My gas gauge is not working. I changed the sending unit no luck any suggestions?
Well, you are supposed to verify the malfunctioning component before replacing stuff. Anyway, the next most likely component is the gauge itself. You can test it and the sending unit as well by measuring or applying voltages.
Is your trip odometer working?
Just reset it each time you get gas, estimate your worst possible range, and fuel up when you’re at no more than 75% of that.
I know people who have had their gauge go and have used that approach without problems for years… Its annoying, but it is cheap!
I am not a mechanic but it needed a fuel pump and the shop suggested that I change the sending unit. How do I measure voltage what type of tools do I need? thanks for your help
It would help if you had a wiring diafram to correctly identify the wires that connect to the sending unit.
Pull apart the connector, turn the key to ACC position, and use your voltmeter to check the following.
One wire from up front should be trying to send +12v to the unit. Check.
Apply +12v to the unit through its connector. Check the other terminal for voltage, something less than +12v. Check.
Send +12v forward to the gauge and have an assistant tell you if it now reads full. Check. If it fails to read full, you may have a bad gauge. Remove it for similar testing on the workbench.
I drove a Toyota with a bad gauge for several years by using the odo to keep track. The gauge started working again eventually. I was too poor to hire someone to fix it and my time was taxed as well.
Mechanic jumped the wires gauge worked told me must be a broken wire he could not find it. Anyone know where I can get a diagram of the wiring so I can trace it?
My brother’s beat-up old Saturn works by this method. Well, most of the time anyways.
He helped me move across the province once. The inside of the car was filled up, and a few big things were put on the roof. Nobody thought about the extra fuel consumption. We ran out of gas about 20 miles past the nearest gas pump. 
Do you know what wires he jumped and where?
From memory here, and this is real fuzzy, it seems to me the gauge lead to the tank unit is yellow/white. If you unplug this connector near the tank (multi-pin and also has the pump leads) you should be able to use a jumper wire or test light and connect that Y/W wire to ground somewhere. With the key on the gauge should read FULL.
If it does, then the connector terminals where it plugs in are faulty or the tank sending unit is no good. A bad ground is also a possibility but I think the fuel gauge ground is also common with several other items such as the temp gauge, tach, etc. and if a ground connection was the problem then you would have a number of inoperative items.
To get diagrams and the complete shop manual on line you can pay $25 for it at alldata.com. Local libraries sometimes have subscriptions to services like this. Someone at www.crownvic.net might post a diagram for you. That will be a good resource for you in any case if you will be doing further work on this car.
When I disconected the plug under the steering wheel on the left side the gauge would go up to full. Now I plugged it back in it went down then slowly went to 1/2 tank but now stays there doesn’t move
You might try filling the tank and then see what happens to the gauge.