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73-87 Chevy _ GMC Trucks => Interior & Equipment => Instrumentation => Topic started by: The1984Silver on May 25, 2015, 04:20:20 am
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Hello everyone, My fuel gauge is not working and i would like to fix it due to the inconvenience it can cause ;D. So once before it stopped working and i replaced the gauge and that fixed it for awhile now its no longer working. The gauge will read 1/4 tank most of the time but sometimes it goes to empty. So the things i have done is:
-Took the gauge back out and test for a good ground which seams to be fine.
-Tested for 12 volts ignition signal when the ignition is On which i am receiving about 12.163 or something so that's fine
-Now i wasn't sure how to test the signal wire so this is where i need help figuring out if its the sending unit/wire or the gauge its self. But if i remember correctly its was 1 or so volt at the signal wire and with the ignition on i got 49 Ohm's and with the ignition off i got low Ohm's.
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I am having the same issue with 76. I hope someone can help..
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see if this (http://forum.73-87chevytrucks.com/smforum/index.php?topic=25339.msg211241#msg211241) helps
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from the link above
TEST PROCEDURE - GAUGES
Using an ohmmeter set to the appropriate scale, "zero" the ohmmeter if the meter requires it. Then, while referring to the subsequent gauge-specific charts, with the ohmmeter connected to the tester leads, adjust the tester to the stated resistance for testing the 'Low' index line of the particular gauge and year vehicle.
Once the tester is adjusted, substitute it for the gauge sending unit by unplugging the wiring harness from the sender, connecting one lead of the tester to the sender wire, and the second lead of the tester to a verified, good ground. Turn the ignition ON. The gauge needle should point to the lowest reading (leftmost index line) on the gauge face.
Similarly test the 'Mid-range' and 'High' index lines of the gauge by systematically resetting the tester resistance according to the appropriate chart.
A correctly reading gauge indicates there are no problems with the gauge or its wiring, limiting any faults to the respective sending unit.
An incorrectly reading gauge calls for further testing and verification of 12-volt ignition power, grounds, connections, and associated wiring.
TEST PROCEDURE - SENDING UNITS
The subsequent charts provide precise sender resistance values for three specific gauge readings (Low, Medium, and High), hence, can be used to estimate sender resistance when actual physical parameters are known (e.g., engine temperature that is verified with a thermometer, actual fuel tank level, engine oil pressure that is verified with a manual gauge, etc). Therefore, a sending unit’s resistance can be measured and “subjectively” compared to its interpolated value between ‘Low,’ ‘Medium’ and ‘High,’ based on the charts and the observable/verifiable physical conditions that actually exist on the vehicle.
FUEL LEVEL GAUGE PARAMETERS
For Truck Years: 1967 - 1990
The Fuel Gauge Needle Should Point to:
Left Line (Empty) when sender resistance = ~1 Ω
Middle Line (Half) when sender resistance = 44 Ω
Right Line (Full) when sender resistance = 88 Ω
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Thank you my RH tank reads 3/4 when full.
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Works :)
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Alright so i get 12 volts at the gauge, it has a good ground, but it reads 104 ohms when i have around a half tank or so at the sending wire. Problem is my stock gauge reads 1/4 and the other gauge i got reads a little past full. Only problem is i thought the problem was fixed because it read full on the 2nd gauge and i knew the tank was full of gas but when i put it all back together it went full empty so now im at a loss for ideas.
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Update: So more weird issues sometimes i get 91-94 ohms other times 104-130ohms but the weird thing is both my gauges read just over 1/4 even though their getting the 91-130 ohms i thought at 90ohms the gauge should read full. I'm now at a true loss for what this issue is. Im thinking the sending unit is bad since i get a reading as high as 130ohms but the two gauges reading 1/4 with 91-130ohms pumping though it makes me wonder if both the gauges are bad or whats happening there.
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im going to let BD or hatzie take over. im still learning off them lol
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Run a jumper wire directly from the stud on the sending unit to the gauge and one from the ground on the sending unit to the battery ground, then check the gauge and see if your getting a good reading or not. If that doesnt help I would pull the sending unit out and ohm it out at the sending unit itself and move the float up and down while watching your volt meter. It should read around 0 at the empty side and around 90 in the full . That way you know if your wiring is bad in the truck or not or if it's your sending unit. I just did this and mine and it was reading 109 on full and 16 on empty which ='s a bad sending unit.
