Where does this wire go?

[Tranz Zam]

It comes off the distribution block on the firewall. Its a heavy gauge wire, so I'm assuming it goes to a direct power source like the battery or alternator, but I'm not positive.

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[Tranz Zam]

Anyone, anybody?

 

[Georgeb]

In many cases it didn't go anywhere. I believe it was for adding additional accessories if desired.

 

[da_raabi]

Get it? Not positive? Get it? Cuz its a 12v hot wire?

 

[Georgeb]

Get it? Not positive? Get it? Cuz its a 12v hot wire?

Im positive about the negative but Im a little negative about the positive.

 

[gmachinz]

You should have a 12 or sometimes a 10 gauge power feed to your firewall bulkhead-this is where your bussed BAT power comes from inside the truck.

 

[Tranz Zam]

You should have a 12 or sometimes a 10 gauge power feed to your firewall bulkhead-this is where your bussed BAT power comes from inside the truck.

So should I run it directly to the battery? I have a wire coming from the battery going to the alternator, and the smaller wire coming off of the distribution block is already going to the alternator. So that block is getting direct power already.

I can run it to the alternator, but it seems redundant.

 

[Rusty Nail]

Hold on.

I got the same deal and everything works fine, so it hangs there. Why does it need terminated so badly? If it ain't broke, don't fix it.
No tellin what will happen!

So I took a quik pic and I see it attached to some wire dealy. I know there isn't a fuse in that holder, but I wonder where it goes?!

Meh. Everything is fine, idgaf where it goes.
See?

 

[Tranz Zam]

Hold on.

I got the same deal and everything works fine, so it hangs there. Why does it need terminated so badly? If it ain't broke, don't fix it.
No tellin what will happen!

Well I didn't want to leave a 10 gauge, constant hot wire hanging around. I guess I'll leave it off for the initial startup, and if it appears I don't need it I'll just remove it completely.

If I'm not losing my mind, there is already power getting to everything, so I shouldn't need it.

 

[gmachinz]

So should I run it directly to the battery? I have a wire coming from the battery going to the alternator, and the smaller wire coming off of the distribution block is already going to the alternator. So that block is getting direct power already.

I can run it to the alternator, but it seems redundant.

No-that big 10ga wire needs to go to your firewall bulkhead if you don't have one there already....where are the two smaller fusible link protected wires going to?

 

[Tranz Zam]

No-that big 10ga wire needs to go to your firewall bulkhead if you don't have one there already....where are the two smaller fusible link protected wires going to?

The bulkhead as in where the harness plugs into the fusebox?

The other wires run to the back of the alternator, and to the starter. The big 10 gauge wire was cut before I got the harness, so I have no idea where it goes.

Its confusing because I have the LS harness running the engine, and I really only need the stock engine harness to engage the starter and provide power to the cab harness.

 

[chengny]

Did the truck come with an auxiliary battery option? If so that wire may have lead main battery power over to the aux. battery relay:

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[Tranz Zam]

Did the truck come with an auxiliary battery option? If so that wire may have lead main battery power over to the aux. battery relay:

Hmmm, possibly? The engine harness is out of an 86 that had a plow, so it might of had dual batteries.

My trucks wiring has been a brain buster. I'm working with the following:

1978 Cab harness
1973 Tail light / Headlight harness
86 Engine harness
2000 5.3 LS harness

Another quick question, my cab harness is from a 78, but my cab is an 87. The 78 harness was a heat only truck, but the 87 cab is an A/C truck. The HVAC harness plugs into the cab harness separately, but the two dont appear to work together. The plugs are different. Any idea on what to do here? I have a feeling I'll be busting diagrams and a test light out for this one.

 

[chengny]

I'm not sure I understand what exactly you are saying here, but I'll try:

Another quick question, my cab harness is from a 78, but my cab is an 87. The 78 harness was a heat only truck, but the 87 cab is an A/C truck. The HVAC harness plugs into the cab harness separately, but the two dont appear to work together. The plugs are different. Any idea on what to do here? I have a feeling I'll be busting diagrams and a test light out for this one.

Is the 87 HVAC (A/C setup) harness still installed? If so - and all the engine side components are still there - don't worry. Wiring the 87 HVAC harness into the truck's main electrical harness and connecting it to the engine side components will be easy.

There are only two leads in that A/C harness that need to be connected to the main electrical system.

One lead (brown) comes from the fuse block and connects to the pigtail on back of the control assembly. This feed is used for the control side (i.e. speed control) of the blower circuit and also compressor clutch activation.

The other external connection for an A/C equipped truck is the big red wire in the engine compartment. It connects to the firewall junction block and is the power supply to the blower. It is lead to the blower through the blower relay.

These wiring diagrams attempt to explain how fan speed is controlled:

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If the cab is set up for A/C (and you intend to use it) you will need an A/C specific harness. There are significant differences between the two years in the layout of the engine side of the systems - both in physical locations and type/number of control devices. These dwgs will give you an idea:

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But, other than that brown lead from the fuse block and the red lead to the JB, the AC harness is entirely segregated from the rest of the electrical system.

You will of course need a late model HVAC control panel, but that should still be mounted in the dash - correct?

On this part I don't follow you at all:

The HVAC harness plugs into the cab harness separately, but the two dont appear to work together. The plugs are different.

Are you talking about the style of connector used to connect the brown wire from the fuse block to the pig tail on the control panel? Like this?

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Those connectors are a pain to make up. If the two halves don't match up you will have to use solderless connectors.

 

[gmachinz]

If you want to install all NEW harnesses, I can provide you with them-I can build to suit your setup while keeping it all GM in appearance! PM me if interested-I've done a lot of "modified" new harnesses just like you're describing!