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Gas gauge not working


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Hello everyone, I am working on a 1998 Chevy Z71 k 1500 with a 5.7 l vortec. I have replaced the sending unit and have proper voltage drop at the tank, 5 volt references good. The gas gauge when I turn the key on jumps past full and then drops to empty. I have pulled the cluster and tested the prongs behind the cluster and cannot find the same voltage as I have at the tank. The gauge reads empty all the time, and I am having trouble figuring out where the sending unit wire runs to before it reaches the gauge. The truck needs to smog but sets a check engine light for a the fuel gauge not reading. Does anybody have any answers to this or a wiring diagram?

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did you replace the whole fuel pump module? can you manipulate the gauge with a test light to ensure the problems in the wiring and not the tank? there was a similar post not too long ago where the new sending unit was hitting the baffle in the fuel tank causing it to read empty but I don't think that would set the CEL.   the wire probably goes to the PCM then the PCM sends the signal to the gauge. my 98 has never set the CEL no matter how screwd up the gauge but it may be because its a 2500hd regular Silverado so may have different electronics

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usually the way the gauge works is the sending unit is a variable resistor between the gauge and ground but if its going to the pcm may be different and have a 5v reference. reading empty would mean its shorted and reading full means the circuits open

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I have tested the wires at the tank and they are working, tested sending unit out of the ohms are reading correctly from zero up to around 240 ohms. It runs through the PCM on this truck and uses a 5V references. Has around half tank in it, when I tested the reference it was good plugged pump in and reads 2.74V. sending unit is working correctly. I'm trying to figure out where the signal wire runs to and where it comes out. I can't find a pin diagram for it.

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so if you unplug the fuel tank connector the gauge stays on empty?  normally it should go to full.  I know theres a PCM pinout on ALLDATA cuz I have used it when my cr sensor wires shorted out

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I found a pinout chart for the PCM, the voltage on Pin's 12 and 13 match what's at the tank. I don't have access to ALLDATA. I need a wire diagram from the PCM to the garage if anyone has one they can share.

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Pin's 12 has 5V 13 has 2.7V which means I a getting correct voltage to PCM from tank the plug for the cluster should be pin 16 with a purple wire but I have no voltage at this pin. Trying to find a wire diagram that shows from PCM to cluster. Can anyone help. I am an experienced mechanic and have taken advanced automotive electrical and electronic systems in college, I know how to test just can't find the correct wiring diagrams.

Edited by OBSGuy1980
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Update, PCM C3 connector pin 12 5V references is good, pin 13 fuel level module  has 2.7V same as tank connector and PCM C1 connector pin 9 Fuel Gauge (Fuel Level Output) only has 1.5V. Does anyone know what pin 9 voltage should be?

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