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I jumped the sending wire to the gauge and got same reading just over 1/4. Whats a way to test the resistor on the gauge because i think both gauges are bad. Even though i know the 91-130 ohms is wrong from the sending unit, i don't see how the gauges read just over 1/4 with that many ohms it should be read past full correct? I'm going to pull the tank out when i wake up and report the results i get.
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Yes yes are correct, andything over 90 ohms should be past full. sounds like a a bad gauge to me. Did you have the ground jumpers as well to the battery just to eliminate that as well??
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Yes i did check everything to make sure it had a good ground so i should be okay their. But guess ill be buying my 3rd gas gauge tomorrow...
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Testing for a inoperative fuel gauge sending unit (http://forum.73-87chevytrucks.com/smforum/index.php?topic=16071.0)
Fuel System Schematics (and fuel gauge troubleshooting) (http://forum.73-87chevytrucks.com/smforum/index.php?topic=9072.0)
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You are making this way more difficult than it needs to be.
...Something i thought i would mention is that if the gauge is installed and i push on it sometimes it will go to full and then when i push in another spot it will go back to a quarter....
This ^^^^ is an indication of an intermittent connection between the gauge and the flexible printed circuit board (PCB) on the back of the instrument cluster. Typically, this is caused by accumulated dust and oxidation between the PCB and the three silver spring clips that protrude through the back of the cluster housing to engage the pins on the back of the fuel gauge; and/or a loss of spring tension between the clips and the gauge.
To correct electrical continuity problems between the PCB and the gauge, remove the cluster from the vehicle. Remove the fuel gauge from the cluster. Carefully remove the spring clips protruding through the back of the cluster housing being careful not to tear or damage the PCB. Using crocus cloth or a Pink Pearl eraser, gently clean the copper foil where the spring clips for the gauge push through the PCB and snap into the back of the cluster. Be very careful while cleaning the PCB to avoid peeling the copper foil loose from its plastic sheet. Next, polish the little contact arms of the clips. Using a pair of needle nose pliers, tweak the spring clips ever so slightly to increase the tension between the spring clips, the PCB and the gauge pins when installed. Smear a sparing amount of antioxidant paste or dielectric grease on the copper foil where it contacts the spring clips and reinstall the clips through the back of the housing so they snap and lock into place, making a firm connection against the PCB. Check the nuts securing the ceramic resistor to the back of the gauge to ensure they are snug, then reinstall the gauge, set the cluster into place in the dash, and reconnect the harness to the cluster. But, don't completely reassemble the dash yet...
Yes i did check everything to make sure it had a good ground so i should be okay their. So i need a new gauge and sending unit then? I don't understand how 2 gauges and the sending unit can go bad. Ill put a new sending unit in and still have a wrong reading because the gauge seams like i cant win. Guess ill be buying my 3rd gas gauge tomorrow...
- With the factory fuel gauge installed and hooked up, disconnect the signal wire from the fuel tank sending unit. With ignition switched on the fuel gauge should climb way past full to about the 3:00 o'clock position. Does it?
- Ground the signal wire leading to the gauge and with the ignition on the fuel gauge should drop to empty. Does it?
- Connect a 45-ohm, 1/2-watt resistor (procured locally from Radio Shack or similar electronics supply) between the sender wire leading up to the fuel gauge and a good, clean, bare metal ground. With the ignition switched on the fuel gauge should indicate 1/2 tank. Does it?
Report your progress and findings then we'll go to the next step. Incidentally, are you using a digital or analog meter?
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- Yes it jumps to about 1 or 2 o'clock position
- I grounded the signal wire and it did drop to empty
- As for the resistor i will go pick one up from radio shack as soon as i can and report my results
I am however using a digital meter.
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The subsequent excerpt from your original post troubles me...
-Now i wasn't sure how to test the signal wire so this is where i need help figuring out if its the sending unit/wire or the gauge its self. But if i remember correctly its was 1 or so volt at the signal wire and with the ignition on i got 49 Ohm's and with the ignition off i got low Ohm's.
- First and foremost, never attempt to measure the resistance of any circuit that is connected to power or otherwise energized!
- Second, don't waste your effort on measuring resistance through the gauge. There is no meaningful spec for it and it will only serve to confuse.
- Third, measuring voltage on the signal wire is similarly meaningless with no useful specification for reference.
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- Do verify ignition voltage to the "gauge" (already done).
