87 R10 Battery/Starter/Alternator Wiring

[mike rust]

I'm swapping the factory 305 with a 5.3l in my 87 R10. I plan to use the 87 batt/alt/starter wiring with the 5.3l and I can't recall how the wires were routed.

The positive battery cable is self explanatory......to the starter and the alt. Its the top red wire from firewall bulkhead that has me confused. Roughly 4ft past the bulkhead there is a ring terminal, then another ring terminal after another ~4ft. Does it go to the alt then to the HVAC junction towards the middle of the firewall?

Thanks in advance for any help you can give me on this one.

 

[Charlie]

 

[75gmck25]

I don't know if the wiring on your '87 is different from my '75, but I still have the original wiring setup in mine.

- Battery post has large cable running to the large lug on the starter.
- At the starter lug there are two small (10 gauge) wires with fusible links. To physically protect them when coming up the back of the engine, they run up through a metal tube that is bolted to the back corner of the bell housing. The wire to the solenoid also runs through that tube.
-- 1st 10 gauge wire runs almost straight vertical, then out across the valve cover (in flex conduit) to the alternator BAT terminal.
-- 2nd 10 gauge wire runs up and then over to the bottom of the power junction that is right next to the brake booster. This is your constant 12 volt power to the truck.

The power junction next to the booster has at least two cables running "out" to power the truck.
-- One runs straight over to the back of the fuse box.
-- One runs the opposite direction across the top of the firewall to the blower relay. This line is fused separately with a 30 amp inline fuse about midway between the power junction and the relay.

On the next generation of trucks (GMT 400) they used a power junction setup that I like better. They ran power from the starter lug up to a power terminal strip on the firewall that has about 6 threaded lugs, and then powered most of the truck from there. I've already pulled one of these from the junkyard, and plan to use it during later projects. I already used the alternator charge wire from the newer truck to get a larger gauge charge wire that already has a fusible link molded on.

Bruce

 

[mike rust]

I don't know if the wiring on your '87 is different from my '75, but I still have the original wiring setup in mine.

- Battery post has large cable running to the large lug on the starter.
- At the starter lug there are two small (10 gauge) wires with fusible links. To physically protect them when coming up the back of the engine, they run up through a metal tube that is bolted to the back corner of the bell housing. The wire to the solenoid also runs through that tube.
-- 1st 10 gauge wire runs almost straight vertical, then out across the valve cover (in flex conduit) to the alternator BAT terminal.
-- 2nd 10 gauge wire runs up and then over to the bottom of the power junction that is right next to the brake booster. This is your constant 12 volt power to the truck.

The power junction next to the booster has at least two cables running "out" to power the truck.
-- One runs straight over to the back of the fuse box.
-- One runs the opposite direction across the top of the firewall to the blower relay. This line is fused separately with a 30 amp inline fuse about midway between the power junction and the relay.

On the next generation of trucks (GMT 400) they used a power junction setup that I like better. They ran power from the starter lug up to a power terminal strip on the firewall that has about 6 threaded lugs, and then powered most of the truck from there. I've already pulled one of these from the junkyard, and plan to use it during later projects. I already used the alternator charge wire from the newer truck to get a larger gauge charge wire that already has a fusible link molded on.

Bruce

Thanks for the reply.....looks to me that this wiring changed slightly from the earlier trucks, but is fundamentally the same.....the junction box on the firewall appears to be powered from the alt rather than the starter on the later trucks.

I would still like to hear from someone with an 87+ square body.

 

[RoryH19]

Try this youtube,

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He has a 2 part bulkhead wiring video.


81-87 is basically the same only difference is the bulkhead has a wire for the fuel pump in 87+ since they are TBI.

I swapped my k5 with a 6.0/4L65e
It's a good reference but I didn't follow exactly. I also have dual electric fans.
Also have drive by wire throttle (DBW) and cruise control.