Technical Electric fuel gauge jumping around. Fixed. Update in OP

I'm familiar with old Smiths gauges, but not repops from India. My guess is they're the same. English cars with Smiths gauges had a voltage stabiliser incorporated into the dash. An oblong, 3/4" or so long item, with a spade terminal at each end. May or may not explain your problem.

Chris

 

Ford installed the CVR because they were still using 6v. gauges in a 12v. system. The CVR reduces the voltage to 6v., a 12v. gauge won't read correctly...

 

Just did some googling to confirm that the Smiths gauges are 10v and require a voltage regulator. Search smiths instruments voltage stabiliser.

Chris

 

Both my old IH tractors were made in England and had smiths gauges. When I did the alternator conversions on them I used these repro gauges made in India. They are clearly marked 12v and have worked good on the tractors. Oil, Temp Volts and Tachometer.I got a set for the truck Oil, Volts, Temp and Fuel. I am only having a problem with the fuel gauge and I think it’s from surface voltage (what I read today) so putting a voltage stabilizer on the 12v feed for the fuel gauge seems like it might fix my problems. The other thing that came up was the sender unit so I will pull it out on the weekend and ohms test it to see if it’s the problem.
Thanks for all the help and input so far. Electrickery is not my thing. I can follow diagrams and if the diagram is correct it works but I don’t know why it works.

 

Instrument cluster constant voltage regulator were used through the Ford 1986 model year , I doubt that was because of 6v gauges

 

The one I am getting tomorrow from the auto parts store is from an 88 ranger

 

That will reduce 12 volts to 6 volts.

 

You are close , what I can find says 5v , wouldn't be of much value wit a 12v gauge. Back to the drawing board .

 

Thanks for that. Saved me some hassle.

 

I thought you were close to a resolution with this post. I'd say you have an alternator problem?

Alternator unhooked, all ok. Hook up the alternator and guage flops.

 

I have been checking on this thread and believe that the issue could be from the alternator. Years ago, some alternator "whine" would sneak into some radios and tape players. Solution was to put an inductor in series with the power lead going from ignition switch to the radio or tape player. If I am correct, the value of the inductor was 1 millihenry. It will pass 12 volts DC, but will filter out the 600 hz alternator frequency that is generated at the typical engine speed before passing through the big Diodes in the alternator.

 

At this point I would just replace the alternator. They are not that expensive, and if it has a bad diode causing your fuel gauge problem it will fail soon anyway.

 

After calculating, the value of the inductor is 33 millihenry. Digikey has one for $4.75 that should work.

 

Replacing the alternator diodes is not difficult or expensive.

 

Ford tried 12v. gauges in '56, they were problematic so went back to the 6v. gauges in '57 and used them for years...

A few mentions of this here... https://www.jalopyjournal.com/forum/threads/change-to-12-volt-keep-6-volt-gauges.1173752/

 

So today I swapped the alternator with both the alternators from my tractors and still the same. I am going to get the smiths voltage regulator and hook it up to the gauge 12v supply and that will drop it to 10v and see what happens.

 

In my experience, 99.99% of this type of problem in hot rods is grounding.

 

BINGO!!!

Ben

 

Do you have a different gauge that would be compatible with the sender you have. If so try it, even if it doesn't match the other gauges. If it works correctly then you will know it's the gauge.

 

Agreed with others who suggested grounding issue. So, even though it might seem redundant and not needed, roughen up the area around the grounding boss on the back of the alternator and attach a hefty ground wire to a good place on the block. Then another ground strap from the block to the firewall. Then yet another ground wire from roughened up areas on the instrument cluster frame and dash even though it seems unnecessary with all the screws already holding things in place. And if you're really anal, wouldn't hurt to run a separate ground wire from the alternator boss direct to the instrument cluster frame and any other places that seem unnecessary. Can't have too many grounds.

 

Thanks for the input. I have everything over grounded now. It’s wired like it is a fiberglass car. All devices are directly wired to the main grounding lug near the battery.

