Fuel gauge troubles

[SlantSixSullivan]

1980 Diplomat, slant six. I started having an issue with my fuel gauge a couple months ago. I would be driving and the needle would plummet down to E, past the empty mark as if the key was off. Sometimes it would come back up to the accurate level where it was before. It did this on and off for awhile, then it stayed on E permanently. Without testing my sending unit, I replaced it and my dented fuel tank. The replacement sender came with a ground wire which I used, and bolted it to a clean bare metal spot on the body. With the new sending unit in, the needle does move now but does not go further than 1/4, even when completely full. A member of this site (and dippy.org) generously sent me a used fuel gauge for free. He knew it to be a good gauge, so I tried it. It does the same thing, only its needle doesn't go as far as 1/4 full. I even ditched the replacement ground in favor of the factory two wire connector, the style used on my distributor. Still no luck. How can I test this problem? I know the Ohms range of a properly working fuel sender, so I will check that out. However, I doubt that is my problem since the old and new both haven't solved it. How about the instrument cluster voltage regulator? I noticed the temp gauge also reads a bit low, and it's light is almost always on. I did install a 160 thermostat, and that may be why.

 

[BudW]

Before I replace a fuel sending unit, I take the sender out of the tank and (and with sender grounded – a clip on radio shack jumper works great) perform a swing test – while someone is watching the gas gauge.

It should go to both range extremes – but will not move that fast. It also will not jerk around on the gauge, during the sweep test.

If it does out of car, but not in car, then look in fuel tank for an obstruction – or something.

If it does not, then follow the directions below.

The fuel gauge uses a 5 volt regulator (on inst. cluster) – and I rarely see them go bad (for FMJ’s). On the 60’s Mopars, I do and - when that regulator goes, it will zap all gauges at once (not fun).
BudW

 

[SlantSixSullivan]

Before I replace a fuel sending unit, I take the sender out of the tank and (and with sender grounded – a clip on radio shack jumper works great) perform a swing test – while someone is watching the gas gauge.

It should go to both range extremes – but will not move that fast. It also will not jerk around on the gauge, during the sweep test.

If it does out of car, but not in car, then look in fuel tank for an obstruction – or something.

If it does not, then follow the directions below.

The fuel gauge uses a 5 volt regulator (on inst. cluster) – and I rarely see them go bad (for FMJ’s). On the 60’s Mopars, I do and - when that regulator goes, it will zap all gauges at once (not fun).
BudW

BudW, when I installed my new sender, I did perform a swing test, which failed. The needle on the gauge did not move from E. Now that I have the new sender and tank in the car, the needle does come up to about 1/4 when the tank is full. I just came inside from testing the sending unit's resistance. Now, tell me if I'm doing this right or wrong. I set my multimeter to 200 ohms range, since this system's range is somewhere between 10-73 ohms. With the ignition switch off, I unplugged and probed my two prong connector at the sender. My meter did nothing. No numbers whatsoever. Could this indicate a faulty connector? I did this with the ignition off simply because with the connector being unplugged, no current would reach the gauge anyway. I can drop the tank to get access to the sender itself to test for resistance, but I'll have to wait until I use some of the gas in it. 16 gallons is pretty heavy!!

 

[SlantSixSullivan]

I confirmed my instrument voltage regulator is working properly. At the temp sender, I connected a lead from my multimeter and used a good ground and saw voltage fluctuate between 0-6 volts with the key on. Knowing the regulator is good, I then tested the wiring from the two prong connector at the sending unit to the gauge. Again, I saw voltage fluctuate from 0-6 volts. I have now tried two gauges, both of which do the same thing - show a level of fuel lower than what is actually present. This tells me the gauge is not at fault, nor is the sending unit. That leaves me with the printed cicuit board in the dash, or the short section of wire which connects my sender to the wiring harness in the car.

 

[BudW]

The voltage regulator/limiter does not put out a constant 5 volts.
Basically what it does is switches 12 volts on/off/on/off to get roughly 5 volts – but is more way more A/C than D/C voltage (if that makes sense). Gauge needles were not designed to move very fast anyway.

Because the 5 volt supply voltage does vary, it makes using “voltage” difficult to use to find the problem, because the voltage is constantly jumping around.

Having a “known good sender” with someone under car moving it, and another person in car to watch the gauge - is almost always the best way to test them.

Getting to a related topic for a moment, the ’69 and down alternator regulators worked the same way. They would switch 12 volts to the alternator field wire on/off/on/off real quick to keep a steady 14 volts (or whatever voltage the regulator temperature allowed for sense voltage regulators are temperature sensitive, on purpose).
Electronic voltage regulators (’70 and up) took care of that problem (and a lot more reliable).

Going back to topic, several years ago, this company http://www.rt-eng.com/rte/index.php/RTE_limiter made electronic voltage limiters which do a number of things:
One, the output was a steady 5 volts except when cold which it bumps voltage up a bit to warm the gauges faster.
Two, it wasn’t limited to only handling 2 gauges (Amp gauge doesn’t count).
Three, it wouldn’t burn the gauges up if something went wrong (a big problem with the older A, B, C and E-Bodies) – which ironically IS still the same voltage limiter we use.
Four, is makes testing a lot easier.
There are more reasons – but that company’s web site doesn’t appear to be working to remind me of the other reasons - which is not good. Hopefully it is a short term hiccup.

I have two of their voltage limiters left (one for each of my cars) – and they were not cheap (but a lot cheaper than replacing your gauges).


The temperature gauge, oil pressure gauge (not used much in FMJ’s) and fuel level gauge all use the same resistances, so a person can use a fuel gauge with jumper wires, to test the temp (dash) gauge (or oil pressure gauge, if equipped) for operation.

I had purchased some parts from Radio Shack many years ago and connectors to make my own tester (like the official Chrysler tester) – but never did get around to finishing that project.
BudW