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Discussion Starter · #1 ·
Hi guys,

I have a few questions, I'm just going to lump them into one post here. So I just installed a new sbc 350 into my 85 GMC truck, and I am able to get the engine to turn on and run, but the alternator isn't charging the battery and it dies after some time. The alternator is brand new, and when I check the terminals on the alternator they read out around 14V, so I think it must be a problem with the wiring. Attaching an image below, but could someone tell me if this is the fusible link for the alternator, and if this needs to be replaced?

Also, I am noticing that the engine seems to almost die when I press the gas. Anyone know what would cause this? There is also an issue with the transmission, nothing seems to happen when I shift gears(I am pretty sure something on the transmission works because it does go into park when I set to park, I can clearly see the shift linkage move when shifting). At one point I put the truck into drive and then pressed hard on the gas, and it sounded like the engine was racing on the highway going very fast. Letting off the gas and applying the brakes didn't seem to do anything, so I shifted the truck into park and tried to turn it off by taking the key out. The truck started violently popping and backfiring out of the exhaust and even off kept trying to run (I'm guessing it was still getting oxygen). It continued like this for a few minutes before finally sputtering to a stop and then spitting out gas out the top of the carb. Is this an engine issue or a transmission issue?? Thanks for the help!!

Automotive tire Motor vehicle Wood Tread Gas
 

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Typical wiring in your vintage of truck with a 10si or 12si alternator -
Alternator charge wire is connected to the threaded BAT terminal on the alternator. Wire runs down the valve cover to a metal tube that is bolted to the back of the engine. It runs down through this protective tube to the large terminal on the starter. The fusible link is on the starter end of that wire. That same starter terminal is where the large positive battery cable is connected.

Alternate simplified wiring- Connect an 8 to 10 gauge charge wire with fusible link to the positive terminal of the battery . Run it straight across and down behind the alternator to keep it away from the belt, and connect to the BAT terminal.

There is also a two wire plug on the back of the alternator. One wire is the sense wire and one is the exciter wire. The red sense wire was originally connected into the wiring loom, but may have been connected to the BAT terminal if someone modified the wiring. The exciter wire should have 12 volts switched through the ignition. This is one simple wiring diagram. 10si wieifn - Google Search
 

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I'm pretty sure the cable you have circled in red on your picture is the power wire going to the fuse box in the cab.

The alternator charge cable usually runs back to the starter. And if there's a fusible link, it's generally at the starter end.

Start checking voltages at the battery and at the starter power post (where the battery cable is attached) with the engine running. It should be close to or over 14V the whole way.
 

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It's been 5 years since I re-wired an 82 C10 pickup, but here are my best recollections:

The wires on the firewall block are probably like this:

Left side: Originates on the starter's BAT terminal and runs through a fusible link.
Right side: Large wire feeds fuse panel, small wire goes to alternator sense terminal. (This is where the regulator looks for 14.2 volts.)
The metal strap is just a way to connect the 3 wires.

On my 82 C10, there was a second large wire (metric, but around 11 gauge) running from starter BAT terminal thru a second fusible link to the fuse panel. So there' plenty of current capacity to the fuse panel -- two 11 gauge wires vs. one 12 gauge wire on older trucks. This wire did not go to the firewall terminal block, but, as I recall was in a bundle lower on the firewall.

I don't remember how the OE charge wire (alternator to battery) was connected, but it sounds like you have a bad connection. I'm not sure if there was a fusible link in that wire.
 

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Discussion Starter · #7 ·
Hey guys, so I think I found the problem - the alternator cables to the starter are pretty fried. I've been looking everywhere online but cant seem to find this - it's 3 cables coming together with two fusible links I think? the black cylindrical things (I think are the fusible links) say 16 and 14, are these gauges for the wire? Does anyone know if there's a part number I can buy that will replace this, or should I just solder three fusible links all together?
 

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Go to the auto parts store and buy the appropriate size fusible link, eyelet, and make that little harness. Fusible link is sacrificial wire that burns up under to much load. The black plastic parts are probably just crimps that are enclosed in plastic.

Keith
 

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If your alternator is an exact replacement for the stock alternator (exact same amperage capacity), making an exact replacement for that would work.

Do both wires go to the same place? If they do, you might be better off upgrading to a Midi fuse bolt down fuse block and fuse on the firewall, next to the junction that the wire goes to, with a single larger wire connecting that to the alternator, and a short larger wire connecting that to the alternator. Fusible links are a PITA, even for someone who does soldered lap or lash splices all the time.

Two cables in parallel = "-4" on the AWG size of the wire. So if that's 2 12AWG wires, it's equal to a single 8AWG wire. And if you absolutely must use a fusible link, you want fusible link wire (it's a special kind of wire) that's 4 gauge sizes smaller (+4 on the AWG number) than the wire it's protecting, and there should be at least a 4 inch +- piece of the fusible link wire in the circuit.

If you've upgraded the alternator to a higher capacity, higher amperage unit, you need to upgrade that wire to a larger size. The size of the wire depends on the amperage capacity of the alternator.
 

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Hey guys, so I think I found the problem - the alternator cables to the starter are pretty fried. I've been looking everywhere online but cant seem to find this - it's 3 cables coming together with two fusible links I think? the black cylindrical things (I think are the fusible links) say 16 and 14, are these gauges for the wire? Does anyone know if there's a part number I can buy that will replace this, or should I just solder three fusible links all together?
The black cylindrical things are the splices encased in molded plastic. The fusible links are the lengths of wire between the splice and the ring terminal.
 

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Discussion Starter · #12 ·
Update -finally got around to replacing these. As soon as I turned the truck on, smoke started coming pouring out near the starter. Still not charging the battery. I'm wondering if there's a short happening somewhere? I used 10 awg for all of it which I think is good. I checked the voltage on the black wire (I think the excite wire?) And that was showing as 12V. Will check the other two wires and see what I get. Wondering if I should hook up the sense and bat terminals on the alternator and see if that works?
 

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"Turned the truck on" ... Is this strictly key on or key on + start? Either way, seems you have a dead short somewhere.

Russ
 

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One thing I find helpful when chasing a "dead short" is a light socket with bulb, with alligator clips on the wires. Disconnect the negative battery cable and clip the light between the battery post and the cable end.

The light bulb will limit current/power to the wattage of the bulb, and it will be bright if there's a dead short. If there's very little current draw, the light will not illuminate, and it will allow low current to pass through with little resistance. This is also helpful (with a lower wattage bulb and matching socket) for chasing current drain issues.
 

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Pretty sure an '85 didn't have points, but if some previous owner once did a conversion, maybe there's an old bypass wire from the starter solenoid supplying 12v to where a coil once would have been that's either grounding or inadvertently hooked up to something it shouldn't be?

Russ
 

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Your starter smoking makes me think it was experencing a heavy load.

Bad grounds can cause all kinds of issues and is the first place you should start. Once you confirm your grounds are good with clean metal hitting clean metal and no broken strands inside the shielding itself you can start testing.

Now you can also have a heavy load from the starter and it can melt the battery terminals or damage the battery internally. This is not always easy to notice as the alternatior can "make up" for a battery that is only holding 8 or 10 volts.

So do your grounds, take your starter and battery to a store and have them tested (it should be free). Then test your wiring double and tripple checking it before installing your new parts(if you bought new parts).
 
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