1. Check the Fuse: Start with the simplest possibility. Consult your owner's manual to locate the fuse for the tow/haul system. Check if it's blown and replace it if necessary.
2. Inspect the Switch Itself:
* Visual Inspection: Look for any obvious damage to the switch. Is it loose, cracked, or showing signs of wear? A broken or worn-out switch is a common culprit.
* Switch Operation: With the ignition on, try pushing the switch. Does it click or feel like it's engaging? Sometimes, internal failure occurs without visible damage. A multimeter can test if the switch is sending a signal when pressed. (See step 4 for multimeter testing).
3. Check the Wiring:
* Connection at the Switch: Locate the connector that plugs into the tow/haul switch. Carefully disconnect and inspect it for corrosion, loose connections, or broken wires. Clean any corrosion with electrical contact cleaner and reconnect securely.
* Wiring Harness Inspection (Advanced): If the connector and switch are okay, a more involved inspection of the wiring harness may be needed. This often requires tracing the wires back from the switch to the transmission control module (TCM). This is a more complex task, and you may need a wiring diagram specific to your truck's year and model. Visual inspection may find broken wires, chafing, or other damage.
4. Test with a Multimeter (Advanced): A multimeter can confirm if the switch is sending a signal to the TCM. You'll need to consult a wiring diagram to identify the correct wires at the switch connector. With the ignition on:
* Check for Continuity: Test continuity (resistance) across the switch contacts when it's pushed. There should be continuity when engaged, and no continuity when disengaged.
* Check for Voltage: Test for voltage at the wires going to the TCM when the switch is engaged. You should see a voltage change when the switch is activated. (Caution: Always disconnect the negative battery terminal before testing voltage).
5. Transmission Control Module (TCM) Issues (Advanced): If the switch and wiring are good, the problem might lie with the TCM. This is the most complex and potentially expensive fix, often requiring professional diagnosis and possibly replacement. A faulty TCM won't interpret the signal from the switch correctly.
6. PCM (Powertrain Control Module): Less likely, but a faulty PCM could also prevent the tow/haul function from working. Similar diagnostics would be needed as with the TCM, often requiring professional tools.
Important Considerations:
* Owner's Manual: Your owner's manual is crucial. It contains fuse box diagrams, wiring diagrams (sometimes), and other helpful information.
* Professional Help: If you're not comfortable working with automotive electrical systems, it's best to take your truck to a qualified mechanic. Incorrect wiring or attempts to repair the TCM can cause significant damage.
* Year and Trim: The specific details of the tow/haul system can vary slightly between different years and trim levels of the 1999 Silverado. Providing this information will help in finding more specific repair guides or diagrams.
Start with the simpler steps (fuse, switch inspection) and progress to the more advanced ones only if necessary. Remember safety first – always disconnect the negative battery terminal before working on electrical components.
