Alternator Light always on

Mike H.; From your wiring schematic, you have 12v going to B+ on the alternator from the battery. To have voltage coming from D+ on the alternator to D+ on the regulator, then to the amp light, you must have voltage crossing from B+ to D+ within the alternator (short/open circuit). With a voltmeter have you tested if voltage exists between B+ & D+ with the various conditions you described? You may also check for continuity between B+ & D+ with no voltage from battery. If continuity exists, then B+ & D+ are connected someway within the alternator.

I found that diagram in here somewhere... so I'm not 100% sure it's accurate, but it's the only thing I had to go on... I just got the alternator back at lunch today, stopped by the house and plugged it all in... saw the light on and knew the problem was still there.

I'll check for continuity between B+ & D+... that makes perfect sense. Of course we're assuming it's not supposed to have continuity... I profess my ignorance on that!!!

Looking at the diagram DF switches the regulator right? If that's the case then it appears when engaged D+ goes to D- (Ground) which illuminates the dash light.

It would also make sense for me to check if D+ is getting voltage when the key is off. If it is then that would likely be the problem.

I think...

Does the alternator start charging when D+ goes to ground?

MH

Is your alternator and regulator similar to the original? (before I can comment)

Alternator is Bosch 0120 400 643 / 55A

Regulator seems to be the original also

The more I look at the diagram the more occurs to me... if the alt light comes on when it's getting power from the ignition switch and grounded @ the voltage regulator then if it's on when the ignition is off the problem MIGHT be in the switch...

Ugh...

MH

sorry, I can't help.
my googling couldn't find a schematic of the internal wiring for a Bosch 4 wire

quote:

Originally posted by M!ke H.:
I found that diagram in here somewhere... so I'm not 100% sure it's accurate, but it's the only thing I had to go on... I just got the alternator back at lunch today, stopped by the house and plugged it all in... saw the light on and knew the problem was still there.

I'll check for continuity between B+ & D+... that makes perfect sense. Of course we're assuming it's not supposed to have continuity... I profess my ignorance on that!!!

Looking at the diagram DF switches the regulator right? If that's the case then it appears when engaged D+ goes to D- (Ground) which illuminates the dash light.

It would also make sense for me to check if D+ is getting voltage when the key is off. If it is then that would likely be the problem.

I think...

Does the alternator start charging when D+ goes to ground?

MH

You are on the right track. I would suspect your ignition switch is failed. Confirm the lamp is extinguished once engine running and also that you can measure about 13,5-14,5 volts on the battery with engine running?

The switch appears to be good... if the D+ wire is grounded then the light goes on an off with the switch as expected... I'll check it again at lunch, but it appears there is continuity between the B+ and the D+... since D+ goes to ground this would explain draining the battery. The part I can't figure out is how the light comes on with the switch off if D+ & B+ are both hooked up. That would seem to indicate that when the switch is off that the wire going to the light is grounded and the B+ is backfeeding through D+.

The most annoying part is that it was working fine, I just took the alternator in for a noisy bearing...

Mike, Ford alts are cheap. Replacing the repaired one with a brand new assembly would be my first move.

I've tried to find an appropriate model, closest I can find is the Bosch AL77X. I think the shop that did the rebuild wants to make it right, I just need to determine what "right" is... According to them it's working correctly, but I think there is more to it...

Mike, while I haven't thought of any thing, I am still thinking about your problem.

Is your regulator a mechanical or transistor type?

I'm not sure how to tell which it is... I'm pretty sure it's original if that helps. That being said, the wire for the light is shared with the wire from D+ as shown on the diagram. So it doesn't seem to make a difference if the VR is plugged in or not.

Here are the wires on the back of the Alternator

Here are the wires going to the Voltage Regulator

quote:

Originally posted by M!ke H.:
The switch appears to be good...
if the D+ wire is grounded then the light goes on an off with the switch as expected...


The part I can't figure out is how the light comes on with the switch off if D+ & B+ are both hooked up.
That would seem to indicate that when the switch is off that the wire going to the light is grounded and the B+ is backfeeding through D+.

