Engine Runs but Battery Dies (1 Viewer)

rkymtnflyfisher

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Definitely driving the 40. Gotta get those shakedown miles in.


It wouldn't hurt to clean all of your fuses and the fuse holders, if it is a parasitic draw on a circuit somewhere it's going to be a little harder to track down.
 
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Yeah, my battery connections are clean and tight.

One thing I noticed yesterday while driving is that the ammeter in my instrument cluster essentially reads 0 or below. For instance, the reading drops each time my turn signal is on. It's read like that since I got the truck but, now that I'm thinking about it, it's probably supposed to be reading well above 0 while the truck is running. Where do properly functioning ammeters run?
 
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12.6 V is a strong battery.

It needs to be over 12.6 to actually charge the battery.

Was it at idle, did you try revving it? Most alternators don't output much at low RPMs, I've hear the GM 1-wire alternators are particularly bad due to having the internal regulator.

And, if you have an alternator that can output over 30 amps, wiring through the stock amp meter might be a bad idea... Though, if you (or the PO) bypassed that bit of wiring from the alternator to the battery (with heavier gauge wiring that wouldn't start a fire with 100 amps), but still used that original wiring from the battery to your fuse panel, it probably would always be negative (only showing the draw from everything running, not charging)?
 
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Was it at idle, did you try revving it? Most alternators don't output much at low RPMs, I've hear the GM 1-wire alternators are particularly bad due to having the internal regulator.
That was at idle.

And, if you have an alternator that can output over 30 amps, wiring through the stock amp meter might be a bad idea... Though, if you (or the PO) bypassed that bit of wiring from the alternator to the battery (with heavier gauge wiring that wouldn't start a fire with 100 amps), but still used that original wiring from the battery to your fuse panel, it probably would always be negative (only showing the draw from everything running, not charging)?
I think this might be what I'm seeing. The wire coming off my alternator is not an OE wire, and it is of sufficient gauge for the purpose, but then it disappears into the OE harness. It's possible (even likely) that the PO opened up the OE harness to run that wire somewhere. The harness is a mess. I have continuity between the wire that comes off my alternator and another wire that comes from the OE harness and goes to the positive terminal on the battery. Looking at the wiring schematic for a '78 FJ40, what I'm probably seeing is the circuit I've highlighted with red.
1978 FJ40 WIRING_2.jpg

The thing is, I don't know where the fusible link is or if it has been deleted. (hope not) It also looks like there are 5A fuses on either side of the ammeter, but I'm not sure where those are either. It does look like I should be okay to omit that circuit completely with a jumper directly from the alternator to the battery, which is what I'll try next.
 
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That was at idle.


I think this might be what I'm seeing. The wire coming off my alternator is not an OE wire, and it is of sufficient gauge for the purpose, but then it disappears into the OE harness. It's possible (even likely) that the PO opened up the OE harness to run that wire somewhere. The harness is a mess. I have continuity between the wire that comes off my alternator and another wire that comes from the OE harness and goes to the positive terminal on the battery. Looking at the wiring schematic for a '78 FJ40, what I'm probably seeing is the circuit I've highlighted with red.View attachment 2415641
The thing is, I don't know where the fusible link is or if it has been deleted. (hope not) It also looks like there are 5A fuses on either side of the ammeter, but I'm not sure where those are either. It does look like I should be okay to omit that circuit completely with a jumper directly from the alternator to the battery, which is what I'll try next.

All the current for the whole system (except the starter) goes through the amp meter, and it's more than 5 amps (rainy day you'll have the heater, headlights and wipers all on at the same time, 55 watt low beams are close to 5 amps alone). So, I'm really not sure how that diagram works... Amp meters, as far as I'm aware, don't work in parallel unless they use a shunt inline.

Fusible links are small sections of wire that are smaller than the rest of the wire, several gauges thinner, and supposed to burn up before the rest of the wire, with extra insulation to contain the burnt conductor. There are apparently some reasons people believe you shouldn't just replace it with a fuse. But as far as I can tell, that's what someone did on my FJ40, and reading all the explinations people have for not doing that, I'm going to continue using a 30 amp fuse right off my battery.