- Do verify that the "gauge" is properly grounded (already done).
- Do verify that the "sending unit" is properly grounded - generally the ground wire for the sender will be black, blue, green, or purple, depending on the sending unit. In contrast, the signal wire running to the gauge is pink.
- Do measure the resistance of the "sending unit" through a full sweep of the float if/when you pull the sender out of the tank - and notice whether there is a smooth transition in resistance from Empty (zero Ohms) through Half Tank (45 Ohms) to Full (90 Ohms). But, be advised that if you remove the sender from the tank, the sealing o-ring may swell excessively and need to be replaced to reestablish a suitable seal.
- When you substitute the 45-Ohm resistor for the sending unit to verify the half-tank calibration of the gauge, be sure that the sending unit is unplugged from the circuit.
[Note: The fuel sending unit resistance does not exhibit a perfectly linear trace when graphed. Instead the graph manifests a very shallow S-shaped curve. The subtle curve in the graph occurs, because the float moves through an arc inside the tank and the sender resistance is engineered to compensate for the arcuate motion of the float].
Keep us posted.
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So I think i found a problem with the ground from the sending unit. Just to make sure the Purple wire in the pic is the ground wire right? Because if so when i disconnect the wire and let it hang the gauge doesn't move at all. Also with the wire disconnected i pulled the gauge did a ohm test to the sending wire and still got the same 91-130 ohms. I then ran a wire from the ground on the battery to the ground wire and the gauge and ohm reading still didn't change.
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So I think i found a problem with the ground from the sending unit. Just to make sure the Purple wire in the pic is the ground wire right?...
If the other end of the purple wire connects to the fuel tank sender, it is the sender ground. Based on the image...
(http://forum.73-87chevytrucks.com/smforum/index.php?action=dlattach;topic=31010.0;attach=29470;image)
...remove the bolt attaching the purple wire to the frame, thoroughly clean the frame to shiny bare metal, and then reassemble with a healthy coating of antioxidant paste available from most hardware stores.
...when i disconnect the wire and let it hang the gauge doesn't move at all....
The sending unit is probably grounded through its attachment to the tank. The ground wire simply guarantees the sender is grounded.
...Also with the wire disconnected i pulled the gauge did a ohm test to the sending wire and still got the same 91-130 ohms. I then ran a wire from the ground on the battery to the ground wire and the gauge and ohm reading still didn't change.
91-130 Ohms is a 40-Ohm range. Are you saying the measured resistance consistently fluctuates within that range? Is the resistance fluctuating as you jostle the truck? Are the meter leads making good connection to clean metal? ???
Moving forward:
Disconnect the pink signal wire from the fuel tank sender. Measure the resistance of the 'sender' between the signal wire stud on the top of the sender (or harness connector if you're lucky) and the ground wire attached to the frame. Post the result.
If the measurement is less than 90 Ohms, with the pink signal wire still disconnected from the tank sender, measure the resistance of the pink signal wire between the "free end" at the tank and the dash gauge. Is it less than 1 Ohm?
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Yes the ohm reading jumps and bounces around in between 91-130 ohms sometimes it goes up to 160ohms. The leads are good and bare. And the resistance moves around so fast i cant tell if it changes when i jostle the truck.
The reading i get when measure from the sending unit stud and ground is the same as when i tested the signal wire from up top i get the 91-130ohms. And the sending wire i get 0.5ohm.
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Replace the tank sender and service the ground connection to frame.
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Update: Its fixed! So i pulled the sender and its fine, i narrowed to the gauge. The gauges i have are bad so i tried putting a 100ohm 1watt resistor from radio shack and it works like a charm. The only problem is the resistor gets really really hot and i have to remove it. I'm not to sure that 1 watt is enough for that kinda of setup although.
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...i tried putting a 100ohm 1watt resistor from radio shack and it works perfect. The only problem is the resistor gets really really hot and about catches on fire in about a min. I was wondering are these resistor only for a test only? or can you use one as a permanent fix? and if so what resistor should be used. Thanks...
So, you had connection problems at the sender....
The resistor heating to the extent you described is dangerous and could ignite a fire! Where did you connect the resistor - where is it in the circuit - between ignition and ground?
Adding 100 ohms to the circuit to make it work 'perfect' indicates there's still a fundamental problem in that circuit.
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Yes the resistor is between ignition and ground. And what do you mean theirs a problem in the circuit? I know i have 12v power, ground, and signal to the gauge. I also have good ohm reading from the sending unit now and it has a good ground what else could be wrong?
