ctgriffi

I've been picking away at this charging issue, off and on, for about 10 days. Pulled my OEM alternator and got it tested (was fine) but picked up another reman anyway (didn't help). This 90 2WD 3-Dot-Slow is in great, running shape with only 98K, but it is definitely not charging one bit—voltage is always just existing battery voltage: 12.6v or thereabouts.

After much testing and troubleshooting, I've realized that the red IG wire to the alternator has zero volts at the alt plug, with Key On Engine Off—while the white wire and the yellow wire seem to be fine, with both showing close to battery voltage. Fuses are all good and have been doublechecked, so I'm 99% sure I have some kind of connection or wiring problem between ignition switch and the alternator. Anybody know a likely spot that the red wire would fail? Can somebody describe what path that red wire takes between ignition and alternator?

Much appreciated!

scope103

The red wire connects to IG1 on the ignition switch through the (oddly named) Engine fuse. Check the fuse with a multimeter. (w/key-on, look for battery voltage to ground on each side of the fuse.)

The (thin) yellow wire is to the charge warning light, and with engine-off should be at ground, not battery. Something is wrong here.

Using small jumper clips, test the voltage to ground for all four wires (the 3 thin and the one fat) while running.

ctgriffi

Thanks for your help, Scope103. Please take a look when you can—let me know if I need to do something differently!

Did some more testing tonight based on instructions, although not with engine running. Battery needs a charge but was at 11.95v.

Tested both sides of Engine Fuse to ground with multimeter and got the exact same reading: 11.7v

Checked all four wires to ground at the alternator (with the alt disconnected/removed) and Key On Engine Off:

• B+ = 11.95v (same as battery)
• White = 11.9v
• Red = 0.0v (open circuit)
• Yellow = 11.47v (should be nothing)

More info: If I jumper the red IG2 wire from 3-pin alternator plug all the way into the Engine Fuse at driver's kick panel, there is no continuity, Key On or Off. Lastly, I never get a charge light on the dash with Key On, but if I jumper the yellow alt wire to ground, it will always light up (bulb is fine).

scope103

You can't tell "open circuit" with a voltmeter. A reading of ~0v could be a disconnected wire, or a wire pulled to ground. (If you want to tell if it's pulled to ground, you use a test light/multimeter from the positive side of the battery to the point in question.) Yellow should be pulled to ground, which is not the same as "nothing." (Get out of the habit of saying "my multimeter showed nothing." It's 0v or open circuit, or dead short with ~0ohms, or something else, but it is never "nothing.")

So when you say there is "no continuity" from the 3-pin connector to the Engine Fuse, that would be with the ohms scale. Which make sense; your red wire is broken somewhere. You should be able to safely jumper the red wire on the alternator directly to battery plus; I'll bet you'll get ~14.1v (charging voltage) on the fat wire if you do.

I also think your IC Regulator is not working, in that it won't pull the small yellow to ground (and your alt light doesn't work). It could be limited to that; I'm going to guess the parts-store alternator tester doesn't care about that lead, and only looked for enough current and the right voltage on the fat wire when red was energized. If you're not a slave to all-original, you could add a voltage meter and use that instead. https://www.aliexpress.us/item/2255800646873229.html

ctgriffi

Thank you, sir. Really appreciate the continued help... and the suggestion to use more precise language. I work on computers all day as a developer, so I get it: it's counterproductive to gloss over details when troubleshooting systems like this.

To be more accurate about the red wire: it was actually showing ~0.3mv (bouncing around close to that on the DVM) which indicates a broken wire or a short-to-ground, like you mentioned.

I like your idea about jumpering the red wire to battery pos, since that's pretty much what I was contemplating doing. Question though: is there any downside to having that wire non-switched? And, related question: would it be much better to run a new red wire from the alt straight to the Engine Fuse in the cab, if I can manage it?

ctgriffi

Ended up tying into the red wire at the Alt plug, ran it back and along the firewall, round to the battery—tied it all up nicely so it's not ghetto. Everything seems to be working great now with the original alternator and charging just fine at over 14 volts. Also, the charge light is working again, so apparently the red wire is needed to "power on" the alt at Key On and tell it to pull yellow to ground.

Thanks again @scope103 for the the help.

scope103

You need a switch in that wire. You don't want current flowing through the alternator all the time.

Even more than that, you need a fuse. 10amp should do.

Since IG1 also powers the wiper, turn, and gauge fuses (which are presumably working), you could just run your new wire back to the Engine fuse, and you'll be that much closer to OEM.

ctgriffi

Good call. Pulled that wire back out and ran it into cab by kickpanel. I used a fuse tap which adds another fused circuit, and this made it pretty easy to tie into the Engine Fuse. Truck is running good and charging again.