QuestionQUESTION: dick i talked with you awhile back about my faulty fuel gauge in my 70 charger, you gave me a few things to check wich i did and helped out alot, you told me to check to make sure the gauge was good by grounding out he wire going to the sending unit and to see if the gauge went to full and it did, you then said to check the insterment regulator with a test light to see if it palsated and it did, so that would indicate that it was indeed good so i then went to the next step that was to check the sending unit out of the tank with a helper in the car i hooked the sending unit up and moved the float arm while they watched the gauge, with the float in the full position the gauge only went to 1/2 tank and would not go any further, also is it normal for the sending unit to get hot, i would think it might spark a fire in the fuel tank.i think we might have this one whipped with a little more work, i am using a 18 gauge wire running from the negetive side of the battery for a positive ground.
ANSWER: I answered this yesterday, but it appears that the system lost it - so here goes again!
You have proven that your instrument regulator and your dash gauge are OK, so the problem has to be with your sender, or perhaps the grounding to the tank. Try grounding the sender with a shorter, heavier wire to do your test - ground it to the rear bumper. That should improve the situation somewhat - but I doubt it is going to bring the reading all the way up to full. Unless it does, I think you have a bad sender. If it does make the gauge read full with the float all the way up, and it still doesn't work well in the tank, your problem is in the grounding of the tank - you may have to scrape the surface of the tank to bare metal and add a wire to the chassis to get an accurate fuel reading.
Dick
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QUESTION: ok i will try a heavier gauge wire to the bumper, but as for the other question the sending unit is getting hot is that normal, its hot enough to cause alittle smoke from it.
ANSWER: OK, that is worrisome! Check to make sure you see the pulsating 12 volt signal at the sender connection from the car's wiring harness - if you do, and the sender is getting outrageously hot, that adds to the suspicion that the sender is bad. The sender should get somewhat warm, especially when the float is all the way up, because it is dissipating about 5 watts of power, but smoking seems a bit extreme.
When the float is at the minimum position, the power dissipated is much lower, and the sender should be getting warm, but not hot.
If some wiring error is causing the sender to see a constant 12 volts, the power dissipation goes up to about 25 watts, which will get it pretty darn hot after a few minutes - probably hot enough to burn your hand.
Even with 5 watts, I guess there is a good chance there is enough debris or dirt in the unit that it might smoke for a while.
Dick
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QUESTION: i am going to try a larger ground wire maybe 12 guage and put it closer to the sending unit, would you believe i wire ambulances for a living and i can't figure this out. thanks for your help doug
AnswerWell, 12 gauge will sure be good enough!
Since you are familiar with electrical things, here is some more information:
The sender varies between around 5 ohms for a full tank to around 80 ohms for an empty tank. The average voltage sent to it by the instrument regulator is about 5 volts, so when the tank is full, the sender is pulling 1 ampere (thus the 5 watt power dissipation estimate). These resistance values are not critical at all, the main thing is that the resistance varies from pretty low with a full tank to much higher for an empty tank - the dash unit can cope with a lot of variation from unit to unit. I've seen them where a full reading is at 10 ohms, and an empty reading is as low as 60 ohms, the dash indication still works fine, so don't be too picky about the reading with an ohmmeter.
The factory wire feeding the sender is 14 gauge, but the ground return is via a short strap or wire from the tank to the chassis, so there is basically very low resistance in the ground wire. Your 20 foot piece of #18 wire would have had a resistance of only a few tenths of an ohm, so I really don't think your test setup was the reason for the low reading - but maybe a little loose connection somewhere, or some contamination on the wire ends could have affected the gauge reading.
If you have access to an ohmmeter, measure the resistance of your sender for full and empty positions - and see if it varies as you swing the arm, and notice if it changes smoothly. If it does, it is probably OK and there is some other problem, but since you observed a full reading on the dash when you grounded the sender wire at the tank, I think there is no other explanation than that the sender is bad or has a poor ground on it. If the resistance jumps around as you swing the arm, the windings inside the sender have probably come loose, or the wiper arm that rubs on the windings has lost its tension - this is the way the senders usually fail.
Let me know what you find out.
Dick
