Reverse the ohms?

[66stepside]

I am not sure if Ohms is the right word, but I was driving the 65 to the gas station because the gas gauge was reading empty. Well, she died a block from the station. Awkward.

The gas station didn't have a loaner can, but they did sell 2 gallon cans. So out of desperation I bought their super expensive can. Awkwarder.

So, the aftermarket gas gauge is working , BUT it reads in reverse. In other words preprinted FULL = Empty and EMPTY = full.

So, a few questions:

1) is there away to "easily" reverse the OHMs reading to get it to read correctly?

2) even though it is hooked up correctly, can I swap a wire, or two to reverse the OHMs?

3) since the sending unit is working, should I just find the stinking stock gas gauge and run the wire to it and forget about the aftermarket gauge?

[Bubba68ss]

Your easiest solution would be to reverse the E and F on the gauge.... Sort of joking. But I don't believe you can change resistance in a single wire. There's no polarity.

[derf]

You either have to replace the sender or the gauge. Or, crack the face off and swap the E an F on the gauge that's in there.

[66stepside]

Yeah, I figured it wouldn't be a simple fix.

Thanks guys.

[Nikkormat]

Who made the aftermarket gauge?

[carnuck]

I have re-bent the float arm in the tank so it's bent backwards. On an old unit it's likely to self destruct.

[Tatsadasayago]

The aftermarket gauge is backward. The stock sender should be a 73 to 10 Ohm unit. In comparison, the typical GM sender from that period was like 0-90 Ohms,
If your sender uses the round mounting plate with the six holes, you can get a VDO adjustable unit from summitracing for around 65.00.
They look like a typical semi-truck tank sender.

[SJTD]

If the gauge has two terminals and they are both isolated from the case it seem to me you can simply reverse the leads. Check restance between each post and the case. Should read open.

It's simply reading voltage across an internal shunt, no?

[tgreese]

The circuit is likely a voltage divider, with the gauge across the upper resistance. The sender is the lower, variable resistance.

The pre-80 gauges are grounded through the gauge body, which goes to chassis ground through the tank. No practical way to reverse that, unless you either isolate the tank (I would not) or go with a plastic tank.

IIRC there are two "standard" ranges, GM and S-W. If the GM gauge is backwards, look for a gauge that uses the S-W standard.

[SJTD]

But he's running an aftermarket gauge.

His circuit is simply a loop. Battery, gauge and sender. Drop across the gauge should be the same wherever it is in the circuit. If it's reading backwards turn it around. Unless there's a diode it the gauge.

What am I missing?

I'm concerned because I plan to use a 1-1/2" pressure gauge for fuel level. The gauge wants 250-43 Ohms for 0-100 psi and fuel level senders are available that read 240-33 full-empty. The plan is to add a 10 Ohm resistor and run the gauge backwards.

[derf]

Electrical circuits don't work like that.

It doesn't matter which direction the current is flowing. It doesn't matter what "side" of the gauge the resistant load is on. It will read the same.

Electricity doesn't work like flowing water when it comes to a gauge. You can't just simply reverse direction to get the opposite behavior. The gauge measures current flow the same way in either direction. And current flow is based on the resistance in the sending unit.

Now, if it was an electric motor, the polarity would matter. But in a gauge, it doesn't

[SJTD]

How embarrassing. After thinking this through some more I see it wouldn't work. Thanks.

For argument's sake say the gauge is looking for 1-2 volts full to empty and the drop across the sender is 10-11. The gauge works backwards. Reversing the gauge gets the "delta V" in the right direction but the gauge would peg backwards. Wired in reverse it sees -2 to -1. So polarity does matter.

Fortunately the setup I came up with doesn't require the small level gauge.

Apologies to 66stepside for the jack.

[tgreese]

I was not clear above. To reverse the sender rate, you would need to modify the sender.

Look here http://www.summitracing.com/search/depa ... 4294927072

Forget voltage - the sender has a resistance. That's all it is, a variable resistor. You have the wrong "sense" of sender where the change in resistance is backwards from how the gauge works. There are plenty of gauges that work with the 73 full / 10 empty sender. Just match the sender to the gauge and it will work fine. Don't make it more complicated.

[Nikkormat]

If you post a picture of the aftermarket gauge, I can look for a SW range gauge with a similar appearance.

[66stepside]

Here's what I have installed by someone other than myself.





I am personally leaning toward wiring it to the instrument cluster gauge and see what happens. There are two severed wires at the base of the cluster female plug that connects to the cluster itself. My guess is one of them is the factory gas gauge.

I wouldn't mind just remembering E=F and vice versa, but it messes with me a bit that the sending unit works, but the gage itself doesn't as it's usually the other way around. Yeah, I'm weird.

[Nikkormat]

Here's what ya got: http://www.autometer.com/2-fuel-level-0 ... eries.html

Here's what ya need: http://www.autometer.com/2-fuel-level-7 ... -4322.html

But, the stock gauge should work just fine, unless it's dead. The stock fuel gauge uses the SW range just like the 70's tank I'm assuming is under your rig..

[66stepside]

I am pretty sure it's a 70's tank- as is the filler neck. If the gauge is non-op I'll mod one from a 70's rig.

Thanks for the links!

[phlorentz]

Reverse the wire at the sending unit.

[tgreese]

Not that easy. One path to the sender is the chassis ground, and you can't easily reverse that.

Plus the thread is about 3 years old. Unlikely you're in time to help the poster.

On a more cheerful note, welcome from Boston!