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This problem is kicking my me stupid.

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Old August 12th, 2018, 5:53 PM
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I have a 1981 4.1 inline 6 with a charging problem. 37 years old might provide a clue. It does use an internally-regulated alternator, I think it is a 10LSI.

The alternator on it would flicker the BAT light at idle, and then the light quit coming on. From my recollection, I think it should show light without the engine running, but with key on. I checked the battery, 12.5 with the engine running or off. I went directly with my meter to the back of the B+ post and had 12.5, which is what the red terminal on the 2 wire plug was also showing. So with that limited knowledge, I bought a replacement unit without taking mine out because I couldn't disable the car where it was. I thought it had bad brushes in it or maybe it was otherwise bad. Later, I took it apart, saw that the brushes were almost new, had it tested, and it has tested good twice, at two different locations.

I swapped in the reman alternator. Now the dash light is bright red at all engine speeds, and the replacement alternator is blasting out 14.5 vdc on the B+ terminal The same voltage is showing at the battery, but I went to the connections that I found, and cleaned any corrosion off them i could find, including all of the grounds.

The battery is less than 6 months old, and the cable ends are new, but I did a load test on the battery to verify I don't have a shorted cell. With the engine off, the battery showed 12.64 at the start of the test, 12.35 during a headlight load test, 11.15 during a starter cranking test (coil disconnected), and 12.33 after stopping all tests and letting the battery sit to stabilize itself.
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I put my original alternator back in, and again, it is flat lined at battery voltage, even though it had tested good off the truck.

I reinstalled the reman unit and again I am met with a bright red light in the dash at any engine speed. and 14.48 output to the battery. The fact that the positive battery post is showing 14.48 indicates to me that there isn't an open to the feed line back to the battery, creating an excessive resistance.

I've been all over YouTube and up and down the Internet, and am wearing out my welcome at the auto parts stores in the area. Yeah, it's that guy, again. I'm not quite ready to surrender this challenge to a shop. Part of owning these niche vehicles is being able to generate your own repairs, and I'm not about to quit on this one. I had to let someone else do the transmission, and i won't go near rebuilding the carb, but this monkey mess of wiring has never been hacked on, so I know that isn't an issue here.

I hope someone can help me with what i might have learned in high School if i hadn't slept through the class.
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Old August 13th, 2018, 4:41 PM
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you old alternator was definitely bad.
the charging system light should do a bulb check at key on and should shut off with the system charging. the problem may be with the indicator circuit.
post the make and model and I will look into it.
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Old August 13th, 2018, 8:28 PM
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Hi tech2, thank you for being willing to help me evict this monkey from under the hood of my truck.

Since my original post, I went back and started verifying connections. I disconnected the 2-wire connector on the alternator and checked the resistance on the regulator wire that travels through the lamp to engine ground and found the resistance to be 9.6 ohms, which I figured was probably about right considering the lamp in the circuit.

I found that even though I thought I had corrected a bad body ground from the battery to the radiator core support, when I checked with a multi-meter it showed between 400-700 ohms from the strap to the core support. That equated to about 7vdc which would bounce up some if I pushed on the head of the securing bolt like a dentist pushing on a bad filling. That, at the time, I thought was the entire issue, as my thought was that without a tight body ground, the amperage back through the ground path of the regulator transistor wouldn't be strong enough to push open the gate in the transistor and break the charging field. I was hopeful that when I started the car last night I would be done. When I turned the switch to the "ON" position, the light didn't come on. When I started the truck, again the light came on, bright red, and the alternator was running at full blast at idle. So that was kind of a let down. I though maybe I had burned up this alternator with having run it with a bad regulator ground. That was last night.

Today, I checked the fuses. Even though the alternator circuits I have looked at don't show fuse-box use, I found two fuses open, so I went ahead and replaced obviously bad parts with good ones, as any decent mechanic should do.

The 20 amp blower motor fuse was open, as was the 20 amp choke heater fuse. So $2.16 cents later, I had two new fuses in place. With the key "ON," I now had the blower fan operational AND the red light on the dash showing with the engine off, something it hadn't done for the week or 10 days I have had this problem. So I have not a clue what caused the red light to now work with a key "on" check.

When I started the truck, the red light went "OFF" (HoRaY!) and in checking the alternator voltage at the battery, with the lights on and fan running at full speed, I am showing a sane value of 13.99vdc, so I think I am out of the woods with part of this. Fix one problem, and two others show up.

I still don't have power to the end of the wire that is supposed to heat the choke circuit. It looks like "maybe" the oil pressure switch which has three wires connected to it, might be bad and not closing to complete a circuit, which may be the choke heater. Don't want that running on a cold, stopped engine, which would continue to heat the choke without the cut-out in place. My next step is to jump out the switch to test that theory.

To answer your question, the truck is a 1981 C10 Scottsdale, with the original 250 inline 6, 2 bbl Rochester Varajet carb. If you need any other information, let me know. The VIN is 1GCCC14D4BJ168101, which AutoZone can't even find during their computer look-ups to attempt to help me. Makes it kind of hard to believe the have the "correct" parts in any case, but that is another story for another time.

Again, thanks for any and all the help.

Jonathan Gilbert
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Old August 14th, 2018, 3:22 PM
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Well, here is today's update. I reinstalled my original alternator, and it is now working. So maybe between a poor body ground and the broken fuse, the alternator just went AWOL but now it is back, and that problem seems to be fixed for the time being. Since this in an electrical thread, I want to keep it open at least until I get the other hair-pullers solved.

I checked the oil pressure switch, and it seems to be working. ( instrument cluster is missing bulbs and has broken out bulb anchors) I thought about it some, and it makes no sense that GM would use an oil pressure switch to feed the choke wire, because with the engine running, the switch would break the contacts, to turn off the oil light, and any other wire that would need twelve volts for the current use would no longer have it available. If I had a factory schematic in front of me I would probably be a lot further along in this detective work.

I also don't understand why replacing either the choke heater or the fan blower fuses had anything to do with the operation of the charge light when the key was turned to the first position. Again, I can't find a quality diagram to become educated with. I'd be willing to make the investment for a FACTORY wiring diagram. The shi**y bits I have found on the web have been worthless. Excuse my poor English. I don't have words to describe my angst regarding this electrical ***** of a nightmare.

Thanks for taking the time to read, ponder and maybe offer some ideaz.
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Old August 14th, 2018, 9:35 PM
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This should be interesting to anyone who ends up running into this same problem. In looking at whatever wiring schematics the Web offered, all of them indicated that a light blue wire feeds the choke heater from the oil pressure switch, and the idle-up shut off is supposed to be fed from a tan/white wire. On my retard of a truck, BOTH wires were the same color, blue and the longer of the two was routed to the idle-up solenoid and the shorter wire would barely would reach the choke heater.

Tonight I jumped out the oil pressure switch connector, turned the key on, and discovered that the longer of the two wires was the current carrier. I was looking at a blue wire the first time, when no voltage was showing during a key on test with the switch turned on. That blue wire goes all the way to the bulkhead connector and I have no idea when it is supposed to be energized to activate the solenoid. I guess GM must have liked blue there in Wisconsin when they built my truck oh so many years ago.

With that issue now fixed, maybe i will gain some MPG now that the choke should be working properly.
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