Disappearing Fuel Level

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j.mcconnell

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1973 Duster, re-wired with a Speedway 21 circuit kit that is like a lot of other universal kits.

With the gauge cluster plugged in and sitting on my lap the gas gauge work fine. When I install the cluster in the dash, the fuel level drops to zero after a few minutes. I have recreated this in my lap by connecting a wire between the metal cluster mounting point to the metal dash. I have a ground wire from the wiring harness connect to the back of the cluster already and the arrangement is similar to the American Autowire classic update harness. I believe that wire was for the light ground. I wouldn't think an additional ground would make the gauge stop working.

https://www.americanautowire.com/PDFLink/92971259 510605 IN 0.0.pdf
 
The gauge cluster (the metal housing) needs to be grounded to the body. I don't think you have a ground anywhere else that does the same thing. I assume you are having an issue with the fuel gauge circuit and need to diagnose that with the cluster grounded.
 
I have the ground wire circled in red at the location pictured and the gauge works correctly while the cluster is out of the dash. When the cluster is in the dash or I use a jumper (red line) from the mounting hole to the dash frame, the gauge goes to zero after 3-5 minutes.

Capture.JPG
 
Thanks for the pic. This is not the factory setup but we can help. I have to question whether that black ground wire is grounded at the other end because that should do the same thing as your added ground wire.
 
Just a heads up. When we properly ground the cluster and the gas gauge reads zero, can we assume it is reading correctly and you have little to no gas in the tank? Just thinking ahead.
 
I have exactly 5 gal in the tank, poured from a VP fuel jug. With the cluster not touching the dash and the circled ring terminal connected the gauge read about a third of a tank. 50’ish ohms on the sender wire seems correct. The gauge reads zero with no power connected.

Once the cluster is installed or the second ground added the gauge goes to zero. I wouldn’t think additional grounds would be a problem. Maybe a short?
 
Is the sender and meter a set, ie designed for each other?

I saw on a reailers website that their guage only works with their sender, dont know brand
 
Sender is a 0-73ohm unit, should be fine.

Update: Circled wire with the ring terminal on the back of the cluster has no continuity to ground. That means I wasnt getting a ground until the cluster was installed in the dash. When checking that continuity with a jumper wire to establish the ground, the meter beeped for continuity with a rhythmic 1 sec on/off (ground/no ground). When connected to the gas gauge pin, it blinks between 35ohms and OL with the same rhythm.
 
Very odd.. fuel sloshing?
Any chance you are seeing the instrument cluster voltage regulator it goes on and off at some frequency?
 
Dana67Dart said:
Very odd.. fuel sloshing?
Any chance you are seeing the instrument cluster voltage regulator it goes on and off at some frequency?
Click to expand...

Lol definitely not sloshing, the car doesnt run for other reasons. The neighbors don't appreciate me trying to get it to run at this time of night, so I'm working on other things.

I'm starting to suspect the VR as well. I have 55 ohms on the sender wire at the back of the cluster which is reasonable. I should add that this cluster in a unknown item. It never had more than the speedo connected prior to this attempt. All I really want is the fuel gauge and the signal indicator and speedo lights would be a welcome bonus. Thats a topic for a different thread maybe as they should be mostly separate circuits.
 
getting in the car could cause sloshing.

is the dash out of the car?
if so you can test all funstions of hte dash out of the car.

you will need some jumpers and a car battery

I dont have the particulars of which pin is which but you can figure that out with the Factory Wiring diagram.

you will need to connect batt negative to the metal housing and batt positive to the propper pin.

Now connect, per the wiring diagram, the proper pin on gauge for temp, or fuel and connect to resistors ( 5 watt should do ) connect the other end of the resistor to batt neg.

(this is just a ROUGH example PINS are NOT correct)
cluster.jpg



(The below links are for reference only I am not recommending these particular items)

This will simulate empty or engine cold (~75 ohms)
Resistors - 5W, Wirewound, Power, Resistance: 75 Ohm 603016355715 | eBay

this will simulate 1/2 tank or engine at normal operating temp (~22 ohms)
30pcs 5W 20 ohm 20R Resistance Ceramic Cement Power Resistor Kit Part 911346346632 | eBay

this will simulate full or overheated (~10 ohms)
10pcs 5W 10 Ohm 10Ω Ceramic Power Resistor Moisture Resistance | eBay


You could also get a variable resistor 0 to 100 range 1 amp or 5 watt capability should do. (don't go under 7 or so ohms or you might cook the gauges)

For lamps in the dash, use the same type of hookup but follow Factory wiring diagram for plus and minus.
 
I've gotten all of the lights to work on the bench with a 12V 5A power supply connected directly to the individual pins and the gauge frame. What I didn't have was a resistor anywhere in the range the tank sender would provide (RIP RadioShack). I dont know or care about the other temp, oil press, or alt gauges as I have electronic transducers tied into my microsquirt. E85 isnt super popular in NC so a working fuel gauge would be great. I do need to clean/tighten the nut on the fuel gauge as I saw the effects of some corrosion when testing the dash lights. It seems like a certain amount of time wasted dealing with electrical stuff is required with these 40yr old cars.
 
if i'm not mistaken, dual ballast resistors have one side that is 5 ohms, two in series would give you 10 ohms and show a full tank give or take.
 
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