Gas Tank sender voltage

[CudaHead]

Helo
      I'm working on my 1971 Challenger.
The fuel gauge is showing about 1/2 with the tank full.
It changes after it uses some fuel, going down.
This is the standard dash.
I was testing the wire at the tank.
I put a cable on the ground and another one to the wire, using a DVM.
The voltage on the wire is showing about 5.7V but it goes away fast for about 4 seconds and then come back.
It keeps pulsing signal on and off.
Do I have a bad regulator on the dash? I installed new one about 6 months ago.

[DeathProofCuda]

An original style dash gauge voltage limiter provides a pulsing 12-volt supply to the gauges, which averages out to about 5 volts.  The pulsing action is provided by a bimetal strip that heats up and then bends away, breaking contact through the circuit.  It then cools, bends back, and reconnects the circuit.  Sounds like this is what you still have.

I believe that the newer digital limiters provide a constant 5-volt signal.

For more information, see this Chrysler reference book on thermal-electric gauges

Your issue is more likely a bad ground connection at the tank, or a faulty sending unit in the tank.

[Katfish]

I'd pull the sender and check the float.
They are made of brass and prone to deteriorate and get pin holes.  This causes the float to "sink", so it never shows full.  Happened to me twice.
If you pull the float, give it a shake and it will be obvious if this is the problem.

[CudaHead]

I'm working on it.
I'm thinking that it's a ground problem. Still testing.

[usraptor]

If it's the original sending unit to the car,or even several years old, I agree with Katfish that it's probably the float.  If not I'll still check the float arm angle.  The repo units are not necessary calibrated correctly as the calibration simply consists of the angle of the float arm.  When I installed a new unit in my Cuda's gas tank it took some fine tuning (bending) of the float arm to get it to register correctly.  Good luck. 

[CudaHead]

 Ok, I'm still working on this. Any ideals?

Changed the gas sender.
Front battery to the blue wire from the tank. full sweeps.
The ground on the clip over the hose, full sweeps.
The ground on the tube on the new sender, full sweeps.
Took the blue wire back on the sender. Moves to about haft and moves back, just like it worked before.

[Jay Bee]

This is my all time favorite mod. I've fixed a few senders doing this, even got a thank you comment.

[Brads70]

I thought your measuring resistance Ohms 70-10  to see they are functioning?

[Jay Bee]

I believe that the newer digital limiters provide a constant 5-volt signal.

The solid state RTEng limiter that's been in my car for decades pulsates, No idea if they've changed. The solid state limiter from Vans, that went into a cluster I sold, was a steady 5volts.

Before selling that cluster I mocked up power to the gauges and a good sending unit. (Bare in mind  I'm using a rallye dash but since the same sender is used for both I believe the results should be the same too.) Then I moved the sender's arm, watched the needle on the gauge and recorded the ohms. Here's the results. You can do a static test of the senders resistor as in the picture and use the chart to tell you what the needle should read. Of course, that's all moot if there's a wonky ground at the pivot point.

[CudaHead]

I did check the Ohms's on the sender before I installed it. They are in the correct range.

I think my next move is to remove the front kick panel and find the blue wire. I will ohm it to see if the wire to the sender are connected. If that works ok, I will try to pull the Dash out and ohm from that to the sender.

Thanks

[CudaHead]

I have a solid state RTEng limiter, but I never used it. I also bought a new Fuel gauge.
I will be installing them.

[CudaHead]

OK, I fixed it. On the back of the dash had the blue wire unconnected.