1989 GMC Jimmy S15 4.3L that when you sit at a red light can watch the fuel gauge move from half tank to an eighth and it keeps moving. What do fix problem short new sending unit?

That sounds like a classic faulty fuel level sending unit. The needle's dramatic and continuous movement while stationary strongly suggests the problem lies in the sender itself, not something in the fuel system causing actual fuel loss. While other things *could* contribute (a very rare short somewhere in the wiring, for instance), a new sending unit is the most likely and cost-effective first step.

Here's why a new sending unit is the best starting point and what to consider before buying and installing it:

Why it's likely the sending unit:

* Consistent, Dramatic Movement: The gauge isn't just fluctuating slightly; it's making a significant drop. This points to a problem with the sender's float or potentiometer, not a minor electrical glitch.

* Stationary Vehicle: The problem only manifests when the vehicle is stopped. If it were a fuel leak or other major system issue, you'd notice the problem while driving too.

Before you buy a new sending unit:

1. Check the Fuel Gauge Cluster: Although less likely given the description, ensure the gauge itself isn't faulty. A bad gauge will give inaccurate readings even with a good sender. It's rare for a gauge to act *this* dramatically, but it's a quick check.

2. Inspect the Wiring: While unlikely to be the primary cause of the drastic drop, carefully examine the wiring harness connecting the sending unit to the fuel gauge. Look for:

* Broken wires: Any fraying, exposed wires, or broken connections can interfere with the signal.

* Corrosion: Check the connectors for rust or corrosion, which can disrupt the electrical connection.

* Loose connections: Make sure everything is securely plugged in.

Replacing the Sending Unit:

This is usually a fairly straightforward job, but the difficulty can depend on your vehicle's configuration. Generally:

1. Safety First: Disconnect the negative battery terminal before working on any fuel system component.

2. Access the Fuel Tank: This usually involves dropping the fuel tank, which is a somewhat involved process. Consult a repair manual specific to your 1989 GMC Jimmy S15. (A Haynes or Chilton manual is a good investment.)

3. Remove the Old Sending Unit: Once the tank is accessible, disconnect the electrical connector and the fuel lines. The sending unit is typically bolted or clamped to the top of the fuel tank.

4. Install the New Sending Unit: Make sure the new unit is correctly oriented (often marked on the unit itself), and securely attach it. Reconnect the fuel lines and electrical connector.

5. Reinstall the Fuel Tank: Carefully lower the fuel tank back into place and secure it.

6. Reconnect the Battery: Reconnect the negative battery terminal.

7. Test the Gauge: Start the vehicle and check the fuel gauge to see if the problem is resolved.

Important Note: If you're not comfortable working with fuel systems, consider taking your vehicle to a qualified mechanic. Fuel is flammable, and working with fuel tanks requires careful attention to safety. A shop will have the correct tools and expertise to handle the job safely and efficiently.