Here's an interesting one - this actually is for a '73 Galaxie, and not a truck. However, the wiring is put together very similarly to the same era pickup trucks, and even many of the wire colors are the same between the two. So, much of the same stuff applies.

I'm rebuilding the car's main dash harness because the turn signal power fused out and smoked everything in its path. I'm doing most of the patchwork by hand, but I do have a harness out of a '77 LTD wagon that I've been able to swap some paths over directly.

Both cars have a row of idiot lights across the instrument cluster and a fuel gauge. While working on the instrument cluster, I expected to find an electromechanical regulator to provide 5 volts to the fuel gauge. What I didn't expect to find, however, was that the power to the regulator isn't a full 12 volts; it's dropped down through a resistor wire (just like the coil). The resistor wire is black with either a green or yellow stripe (it's so aged that it's hard to tell). Looking at wiring diagrams for my '79 truck, it appears this was used in trucks as well. So maybe someone else here has seen it too.

I need to repair or replace it since the insulation is gone in several places - but since it's a resistor wire and it builds its resistance over its length, I figured it would be better to just swap the entire resistor wire from the other harness in (it can be removed without cutting by pulling out the crimp terminals on each end). That's all fine, but I happened to measure them both and found that the resistor wire out of the '73 harness has a total resistance of 12.5 ohms, while the resistor wire out of the '77 harness measures 8.3 ohms. This surprises me since I figured the regulator was pretty much the same between the two years, but perhaps I'm wrong. Does anyone have any insight as to why two different resistances were used, or why the power to the regulator is dropped down in the first place?

What I figured I'd do is install the '77 wire, and then while I'm testing the harness before I put it back in the car, just put 10 and 70 ohm loads (to simulate the sending unit) downstream of the gauge and make sure that the fuel level still reads correctly with my '73 gauge and regulator. I'd also check to see that nothing gets too hot with the 10 ohm case. Another option I'm considering is doing away with this setup and running a full 12 volts to a 7805 regulator, and powering the fuel gauge off that. I've got a few other things to finish before I make it to this, so I haven't decided yet.

Sorry for the length - since this is such an obscure observation, I don't expect to get much traction in this thread, but I figured I'd see if somebody knows something I don't. Thanks in advance.

 

I have pretty good diagrams for the 80-86 trucks, and they don't show a resistance wire in series with the gauge regulator. I didn't know they used one in the earlier years.

If you are worried about the insulation of the wire, (and this wire needs to stay in place it sounds like), I would find a piece of shrink tubing, a small piece of vinyl tubing or even a piece of very small vacuum line, and slip that over the original wire to insulate it, and re-use this special wire.

To test the gauges, I just ground the sending unit wire, or let it hang in the air(zero resistance or infinite resistance). This will exercise the gauge enough full scale both ways to see if its working without harming it.

 

Thanks for the input. After reading up a little more, I think the resistor wire might actually be the shunt for the ALT light, and not gauge regulator power. That would make a lot more sense. I need to take a closer look at the traces on the printed circuit behind the cluster and I'll follow up.

 

Here's a picture of the resistor on a 80-up cluster with the "gen" light.