1. Check the Simple Stuff First:
* Fuel: This sounds obvious, but ensure you actually *have* fuel in the tank. A low fuel level might not register accurately, or the gauge might be stuck low.
* Grounds: Poor grounds are a common culprit in older vehicles. Inspect the ground wires connected to the fuel sending unit (inside the fuel tank) and the instrument cluster. Clean any corrosion or loose connections. A bad ground can cause erratic readings or a completely dead gauge.
* Fuses: Check the relevant fuses in your fuse box. Your owner's manual will show which fuse(s) control the gauge cluster.
2. The Fuel Sending Unit:
This is the most likely suspect. Located inside the fuel tank, it's a float mechanism that moves with the fuel level, sending a signal to the gauge.
* Testing the Sending Unit (Most Difficult): This involves dropping the fuel tank (a major job). Once out, you can test the sending unit's resistance with a multimeter. With the tank empty, it should read near zero ohms. With a full tank, it should read near its maximum resistance (check your repair manual for the specified range). If the resistance doesn't change with the fuel level, the sending unit is faulty and needs replacement.
* Consider Alternatives (Without Tank Removal): There's no guaranteed way to test this without removing the tank, but you can sometimes get hints. For example, lightly tapping the top of the tank *might* free a stuck float if that's the problem. This won't solve a damaged unit, but it could provide temporary indication of the unit's problem.
3. The Gauge Cluster:
* Gauge Cluster Itself: A faulty gauge cluster is less likely than a bad sending unit, but possible. If the other gauges in the cluster are working, this is less probable. Testing the gauge cluster requires specialized equipment and is often more costly to repair or replace than the sending unit.
* Wiring to the Gauge Cluster: Inspect the wiring harness for breaks, corrosion, or loose connections between the fuel sending unit and the gauge cluster. A wiring diagram (available online or in a repair manual) will help you trace the circuit.
4. Wiring and Connections:
* Trace the Wiring: Use a wiring diagram to trace the circuit from the sending unit to the gauge cluster. Check all connections for corrosion, loose terminals, or broken wires.
5. Repair Manual:
A repair manual specific to your 1985 Chevy S10 Blazer is invaluable. It will provide detailed wiring diagrams, specifications for resistance readings, and step-by-step instructions for removing the fuel tank and testing the components.
In Summary:
Start with the simple checks (fuel level, grounds, fuses). If those don't work, the fuel sending unit is the most likely culprit. Replacing it is a significant undertaking requiring tank removal, but often more cost-effective than other potential repairs. If you're uncomfortable working on your fuel system, consult a qualified mechanic.
