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Anyone ever replace a sending unit power wire ?

pearljam724

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Ive been messing with my fuel gauge only reading 3/4 when full. I tried two different sending units. 1 original and 1 new aftermarket and they both have the same readings. All of the sudden I have no power to the blue sending wire Verified by gauge reading empty now and I tested blue wire output via voltmeter located at tank.
I know a blue wire connects to the speedometer round connecting harness. Assuming that’s the sender wire that goes to the tank. Does anyone know for sure if that blue wire connects anywhere else in between tank and round gauge connector ? I think this wire had slightly too much resistance and that’s why the gauge would only read 3/4 when both sending units are at full mark. I’m not getting power to that blue wire now. I might have to run a new one up under the car. Because I don’t want to rip the carpet out again if I want my gauge to work again. Anyone ever replace that blue sending wire before ? Can you please share what to look for under dash to accomplish ? I’m thinking I can run a new wire directly to the round speedometer connector. Not sure if that will work.
 
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Lower left kick panel.I think its an 8 wire connector. That fuel gauge wire passes through it as well. Then rides under the wire protector down the driver side of the car. Under the rear seat and over the hump into the trunk. I assume you have the ground strap hooked up as well.
 
That is not a power wire, it is a ground signal from the sender to the gauge.

It runs from the sender to the round connector and doesn’t connect to anything else. There is a plug at the kick panel where the rear harness connects to the dash harness.
 
Lower left kick panel.I think its an 8 wire connector. That fuel gauge wire passes through it as well. Then rides under the wire protector down the driver side of the car. Under the rear seat and over the hump into the trunk. I assume you have the ground strap hooked up as well.
Thank you, very much. Tank is off. Yes, I hooked both sending units to the tank wire and grounded it very well without tank installed. Everything worked fine during my testing. Came back a day later and no power to blue wire now. Power everywhere else.
 
That is not a power wire, it is a ground signal from the sender to the gauge.

It runs from the sender to the round connector and doesn’t connect to anything else. There is a plug at the kick panel where the rear harness connects to the dash harness.
Ok, thank you. So the sending unit is grounded twice ? Or is blue wire a ground for the gauge ? I wonder why gauge stopped working all of the sudden ? Before the tank was removed. This would happen once in a while. But, the gauge always worked again eventually. Now it won’t. Has to be a bad ground somewhere. The blue wire being a ground. Might need replaced ? It’s a very small gauge wire. Maybe it’s not the greatest ground due to its gauge and age. Internal oxidation, etc.
 
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The blue wire is a variable reference to ground. Make sure that the ground strap from sending unit nipple to fuel line has a low resistance connection.
 
The blue wire is a variable reference to ground. Make sure that the ground strap from sending unit nipple to fuel line has a low resistance connection.
Yes, thank you. I understand it needs a ground strap as you mention. I grounded both units without tank installed to test their gauge readings. Both worked fine with a gauge reading yesterday during this testing. Came back today and I couldn’t get a reading at the gauge and can’t understand why ? I did all of this testing without tank installed. Good yesterday, no fuel gauge movement today.
 
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Ok with key on very briefly ground the blue wire and see if you get gauge deflection. If you leave it grounded much past once the gauge goes to full you risk burning up the gauge. If no gauge response pull
The driver side kick panel and in the connector ground the blue wire going to the instrument cluster and see if you get gauge deflection. Your voltage limiter could be bad
 
Ok with key on very briefly ground the blue wire and see if you get gauge deflection. If you leave it grounded much past once the gauge goes to full you risk burning up the gauge. If no gauge response pull
The driver side kick panel and in the connector ground the blue wire going to the instrument cluster and see if you get gauge deflection. Your voltage limiter could be bad
Ok. I will remove kick panel and try grounding the blue wire there. I grounded the blue wire at the tank as you stated. And was aware to turn key on very briefly. But, gauge didn’t move.
 
do Your other factory gauges work?
I’m only using factory fuel gauge and speedo. I have aftermarket alternator/temp/oil pressure separate. All lights are on original cluster. I also tried another voltage limiter. But, I think if limiter was bad the fuel gauge would be pegged at full. So, I don’t think that’s the problem.,
 
The limiter can fail two ways. Stuck closed or just broke and don’t cycle on and off at all. The circuit is very simple 0-5 volts to the gauge depending on fuel level (resistance to ground) to an nichrome coil in the gauge so the coil in the gage is
open, no ground or limiter is bad that’s assuming the wires and connections are
good. the needs 12 volts to the primary side also so with ignition on you could verify 12 volts to the limiter.
 
The limiter can fail two ways. Stuck closed or just broke and don’t cycle on and off at all. The circuit is very simple 0-5 volts to the gauge depending on fuel level (resistance to ground) to an nichrome coil in the gauge so the coil in the gage is
open, no ground or limiter is bad that’s assuming the wires and connections are
good. the needs 12 volts to the primary side also so with ignition on you could verify 12 volts to the limiter.
I tried two limiters, same results. I wonder what effect the condenser has if it were to be defective ? Odd thing about the condenser. That is the only mechanical part on a 69 b body that is not obtainable new., I have never seen them new. I have one last thing to check. Is perhaps the round connector on back of cluster. Sometimes those wires pop out the connector. But, like I mentioned. Worked yesterday, not today. Nothing changed since then. Weeks prior, gauge would sometimes not move when I first turn ignition on to run only without starting. Then its fine a day later. Weird. Believe it or not. Seems starting the engine compared to just turning the key to run without starting seems to affect it ?
 
The condenser is just for noise suppression for the radio. I’m sure if you prove the circuit you’ll find the problem. Check for 12 volts to the limiter to make sure it getting power.
 
The condenser is just for noise suppression for the radio. I’m sure if you prove the circuit you’ll find the problem. Check for 12 volts to the limiter to make sure it getting power.
Thank you, I appreciate your insight. You were a lot of help. If my aftermarket ammeter gauge is only reading around 10.5 volts with key in running position, but engine not running. Would that not be enough voltage to make the gauge sweep ? If that’s the case. There is my problem.
 
Yes that voltage is low . What is the voltage at the
battery terminals?
 
Yes that voltage is low. What is the voltage at the
battery terminals?
I ll check that tomorrow. If it needs 12 or more to sweep. That’s the problem. That must be the reason why it doesn’t sweep once in a while. Because I may have noticed it without starting the engine, which in turn the battery is charging at or above 12 volts when engine is running. I can’t start the car because it has no gas tank momentarily. But, I can add a little charge to the battery to see if that works. I don’t want to put the tank on until I sort out the gauge. Whole reason for removing tank.
 
Ok, thank you. So the sending unit is grounded twice ? Or is blue wire a ground for the gauge ? I wonder why gauge stopped working all of the sudden ? Before the tank was removed. This would happen once in a while. But, the gauge always worked again eventually. Now it won’t. Has to be a bad ground somewhere. The blue wire being a ground. Might need replaced ? It’s a very small gauge wire. Maybe it’s not the greatest ground due to its gauge and age. Internal oxidation, etc.

The blue wire is a ground signal from the sender to the gauge. The sender is grounded to the body through the fuel gauge strap that clips on the fuel line to the sender right at the front of the tank. See photo.

Every load device needs power and ground to operate whether it’s a light bulb, electric motor, or a fuel gauge. The ground signal from the sender is variable, that’s how the gauge changes it’s reading. The power is from the voltage limiter on the back of the cluster.

Just like dash lights can change their brightness from the dash light dimmer switch.

48A451DF-398F-4A60-A528-41C7F56A1B0B.jpeg
 
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