 

At this point, that is what I would do.

It may also be worth while to run a separate wires to the gauge, keeping it separately from everything else. In other words, isolate the gas gauge wires from other wire bundles. Run two simple wires, 1 directly from the sending unit to the gauge, and 1 from the gauge to ground. The gauge power supply drop should cover any "noise" on the gauge power end.

I'm thinking the gauge may be picking up "noise" from bundled wiring when the alt is charging.

 

You need to confirm the voltage of the gauge
If it is 10v use an LM7810 voltage regulator [approx $3-4]
If it is 6v use an LM7806 voltage regulator
If it is 12v use an LM7812 voltage regulator

These things smooth out the output voltage regardless of input [9v -35v]

 

Thanks for the input.
I have ordered several different voltage regulators. I am going to try them first before I get another fuel gauge

 

You may want to place capacitors on the input and output pins to ground per the diagram.

 

I need an electrical engineering degree.
I got the voltage stabilizer wired in to the gauges 12v input. That’s the 12v supply for fuel oil and temp and nothing worked. So I started checking voltages. I just have it running off the battery. I have 12.5 at the battery into the regulator and 1v at the gauge terminal. I pulled it apart and tested the regulator not hooked to anything on the output side I have 10v so I started checking the gauges. On the 3 gauges there is 300 ohms between the power in and the ground and 100 ohms between power in and signal. So thats 900 ohms on the power supply which is dropping the incoming voltage to 6v and heating up the supply wire.
I am ready to chuck these gauges and have no gauges just idiot light for oil and alt.

 

Just calm down and try and approach this bit by bit.

You need to confirm what voltage your gauge is!!!!!!! [some are 6v or 10v or 12v]
If your gauge is 12v you need to test if there is a power surge or not.

Disconnect the 12v feed to the gauge [TAPE IT UP]

Make a "bypass" test wire and connect it from the gauge directly to the battery [ with alligator clip, and Inline fuse]
Take it for drive
If it is still jumping around, it is highly unlikely to be the harness [but the sender or gauge itself]

Fuel /Temp gauges are stupid-simple! they are just 2 electromagnets fighting each other.
I have repaired the coils in them [when my eyesight was younger]

You cant really damage them unless you "dead short" them unfused.
different voltage [less] and reversing polarity won't damage them. They simply don't work properly.

Make a test wire [with and eye for the gauge, an inline fuse, alligator clip for the battery] and go and test it.
It will save you weeks of frustration.

Also the sender is just a rheostat [that grounds] a bit of Isopropyl alcohol cleaning it really helps. [I saved a friend $500 on a Porsche by doing this]

You can also test your gauge by disconnecting the sender wire at the gauge, and connecting another test wire to a "potentiometer" [that grounds]
Dial it up/down and test it around the block for stability.

With a few tools [like test wires, and a potentiometer] you can "go through the motions" and actually get a result.
Some things can be annoying like a fuel tank that has new mounting isolators that won't ground properly [a test wire also can eliminate this]

Everthing above ^^^ is what old timer auto electricians use to do

 

I ran the alternator to the battery and capped the wire that went to the power distribution/fuse block. Result is the output has stabilized at a consistent 14.5v when charging.

 

with a 3 wire internal regulated alternator [eg: 10 si] the charge wire direct to the battery is best.
Another common "mistake" is to bridge the T1 wire to the charge wire [so the regulator gets a reading]

You can usually do this ^^^ safely but it is best to run this wire back to somewhere in the harness so the regulator gets a true reading.
I had a Falcon with a 3 wire Bosch Alternator that the Fuel gauge moved in unison with the turn signal flasher [only when the engine was idling]
This was caused by an "electrical repair" in the past [over charging, so they bridged this wire]

 

Got it working.
After rewiring the dash and trying a bunch of ideas and suggestions the one that fixed it was running a ground from the sender unit to the ground lug on the frame rail. All my grounds are wired back to the battery. I am not relying on rusty panels to make a good ground. So in theory the sender is now grounded to the gauge.