I've read through a bunch of charging system threads here but I cannot find any that describe the issue I am having. The truck is a 1990 F250, motor is a 7.3 IDI with a 1G alternator and external regulator. The alternator recently died so I pulled it an took it to O'Reillys to have it checked. They confirmed it was bad so I bought a remanufactured 1G and installed it. The alternator still had no output so I replaced the voltage regulator (and cleaned all the regulator grounds). Still no charge.

I have a Ford factory service manual for the truck so I ran through the charge system diagnostic tests. The results said bad alternator so back to O'Reillys for a test. The new alternator was good on their tester. They also charged and load checked both batteries and they passed.

At this point I did a bunch of repair and upgrades to wiring and ran through the diagnostics several times looking for clues. Here's the list of updates:
Both batteries and all battery cables were new 6 months ago. Also cleaned all the grounds on them again.
Alternator is new. After multiple failed tests O'Reillys swapped it for another one but I see the same results.
Voltage regulator is new.
The large BK/O wire from alternator to starter relay checks ok with an ohm meter. I replaced fuse link J at the starter relay just to be sure.
I replaced the voltage regulator connector, Field wire (O/LB), Stator wire (W/BK), and alternator connector.
I added a ground wire from the alternator to the battery negative terminal (there was not one in the harness).
I added a ground wire from the voltage regulator case to the battery negative terminal.

Even after all of these changes I get no output from the alternator. Engine on, no load, 1500 rpm I see 11.54 volts at the battery and 11.85 volts at the alternator output. Engine on, loaded with lights and AC, 2000 rpm I see 11.46 volts at the battery and 11.85 volts at the alternator output. Unplug the regulator and jumper the A terminal to F and I see no change in these numbers. Checking the I and S terminal voltages at idle shows S = 5.77 and I = 11.09, as desired. Resistance of the F circuit wire from the regulator connector to ground is 4.6 ohms which is good.

I'm out of ideas and tests to run. Where do I look next?

 

Iirc you need to ground the F wire (field) and it should put the alternator into wide open mode to test it. The other wire to the alt is power in iirc but its been awhile.

I did the 3g swap and the only reason i did it was it puts out most of its power at idle which was a big thing for me since my rig does a lot of low rpm traveling

If they'll refund that alternator towards the price of a 3g (which they've done stuff like that before for me) then that would be the route I would take. Its such a simple install and there are plenty of write ups on it.

You'll gain 20 charging amps and get it easier. Mine came off a mid 90's van with the inline 6 if that helps you see if they offer one for close to the same price

 

With the engine off, check you have battery voltage at the Bat terminal of the Alternator. If not you have an issue there. Start truck. Feed the field, battery voltage with a jumper wire from bat positive. You should get an increase of at least a couple of volts. If you see an increase the alternator is doing it's job. Your problem is on the regulator side of things.
From your information, to me looks like wiring. Jumpering right to F from battery eliminates all that.

 

That's a definite no-no, who knows what damage you'll do to diodes or regulator. As Don Senkow posted above, jump from the battery positive post to the field terminal on the alternator which forces the alternator to put out its max voltage and amperage to tell if the alt is working or not. It should jump to the high 14+ voltage range.

 

The best way you can help yourself is to make the 3G swap.Even an old used 3g from the yard is far superior to a new 1g from the store.It's actually much cheaper too.Consider taking it back and just doing the swap.You'll have a far better system for pennies on the dollar.They come with an added bonus too........They wont burn your truck to the ground.

 

I have battery voltage at the BATT terminal and at the regulator A terminal with the key off. I've tried unplugging the regulator connector and jumpering the A terminal to the F terminal (which goes to the alternator Field) and I see no change. The wiring from the regulator to the alternator is new, I just replaced it over the weekend. I'll try jumping the alternator field straight from the battery and see if that does anything.

 

From what I've read here the 3G swap sounds like a good idea. My only concern is that I do not understand why the truck is not charging now. If the issue is elsewhere in the truck I'd like to find it before I drop a 3G into the mix.

 

The 3G wiring is 10x simpler. If you are looking to reduce variables, I agree with @FORDF250HDXLT and doing the upgrade now. If you can get the alternator and the wiring harness from a donor car in a junkyard, that's the best, but they sell the 3G wiring on ebay now. Check this out: https://www.ebay.com/itm/119851-ALTE...frcectupt=true

 

1. Do the 3g swap!! This is my first upgrade once the weather breaks in northern Indiana. I have had to replace my 1g alt too many times.

2. Don't trust parts stores!! Jump the voltage regulator on your 1g to see if it charges. Get a cheapo voltage gauge to plug in to your 12v outlet in the vehicle. Doing this will tell you if your alternator is good or not. If you jump it and still no charge go back to the store and tell them they are full of crap.

Seriously, do your own testing vs believing your parts store. I have had them tell me that my batteries were fine, that my alternator was fine, etc etc.......and each time they were wrong. Do your testing and then demand they honor their warranty. In my case I am lucky. There are enough gear heads that work at my local shop that I have talked to that will listen to me and replace what I need if I have done the tests.

 

I am going to do the 3G swap. I've collected all the bits but an alternator and have the old 1G and wiring out of the truck. Tomorrow I hit the wrecking yards for an alternator.

I did bypass the regulator, with no change in the alternator output. I've tried two rebuilt alternators with no success. Even after all the wiring and connector changes and added grounds I still never saw more than 0.1 volts over battery voltage at the alternator output. Hopefully the 3G swap will be the silver bullet I need. I'll share the results when I get it done.

 

Good luck, my junkyard 3g has been going strong for 2+ years. I let an alternator shop test it first though.

 

I did the 3G swap today. It went in pretty easily. Clocking the alternator was a bit harder than I expected as the case did not want to turn. I ended up tapping with a rubber mallet to get it moving - it looked like the case had to separate a little bit to allow things to turn. After it was all installed I fired up the truck and checked the output. Still nothing much over battery voltage!

While doing the wiring I had found a damaged wire from the starter relay to the two fuse links that feed the glow plugs. It was badly melted and black. I had replaced that wire while I did everything else. On a hunch I un-hooked the glow plug circuit and re-tested the alternator output. Success! Lots of charge going to the batteries.

Which means I have a big current drain somewhere in the glow plug circuit. I've already replaced all the the glow plugs and built a new glow plug harness from the timer relay to the glow plugs. It looks like I get to dig further into the wiring loom to the timer.

Here are some pics of the alternator swap: https://jdwxly.smugmug.com/1990-F250-IDI-Diesel

 

Judging by your pics that O'Reilly alternator is the same 70amp bull**** they sold me. It never would get much over 13 volts and I dealt with low batteries all the time. I was so happy to get rid of it.. Haven't looked back since I did the 3g.

 

Disregard, wrong thread.