I think the gauge is for a 911 but not sure. I picked up a 25-ohm rheostat thinking I could use that to determine what signal was empty, quarter, half 3/4 and full but I get the same voltage out of it (rheostat) all of the time. Do I have the wrong rheostat or am I do something else wrong. The guy at radio shack didn't even know what a rheostat was so no sense asking him.
The sender is sending about 3 volts at about a quarter tank (red light is on) will fill it up and recheck the signal at full.
TIA
Full Version: testing gas gauge voltage range
The slide gets stuck which of course, isn't an electrical problem at all...
was not a problem with original gauge, this is a modified gauge and will go from full to to 5/8 and then the red light comes on. I am on the second tank with this gauge. so I really don't thionk it is the sending unit.
Is the signal from the sender in volts or amps? I am reading volts.
Is the signal from the sender in volts or amps? I am reading volts.
ok I hooked the old gauge up and it reads near empty so it isn't the sender. 
Is there anything I can do to convert the new gauge?
I still cant get the rheostat to move the gauge. I tried hooking the 12v to the center post and the each of the outer posts (one at a time) to the signal connection but it didn't move.
Was really hoping to use this gauge as it is 3 gauges in one.
really hoping one of the electronic experts will jump in.
If not I guess I will just have to go back to the old one or gas up at every even hundred mile reading.
Is there anything I can do to convert the new gauge?
I still cant get the rheostat to move the gauge. I tried hooking the 12v to the center post and the each of the outer posts (one at a time) to the signal connection but it didn't move.
Was really hoping to use this gauge as it is 3 gauges in one.
really hoping one of the electronic experts will jump in.
If not I guess I will just have to go back to the old one or gas up at every even hundred mile reading.
The gas sender changes resistance compared to ground.
You need to include pictures of the gauge and the connections on the back. It's hard to help if we don't know what you're looking at.
You need to include pictures of the gauge and the connections on the back. It's hard to help if we don't know what you're looking at.
Click to view attachment
This is the front of the gauge, not able to get picture of the back at this time but it has the late round black style electric connection.
the rheostat I got doesn't have a big enough range but now that I know what I was doing wrong I was able to get the needle to move.
Based on the info I was able to find there should be no difference between the 914 and 911 gauges so far as signal; I will get a different rheostat to verify but suspect that the gauge needs repair.
Am I thinking completely wrong or would the most likely thing wrong be the resister from the 12v. With key off gauge goes to zero and with key on doesn't go below 5/8. That is what I think I need to test next (minimum and maximum range) with a rheostat with the proper span to determine if gauge is messed up.
thanks for pointing me in the right direction for testing. Am open to any other suggestions anyone may have.
This is the front of the gauge, not able to get picture of the back at this time but it has the late round black style electric connection.
the rheostat I got doesn't have a big enough range but now that I know what I was doing wrong I was able to get the needle to move.
Based on the info I was able to find there should be no difference between the 914 and 911 gauges so far as signal; I will get a different rheostat to verify but suspect that the gauge needs repair.
Am I thinking completely wrong or would the most likely thing wrong be the resister from the 12v. With key off gauge goes to zero and with key on doesn't go below 5/8. That is what I think I need to test next (minimum and maximum range) with a rheostat with the proper span to determine if gauge is messed up.
thanks for pointing me in the right direction for testing. Am open to any other suggestions anyone may have.
The three connections on the back should be (in no particular order)
+12v
Chassis Ground
Fuel Level Sender
Confirm that the wires are going to the right places first. Even better pull the connection on the back of the gauge and make some temporary jumper wires to take everything out of the equation. The best way to approach electrical issues is to isolate the problem as much as possible, and then widen your search from there, little by little. So if the gauge works with known +12v, known ground, and a 100ohm potentiometer/rheostat to ground, then you can suspect the chassis wiring, or the sender.
The next step if the gauge works is to pull the connection at the sender and use your rheostat between the green wire and known ground. Also check the brown wire to ground and make sure it's around zero ohms.
I suspect the wiring on the back of the gauge is wrong.
