How do you replace the heater core in a 1990 GMC Sierra?

Answer 1

1. Disconnect the negative battery cable.

2. Remove the coolant overflow bottle.
3. Drain the cooling system.
4. Disconnect the heater hoses at the core tubes.
5. In the engine compartment, remove the heater case-to-firewall screws.
6. Disconnect the antenna cable at the mast.
7. Remove the glove box.
8. Disconnect the wiring harness at the engine's Electronic Control Module (ECM).
9. Remove the ECM and bracket.
10. Remove the right side kick panel.
11. Remove the right side lower dash panel bolt and nut.
12. Remove the heater case mounting bolts.
13. While lifting the instrument panel slightly, remove the case assembly.
14. Lift the core from the case.