Wiring upgrades

[Ebert89]

Hey guys, having issues with the volt gauge on the dash being incorrect. My multimeter shows 14.1 volts at the battery with AC, radio, all head and lights so I am assuming the alternator is doing its job. Looking at some threads I'm considering replacing the Big 3, already replaced the starter last month with the cable running it to the battery. Any issues I can run into with the link to the fuse box or the little alternator cable? Thanks

 

[Donald Mitchell]

It's probably the gauge. Welcome to the forums!

 

[Ebert89]

It's probably the gauge. Welcome to the forums!

Thanks! The gauge dances with my turn signal too it probably is busted.

 

[1998_K1500_Sub]

Check the ground straps on the passenger's side, one from the frame to the engine and another from the frame to the body firewall. They can get gnarly with age. They're both located near the firewall.

I'm not saying this is the problem with your gauge.

 

[Ebert89]

Check the ground straps on the passenger's side, one from the frame to the engine and another from the frame to the body firewall. They can get gnarly with age. They're both located near the firewall.

I'm not saying this is the problem with your gauge.

Just checked, they're a bit torn up I'm gonna replace them tomorrow thanks

 

[1998_K1500_Sub]

I should have mentioned a few other, typical trouble spots (do not assume this is a complete list):

- the ground connection for the fuel pump to the frame. On the Suburban it's near the rear bumper (just look for the wire bolted to the frame). On the trucks, I'm not so sure... presumably near the fuel tank.

- the ground strap from the firewall to the hood, which may not be present on all GMT400s. It was on mine, perhaps because I have the automagic light on the underside of the hood


Then, you might take your DVM and start making some measurements while the electrical system's under load (lights on, blower fan on high, AC on, etc.), e.g.,

- alternator ground (engine block) to (a) frame, (b) firewall / body, (c) battery neg terminal, (d) radiator support (many lights ground here)

- alternator pos to (a) stud on fuseblock (on firewall driver's side, at least on the 1996 and later models), (b) battery pos, (c) other locations of interest

If you find anything odd, investigate. Post questions here if you have them.

IMHO an acceptable voltage drop across a high-current conductor when under max load is ~0.1-0.2V.

The engine starter circuit voltage drops will be higher.

Maybe I'll come up with some other ideas after dinner.

 

[Ebert89]

I should have mentioned a few other, typical trouble spots (do not assume this is a complete list):

- the ground connection for the fuel pump to the frame. On the Suburban it's near the rear bumper (just look for the wire bolted to the frame). On the trucks, I'm not so sure... presumably near the fuel tank.

- the ground strap from the firewall to the hood, which may not be present on all GMT400s. It was on mine, perhaps because I have the automagic light on the underside of the hood


Then, you might take your DVM and start making some measurements while the electrical system's under load (lights on, blower fan on high, AC on, etc.), e.g.,

- alternator ground (engine block) to (a) frame, (b) firewall / body, (c) battery neg terminal, (d) radiator support (many lights ground here)

- alternator pos to (a) stud on fuseblock (on firewall driver's side, at least on the 1996 and later models), (b) battery pos, (c) other locations of interest

If you find anything odd, investigate. Post questions here if you have them.

IMHO an acceptable voltage drop across a high-current conductor when under max load is ~0.1-0.2V.

The engine starter circuit voltage drops will be higher.

Maybe I'll come up with some other ideas after dinner.

Gonna try to find one of those service manuals and clean up all grounds I can, will perform measurements soon

 

[1998_K1500_Sub]

There are many years' GMT400 service manuals accessible as .pdfs, but unfortunately I don't have a link to them handy. Perhaps @df2x4 will rescue me and post the link.

Measurements will tell you where to spend your time and money. Big-3 is neat but I'm not convinced it's a necessity in the general case.

I do think there's one area that should be addressed on every GMT400 (and many older GM vehicles): The headlight wiring.

The headlight circuit runs through the firewall into the cab, through the dash on/off and high/low switches, then back through the firewall into the engine bay and up to the radiator support. There is a lot of resistance in that path. A better, not necessarily best, simple remedy is to fly a couple relays in under the hood on the driver's side near the wheelwell, trigger their coils from the existing LtGrn and Tan wires from the headlight switch (gain access to them right there in the loom that goes along the driver's fender, cut them there), and ground the other side of their coils. Add a power supply lead from the fusebox stud to the relays, and connect the relays' outputs back to the existing LtGrn and Tan wire segments that lead to the headlights. This solution involves minimal wiring and garners most of the benefits of the fancier "harnesses" you'll hear about / find for sale, if you're using stock wattage incandescent bulbs. Higher wattage bulbs warrant other improvements and your DVM can tell you what needs attention.

 

[df2x4]

There are many years' GMT400 service manuals accessible as .pdfs, but unfortunately I don't have a link to them handy. Perhaps @df2x4 will rescue me and post the link.

Gotcha!

https://www.gmt400.com/threads/88-98-service-manuals.43575/

FYI, that's a stickied thread in the "Engine Performance + Maintenance" section. Should be one of the first 10 or so threads that pop up on that page every time.

 

[Ebert89]

There are many years' GMT400 service manuals accessible as .pdfs, but unfortunately I don't have a link to them handy. Perhaps @df2x4 will rescue me and post the link.

Measurements will tell you where to spend you time and money. Big-3 is neat but I'm not convinced it's a necessity in the general case.

I do think there's one area that should be addressed on every GMT400 (and many older GM vehicles): The headlight wiring.

The headlight circuit runs through the firewall into the cab, through the dash on/off and high/low switches, then back through the firewall into the engine bay and up to the radiator support. There is a lot of resistance in that path. A better, not necessarily best, simple remedy is to fly a couple relays in under the hood on the driver's side near the wheelwell, trigger their coils from the existing LtGrn and Tan wires from the headlight switch (gain access to them right there in the loom that goes along the driver's fender, cut them there), and ground the other side of their coils. Add a power supply lead from the fusebox stud to the relays, and connect the relays' outputs back to the existing LtGrn and Tan wire segments that lead to the headlights. This solution involves minimal wiring and garners most of the benefits of the fancier "harnesses" you'll hear about / find for sale, if you're using stock wattage incandescent bulbs. Higher wattage bulbs warrant other improvements and your DVM can tell you what needs attention.

Will look into this! I do have issues with the headlight switch not working at times, it's come loose off the dash panel