1984 Silverado Fuel Gauge Issue

[The1984Silver]

- 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.

[bd]

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.
  
  
  --------------------
  
  
• 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.

[The1984Silver]

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.

[bd]

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...



...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?

[The1984Silver]

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.

[bd]

Replace the tank sender and service the ground connection to frame.

[The1984Silver]

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.

[bd]

...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. 

[The1984Silver]

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?

[bd]

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.

[The1984Silver]

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.

[bd]

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?

[The1984Silver]

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.

[bd]

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.

[The1984Silver]

Okay so the heat is normal that's good to hear.