‘66 Barracuda Fuel Gauge Pegged!

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I installed the RT-E regulator and bench tested with a cheap Chinese sender (see my post #13 with a picture). As I moved the sender float, the gauge would move up and down. The back of the fuel gauge has thin dense cardboard to prevent it grounding. It was fit snugly between the posts.

I have the cluster loosened and ready to come out again. I might try what Bob said in post#20, to use a separate wire outside of the car from the sender straight to the gauge to see what happens. I’ll double check to make sure the cardboard is in place behind the fuel gauge, too.
You can isolate the limiter with electric tape. It will stay where you put it. It needs to cover all the metal including the 2 little bent tangs that hold the board. The switched 12 volt wire should be moved from harness connector to the regulator. If you simply tapped the wire or something similar, you still have 12 volts going into that gauge. You have to be careful with how/where you mount the regulator also. If on the back of the panel, it cant sit on top of a copper trace. It could short that trace to ground. Post a pic of your completed assembly?
 
The new limiter is attached to a copper trace which is a ground for the bulb. The limiter clears any metal around it as it sits inside the dashboard.

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This photo is from RT-E’s website also showing it mounted to a negative tracer to the bulb.
 
And I am showing good continuity in the upper harness, test probes in the bottom plug and the round plug. Tells me the problem has to be in the cluster, yes? Or when the cluster is mounted, maybe somehow, something grounds when installed?

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I'm not seeing a fault. You're still putting 12 volts in the fuel gauge which I don't like but... I don't know if that is the problem. If you want to open the fuel gauge and bend the thingy per RTE instructions, or completely amputate the original limiter, I can type out how I go about opening these gauges.
As for your test, that shows the wire isn't completely broken. To test is the wire is shorted to ground, one tester lead should be on ground.
 
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Question... when you were testing at workbench, did you have the 12 volts supplied at the RTE regulator or on the board pin?
 
The upper harness shows no continuity with ground, reads “I”.
My bench test was with 12v to the IVR, with the Chinese sender, was working like it should. Prior to that, I tested only the fuel gauge with three 1.5 AAA batteries from a flashlight and the gauge would move up and down.

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Just repeated the benchtest a few minutes ago. Works like a champ out of the car. Photo and 20 sec film. My son filmed...

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I installed the RT-E regulator and bench tested with a cheap Chinese sender (see my post #13 with a picture). As I moved the sender float, the gauge would move up and down. The back of the fuel gauge has thin dense cardboard to prevent it grounding. It was fit snugly between the posts.

I have the cluster loosened and ready to come out again. I might try what Bob said in post#20, to use a separate wire outside of the car from the sender straight to the gauge to see what happens. I’ll double check to make sure the cardboard is in place behind the fuel gauge, too.
Now that @RedFish has identified the wire in the harness connector you should be able to clip onto the other side of that connector and check to see if that wire is shorted/grounded. Maybe disconnect it from the sender as well.
 
You have proven everything is good until the car harness puts 12 volts in the original limiter. My instructions were, isolate the gauge from chassis ground. Move the 12 volt supply from circuit board directly to a substitute regulator. You did not follow my instructions. You did what you saw at the RTE website. you want to open the gauge to interrupt that 12 volts per their instructions or not?
Your original limiter is still getting 12 volts and still attached to the fuel gauge and that path to the sender is the only path to ground that limiter now has, and it will use it. So in simpler terms you are running a regulator and about half to 3 fourths a limiter, depending on how much resistance at the fuel sender. And you shouldn't have needed so much explanation.
 
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When the car puts 12 volts at the red arrow path, the substitute regulator gets 12 volts. So does the internal limiter. the current path inside the gauge that you can't see is marked with pink arrow. So dont put 12 volts to that gauge post. put it directly to the substitute regulator. The 5 volts out of that regulator will feed into the original limiter and out to that contact pin also but that is of no consequence if there is no wire attached there, or a chassis ground. The only current path for the 5 volts is where it is supposed to go.... through the gauge.

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