7 replies to this topic

[sdjones121]

Driving home late last night, about 6 miles from home, my charging/battery light came on at full brightness. Revs didn't impact the light, and it remained on for the rest of the drive home. This afternoon, I've ran a few tests and am leaning toward a dead alternator but I do have one question mark hanging over that diagnosis. According to KernowCooper's "Alternator & Charging Wiring-Basic Tests" post (http://www.theminifo...test alternator), everything goes along with his steps until step 7.

 

At step 7, he states that with the ignition in the ON position, the third (small) wire on the back of the alternator should read 12+v. I get a around 1.4v, and that is what is confusing me. Should I indeed be getting 12v or more on that connection with the engine off but ignition set to ON?

 

With the engine at 2k RPM I'm only getting 12.4v at the battery (12.56v with the engine off), which is obviously what is pushing me toward having a bad alternator. The battery is about a month old, FYI. On the back of the alternator, with the engine running, I get the same 12.4v on both of the large connectors.

 

So, do I have some sort of wiring issue somewhere, or is the 1.4v reading on the small connector not important to the issue. For the record, the car is an '87 Advantage with a pre-84 Metro 1275. As always, any help offered is greatly appreciated!

 

Seth

 

[KernowCooper]

The bulb is bright because there is 12v from the cars wiring to the alternator, if the alternator was charging then the alternator would provide 12v and raising to 13.7-14-2v back up the wire, once the voltage in the alternator goes above the battery voltage the 2 equalisations of voltage puts the ignition light out. (In fact there is a voltage flow from the alternator fro the light but the voltage is approx 1v above battery voltage, that is until very quickly the battery voltage raises and they both cancel each other and the light remains out.

 

So to answer your questions there should be above battery voltage on the small trigger wire from the alternator with the engine running at 2000rpm and with the engine off there will be as you describe almost no voltage as the ignition light then finds a path to earth to bring the ignition light on, all your seeing is voltage from after the resistance of the bulb to earth

 

at 12.4v the battery is not being charged, double check the connections on the alternator for corrosion, but I strongly suspect from what you have said your alternator is at fault.

[sdjones121]

Thanks for the quick reply! As I said, I was leaning heavily toward it being the alternator itself, I just wanted to do a reality check before I pulled it. But, based on your reply, I pulled it out and popped down to Autozone (US auto parts store) to have them test it. As expected, it failed completely. Looks like the Mini is stuck in the garage for a bit.

 

Thanks again for your response, and thanks for the great testing write-up too!

[Captain Mainwaring]

The bulb is bright because there is 12v from the cars wiring to the alternator, if the alternator was charging then the alternator would provide 12v and raising to 13.7-14-2v back up the wire, once the voltage in the alternator goes above the battery voltage the 2 equalisations of voltage puts the ignition light out. (In fact there is a voltage flow from the alternator fro the light but the voltage is approx 1v above battery voltage, that is until very quickly the battery voltage raises and they both cancel each other and the light remains out.

 

So to answer your questions there should be above battery voltage on the small trigger wire from the alternator with the engine running at 2000rpm and with the engine off there will be as you describe almost no voltage as the ignition light then finds a path to earth to bring the ignition light on, all your seeing is voltage from after the resistance of the bulb to earth

 

at 12.4v the battery is not being charged, double check the connections on the alternator for corrosion, but I strongly suspect from what you have said your alternator is at fault.

 

Bit more than that too - If the bulb blows or there is no return on that cable for any other reason, then you'll lose excitation voltage for the alternator and it won't charge.

 

There has to be a +ve supply on that wire - if you lose it the alternator won't charge (in saying that, I suspect that the alternator has gone pair shaped too, but suggest you make a temporary supply using a test lamp from a battery +ve source through the lamp to the "ind" terminal. Start the car as normal (take care of course) and see if the light goes out and the alternator charges.

 

Edit - see I was slow in typing and you've tested it.....do however make sure you've got a supply on the "ind" wire.

Edited by Captain Mainwaring, 11 June 2013 - 02:42 AM.

[sdjones121]

Mainwaring, it seems like you're spending half your life helping me with my issues. :) As luck would have it, the alternator died yesterday and my new radiator showed up today. GRRRRRRR. Oh well.

Edited by sdjones121, 11 June 2013 - 02:50 AM.

[Captain Mainwaring]

Hahaha...stand at ease...it's the time difference you know.....   Get that rad in, I'm desperate to see how it goes.

Edited by Captain Mainwaring, 11 June 2013 - 09:55 AM.

[sdjones121]

My plan was to get the rad in tonight, but this has sort of put a damper on that. The money we'd budgeted for car parts for the month went towards the radiator so now I need to wait to buy the alternator. So, for now, the radiator is just going to sit in it's box until I can do it all at once. The military is sending me away for a bit soon too, so it may will be September before I can actually get the Mini back on the road. I'd been planning on meeting up with the local British car owner's club in two days time, but that's now a no go.

[Captain Mainwaring]

I'm in the same boat, but it's the judge sending me away for a bit, assuming he doesn't accept the bribe of course.

 

Really suggest you just get the rebuild bits for the alternator (depending on what it is of course). Unless it's cooked windings (which is rare) then the bits won't cost much and the job is enjoyable, loads of guides on youtube.