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Old 07-02-2006, 12:43 AM
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Fuel guage

OK guys I've got kinna a problem.
My car was wired when i got it with a 1977-78 chevy truck harness.It has older Autometer guages in it and the original Volvo tank.
Becuase I found so many disasters in the wiring I had to go through the whole harness and resolders a bunch of splices and fix a bunch of stuff,I feel safe with it now cept for the fuel guage.I left it wired the way it was as I don't know how its spoed to be wired.
I have not hooked up the wire to the fuel tank in fear of blowing a cratter in the concrete...ya know??I don't know if the original car was 6v or 12v.
1)How do i know the gas guage was wired right.
2)How do i know if the sending unit will work with the guage.
3)If the car was originaly 6V(not sure yet)can I use a resistor in line??
Does it even matter if the car was a 6V system being as the tank is a measure of resistance??
I do know not to send 12V back there.

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Old 07-02-2006, 02:02 AM
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Doc here,

Well if you are not sure if the gauge is 6 or 12 volts,that would be your first hurdle..After re~reading the post, they are auto meter..the chance is pretty good that it is 12 volt..If you can go to the auto meter site and try and confirm..If it's that old..you may need a photo of it..

If you have access to a variable power supply with current and Voltage meters, you could try and fuse it at like 3/10 an amp and run the supply up to 6 volts watching the current meter as you do..If that needle shoots up after 7 or so volts..or blows the fuse, It's a 6 volter..If the current stays low all the way to 12 volts , it's a 12 volter..

Most gauges are marked "S", "G", and "I"..the "I" is switched power, the "G" is ground..and the "S" is sender..

You need to find the resistance range of the sender on the gauge..get a hand ful of resistors from Rat shack..1/2 watter's, 30 ohms, 60 ohms, 90 ohms, 120 ohms, up to 290 ohms..

put each to the "S" terminal with it on, and note the reading..as you ground the other side of the resistor..

For instance if your gauge climbs at a reasonable rate to "F" as you add 60 ohms resistance, you need a 0 to 60 ohm sender..The reverse also can be true..it could be a 60 to 0 ohms sender..just note what way the needle goes when you ground a resistor, that will tell you what way you need the sender..

Then to check the sender, remove it from the tank, hook the egress panel to the black probe on your DVOM, set for ohms, R X 1, calibrated 000, and note the reading..if it's 000, for instance, raise the float to the uppermost level..note that reading..If it is for instance 60 ohms (or close to it) that is the gauge it will play with .. a 0 to 60..If you need something different, bring it in and get the proper resistance value for the gauge..

A Fuel tank sender is NEVER referenced to power..EVER..it is all ground through a variable resistor... It's not a good thing to have spark potential with 25 gallons of fuel...(even electric tank pumps make me nervous.. )

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Old 07-02-2006, 02:14 AM
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Here is a site that has a bunch of downloadable instructions for various gauge mfgs. http://www.egauges.com/AM_Instr.asp I believe autometer makes gauges to match the sender's resistance. The number on the back of the gauge would tell what the matching sender woud need to be. here is the autometer chart that might have a gauge number to look up and get the sender resistance value from.
http://hp.autometer.com/charts/gauges.html
I wouldn't second guess doc for a second-but sometimes having a picture helps.
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Old 07-02-2006, 02:24 AM
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Doc here,

Good site Mikey.., Thanks!

It has a range of years and type vehicle on it , if you click on the "What Resistance sender do I need" link in FAQ, under auto-meter..

So If you know what Car it came from before..It'll save ya a bunch of work..

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Old 07-02-2006, 02:33 AM
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I usually check the sender with the vom also.
It is always a challenge to dicscover what someone cobbled up, and then left for you to figure out. I use all the clues I can find.
I just picked up a resistor pack with 2 leads and about 50 resistors. Packaged in a plastic can about as big as a chew tin. Turn the top and get a different resistance value. I found it at a swap meet and when I saw it I realized it could be used for checking fuel gauges , among other things. I love swap meets.
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Old 07-02-2006, 02:42 AM
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Doc here,

Yeah , I hear that!!...I built a resistor sub box from a plastic project box I got at rat shack, along with the same pack of 50 1/2 watt resistors, a couple of rotary wafer switches, and chassis mount terminal bars, and a switch between the input of the wafer switches (to change scales) and a set of probe binding posts coming out one end ..just for Gauge / Sender values..really handy! helps quickly when you stick it in line..Nnow if I ever get around to marking the switch positions...I'll have it made..

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Old 07-02-2006, 11:54 AM
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Yea the guage is a 12Volter.
So I take it it does't matter if the original car was 6V.Cause the sender doesn't take line voltage,its only a resistance path to ground for the sending side of the guage??
If so,I just need to find out what resistance sender my guage needs and what resistance the existing sender is??Using the resistors from the shack,right?
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Old 07-02-2006, 08:36 PM
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Doc here,

OK, I think your confusing 2 issues here..

The Sender is NOT voltage dependent..it will not care if the last system was 6 or 12 or even 24 volt..It will operate fine in either car..

THE gauge WILL care..If it is 6 volt (which I doubt unless it was a super, Super old gauge..) and you hooked it up to 12 volts....You'd let all that Day ~ Um factory packaged smoke get loose..And the Queertrons would blow the fuses..

The Gauge Power is what you need to be sure of the voltage on..(some are in fact not voltage dependent also..they will operate over a range of gauge power..like 5 to 15 volts..)

For testing I advocate this method IF you have access to the supply..

Quote:
If you have access to a variable power supply with current and Voltage meters, you could try and fuse it at like 3/10 an amp.

Then, run the supply variac up to 6 volts watching the current meter as you do..

If that CURRENT needle shoots way up QUICKLY after 7 or so volts..or blows the fuse, It's a 6 volter..If the current stays low (an amp or less) all the way to 12 volts , it's a 12 volter..
That should tell you right away..(without damage to the gauge)

Happy 4 Th!

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Old 07-03-2006, 04:46 PM
E.T. divided by $ spent= Speed
 
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Thanks Doc,
Yea the guage is definetly a 12volter. I just didn't know if the sender had to be 12volt compatible.But you answered that question allready for me.
So basically I just gotta finger out what resisance sender the guage needs and what resitance sender I have in the tank,right?
THKS and happy 4th
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Old 07-03-2006, 08:56 PM
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Doc here,

Yuppers!

Just match the resistance and direction and your good to go!

Happy 4 Th of July...

Hotrodders are the
Most independent
group of individuals
today..
So I guess It's our
Day too...

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