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Fuel Gauge Mystery Continues...

DeadParrot

Active member
213
47
28
Location
oklahoma city, ok
Found my old gauge. To test if the 'damper' is still working, gently blow on the gauge needle while holding the gauge in your hand. If the needle easily moves, bad gauge. The needle of a good gauge will barely move even if you blow hard. A further search of the Chevy PU forum suggests the damper is simply a tight fit on the gauge bearings and as they wear the gauge gets more bouncy.
 
231
5
16
Location
Mount Laurel NJ
So here is what's up. My damper is good, I even had another gauge to spare and it's good too. Here's my final verdict. I disconnected the sender wire on top of the fuel tank completely, and kept the ground on. When measuring VOLTAGE between the ground and sender terminal, I get 0V as it should be. Find for some reason when I turn on the headlights, the sending terminal becomes POSITIVE and outputs 3.7mV. how is that even possible when there isn't any other wires going to it??
 

DeadParrot

Active member
213
47
28
Location
oklahoma city, ok
Does the stray voltage happen with just the running lights on and headlights off? Bad headlight grounds can send voltage down the off side of the headlight circuit. If low beams on, the high beam side can become energized if the ground is bad because both filaments share a common ground inside the bulb. Not sure how it would get to the pink wire unless the headlight switch has a leak. It does have the dimmer rheostat that controls the dash panel backlights.

What are you using as your ground reference while taking these measurements? Wondering if your frame/body to battery ground has enough resistance to cause a voltage to show up via the sender if you are using the negative side of the battery as reference. Wouldn't take much corrosion there to cause a frame voltage when the headlights are on. Headlights + running lights draw a fair current.
 
231
5
16
Location
Mount Laurel NJ
The Stray voltage occurs either when the headlights are on, on the 2nd Notch of the headlight plunger, or when pressing the brakes. Thus, I have sort of determined that the short must be occurring on the harness that goes to the back of the vehicle. However, I I peeled back the insulation and looked all over the place, and I was not able to find anything conclusive. I followed every single wire, gave every single Shake test on every single spot, and still nothing. No changes. I traced it all the way to the firewall, and even behind the fuse block again, stI'll no changes. also, I removed the headlights, and even the headlight pigtail, from the circuit. Which I was in need of replacing the headlight terminals anyways. For the ground, I am using the one underneath of the tailgate, to the left, where the tail light ground is. And another reference to a ground can be either a clean spot on the exhaust pipe, or the body tub, anything other than the frame. the body to frame ground, at least in the front where the core support is, to the passenger side frame is totally clean also. And I did a continuity test without any signs of resistance.
 

porkysplace

Well-known member
9,604
1,494
113
Location
mid- michigan
The Stray voltage occurs either when the headlights are on, on the 2nd Notch of the headlight plunger, or when pressing the brakes. Thus, I have sort of determined that the short must be occurring on the harness that goes to the back of the vehicle. However, I I peeled back the insulation and looked all over the place, and I was not able to find anything conclusive. I followed every single wire, gave every single Shake test on every single spot, and still nothing. No changes. I traced it all the way to the firewall, and even behind the fuse block again, stI'll no changes. also, I removed the headlights, and even the headlight pigtail, from the circuit. Which I was in need of replacing the headlight terminals anyways. For the ground, I am using the one underneath of the tailgate, to the left, where the tail light ground is. And another reference to a ground can be either a clean spot on the exhaust pipe, or the body tub, anything other than the frame. the body to frame ground, at least in the front where the core support is, to the passenger side frame is totally clean also. And I did a continuity test without any signs of resistance.
The exhaust isn't a good place for a ground , every joint in it can cause issues.
 

Tinstar

Super Moderator
Super Moderator
Steel Soldiers Supporter
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Location
Edmond, Oklahoma
Any other issues that you’ve found while chasing the fuel gauge issue?
 
Last edited:

DeadParrot

Active member
213
47
28
Location
oklahoma city, ok
On the coolant voltage. Could be simply your meter probe is dissimilar enough from the radiator and you have enough impurities in your coolant that with the probe in the coolant, you have a primitive battery. Variation on the two different metals in the lemon trick. If you are using a DVM, those usually have a high input impedance and will read volts from almost anything.
 
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