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kipps 05-16-2019 08:23 PM

Wire gauge question
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Looking at this 1988 manual, here it's specifically showing the R/V(squarebody) trucks. Am I reading this correct, that the first number is a circuit number, and the second number is a square mm cross-section?

If so, I can cross-reference that size number and come up with a AWG size. Link here.

Reading those five wires in the middle of the screen, I come up with 22, 12, 12, 12, and 18 gauge respectively. Is this the correct way of interpreting it? Why did GM not list that 22 gauge wire according to its metric equivalent?

Another unrelated question. Did GM ever install a factory trailer plug or brake unit in the squarebody trucks, or was it just up to the owner to set up his truck for pulling a trailer?

ray_mcavoy 05-16-2019 09:18 PM

Re: Wire gauge question
Yes, you are reading the diagram correctly with the first number indicating the circuit number and the second being the cross sectional area of the wire in square mm.

Many of the factory manuals also included a wire gauge conversion chart similar to the one in the link you posted.

The circuit #130 (22 gauge brown/white wire) is a special resistance wire. So that's probably why it continued to be specified by AWG rather than the metric equivalent.

kipps 05-17-2019 05:40 AM

Re: Wire gauge question

Originally Posted by ray_mcavoy (Post 8526860)
The circuit #130 (22 gauge brown/white wire) is a special resistance wire...

That's a new one to me. Can you explain further?

That brown/white wire is providing a ground for the brake light as a test in the key-start position. Why would anything special have been required?

I'm replacing all the wiring on the truck with my own custom harness. So I will need to deal with this wire.

ray_mcavoy 05-18-2019 09:57 PM

Re: Wire gauge question
Circuit #33 (tan / white) provides the key-start ground for the brake warning light bulb check. That one is a regular wire.

Circuit #130 (brown / white) is the alternator "exciter" circuit. It provides some current to the alternator field when the ignition is switched on. The resistance wire is used to prevent the alternator's diode trio from being connected directly to the battery (through the ignition switch). And it also prevents feedback that can keep the ignition powered via the alternator after the key is turned off.

On vehicles with an "ALT" or "GEN" light, the bulb is wired into this circuit and provides the necessary resistance. Some applications have the resistance wire connected in parallel with the light to serve as a backup in case the bulb burns out. And vehicles with a factory ammeter or voltmeter (and no light) usually just have the resistance wire.

If you can't salvage & re-use the resistance wire from the old harness, it can be replaced with a resistor. The value required will depend somewhat on the alternator you're using. The older SI series alternators were okay with as little as 10Ω but the newer CS series need a minimum of 35Ω. But I believe both of them will work with up to around 150Ω or so and the higher resistance reduces the current so you can use a lower wattage resistor. And if you're using a CS alternator that has an "I" terminal (not all of them do), that can be used instead of the "L" terminal and connected directly to the ignition switch without the need for a resistor or resistance wire.

kipps 05-19-2019 12:51 PM

Re: Wire gauge question
Sorry, my mistake. I didn't look up the circuit number when I wrote post #3. I went off of my memory, which was faulty.

I don't know if this wire still applies in my situation. I'm installing a LS engine, complete with alternator, ECM, and underhood fuse box; all from a 2010 tahoe. I think the alternator is handled completely within that complex, but I need to check to be sure.

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