Gas gauge works @ 1/2 way

[36coupe]

The sending unit does starting showing a level until half a tank. For the 1st half it show 100% full then after 100+ miles it drop to 1/2.
Is there something simple wrong with the sending unit in the tank. It looks like a new after market unit.
Thanks

[JSeery]

That's most likely the problem. You could try adjusting the arm. Some people claim to get them working but a lot can't. No idea what the difference is, but the repro works completely different than the originals.

[36coupe]

I think the original is in a box somewhere
I should dig that out and see if I can fix it.

[FireEngineMike]

Mac's sells a sending unit they claim can be used with only original dash gauges. Do you know if they do indeed work?

[JSeery]

I be almost certain it was a resistance unit. Everyone that sells them claim they will work and some users claim they get them to work. I have no idea how they could really work correctly, but whatever.

[flathead 53]

Sponsored Links (Register now to hide all advertisements)

get the original repaired, the aftermarket ones do not work well if at all

[cordbob]

Is it possible you have a 6 volt sender in a 12 volt system.

[Jembow]

I set my arm so it is accurate when empty but right now with about half a tank the needle is way up out of sight. The answer seems to be finding an original sender.

[Andy]

The gauge is an ammeter. It measures the current going thru the sender. It starts off with a high sender resistance when the tank is empty to show low current and low fuel level. The resistance drops off as the float moves up to show more fuel in the tank. I would put a 100 ohm resister in the line going to the sender and see how it works. Try other resisters and see if you can get it to read zero when empty. That is all you really need to know. You probably will have errors when the tank is full.

[flatheadmurre]

The meter can be adjusted to, you have two settings for zero and the span of it.
Small screwdriver and adjust what looks like cogs in a hole on the back of it.

[JSeery]

Originally Posted by Andy

The gauge is an ammeter. It measures the current going thru the sender. It starts off with a high sender resistance when the tank is empty to show low current and low fuel level. The resistance drops off as the float moves up to show more fuel in the tank. I would put a 100 ohm resister in the line going to the sender and see how it works. Try other resisters and see if you can get it to read zero when empty. That is all you really need to know. You probably will have errors when the tank is full.

Andy, the original doesn't work on resistance, it works on pulses. The original sender is a points arrangement that vibrates. The rate of vibration varies the current throught a coil in the gauge. The repro sender attemps to duplicate this with a variable resistor controling the current to the gauge.

[Andy]

Learned something new today. Thanks.
I helped a guy get a Drake sender more in range by adding a resister. He has 12v and some non stock gauge.

[JSeery]

Some people seem to get them working!

[flatheadmurre]

Originally Posted by JSeery

Andy, the original doesn't work on resistance, it works on pulses. The original sender is a points arrangement that vibrates. The rate of vibration varies the current throught a coil in the gauge. The repro sender attemps to duplicate this with a variable resistor controling the current to the gauge.

The original sender adjust the location of one of the points.
Then the time for heating the bimetal up enough to open the points gets longer or shorter.
When the points open both sets of heaters start to cool down until the points on the sender close and start heating both the meters and the sender again.
So not vibrating just heating and cooling, time depends on the setting of points on sender.
Same type of bimetal in both sender and meter and the adjustment of the points in the sender gives the same amount of movement of the needle in the meter.
A variable resistor should in theory be able to do the same job but since you then constantly feed it a lower current instead of a pulsed one it needs fine tuning.

[JSeery]

Originally Posted by flatheadmurre

So not vibrating just heating and cooling, time depends on the setting of points on sender.

Don't disagree, guess it depends on your definition of vibrating. To me opening and closing of the points is what I refer to as vibrating. But I agree 100% with your description. This comes up weekly it seems and I get lazy on the long winded explanation being repeated over and over, but guess that is the better approach as it a new question to the one asking it.

I'm really not sure how quickly the points open and close, I always refer to it as a pulse or vibration, but if it's slow enough (whatever rate that is) I guess it falls out of that category and into just opening and closing.

[rotorwrench]

A resistance based sender could work the indicator in theory but the range of resistance that would be required is way outside of the currently available resistance based senders that are more in tune with current type indicators. It would almost be easier to get someone to manufacture an OEM King Seeley type sender than to get them to make a resistance based unit that would actually work the indicator. The demand for such parts is just not enough to get anyone to step up to the plate. It's a shame too since the King Seeley systems would outlast the resistance units by at least a 2:1 ratio.

The most plausible recourse would be to manufacture an indicator that looked original but worked with one of the currently available resistance based sending units. If they designed it well, they would have different face plates that would work with the same meter in order to satisfy all the needs better.

[Brooks]

Andy, what size & ohm resister did you use to make it work. I have the same problem. Thanks

[Andy]

I had no idea what was needed. I bought a pack of 5 22 ohm resisters from Radio Shack for $1.50 I kept adding them in series untill the gauge read near zero. I used 4 for 88 ohms. I went and got a pack of 100 ohm resisters and one works fine. They are 1/2 watt.

[Brooks]

Thanks Andy, that is what I needed to know. Have a great night!