If that is the case, that is one of the rectifing diodes has failed and allowing current to leak through, the back feed to make the lamp come on could be through a normal "running" device.

Makes sense... I take it back and go over it with them tomorrow... I'll let y'all know what they say.

Thanks again,

MH

I wonder if you pulled fuses (to remove all RUN device) that the light would go out

a drawing would would help. for example on the Pantera, even if the two fuses are pulled, the power to the tachometer would provide a feed back path to ground

If JFB is correct (failed diode in one of the legs), you should be able to find this with a voltmeter (actually an ohmmeter) with the alternator in the car.

If all the diodes are good - you should be able to measure resistance between B+ & D+ in one direction, and basically, an open circuit by just switching your meter leads.

If one of the diodes is failed, and is conducting, you should see a similar resistance in both directions.

Or you could use the "diode-checker" setting of your Harbor Freight meter to evaluate it.

Rocky

I just went through hoops chasing a similar issue on my excavator. Wouldn't crank, battery relay wouldn't release and charge light stayed lit when the key was shut off. Batteries stayed charged though.
I thought critters had nibbled at the wires. I was unwrapping sheaths, chasing wires, testing connections. Finally took alternator to my auto electric guy for a test-N/G. He rebuilt it, I reinstalled, gremlins gone. All were backfeeds through the alternator.

The alternator is back in the shop with wiring diagrams and details...

I was getting resistance in one direction only between B+ & D+ which indicates (from previous comments) that the diode is working correctly. I asked if it was possible the diode was somehow reversed, they said they didn't see how. But would look into it...

From what I can tell D+ goes to both the VR and the Gen light, then to the ignition switch. If the switch is on it gets power, if the switch is off it goes to ground. Somehow B+ is backfeeding power to D+ and with the switch end of it grounded the light comes on.

What is perplexing to me is that with the key off D+ (wire to switch) is grounded... if B+ is backfeeding power to it then this explains not only the light coming on, but also the load being placed on the system which drains the battery. But the load seems to be far more than just a little light bulb...

Hopefully they'll figure it out...

Thanks for the thoughts and suggestions...

MH

quote:

Originally posted by M!ke H.:

What is perplexing to me is that with the key off D+ (wire to switch) is grounded...

Maybe just my obsession with terminology, but if this is what you are measuring, I would not expected grounded. BUT, I would expect to be reading the sum of the load resistance on the "Running" fuse. Now that resistance could be very low, but not "grounded"

Perhaps my understanding is as bad as my phrasing... if the ignition switch is off and you put power to the D+ (backfeeding) the light will come on, to me that means the ignition switch is going to ground when off. But there may be more to it than such a simple answer...

if you ground D+ and turn the switch on then the light comes on as expected.

If everything on the alternator and VR are connected normally EXCEPT the B+ then all works as expected. As soon as you connect B+ the light comes on regardless of where the ignition switch is and there is a small spark when you touch B+ wire to the B+ terminal.

I'll have to verify tonight, but based on what I've seen, using your diagram the VOM would read a ground (continuity) with the key off and 12V+ with the key on.

I'm tempted to just cut my losses and go to one-wire alternator configuration, but I'm $200 into this issue that started off as just a noisy bearing... I wanna see how it ends!

MH

the Bosch number for the Alternator that is suitable for a Pantera is 0120 469 779. (source Ronald Jaeckel in Hamburg DE)

quote:

0120 469

Well... if the shop isn't able to figure it out I may go this route... it looks like this one has a built in VR... I'm fine with that... at this point I just want it to work!

and it's much less $$$ than I've already spent!

Thanks

MH

quote:

Originally posted by M!ke H.: if the ignition switch is off and you put power to the D+ (backfeeding) the light will come on, to me that means the ignition switch is going to ground when off. But there may be more to it than such a simple answer...

I am using the Pantera drawing of the ignition switch, my minor point is the switch should not be grounding, BUT you would measure very low resistance because of the multiple loads on the run side going to ground

With the ignition off the wire going to D+ seems to be a fairly solid ground, adding +12v to it turns on the light. Grounding the D+ wire and turning on the ignition on turns on the light.