+12v
Chassis Ground
Fuel Level Sender
Confirm that the wires are going to the right places first. Even better pull the connection on the back of the gauge and make some temporary jumper wires to take everything out of the equation. The best way to approach electrical issues is to isolate the problem as much as possible, and then widen your search from there, little by little. So if the gauge works with known +12v, known ground, and a 100ohm potentiometer/rheostat to ground, then you can suspect the chassis wiring, or the sender.
The next step if the gauge works is to pull the connection at the sender and use your rheostat between the green wire and known ground. Also check the brown wire to ground and make sure it's around zero ohms.
I suspect the wiring on the back of the gauge is wrong.
my 12v hot goes to the + terminal, the green sender goes to the G terminal and the ground goes to the terminal with the ground sign.
I know what you mean about the wiring, when I got the car the original oil temp gauge didn't work; turns out the neutral (ground) and signal wires were swapped at the connector. I switched them and it worked fine after that.
I will test it with a 100 ohm first and compare it to the old one to verify the results, but since the problem started when I installed this gauge and my old one still reads right when I hook it back up I am pretty sure it is the gauge. I might open it up and make sure the internal connections go to the correct terminals, not sure why they would be wrong but I have seen stranger things.
I bought it from someone who bought it from someone and suspect I may be the first to actually hook it up. lucky me
I know what you mean about the wiring, when I got the car the original oil temp gauge didn't work; turns out the neutral (ground) and signal wires were swapped at the connector. I switched them and it worked fine after that.
I will test it with a 100 ohm first and compare it to the old one to verify the results, but since the problem started when I installed this gauge and my old one still reads right when I hook it back up I am pretty sure it is the gauge. I might open it up and make sure the internal connections go to the correct terminals, not sure why they would be wrong but I have seen stranger things.
I bought it from someone who bought it from someone and suspect I may be the first to actually hook it up. lucky me
I did'nt have a chance to test the gauge. yes it was purchased from someone here who may have not tested it as well. i believe that i sold it back to you for the same price I paid for it. According to the seller, it was rebuilt by north hollywood speedo.
If for any reason you need to send it out for repair, I will help out.
I feel bad that you are having issues with the gauge.
you are a great guy!
PM me if you need my assistance
If for any reason you need to send it out for repair, I will help out.
I feel bad that you are having issues with the gauge.
you are a great guy!
PM me if you need my assistance
914 that is what I figured happened, so no big deal as far as I am concerned. 
I just look at it as a challange and a learning experience. Hey I am learning how to use a rheostat
my thinking right now is either something is hooked up wrong inside or the resister(?) inside is bad, the wrong one, or something is touching where it shouldn't.
I am pretty sure it will have me scratching my head and then all of a sudden I'll go "that's it".
The gauge "works", just at the wrong range; reads slightly over a half tank near empty but goes to empty when key (power) is off. now all I have to do is figure out why. The problem is I don't know electronics, so I will be learning. Will be testing both this and the old gauge and recording results to compare.
also thinking that if I add another rheostat on the 12v side (for testing) I can then simulate different resisters to see if that makes a difference and possible figure out which size is needed.
I just look at it as a challange and a learning experience. Hey I am learning how to use a rheostat
my thinking right now is either something is hooked up wrong inside or the resister(?) inside is bad, the wrong one, or something is touching where it shouldn't.
I am pretty sure it will have me scratching my head and then all of a sudden I'll go "that's it".
The gauge "works", just at the wrong range; reads slightly over a half tank near empty but goes to empty when key (power) is off. now all I have to do is figure out why. The problem is I don't know electronics, so I will be learning. Will be testing both this and the old gauge and recording results to compare.
also thinking that if I add another rheostat on the 12v side (for testing) I can then simulate different resisters to see if that makes a difference and possible figure out which size is needed.
Why not give North Hollywood a call? Tell them what it is doing & they might know exactly what it is. May save you a lot of time.
QUOTE(rmdinmd @ Mar 26 2014, 08:34 AM) 
914 that is what I figured happened, so no big deal as far as I am concerned.
I just look at it as a challange and a learning experience. Hey I am learning how to use a rheostat
my thinking right now is either something is hooked up wrong inside or the resister(?) inside is bad, the wrong one, or something is touching where it shouldn't.
