Rear Hatch Defroster Troubleshooting and Repair

Overview

This guide provides step-by-step instructions for troubleshooting and repairing the rear hatch defroster on a Porsche 944 with a 16v engine. The rear hatch defroster is a resistive grid that heats the rear window to clear fog or ice. Proper operation requires +12 VDC on the left side of the grid and a solid ground on the right side. This procedure helps diagnose issues like faulty connections, blown fuses, or damaged grid lines and provides repair options using adhesive or grid repair kits. Following these steps ensures your defroster functions reliably, improving visibility in adverse weather.

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Tools Required

• Multimeter (with voltage and resistance measurement functions)
• Spade connector jumper (for relay testing)
• 10mm (3/8 inch) wrench (for accessing fuse panel)
• Cleaning cloth and isopropyl alcohol (for preparing surfaces)

Parts Required

• Fuse, 30A, 999 180 001 90 (if replacement is needed)
• Defroster relay, 944 615 117 00 (if replacement is needed)
• Electrically conductive rear window defogger tab adhesive, Permatex 21351 or Loctite 21351 (for reattaching disconnected tabs)
• Rear window defogger repair kit, Permatex 15067 or Loctite 15067 (for repairing damaged grid lines)

Step-by-Step Procedure

Step 1: Verify Defroster Operation

1. Turn on the ignition and activate the rear hatch defroster switch.
2. Set your multimeter to measure DC voltage.
3. Connect the multimeter’s negative lead to a clean, unpainted metal ground point on the car (e.g., a bolt near the rear hatch).
4. Use the positive lead to measure voltage at the left side of the defroster grid (driver’s side on LHD vehicles). You should measure approximately +12 VDC.
5. Measure voltage at the right side of the grid (passenger’s side on LHD vehicles). You should measure 0 VDC, indicating a proper ground.

Step 2: Check Ground Connection

6. Set your multimeter to measure resistance (ohms).
7. Connect one multimeter lead to the right side of the defroster grid and the other to the same ground point used in Step 1.
8. You should measure near 0 ohms, confirming a good ground connection.

If you measure high resistance or no continuity:
Inspect the ground connection on the right side of the grid for corrosion or looseness.
Clean the connection with isopropyl alcohol and a cloth, then retest.

Step 3: Diagnose Voltage Issues

9. If you measured 0 VDC on both sides of the grid in Step 1:

Check the electrical connector on the left side of the grid for poor contact or corrosion.
Clean the connector with isopropyl alcohol and ensure it is securely attached.

10. If you measured +12 VDC on both sides of the grid:

The ground connection on the right side is likely faulty. Recheck the ground as described in Step 2.

11. If you did not measure +12 VDC on the left side:

For Porsche 944 S2 (1989–1991):
Check Fuse #13 (30A, 999 180 001 90) in the Central Electric Panel, located under the hood.

12. Use a 10mm (3/8 inch) wrench to access the panel if necessary.
13. Replace the fuse if blown and retest the defroster.

Step 4: Test the Defroster Relay

14. If the fuse is intact but the defroster still does not work, test the defroster relay (944 615 117 00).

Locate the Central Electric Panel under the hood.
Remove the defroster relay from its socket.

15. Turn the ignition switch ON.
16. Use a spade connector jumper to connect relay contacts 30 and 87 in the relay socket.
17. Retest the defroster operation as described in Step 1.

If the defroster works with the jumper:
Replace the defroster relay with a new one (944 615 117 00).

Step 5: Repair Disconnected Grid Tabs

18. If the electrical connector tab is disconnected from the defroster grid:

Clean the damaged lug and the glass surface with isopropyl alcohol and a cloth.
Apply electrically conductive adhesive (Permatex 21351 or Loctite 21351) according to the product instructions.
Allow the adhesive to cure fully (typically 24 hours) before testing the defroster.

Step 6: Repair Damaged Grid Lines

19. If the defroster grid lines are scratched or broken:

Use a rear window defogger repair kit (Permatex 15067 or Loctite 15067).
Follow the kit’s instructions to apply conductive paint to the damaged areas.
Allow the repair to cure as specified (typically 24 hours) before testing.

Post Procedure Checklist

• Verify the defroster heats the rear window evenly within 5 minutes of activation.
• Check that all electrical connectors are secure and free of corrosion.
• Ensure the fuse and relay are functioning correctly.
• Confirm no warning lights appear on the dashboard related to the electrical system.
• Test the defroster in cold or humid conditions to ensure it clears the rear window effectively.

Sources

13. • Clark’s Garage
    • Pelican Parts
    • Porsche 944 Workshop Manuals
    • YouTube