Fuel gauge/sending unit?

[85 Jimmy]

I've been driving my blazer a lot more then I used to and want to fix this fuel gauge issue I have. It reads way past full, then it is completely empty when it's somewhere between 1/2 tank and 3/4 tank. It can be deceiving when I think I have 3/4 a tank of fuel only to find out I'm on empty.

Anyway, I have an autometer gauge and a sending unit for a fuel injected blazer from LMC. Is it possible that when I installed the sending unit it got caught on the top side of the baffle, giving me a false reading? Or is the sending unit more than likely bad?

I'm going to drop the tank this weekend and check the resistance with the float at empty and at full to see if it's reading right.

 

[clandr1]

Just spitballing here:
LMC sending unit isn't calibrated properly
Autometer gauge isn't compatible w/ LMC unit

Try testing the voltage from the sending unit, and comparing it to your service manual. See if the readings are right.

 

[rampage]

Make sure the Autometer gauge is designed to read between 0 and 90 ohms. You can easily check to see if the gauge is bad or if it's the sending unit. Disconnect the sending unit at the connector by the tank. The gray wire is power for the fuel pump and the purple wire goes to the gauge. With the key in the on position (don't start it!) short the purple wire, the gauge should read either empty or full, I forget which. If you have some small resistors under 90 ohms you can connect them between the purple wire and frame to see the gauge at various levels (can get them from radio shack for pennies). Or with the purple wire not connected it should read the opposite of the shorted scenario (either full or empty). To check the sending unit you need an ohm meter (multimeter). You should see readings between 0 and 90 ohms depending on how full the tank is. I suspect that you accidentally bent the wiper of the rheostat on the sending unit when you installed it.

 

[85 Jimmy]

Ok, so here's an update 1 year later.

The autometer gauge I have is part # 3514 which is supposed to read between 0-90 ohms. So I know I have the right gauge.

I dropped the tank and pulled the old sender, with it out of the tank it reads empty when the float is down and gradually goes to full as I move the float up. So I reinstalled the sender and it still isn't reading right. There is currently enough gas to just barely cover the bottom of the tank, and the gauge reads between 1/4 and 1/2 a tank of fuel. I droped the tank again and check the resistance on the purple wire and with the float down it reads 4 ohms, then jumps between 46-456 ohms until I get the float all the way up where it reads 110 ohms. There is also resistance in the power wire for the pump and the ground wire/sending unit itself. Also, when I reinstalled the sender, I took a flashlight and looked in the tank just to make sure the float wasn't catching on the baffle, it's not.

So I got a new AC Delco sending unit and checked the resistance before installing it and it reads 0 ohms when empty and 90 ohms when full. It also smoothly goes up in resistance as I move the float. I hooked it up outside of the tank and when I turn the key on the gauge just goes all the way to the right so it's horizontal. Yes, the ground wire is hooked up.

Do I need to go ahead and install the sending unit to see if it works, or should it read correctly out of the tank?

 

[85 Jimmy]

I also did what rampage suggested for checking the gauge.
With the gauge grounded out it reads just under empty, with it disconnected the needle goes all the way to the right and is horizontal.

 

[Carcinogen]

Try grounding the sending unit with it out of the tank, it needs to be grounded and should then work just the same as when it is in the tank

 

[85 Jimmy]

I grounded it while it was out of the tank and it didn't do anything different.

Then I installed the fuel pump on to the new sending unit and it magically started working.

 

[Carcinogen]

I totally spaced that 87+ has the intank fuel pump... good call.

 

[85 Jimmy]

I didn't think that having the pump hooked up would have mattered, but I guess it does.

Right now it reads empty, and it is. It better not start acting funny on me when I fill it with fuel (when stuff like that normally happens).

 

[rampage]

Sounds like you got it figured out.
FYI: The resistance that you measured on the power wire for the pump is a good thing - you're measuring the resistance of the windings on the motor which means it's good. And you might have been able to fix your old sending unit by cleaning the built-up varnish on the wiper of the rheostat so it reads smoothly.