getting 12v at pin c, but the wiring continuity and bulb are ok, then maybe the connection from the ignition switch to the bulb is broken?

As above, the alternator needs the current through the bulb to "excite" the alternator into action (whatever that means!). Then the bulb goes out when the alternator is working, because you will have 12v on both sides of the bulb.

Also, have you checked the earth from the battery to the chassis and the engine? The alternator is earthed through the bracket that is attached to the engine.

[matthew6974]

Hi
Thanks for your posts. There is certainly a problem in the bulb circuit somewhere and I agree this acts as the 'exciter' for the alternator electromagnet etc. So if the bulb circuit doesn't come on at ignition then the alternator can't work.

I've checked the earth for the alternator - no probs there.
I've checked the contiutity for the bulb wire, no probs and the bulb works ok.

Part of the problem for me is that I just don't understand how the ignition key is wired in to the alternator - ie how the alternator knows that power to the ignition is being supplied. The wiring diagram (Cortina 79!) isn't helping me much! I'll attach it in case it makes sense to some bright spark out there :-)

Cheers

[fake7]

my kit is '86 Sierra - so similar but probably not the same!

on mine the 12v from the battery comes into the ignition switch on pin30 and out on pin15 with key in on position. Then through a fuse and then to one side of the bulb. Other side of bulb then goes to the alternator. It's the same circuit as the oil pressure switch and the hand brake switch, so do these work on yours?

sorry i can't read your diagram on my rubbish laptop

[thescamper]

how about a temporary re wire direct from a known 12 v supply to the bulb just to ensure that everything else actually works.

Then all it is is a simple case of identifying where the supply comes off of the ign switch, it must be from an ign controlled fuse in the fuse box so shouldn't be that difficult to identify.

If you cant identify it then rewire the bulb supply to a known ign controlled fuse.