Verified a few minutes ago.

Here an explanation of how the idiot light works a friend got on line and sent to me which may or may not help you. He goes further into troubleshooting which I hope doesn't confuse things further.

Good luck


--—---------

Yes, the alternator light does work by a balance. When you turn on the key, 12 volts is supplied to the alternator's field windings through the idiot light. This gives an indication that the alternator's field circuit is intact, and it also provides the current to start the alternator charging. In fact, the alternator may fail to charge if that light bulb is burned out. When the engine starts and gets up to a high enough rpm for the alternator to start putting out voltage, there is a diode inside that takes some of the output voltage and featured back to the terminal that goes to the idiot light. The net result of this is that when the engine is running and up to proper rpm, the idiot light is receiving 12 volts at both ends, which means that the total voltage drop across the light is the zero volts and the light doesn't light.

The problem you're describing usually occurs when there is a break in the wire from the alternator's output terminal back to the battery, it does not have to be a complete break, a high resistance connection is quite sufficient to cause a light to come on. What is happening is that the alternator is full output voltage is getting sent to one terminal of a lightbulb, and the other end of the bulb is connected up to the battery positive terminal through the ignition switch, which is now at a lower voltage because of the voltage drop in the positive wire. This voltage drop causes the light to light, and also results in the battery charging poorly if at all. Usually when this is the case, the light will be very dim at idle and will get brighter as you rev the engine.

To diagnose the problem, put the positive wire of your volt meter on the alternator output terminal and the negative wire of your volt meter on the battery plus terminal. Put some load on the battery, such as lights or something, rev the engine, and see how much voltage is getting dropped between the alternator output and the battery terminal. If everything is good, it will be well under 1 volt. I suspect you will find you are dropping several volts in this wire, which indicates a breakage or high resistance connection somewhere along the wire. Probe between both ends of each section of the wire to find out where the high voltage drop is, then repair the problem when you find it. If you want to be half ass and not go through the trouble of tracing the wire, simply run a very heavy, about eight gauge or so wire from the alternator's output terminal back to the battery plus and your problem will probably be solved.

While you're checking things, put the negative terminal of your meter on the alternator case and the positive terminal of your meter on the battery minus, apply a load, and rev the engine. If you have much voltage drop here, start checking the voltage drop from point-to-point once again until you find the problem, then repair it. If your alternator is rubber mounted, there may be a grounding strap between the alternator case and the block which needs to be present and in good condition. Also check the grounding strap between the engine and the body or battery negative. I don't suspect you have a ground side problem here, but when I've got the meter out and I'm looking into things I always check both ends of the situation.

That all makes sense... I figured that once the D+ pushed power back to the light that the power would effectively cancel itself out and the light go off. What seems to be the problem is that it's getting power even when the ignition is off and the ignition switch seems to be functioning fine in all other ways. I just can't figure out why the D+ on the alternator would be live when the ignition switch is off.

Perhaps if someone else could pull their D+ wire and put a test light or meter on the terminal to see if theirs is also showing power when the ignition is off.

No word yet from the alternator shop...

Thanks for the input and trouble shooting suggestions.

MH

Just heard back from the shop. They found a piece of solder on one of the diodes that was causing it to backfeed. and of course it managed to fry the VR in the process...

Nice to know I wasn't crazy !!!

MH

your original VR got fried due to their error in rebulding the alternator.

did you pay by credit card? I think the shop should make things right for these past two days of your effort when they should have caught the error in testing prior to giving it to you

now that I think about it, they SAID the original was bad and sold me a new one... I never hooked the old one back up... so it should still be good! They are replacing the replacement. If nothing else I'll have a spare.

Either way I'm just glad to check this item off the list!

Thanks again for everyone's help

MH

Is it installed and WORKING?

Yes, just got it back on and working as it should... original VR works fine as well.

Thanks to all for the input and suggestions.

MH