afu45,
I posted this last September - it might be helpful.
Tim
These directions most likely work on earlier gas gauges but it has been a while since I have looked at a 47-53 gauge.
The principles are the same whether the gauge is on the bench or in the gauge assembly/housing in the truck.
The gauge has two posts on the back. One should be labelled with a red tag that says to connect that post only to the fuel tank sender unit. There is also the word "SENDER" pressed into the back of the gauge housing near the sender post. The sender post is on the "Full" side of the gauge.
The post near the "Empty" side of the gauge is connected to a 6v hot/positive lead. The two posts are insulated from each other and the two posts are insulated from the gauge housing.
The gauge housing is grounded. The gauge is grounded through the metal "plate" that is between the two posts on the rear of the gauge. There should be bare metal on the inside of the gauge housing where the gauge attaches to/through the housing.
If the gauge is out of the truck, the gauge must be in an upright position (the orientation should be same as if it was installed in the gauge housing - gravity affects the movement of the needle).
First test: disconnect the sender lead from the sender post. Then attach 6v hot/positive source to the hot/positive post on the gauge. Attach a 6v ground to the gauge body or to the flat metal "plate" on the "backplane" between the two posts. If the gauge is in the truck, the gauge assembly/housing is already grounded (the outside of the body is bare and it is grounded by connection at the three attachment nuts). The gauge should read "full".
If the gauge does not read full, make sure the ground connection is good. If the connections are not good, a good gauge will appear to be bad (no movement of the needle to full).
Second test: Leave all the connections in place from the first test. Attach the sender lead to the sender post on the gauge.
1. If you have the sender on the bench (out of the truck), the sender must be well grounded (run a ground lead from the top of the sender unit to the ground lead that is connected to gauge assembly body (or to the ground backplane on the fuel gauge, if the gauge is out of the truck and out of the gauge assembly). Move the float arm up and down and the gauge should change from full (float arm up) to empty (float arm down).
2. If the sender is in the gas tank, diagnostics become more complex because you do not know how much gas is in the gas tank (and thus you do not know the float level and the correct reading). Also, the sender lead might be bad/broken somewhere between the gas tank and the fuel gauge). And, the sender body may not be well grounded. If the gas level in the gas tank is less than full, the gauge reading should lower from "Full" when you connect the sender lead.
If the sender is in the gas tank and you get no response from the fuel gauge, you should run a separate test-lead from the sender to the gauge (to be sure that there is not a break in the sender lead). In addition, you should run a new/clean ground wire lead from any sender-to-gas-tank screw and connect this lead to the chassis (clean the connection to the chassis/ground and clean the metal around the sender attachment screw). If the needle on the gauge still does not change from "Full", something is wrong - most likely a bad gauge.
If the gauge is in the truck and it does not test properly, you should remove the gauge assembly and then remove the gauge from the assembly. Then, before buying a replacement gauge, bench test the gauge (the leads/connections on the truck, especially ground connections, might have been bad).
Warnings:
1. Do not touch a hot lead to the sender post.
2. Do not use a 12v source with a 6v gauge (however, you may use souce less than 6v - I have used 1.5v and 3v continuity testers).
3. Make sure your sender and gauge are grounded (to the same ground).
I hope the above description is not too confusing. I trust that others will correct any mistakes and/or offer clarifications.
This site might be of interest:
http://www.howstuffworks.com/fuel-gauge.htm
