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Posted by Keith in Washington on Sunday, May 11, 2008 at 01:10:48 :
The gauge has never worked on my 51 PW in the 22 years I have owned it. It is aways pegged at full as soon as I turn on the switch. I occasionally run out of gas. I have looked at it many times and never did figure it out. Well between last night and today I finally got it working. There were two problems and it appeared that there was a short in the system. The short story is that the sender was shorted and one of the coils in the gauge had a bad ground. See details below it you want.
Problem 1:
I have a single wire going to the sender which is correct for the truck. I have checked it several times and have actually replaced it. Last night I was looking for some stuff and came across the original sender. It often reads 0 ohms from the screw to the frame which means that it is shorted at times it will read something. I took the little can like cover off the coil and contact. The contact and windings are fine and I can get from 0 to 90 ohms depending where I have the probe. I was also getting correct readings from the post and ground so it is not shorted. I looked again and noticed that the inside of the cover has a shinny spot. It matched up with the top of the winding post which would give 0 ohms therefore a full tank. The winding post is on a short piece of brass that the gauge wire attaches to. It was slightly bent down from years of vibration and was shorting on the cap. A quick push and the winding post was back in position. I put the cover on and I got the correct ohm readings. So I installed it and I got a reading of 48 ohms which is about right with the amount of gas in it. I connected it and turned on the key and it pegged it self on full again.
Problem 2:
I checked all the wiring and ran a wire from the gauge to the tank still the same problem. I pulled the gauge and a friend and I look at it and tested it with a ohm meter. It took awhile but we figured it out. The gauge has two coils. Both are feed from the ignition switch through one of the terminal posts. One coil is connected to the other terminal and read 25 ohms (across the terminals). The second coil goes to ground in the gauge. It was reading 6=- ohms from the power terminal to the point were it is soldered to the gauge case. However it was difficult to get a reading and I could never get a reading from the terminal to the gauge case. The coil that goes to the gauge moves the needle towards full. The coil that grounds to the gauge case retards/controls the needles movement to full. We noticed when going to the sender that was reading 48 ohms the needle move slower to the full mark. It we ran then sender wire to ground (simulates a full tank) the needle banged over very hard. That told me that the gauge was bad.
Solution:
Many years ago I bought a 12 volt gas gauge as I was going to convert the truck to 12 volts. But never did and the face plate and needle on the 12 volt gauge got smashed one day. I found the gauge in about 30 seconds and I straighten the needle to be somewhat straight. We hooked it up and it read just under 1/2 tank which was correct. The coils are quite different and have more windings and the ohm reading are much higher. But it was reading correctly and would read both full and empty correctly. I spent a little more time getting the needle straight and I switched face plates since my truck has gray gauges and not black. I installed it and went for a ride. It is nice to have that gauge. It bounces around a lot but that is due to their being no baffles in the tank so the gas sloshes.