I figured out what caused my truck to lose power, but I don't want to fix it by repeating old (incorrect) wiring!

[RabbitMan]

Hello everyone,

I've always wanted a squarebody, and I finally snagged a sweet 2WD 1988 R20 Suburban with a 454, full float axle, and brown/gold two tone paint. This thing is the best!

I only got to drive it around a bit before the engine unexpectedly quit. I poked around and eventually pinpointed this wire as the culprit (see pictures). It looks like it carries 12V from the battery/alternator to the junction (pin 24, by the looks of it, based on this wiring diagram booklet). If I jiggle this wire a bit while the engine is running, I can get the engine to cut out.

Now, I could just cut this out and replace it with fresh wire, but I suspect that this is not the correct way to wire power to the junction on the firewall given how it's kinda loose in the engine bay and haphazardly zip tied to stuff. Or maybe it is? What do you guys think?

Also, during troubleshooting, I only found a fusible link at the battery, and I think one at the junction. I assume there are more. Does anyone have any good pics of where these fusible links would be on a '88 3/4 ton?

You must be registered for see images attach

 

[YakkoWarner]

I am very confused here - is your alternator on the passenger side? I have the 1989 version of pretty much this same truck (3/4ton Suburban/454/TH400/FF 14 bolt) and my alternator is on the driver side with the AC compressor on the passenger side. That wire looks pretty thin to be the main power output as well.

 

[RabbitMan]

I am very confused here - is your alternator on the passenger side? I have the 1989 version of pretty much this same truck (3/4ton Suburban/454/TH400/FF 14 bolt) and my alternator is on the driver side with the AC compressor on the passenger side. That wire looks pretty thin to be the main power output as well.

Hey Yakko. Yep, my alternator is on the passenger side (see pic of the engine bay). Note that I removed the intake tube which leads to the front of the engine bay because I needed more access while troubleshooting that electrical issue.

What wire gauge do you recommend? I'm thinking 12ga or thicker, silicone jacketed, routed around the engine in a pre-existing wire bundle on its way to the positive terminal of the battery.

 

[YakkoWarner]

If thats the primary power output, I'd consider 10 gauge or maybe even 8 (10 is normally good for 30 amps, 8 is good for 40 amps but in a hot engine bay wire resistance may become a factor and shorter is better). If that wire is only the exciter (gets 12V from key on power source to energize the coils in the alternator) than a smaller gauge is probably fine. It probably should also have a fusible link in there somewhere.

On my truck the alternator is where the A/C compressor is on yours, so the power wire just runs inside the valve cover along the intake straight back to the firewall (I used some old heater hose as a conduit to protect it). There is a second wire going from the back of the alternator over to the battery. I belive both are 10 gauge but I wouldn't claim either of them to be original - I'm also not convinced my alternator is original either.

 

[H2OnSnow]

Have you looked at the wiring diagram for your truck? The one you showed is an '88. You can find it here: https://www.gmsquarebody.com/threads/1973-1991-wiring-manuals.14498/

That has where all of the connections are supposed to go, including the pin on each connector it hits (but not the routing), and what size wire they designed for that. It takes a little practice to figure everything out, but it is something that is required for owners of old vehicles. You showed that it goes to the bulkhead connector, but not what it feeds on the far side of that. That point is not what the 89 schematic shows. Figure out what is at the far end of that run. That might inform what you should or shouldn't be doing, or fixing. It might not be that wire, but instead, whatever is on the far side of the firewall. Look full end to end before trying to decide what to do.

 

[RabbitMan]

Have you looked at the wiring diagram for your truck? The one you showed is an '88. You can find it here: https://www.gmsquarebody.com/threads/1973-1991-wiring-manuals.14498/

That has where all of the connections are supposed to go, including the pin on each connector it hits (but not the routing), and what size wire they designed for that. It takes a little practice to figure everything out, but it is something that is required for owners of old vehicles. You showed that it goes to the bulkhead connector, but not what it feeds on the far side of that. That point is not what the 89 schematic shows. Figure out what is at the far end of that run. That might inform what you should or shouldn't be doing, or fixing. It might not be that wire, but instead, whatever is on the far side of the firewall. Look full end to end before trying to decide what to do.

It looks like I was looking at these diagrams incorrectly the first time. My red wire actually connects to pin 2, and it appears to supply power to the following circuits:

ECM B
TAIL LPS/ CTSY LPS/ PARK LPS
HORN/DM

This wire appears to be named "2-3.0 RED - IGN SW - BATT". It also looks like it should be spliced to a fusible link connected to the "junction block" (see diagram). From here it looks like the connection proceeds to the starter solenoid, but I have to check this later with a multimeter because when checked visually on the truck, the junction block does not seem connected to the starter solenoid at all.

I'm still not sure about the wire gauge, but I now have much bigger fish to fry. I disconnected the firewall bulkhead connector and discovered that there is significant heat damage on the entire connector! I circled contact #2 in the pic below, and ironically, it appears to be one of the less damaged ones.

While my red wire is incorrectly connected, it wouldn't be responsible for all this melting, right? I'd assume the issue is more related to a ground connection these circuits all have in common, probably the battery's connection to the sheet metal. However, I checked the resistance between the battery (-) and sheet metal and measured 0.4 ohms, which doesn't seem too bad. At the end of the day, I need to replace this connector (and the fuse box, assuming the bulkhead connector is integral with it). I'm not finding any vendors online who sell this. Got any tips?

You must be registered for see images attach

You must be registered for see images attach

You must be registered for see images attach

 

[H2OnSnow]

I haven't looked inside one of those connectors in a while, but I'm guessing that's waterproofing grease.

The circuit you listed is a 12ga wire. The first few pages in that wiring diagram are very valuable for reading and troubleshooting from those diagrams.

You must be registered for see images attach

You must be registered for see images attach

 

[Terlingueno]

It looks like I was looking at these diagrams incorrectly the first time. My red wire actually connects to pin 2, and it appears to supply power to the following circuits:

ECM B
TAIL LPS/ CTSY LPS/ PARK LPS
HORN/DM

This wire appears to be named "2-3.0 RED - IGN SW - BATT". It also looks like it should be spliced to a fusible link connected to the "junction block" (see diagram). From here it looks like the connection proceeds to the starter solenoid, but I have to check this later with a multimeter because when checked visually on the truck, the junction block does not seem connected to the starter solenoid at all.

I'm still not sure about the wire gauge, but I now have much bigger fish to fry. I disconnected the firewall bulkhead connector and discovered that there is significant heat damage on the entire connector! I circled contact #2 in the pic below, and ironically, it appears to be one of the less damaged ones.

While my red wire is incorrectly connected, it wouldn't be responsible for all this melting, right? I'd assume the issue is more related to a ground connection these circuits all have in common, probably the battery's connection to the sheet metal. However, I checked the resistance between the battery (-) and sheet metal and measured 0.4 ohms, which doesn't seem too bad. At the end of the day, I need to replace this connector (and the fuse box, assuming the bulkhead connector is integral with it). I'm not finding any vendors online who sell this. Got any tips?

You must be registered for see images attach

You must be registered for see images attach

You must be registered for see images attach

First thing I would do is clean all that mung out of those connectors.