Sending unit not sending...

[qwik1320]

Yes the gauge worked before the accident... I was rear ended last year and pushed into another car but I think the gauge still worked after the accident, honestly I can't remember for sure though.

Is there a way to check the gauge to verify it works and then work my way back to the sending unit? The last thing I want to do is syphon the gas again and drop the tank.

Thanks for the link to that site, there's a lot of neat stuff on there!

[next69]

Todd,

If you look at the illustration on the link I posted you see with a full tank of gas the resistance is very high thus a low amount of voltage is sent to the gauge and the lower the fuel gets, the lower the resistance gets, thus a higher level of voltage makes it to the gauge. So based on the illustration whith nothing connected the gauge should read full. If you wanted to test the gauge you could take 12 volts and a potentiometer and hook it up to the gauge and reproduce variable levels of voltage and see if the gauge responds accordingly.

[derekf]

The HSW site oversimplifies some of it, I think.

The gauge receives +12v when the ignition is on. The wiring diagram for my '65 says this is a 20g B/P (blue/purple?) wire - but your colors may vary, if you've got a wiring diagram for yours you can verify pretty easily.

The gauge is grounded through a tan wire (at least in 65 it was, your colors may vary. I'll be describing this wire as tan, anyway), which is connected to the sender in the tank. The sender, as described on the HSW site, is a variable resistor. When the tank is full, the resistance of the sender should be 88 ohms and when it's empty it should be 0 ohms. The other side of the sender connects to a chassis ground.

From what your troubleshooting has already determined, we can figure this:

1. When you've got both the tan line and the chassis ground connected, the gauge sees a direct connection to ground (close to 0 ohms) and reads E - if your tank was empty, this would be correct, but it's not.

2. When the tan line is connected and chassis ground is not, the gauge still sees the direct connection to ground (again, close to 0 ohms) and still reads E. This isn't correct with any amount of fuel.

3. When you disconnect the tan line, the gauge no longer sees the ground (open circuit) and reads way past F. This is expected behavior but confirms that it's not your gauge stuck on E.

Sounds like you've got a short to ground somewhere after the tan wire you're disconnecting (I'm assuming you're disconnecting back by the tank and not at the gauge). Verify by unhooking the chassis ground from the sensor, unhooking the tan wire, and measure the resistance between the tan wire going to the sensor (not the one going back to the gauge) and the chassis ground. I imagine you'll see a very small resistance.

If you'd rather check the sensor, give it 12v in at the blue/purple wire (or whatever color it is in your car) and hook a ~100 ohm variable resistor in the ground.. That should get you pretty much the full sweep of the gauge.

Hope this helps, if I over- or under- simplified things please let me know.

[qwik1320]

Derek,
OK, I checked the tan wire that goes to the sending unit. I put a meter between the ground wire off the sending unit (disconnected from the chassis) and the tan wire (unplugged from the harness). I get about 5.6 ohms resistance. The tan wire is free from where it enter the trunk back to the sending unit and I visually inspected every bit I can see - I thought maybe it got pinched between the tank and trunk floor. I'm trying to see if the actual connection point for the tan wire to the sending unit (the brass stud with the rubber boot) is pinched...maybe that's the problem? Do you think it could be something that simple?

[derekf]

I do.

To my mind, the next step is to try and isolate where the short to ground is.. either it'll be a wire pinched/touching, or it'll be something inside the sender.

I don't know which way I like better... at least if it's a bad sender, once it's fixed you know that it's really fixed... there's nothing worse than an intermittent problem.

[qwik1320]

Derek,
Well there's room between where the tan wire connects to the sending unit and the trunk floor. I took a piece of plastic milk jug and slid it in between pretty easily. I have my old mangled sending unit here and I measured between the 2 contacts and that reads open, which I guess is what this should read.

I watched the gauge again as I had only the ground connected, only the tan wire connected, etc. and now it's not moving at all no matter what's connected.

Maybe it is the gauge but the 5.6 ohms between the tan wire and ground isn't right either. My gauges have that plastic printed circuit board on the back. Any other thoughts?

I think I'll get the guys on the 2GCOG site thinking about it too with a post there.

[derekf]

The contacts should not read "open". There should be a resistance (as described above).

The gauge should move if the tan isn't connected - it should move way past F as you were describing before. (Silly question - verify you're testing with the ignition on)

5.6 ohms between the tan and the ground is correct if the tank is almost empty.