This post on the Binder Planet talks specifically about a problem I’m having with my gas gauge, and has steps to fix the problem. Now, to find an incandescent test light.
Here’s another good update:
There’s only one wire from the gauge to the sender. This is the signal wire. It’s not a positive wire; it will however, show what I call “ghost positive” using a test light. It will appear as a slow flashing at the test light. This “ghost positive” is the CVR power flowing through the guage itself to ground. I call it ghost positive because it’s not really there. You will see the same thing in a two wire light bulb circuit with an open ground wire. Positive is present at one side the bulb but has no place to go; so if you hook a test light to the open ground wire side of the bulb, it will look like that’s a positive wire, it’s not, it’s a ghost. I used to think gauges were smarter than light bulbs but they’re not. The useful thing about this flashing ghost at your signal wire is that it does tell you that you actually have the signal wire, it is making its way to the guage and your CVR is working.
The signal this wire carries is a ground signal to which the gauge responds based on ohms of resistance in the signal. As I said, the sending unit provides variable resistance to ground. It does this via a wiper connected to the float. As the float moves up and down the amount of resistance to ground in the signal changes from 73 ohms when the tank is empty and the float arm is all the way down to 10 ohms resistance when the tank is full and the float arm is all the way up. This is why when you connect the signal wire to a 00 ground, the gauge pegs over “F”.
Update: Here are some other good posts on the subject:
…the ammeter gauge has a direct battery feed. That’s why its still working. The other gauges all receive power from the fuse block when the ignition switch is in the ON or ACC positions through circuit 28. This wire runs to the plastic gang plug and becomes 28A which is connected to the multi-pin on the back of the OIL/TEMP printed circuit board (PCB). From there it runs through a small, rectangular device screwed to the back of that PCB called a constant voltage regulator (CVR). This device regulates the 12 volts into an average of 5 volts which will display as a pulsing light when probed with an incandescent test light. From there it passes back through the gang plug as 28B which daisy chains over to the gang plug on the back of the ALT/FUEL PCB. One must be very careful when disconnecting those plastic gang connectors as the pins are very delicate and easily loosened/dislodged from the PCB. The other common issue with these gauges is the stamped steel nuts that secure the individual gauges to the PCB’s work loose and the ground is lost. Best fix is to replace them with brass nuts, but at the least you need to make sure the steel nuts are snug. Now you have the current path for the instruments, so you should be able to trace it out with your test probe.
This thread has more good info about plastic tanks and grounding:
…I was wondering if anyone knows the proper wiring of the fuel sender on a 1973 scout II with a new plastic 19 gallon tank the PO put in. I have a brown wire on the center screw ” I’m assuming this is the signal wire to the gauge” a red wire with a blade connector attached to the top side of the fuel sender ” My Manual says ground” and an extra black wire with a blade connector going nowhere “was this added for some kind of ground because its a plastic tank?” I read a bunch of other threads and they seem to say the red is the signal going back to the gauge and the brown being ground, but my manual, if I’m reading it correctly says the opposite and there is no mention of an extra black wire. Any help would be appreciated. Thanks
The center pin is the one that goes to the gauge. The spade connectors are for the sender ground and the tank ground. The sender needs the ground to complete the circuit and the tank needs a ground to prevent static build up since it is plastic. To determine which wire is the one that goes to the gauge get an incandescent test light. Connect it to a good ground and with the ignition on test the wires. The one going to the gauge should make the light blink.
And here’s a very simple, perfect bit of info:
The wire in the center is the wire from the gauge. The other wire towards the edge is a ground. A steel tank is grounded to the frame, so it’s not critical, but if there is a lot of rust, it’s better to have a ground on the sender.
I have studying to do.
