Technical Electric fuel gauge jumping around

I converted my 39 ford truck over to 12v and the fuel gauge shows the correct level when sitting still at idle but if I rev the engine any the needle fluctuates. When I am driving the fuel gauge needle looks like a miniature windshield wiper. I used a sender unit from a 90’s ford the was the right depth for my tank and was the correct resistance range for the new gauges. The gauges are repro smiths made in India. I have used these brand/makers gauges on my tractors when I converted them to 12v but this is the first fuel gauge I have used. I have the temp oil and volts gauges on the tractors and truck (they work with out any problems when they have the correct sender/sensor)
On the 39 ford truck I used the generic GM style wiring harness that has the alternator run to the fuse block then to the battery. Power for the gauges comes from the same fuse block.

 

does it look like the gauge is showing the fuel sloshing in the tank? like it doesn't have enough damping?

 

Looking forward to info on this. Auto Meter does this.

 

Possibly when it’s going down the road but when is sitting still the gauge moves with the engine speed

 

What does a volt meter say about your system voltage when you rev the engine?

 

Does this have to do with the original King Seely gauge and sending unit?

 

I have a digital volt meter direct off the battery it sits at 12.7 engine on or off and I have a voltmeter in the dash that the needle points at 12 with the key on and 14.5 with the engine on. I have seen the needle move up to 16v a few times while the digital gauge stays at 12.7
this is a new build with all new wiring for everything. All grounds are hardwired back to the battery. On the firewall there is a ground strap bolted on the outside and on the inside of that same bolt is the grounding point for all the wiring inside the cab. There is also a ground wire from that lug that goes to the rear lights. It’s lugged on the frame rail near the fuel tank and the fuel sender is grounded to that lug. So everything is directly grounded by wire to the battery. No relying on rusty metal for grounding.

 

Sounds like perhaps he isn’t running a voltage regulator in the system and voltage changes with rpm.

 

If you are using the OEM King Seely gauge your set up will not work accurately as the OEM King Seely gauge
only works with the OEM King Seely gas tank sending unit. They both operate on heating a bi-metallic spring inside both units NOT on the OHMS LAW Principle. This is discussed at length on Fordbarn.

 

something is wrong. it's not charging the battery, is it? The battery voltage should be close to the voltage on the rest of the system.

How is the alternator output connected to the battery?

 

you missed the bit where I said the gauges are 12v reproduced made in India

 

Alternator is a GM 10Si internally regulated. The output wire goes to the power distribution panel aka fuse panel then to the starter solenoid then the battery. This is supposed to be GM style of wiring according to the instructions that came with the 21 circuit universal harness. All I have running off the power while I have been driving is the ignition coil and the dash gauges unless I have my foot on the brakes

 

but is there really 2 to 4 volts difference from the alternator to the battery when it's revved up?

 

The problem only happens with the engine running. With the key on/engine off the fuel gauge gives a constant reading of 1/2 full which is what’s in the tank. I can bounce the truck around and the gauge says at 1/2 but as soon as I start the engine the gauge goes crazy.
I will check the alternator output and I will disconnect the output wire from the alternator and run a new wire straight to the battery and see if that makes a difference.

 

Has it always done this or a new issue? A good test would be to disconnect the alternator output wire to fuse block and run a jumper from the alternator to the battery and then another jumper from the solenoid/battery connection to the fuse block wire. That would make the battery the damper point and supply a more consistent voltage to the fuse block.

 

no need to disconnect wires, just make sure there is a good path from alternator output to the battery. If it goes through an ammeter, make sure the ammeter is actually able to carry the current (this was a big problem with Mopars in the 60s-70s due to wiring and ammeter failures)

 

It’s a new build with new wiring new alternator new gauges new battery

 

Me thinks the gauge just wants a GOOD ground.

Ben

 

A picture is worth a thousand guesses...indeed.

If the alternator is working properly, and voltage isn't fluctuating while it's spinning, there may be some inductance into the fuel sender circuit, possibly from being next to a high voltage source, like a plug wire...?

 

Today’s test. I disconnected the wires for the alternator and went for a drive. Fuel gauge showed just under half a tank. Did not fluctuate for bumps or stopping suddenly. I have not run a wire from the alternator to the battery. That would involve cutting into the wiring harness which I am reluctant to do after spending hours wrapping it.
Tested the alternator output. Off it’s 12.7 from the battery. Idle it’s 13.8 and 2000rpm it’s 14.7 the battery stays at 12.7
from the picture I have the battery word going to thd fuse panel and the ign/acc wire going to the idiot light.

 

you need to connect the alternator output to the battery!

 

Running a wire to the starter post works on most wiring , if that's closer .

 

It’s a 39 ford with the battery on the firewall and the starter solenoid under the battery.
The 2 red wires at the alternator are crimped in 1 terminal and to separate them I would have to cut into the harness wrap.
I used this (see pic) 21 circuit wiring harness and it’s pre wired for the alternator to feed the fuse block power distribution bar with another wire from the other end of the power bar to the battery.

 

Run an unbundled wire, as needed, to see if the fix you discovered works.

If it works, you seem to know what comes next.

 

Something to consider, bad grounds can cause electric feedback. May try grounding the tank to the battery with a jumper wire and see if that helps?

I bought a new Stewart Warner fuel gauge and it never read right, so I got tired of the weird readings, took the gauge out, pulled the second nuts holding the back cap on the gauge, and found the cap was touching one of the terminals, Yep that will do it. Now it reads right all the time.

 

Doesnt that cause the engine to continue running after the ignition is turned off?

 

No, they have diodes in the alternator...

 

Did the rewire of the alternator. Now have the alternator direct to the battery cable clamp at the + post. The fuse panel wire now starts at the battery side of the starter solenoid. Made no difference to the fuel gauge. Still reads good while driving with battery only and flips and flaps with the alternator hooked up. In my research I have seen references to an instrument cluster voltage regulator. So thats my next move if I can find a small 12v output regulator.

 

I believe you stated that you had seen 1- v from the alternator ? That means there may be a fault in the alternator .. Ford used to have a CVR ( constant voltage regulator)

 

Okay, it is possible that the alternator has a diode problem and you may be seeing a stray AC signal (ripple) coming from it. The quick way to test involves a scope however, you can set a voltmeter to AC and connect it from the alternator output terminal to battery ground. A reading of over 0.5 volts suggest there is a diode problem in the alternator. Don't know if that is your problem, but it is a possibility.