* Unrelated Systems: The 4WD system and the heater system are largely independent. They share power from the battery, but have separate circuits and fuses. The fact that engaging 4WD blows the heater fuse points to a short circuit where the two systems somehow interact.
* Possible Causes:
* Wiring Chafe or Rub: The most likely culprit is damaged wiring. When you engage 4WD, something in the drivetrain or related components might shift, causing a wire harness to rub against a sharp edge or metal part. This could create a short circuit, drawing excessive current and blowing the heater fuse. Pay close attention to wires near the transfer case, front differential, and any linkages associated with 4WD engagement. Look for frayed insulation or wires touching metal.
* Loose Connection: A corroded or loose connection somewhere in the heater circuit could be exacerbated by the vibrations from engaging 4WD. The increased vibration could momentarily make a bad connection worse, leading to a short.
* Faulty Switch/Actuator: While less likely, a faulty 4WD engagement switch or actuator *could* be causing a short circuit that somehow affects the heater circuit. This is a long shot but possible if the short is related to the voltage spike during the 4WD engagement process.
* Ground Fault: A poor ground connection somewhere in the heater circuit could cause a voltage surge or fluctuation, and the added load of the 4WD system might push it over the edge, blowing the fuse.
Troubleshooting Steps:
1. Safety First: Disconnect the battery's negative terminal before working on any wiring.
2. Visual Inspection: Carefully examine all wiring related to the heater and the 4WD system. Focus on areas where wires might move or rub when engaging 4WD. Look for frayed insulation, burned spots, or anything out of the ordinary. Pay particular attention to the area around the transfer case and front differential.
3. Test the Fuse: After replacing the blown fuse, see if it blows immediately upon engaging 4WD. If it does, the short is likely near the engagement mechanism.
4. Systematic Tracing: If the visual inspection doesn't reveal anything, you'll need to systematically trace the heater circuit and the 4WD circuit to pinpoint the short. This may require a multimeter to check for continuity and voltage drops. A wiring diagram for your specific 1988 Silverado will be immensely helpful here.
5. Professional Help: If you're not comfortable working with car wiring, it's best to take it to a qualified mechanic. This problem is tricky and requires patience and a methodical approach.
The unlikely connection between the 4WD system and the heater points to a wiring fault, possibly involving a damaged wire or a loose connection that gets stressed when the 4WD is engaged. A thorough visual inspection and careful tracing of the circuits are crucial to finding the source of the problem.
