I found my fuel buffer module today and decided to take a look at the wires and see what the actual problem with my gas gauge is. It only reads correct when you first fill up the tank, as soon as its under F its nearly a 1/4 of the way below the reading until it hits the 1/4 mark then just falls to E. I read in the online manual that when the sender reads 90 ohms its full, 0 ohms means empty. Correct me if I'm wrong please. I have the wiring diagram of the module in front of me so I would just explain what I found. It is recieving 12 volts from the fuse, 5 volts from the reference wire, the output(D, PPL) and return(M, BLK/WHT) wires to the VCM have .3 ohms. I didnt check the wire from the module to the gauge because I found something more profound. The Fuel level input wire that goes to the sender in the tank to the VCM for a ground, is reading open, infinite resistance. (I dont really understand how this wire works but im guessing the module sends power through this, and it determines the fuel level by looking at the resistance changes/voltage drop through the sender grounded by the VCM?) But the thing is, my gauge is reading E. When I first turn the key KOEO, the guage jumps way past full and then back the E in the blink of an eye. If I decide to start the truck it doesnt even lift off E. I pretty sure the gauge works because when I disconnect the buffer chip it reads past full, but I didnt ground it to see its reaction the other way. Why is it reading E when the sender is reading infinite resistance?
EDIT: Gauge does go to E when grounded.

 

Hello I am a newbie here I have 1999 Blazer LS and have the fuel gauge bouncing problem I have read a lot of the threads. mine is a little different. mine reads full until about 150 miles or there's about then will drop to E and low fuel light comes on. I never get much of any movement of my gauge.

So my question is where is the buffer module at everyone talks about on a 1999 blazer it's not behind the glove box.

what does it look like anybody got a picture of one?

 

EDIT: Added Later - You know there was a lot to be learned in the original post on this thread.

Too bad nobody read it because some people don't know WTF the enter key and paragraphs are for!!!

So here's a thought. .. some people just wont take the time to pick out your thoughts from a pile of words.

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One thing I have found out.. there is a buffer module in earlier trucks, and evidently 97. There is none in the 99.

I'd thought it was in the cluster. But the PCM actually does it.

Will look up the circuit to it, but your best bet is to find the purple wire leading to the sender and check that out.
Here's the schem from another source, and the connector view
so you go to the PCM: C3 the clear {or natural} connector find purple and either read the voltage/key on, or ohm it out with key off.
Ref voltage will be 0 - 5v. Dont recall ohms but rel low...Suggest reading with half tank or so then have someone rock the vehicle

 

My guage will say empty, if I rock the truck it says full, when the gas levels off it goes back to empty

 

At all levels of gas in the tank?
If so definitely a floppy sender resistor pickup.
5 v goes in one side of resistor Grd the other. The arm moves a pickup going into the VCM. If it reads 0 it thinks full. If it reads 5 it thnks empty. - I guess. It thought ga reacted opposite but been a long time
But along that path, the voltage will vary as you see a different part of the 'voltage divider'

If the resistor board is broken at one spot, Then there is no voltage variation along the resistor track and the pickup will see 5v on one side of the break and grd on the other.
Update: hmmm that cant be exactly right. But anyway it gives you the idea.

 

So where is this floppy sender resistor?

 

It's on the Fuel Pump / Fuel sender assy actually in the tank. There's another sensor on that as well, Tank Fuel Pressure - which is mounted atop the assembly.

And all answers are supposition based on logical conclusion, not guaranteed accurate in any given case.

 

Thanks, not gonna tackle it. Right now anyway, just keep track of mileage