I am pretty sure it will have me scratching my head and then all of a sudden I'll go "that's it".
The gauge "works", just at the wrong range; reads slightly over a half tank near empty but goes to empty when key (power) is off. now all I have to do is figure out why. The problem is I don't know electronics, so I will be learning. Will be testing both this and the old gauge and recording results to compare.
also thinking that if I add another rheostat on the 12v side (for testing) I can then simulate different resisters to see if that makes a difference and possible figure out which size is needed.
I was thinking , just thinking ... What if you fill up the car, them move the needle up to the full position with your finger. Them watch. The gauge as you drive and burn the gas. That is of you have it open. More like a needle calibration.
The gauge is expecting to see a changing resistance to GND on the 'G' line.
As the fuel sender float moves up and down it presents a different resistance to the gauge from the G pine to Ground. It is a variable resistor.
The rheostat (or potentiometer aka POT) you have is also a variable resistor.
It will have three connections on it.
We can call them +, -, and wiper.
The resistance between + and - is fixed and will represent the full range that can be dialed with the wiper. If the POT is a 10kOhm, then you can get 0-10,000 ohms on the wiper.
If you put a meter between wiper and - and turn the knob you will see the resistance change. You will get the exact same effect if you measure wiper to +.
So to test, you do not put any voltage on the pot.
Connect the gauge to +12V & GND.
Connect the G pin to the wiper on the pot.
Connect the - pin on the POT to GND.
Now as you turn the knob on the POT you should be able to get the gauge needle to move.
As the fuel sender float moves up and down it presents a different resistance to the gauge from the G pine to Ground. It is a variable resistor.
The rheostat (or potentiometer aka POT) you have is also a variable resistor.
It will have three connections on it.
We can call them +, -, and wiper.
The resistance between + and - is fixed and will represent the full range that can be dialed with the wiper. If the POT is a 10kOhm, then you can get 0-10,000 ohms on the wiper.
If you put a meter between wiper and - and turn the knob you will see the resistance change. You will get the exact same effect if you measure wiper to +.
So to test, you do not put any voltage on the pot.
Connect the gauge to +12V & GND.
Connect the G pin to the wiper on the pot.
Connect the - pin on the POT to GND.
Now as you turn the knob on the POT you should be able to get the gauge needle to move.
keep in mind the pot i picked up has too large of a range and swings too much to be precise but the numbers work for a rough look.
gauge known to work.
10 Ohms = full
32 ohms = 3/4
45 ohms = 1/2
63 ohms = 1/4
80 ohms = R
90 ohms = walking
gauge in question
10 ohms = full
32 ohms = 3/4+
47 ohms = 3/4
62 ohms = 5/8
90 ohms = 1/2 and you are now walking
edit
220 ohms = 1/4 tank
440 ohms = empty
I plan on picking up a pot with a smaller range and try using it as a resister on the 12v side to see if I can change the range.
My thinking is if I can determine what resistence is needed to make the gauge work I can fix it myself. I will have to determine what the "total" resistance is needed through both the current resister and what I add to make it work. I should then be able to replace the current resister with a new one of the proper resistence.
with 3680 ohms added to the 12v side it went to a quarter tank.
Am I thinking right or am I completely off base?
gauge known to work.
10 Ohms = full
32 ohms = 3/4
45 ohms = 1/2
63 ohms = 1/4
80 ohms = R
90 ohms = walking
gauge in question
10 ohms = full
32 ohms = 3/4+
47 ohms = 3/4
62 ohms = 5/8
90 ohms = 1/2 and you are now walking
edit
220 ohms = 1/4 tank
440 ohms = empty
I plan on picking up a pot with a smaller range and try using it as a resister on the 12v side to see if I can change the range.
My thinking is if I can determine what resistence is needed to make the gauge work I can fix it myself. I will have to determine what the "total" resistance is needed through both the current resister and what I add to make it work. I should then be able to replace the current resister with a new one of the proper resistence.
with 3680 ohms added to the 12v side it went to a quarter tank.
Am I thinking right or am I completely off base?
This is a "lo-fi" version of our main content. To view the full version with more information, formatting and images, please click here.