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The light is beginning to shine. Did you replace the ceramic resistor across the back of the fuel gauge with the one from RS? At 1 watt that resistor is underrated for dissipating the generated heat by ~50%. No wonder it's getting hot. The original ceramic resistor is probably rated closer to 4-5 watts.
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Okay i went and picked up 3 original GM fuel gauge resistors from a local truck shop for under 5 bucks. It works! Although i still have a question. Even with the original style resistor when i pulled the gauge off the back of the gauge is on the higher side warm. Is this normal? Ive about had enough of this issue so i may just run the signal wire straight to the gauge as well as power and ground to eliminate the circuit paper or any other interferences.
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I doubt you'll gain anything by running B+, ground and signal directly to the gauge. The worst the PCB will do is introduce resistance - and what you've described isn't the result of added resistance. What's your best guesstimate of resistor temperature?
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Well i almost think that the circuit paper it is introducing resistance due to the heat thats why i want to run the wires directly to the gauge to be sure. As for the resistor temp range they are the original 0-90ohm GM ceramic fuel gauge resistors i wouldn't know the temp off the top of my head. I just never noticed if it got that warm before so it kinda worry's me.
To make it easy on you bd ill go put the gauge back in for awhile and get the peak temp on it with my laser temp gauge to give you an idea.
UPDATE: So i left the gauge in for about 20-30 minutes and got a peak temp on the back of the gauge at (121.3°F) . I guess that's not crazy high considering my black paint is sitting at (111°F) but for a gauge maybe it feels hot to me.
Anyway Ive about got it nailed just the heat is worrying so need to figure that out. But i wanted to thank you bd you are the man. You've helped me every step of the way since i posted the problem and for anyone that this thread helps all is due to bd.
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Can you post a pic of the ceramic resistor showing how it's mounted on the back of the gauge?
The resistor on the back of the fuel gauge should measure ~90 ohms ±10%; the sending unit varies from 0 (E) to 90 ohms (F); and each gauge coil (there are two) is ~100 ohms. The gauge is wired into its circuit to create an impedance bridge between the two coils, the ceramic resistor and the sending unit. The net power effect on the resistor is roughly 2 watts of heat dissipation at Empty and 0.5 watt of heat dissipation at Full. The resistor will become warm at Empty.
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Okay so the heat is normal that's good to hear.
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Ran some controlled tests using a contact K-thermocouple with simulated charging voltage regulated at 13.8 volts and ambient temperature stabilized at 77° F.
After adjusting the fuel level sender resistance to 88 ohms, so the fuel level gauge indicated FULL, the net current flow through the gauge circuit measured 0.143 amp while the temperature of the ceramic resistor on the back of the fuel gauge stabilized at 93.2° F (barely warm).
Readjusting the fuel sender resistance to 1 ohm, so the fuel gauge indicated EMPTY, the net current flow through the gauge circuit increased to 0.291 amp while the temperature of the resistor increased and then stabilized at 123.8° F (quite warm to the touch and similar to what you measured independently).
So, now we have some quantitative data. The fuel gauge and resistor DO GET WARM! It's normal! So, there's no cause for concern. ;)
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Well thank you for that i was still wondering and worrying a little but its makes sense that they get hot.
Note: Alot! of info online is conflicting as far as what resistor to use for the gas gauge resistor test. I'm not a genius but from my experience!! I do not recommend using a simple radio shack resistor, is not a safe way to go about testing the gauge. Radio shack "my radio shack"doesn't make a resistor like the original that can truly deal with the low resistance which can mean high current, and that means heat! So what i mean is using a radio shack resistor could result in smoke and fire folks so be warned. Also if you think your sending unit is bad try to get a true ceramic GM fuel gauge resistor if possible before you pull it. Reason being my sending unit was functional but 2 gauges well make that 3 were crap and didn't work which leads me to believe with this many years on these trucks those ceramic resistors on the fuel
gauge are on their last leg or close to it.
Another thing: Not all bad fuel gauges will stay at 1/4 like stated in many repair manuals. Sometimes they can go way past full, to 3/4 and yes most of the will stick around 1/4. I know this because Ive tested 2 bad fuel gauges from the salvage yard plus my 3 and all were a little different even know they didn't work. As soon as the ceramic resistors in them however my 3 gauges work like a charm.