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Virtual Wrench
Virtual Wrench, Ford Mechanic
Category: Ford
Satisfied Customers: 14620
Experience:  35 years Master Mechanic
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89 F250: 7.3L..charging..about 11 volts, alternator charge idiot light

Customer Question

I have an 89 F250 with 7.3L diesel. Alternator is not charging. Dash voltmeter reads about 11 volts, alternator charge idiot light is on. I had alternator rebuilt about 3 months ago, replaced regulator a couple times.

There is no current to I terminal on regulator when key is on, whether or not engine is running. There is current to A terminal (always hot), and S and F wires from regulator to alternator carry current.

When I run the truck with regulator unplugged and F terminal on alternator directly jumpered to battery +, dash voltmeter still says 11 volts. Hand held voltmeter on alternator S terminal shows no output. So it looks like the alternator is bad and needs replacing.

But I am assuming that no current to I terminal is also part of my problem, and I need help figuring out how to deal with this.

I have a Chilton manual with wiring diagram, but I can't figure out where wire #904 goes from the I regulator terminal. I can trace it on the diagram through a couple of connectors and through the multiwire passage through the firewall, to "Cluster plug B" (E24B-14489-HA). Then I don't comprehend where it goes.

Can you tell me how to trace the wire from the I terminal so I can find where it is shorted or disconnected? I assume it is somehow connected to the ignition switch and the idiot light, but I have not torn into the dashboard or dropped the steering wheel.

Submitted: 7 years ago.
Category: Ford
Expert:  Virtual Wrench replied 7 years ago.



You are correct in assuming that wire from I terminal comes from the ignition switch, then thru indicater light then to regulator. It would appear that the ignition switch, or the wire is at fault. See diagram below.



Customer: replied 7 years ago.
Cool. Thanks much for the diagram.

Since the indicator light is working, it would seem that the problem is not in the indicator light or the wire from there to the alternator. So, it must be in the switch itself or the wire from switch to indicator.

I will start with the switch, since I know how to get at that (lower the steering column). If I have to trace the wires all the way to the indicator light, will I have to pull out the whole instrument cluster to get at it? Sigh.

Expert:  Virtual Wrench replied 7 years ago.



I dont think you will have to go that far. If there isnt any power coming from the switch to that wire, when in the run position, its the switch.


Best of luck my friend!

Expert:  Virtual Wrench replied 7 years ago.



Do you have any further questions?

Customer: replied 7 years ago.
Hi Virtual Wrench,

Thanks for the diagram, but we ain't out of the woods yet.

I took out the switch and tested it with tester made from an 1156 turn signal lamp. In the LOCK and OFF positions, no juice goes from the yellow circuit 37 [BATT pins] to any of the pins. Check.

In the ACC position, I get juice to both Bk-Lg circuit 297 [A1 pins], no juice to any other pins. I think that is good.

In the RUN position, I get juice to both 297[A1]; R-LG circuit 16 [pin 11] (which is identified on your diagram as feeding the alt. indicator light); and both GY-Y Circuit 687 [pins A2].

In the START position, I get juice to BR-P circuit 262 [pin 12] (which is also identified on your diagram as feeding the alt. indicator light); and to R-LB circuit 32 [pin ST], which goes to starter relay.

In no position of the starter switch do I get any current to pin P1, circuit 977, which feeds a wire that is labeled P-W in my Chilton manual but looks mover like brown and white to me. This seems sorta suspicious. In the manual I can trace 977 to a cryptic little figure that is not labeled, but it has a zigzag symbol in it like the one in your diagram's alternator warning indicator figure. Could this be the wire feeding the indicator? The other wire going to this figure is circuit 640, elsewhere labeled as "warning lps feed."

Also in no position do I get any current to BK-LB circuit 41 [pin P2}. And I don't know where this circuit goes.

After testing the switch, I moved to the ignition switch wiring harness connector. I re-hooked up the battery cables, then jumpered from a hot BATT pin to R-LG circuit 16 [pin 11]. The alternator light comes on but NO current to regulator I terminal.

I removed that and jumped from BATT to BR-P circuit 262 [pin 12]. Same result: alt dash light on, NO current to I terminal.

I thought, maybe circuit 977 is relevant. So I jumped from BATT to pin P1, circuit 977, Nothing. No idiot light, no current to terminal I.

I then jumped BATT to both circuit 16 and 977. Yes idiot light, no current at terminal I.

Back before I took the switch off, I hooked my test lamp between the battery + terminal & the regulator harness I connector. With key off, the test lamp glowed and the dash alt indicator was on. With key on, the test light glowed much more dimly, and the dash light was off. This would seem to indicate the wire between the I terminal and the dash light is OK.

But, the jumper test at the switch harness connector would seem to show that the wire between the switch and the dash light is also OK.

So, I am now thoroughly mystified. Why the hell is there no current getting to the I terminal??? Any ideas on how to proceed from here?

Sorry this is such a can of worms. It is getting just a wee bit frustrating.

Customer: replied 7 years ago.

I hope you had a good 4th of July weekend. Now, I am sure hoping you are back in the saddle and can give me some idea on how to get my charging system operational.

A local mechanic suggested that some vehicles are wired so that there has to be oil pressure before there is juice supplied to I terminal of regulator. So you would have to test it with engine running. This makes no sense to me, and I think I remember there being juice to I terminal with engine off when I was testing this a couple months ago.

Any suggestions?

Expert:  Virtual Wrench replied 7 years ago.

