Gas gauge not working after tank swap

[jeryst]

Hi all, I need some advice. I bought an all original 74 Challenger Rally. The gas gauge was working. I noticed that the corner of the tank was sleeping, so I bought a new gas tank, and took it to a garage to have it installed. I had read that the new sending units are not very accurate, so I told them to use the original sending unit since it was working.

When I got the car back, the gas gauge was not working. The mechanic said that he put all the wires back as they had come off. They are saying that maybe the gauge is no good, but the gauge and wiring worked before, so I have to assume the problem is with the tank and/or sending unit because that is all that was touched.

I don't want to take it back to them and would like to do some troubleshooting of my own, but I don't have the means to remove the tank, so I am looking for suggestions as to what the problem may be. The car has somewhere between half and full tank of gas.

Disclaimer: I am a total noob concerning electronics. I've tried to read up on the issue but they keep referring to ground and resistance to ground, and my brain just seems to have problems grasping those concepts. I am assuming that the wire that connects to the sending unit goes straight to the gas gauge.

1. Could the original sending unit be incompatible with the new tank, or could it be installed incorrectly somehow?

2. The mechanic said the ring on the original sending unit is rusty, so could that be causing a ground issue?

3. Could the sending unit have broken during the switch?

4. What has to be grounded, the tank, or the sending unit, or both, or neither? I've run a ground wire to the tank, with no effect.

5. Could they have shorted out the gauge or voltage limiter when doing the install?

6. When measuring resistance, do you measure it on the post that the gauge wire attaches to? I had a friend come over with a meter, and he said there is resistance on the post that the gauge wire attaches to, but he didn't give me a number.

Thanks for any suggestions anyone can offer.

 

[moparleo]

Number one, You paid them , so take it back to them. Unless they are a backyard repair place ( I hope you went to a reputable shop) they should have verified the repair before they gave the car back toy you. If they don't want to help, go to the BBB or whatever you have in your state. Do this Before you start to do anything to your car or they will just blame it on you.
Please don't take this the wrong way, but gasoline is very dangerous. Get help. Be safe.
Work outside. A fire in a carport or garage can turn catastrophic.
Always have the proper type of fire extinguisher handy. Should be part of your tool kit.
Download a Factory Service Manual for your car and go to the fuel section. The gauges are the same for all years. MyMopar
Have someone help you drain the tank. Before you start, disconnect the battery, negative side first always on a post 1955 US made vehicle.
after you get the tank lowered and remove the sender here is link for what to do next and how to test it before it goes back in. This is what the repair place should have done before they charged you.

 

[Mopar Nut]

My 70 Challenger’s sending unit had a pinhole in the float, so it would show empty. I’m not saying that’s your problem but it could be when they installed it back in the new tank. Keep your old sending unit, it can be rebuilt. Could be a ground problem. Have you checked your fuel gauge wire? Wiggle it around on the plug. Does yours have a ground strap on it?

 

[benguin]

I second Mopar Nut's ground suggestion. It's critical, and likely they didn't put the strap back on, or if it was missing to add whatever was being used, even a wire from the sender to a body ground would confirm if that's the issue.

 

[EW1BH27]

The good news is your buddy's meter showed resistance so the sending unit is working BUT, I'd like to know the number. Somewhere between 10 and 70 ohms. If it reads the 70, the float is at the bottom of it's travel so obviously your gauge would read empty. The resistors aren't linear but you should read something other than 70 (+ or -) with that much gas in your tank. If the meter reads closer on the low end then my vote is for a bad ground too or the blue wire got cut/damaged during installation.

 

[413]

Don’t take it back! You told them it worked and they didn’t care enough to make sure the gauge work after they worked on it. So this proves they don’t care about you Or your car. Also they will be working for free on the comeback so how good will it turn out?

move on and do it yourself. check that ground strap, they probably tore it off, broke it and threw it on the floor For someone else to pick up.

 

[moparleo]

Take it back first. You paid for a repair. What is the worst that can happen. They blow you off and you let others now not to go there. Don't just accept failure and then also pay for it. They will just do it to someone else. That is why Consumer Affairs, The Bureau of Automotive Repair and other organizations exist.
Contact whoever is in your area and complain if after you give the repair shop the opportunity to fix it they refuse.
You do have rights when you pay money. Also if you paid with a credit card you can contact them and dispute the bill.

 

[413]

Just like sending your steak back to be cooked more, they are gonna spit all over it or wipe up the floor with it.

don't take the car back, the shop sucks, move forward not backwards. They gonna take the car out and run the wheels off it.