Have you tested voltage with the engine at 1500 - 2000 RPM?
 
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A fusible link is probably much slower to blow than a fuse. That's probably okay on less sensitive circuits.

I have not tested at higher RPMs yet. I'll see if I can get to it tonight or tomorrow night. I'm essentially working two jobs at the moment. The IT department at the University my wife teaches at is not terribly responsive at dealing with the issues of virtual lectures. That type of support is a small part of what I did in the Army, so I've been busy helping her colleagues.
 

Zjohnsonua

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Something that doesn't appear to have been discussed yet...

Looks like you've already swapped the alternator, but if you installed the same type unit you may be having the same issue again...GM 1-wire alternators have a nasty habit of losing their field if they spend long periods at rest. Luckily they can be re-energized with a quick zap of 12VDC to the field. This guy shows you how.
 

John Smith

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Something that doesn't appear to have been discussed yet...

Looks like you've already swapped the alternator, but if you installed the same type unit you may be having the same issue again...GM 1-wire alternators have a nasty habit of losing their field if they spend long periods at rest. Luckily they can be re-energized with a quick zap of 12VDC to the field. This guy shows you how.

I have never heard of this being a thing. But yeah, direct from the DelcoRemy folks Tech Tip: Flash the Field | Delco Remy


If you’ve installed an alternator and it won’t charge, don’t remove it just yet. Here’s why: most Delco Remy alternators have "Auto Start" capability that doesn’t require a separate wire to excite the alternator to charge. During the manufacturing or remanufacturing process—when each alternator is tested for performance—a residual magnetism is generated in the alternator, so it begins charging as soon as the vehicle starts.

On rare occasions, the alternator can become demagnetized when it’s been on the shelf for a prolonged period of time. When that happens, the voltmeter indicates battery voltage instead of the typical alternator charging voltage.

Before you remove the alternator, first try to determine if a loss of residual magnetism is causing the loss of voltage. You can do this by flashing the field:

  1. With the vehicle ignition in the “off ” position, remove the rubber boot from the alternator battery terminal and the rubber grommet from the small “R” terminal.
  2. Take a small piece of wire and touch one end of it to the battery alternator terminal. Then touch the other end to the small "R" terminal. (Note: This may create a small spark, which is normal.)
  3. Remove the wire and start the vehicle. The voltmeter should now increase its voltage reading to around 14 volts.
  4. If the alternator isn’t charging, then re-check all connections and belt tension before removing the alternator.
  5. Put the rubber boot and the rubber grommet back on.
 

Zjohnsonua

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We don't hear about it as often anymore, as there aren't as many of them in every day use, but the alternator shop loves it when people bring these in. Easiest repair dollars they make all day...
 
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I’m running a 120 amp alternator out of an old cop car. There’s no way I’d run it through the stock wiring. The 1/4” thick GM wire has intermittently gotten warm enough to cause the insulation to shrink away. It’s not a one wire, and for what it’s worth, I’d not bother with a one wire, the ignition + connection isn’t hard to wire and then wire it directly to the battery.
 
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Last night I disconnected the line from the alternator that I believe goes to the ammeter before coming back to the battery. I made a jumper to go from the alternator directly to the battery. Started the truck up and checked with the revs up a bit: nothing at all. Shut everything down. Put it back the way it was before. Started the truck up and checked with the revs up a bit: nothing at all. Did the flash the field thing. Tested again. Nothing. Got mad. Went inside. Pet dogs. Felt better.

I'll pull the (new) alternator off and take it in to have it tested tomorrow. Maybe I killed another one?
 

pb4ugo

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When you ran the wire directly to the batt did you test it with a multi-meter? Hooked up that way, The amp meter probably wont work. Hooking up a 1 wire GM alt you may want to check out the Mad Electric website. It describes the various ways to wire the alt.
 
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Yes, I've been using a multi-meter from the beginning. I'll check out that website, but I'm at the point now that I may just covert it to 3 wire.
 

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