Hi there!


Man i got dizzy reading your last post above :-)


The oil pressure doesn't make any sense to me,,,NEVER heard of such a thing with the charging system.....


If you jumper power to the " I " terminal when running does it charge?


Customer: replied 7 years ago.
I did try jumping power to I terminal with engine running, back before I took the switch off. I jumped with the 1156 test lamp, one wire to Bat +, the other to I terminal.

With key off, the test lamp glowed brightly, the alt indicator light was on, dash voltmeter needle pegged off bottom of scale.

With key on and engine running:
The test lamp glowed dimly. The alt indicator light on dash went off. Voltmeter needle was on a wimpy 11 volts, so apparently it was not charging.

I mentioned earlier that alternator does not show a charge when I run engine with alternator unplugged from regulator and jump directly from F alt terminal to Bat +. So I am assuming alternator needs to be replaced. But I cant't figure out why the heck I am not getting current to I terminal.

Customer: replied 7 years ago.
On another note, do you know what amp rating my alternator has? As far as I know it's whatever Ford normallyt put on their 89 7.3L diesel.
Expert:  Virtual Wrench replied 7 years ago.



You assumed correct. If the alt doesn't charge when you full field it, it will need replaced. Do that first, then check it and let me know. The current works weird if something in the alt is shorted and may be preventing current to that terminal,,,not sure as of now....


Depending on the color of the "motorcraft" trademark on the alt will determine amp output. See choices below. I would think a 70 or 100 amp is what it came with. Probably the 100





Customer: replied 7 years ago.
I will check my alternator color, and try to replace it with one of similar output. I know I need to replace it. But I suspect the I terminal wire dysfunction is a separate issue, and I still don't understand what is going on there. I will get back to you when I have a good alternator to test.


Expert:  Virtual Wrench replied 7 years ago.
Customer: replied 7 years ago.

Here is the latest chapter in my sordid tale.

First, I was unable to find any kind of color coding on my Motorcraft alternator per the amperage chart you sent, but I did find a serial number stamped on the body (E9TFAAE319B), Preceding these digits it reads "70A", so I am assuming it is a 70 amp alternator.

I have an '85 F150 in my boneyard. I took the alternator off that. It is stamped "40A" so I assume it is a 40 amp. I also removed the external voltage regulator. Interestingly, this regulator has 4 terminals (I A F S) like the one on my '89 F250, but there is no wire at all running to the I terminal. No plug in that socket of the harness connector. So maybe this is some kind of self exciting regulator? ( from the A terminal?)

I then took both alternator and regulator from the F150 and temporarily installed them on the F250. I had to jumper some wires, since the connectors are different, but I hooked A, F and S on regulator to corresponding connections on alternator, and I wire to regulator.

The result: it charges. I start the truck and, it takes a couple of minutes, but the dash voltmeter gradually moves up to about 14 volts. The needle sits there sort of twitching at 14. My digital voltmeter on battery terminals shows the voltage flickering back and forth between, say, 14.5 or so up to 15.08 or maybe 15.12. I assume this shows the regulator doing its thing.

When I turn on lights and heater blower, dash voltmeter drops to a little under 13 volts.

I had the I terminal at regulator jumpered through my test lamp. The 1156 bulb still shows no current coming to this terminal.

For some reason, the dash alternator indicator lamp also now seems to have stopped working altogether. It is not on when the engine is running, which would be good... except now it does not come on when engine is off and the key is in run position. The "engine" indicator light does work, though.

O mechanical guru, how do you read these signs? I am tempted to give up my DIY pretensions and take this ornery puppy to someone who knows what they are doing. Except, I have already stumped a couple of local so-called experts with this [email protected]#$%!! truck's weird charging problems.

Expert:  Virtual Wrench replied 7 years ago.

Hi there,


The main thing is that it's charging ok if it's between 14-15 volts. To be honest with ya, i would really have to be there to look it over to get a better grip on this. Kinda hard to wrap my noodle around it from here :-)


Best to ya my friend!

Customer: replied 7 years ago.

Me again.

A local guy who used to run an alternator shop told me the wire from alternator indicator light to regulator I terminal probably has a diode in it, so it may be normal not to have current to that terminal when the regulator is unplugged. Maybe all that head scratching on my part was a red herring.

I replaced the alternator with a remanufactured 70A Motorcraft unit. And the regulator with the one that cost $50 instead of the one that cost $15.

Voila. I get 14 to 15 volts, and the idiot light goes out. Problem solved, and great celebration all around.

Or so I thought.

The next day I am driving the truck, and suddenly, out of the corner of my eye, I see the [email protected]#[email protected]# XXXXX red alternator idiot light flickering. The voltmeter says the alternator is putting out 14.5 volts, fine and dandy, but the light flickers. I turn on the headlights, wiper, or heater blower, and the flickering light goes out.

This is exactly what the truck was doing for the last three months before the alternator died.

I am stumped. Any ideas?

Customer: replied 7 years ago.
A local retired alternator shop owner told me the flickering idiot light that goes out when I turn on an electrical load sounds like it might be a bad stator wire. But I disconnect the S wire from alternator and regulator, jumper 12 volts from battery positive at one end, ground other end thru 1156 test lamp, and lamp glows nice and bright. So that wire appears to be OK.

Expert:  Virtual Wrench replied 7 years ago.

Hi there,


Sounds like you are really having fun now.....i'm out of ideas, but i will ask some of the other experts if they might know.