Rear Hatch Defroster Troubleshooting and Repair – 2.5L NA

Overview

This procedure helps diagnose and fix issues with the rear hatch defroster on your Porsche 944 (8-valve engine). The defroster grid acts like a large resistor, with 12 volts on one side and ground on the other when working correctly. Troubleshooting involves checking voltage, ground connections, fuses, relays, and repairing damaged grid lines or connectors. This guide is specific to the 1983-1985.5 (pre-85.5) and 1985.5-1988 (85.5 onwards) models.

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Tools Required

• Multimeter
• Spade connector jumper

Parts Required

• Fuse, N 017 131 12 (25A for pre-85.5 models, Fuse #4)
• Fuse, N 017 131 14 (30A for 85.5 onwards models, Fuse #13)
• Defroster relay, 944 615 117 00 (if replacement is needed)
• Electrically conductive rear window defogger tab adhesive (e.g., Permatex, 21351 or Loctite, 21351)
• Rear window defogger repair kit (e.g., Permatex, 15067 or Loctite, 15067)

Step-by-Step Procedure

Step 1: Check Voltage Across the Defroster Grid

1. Turn on the ignition and the rear defroster switch.
2. Set your multimeter to the voltage measuring function (DC volts).
3. Connect the multimeter’s negative lead to a clean ground point on the car (e.g., a bare metal bolt).
4. Use the positive lead to measure voltage on the left side of the defroster grid (driver’s side). You should see approximately 12 volts DC.
5. Measure voltage on the right side of the grid (passenger side). You should see 0 volts DC (ground).

Step 2: Verify Ground Connection

6. Set your multimeter to the ohmmeter (resistance) function.
7. Measure resistance between the right side of the defroster grid and a ground point on the car. You should read 0 ohms, indicating a good ground connection.

Step 3: Diagnose Voltage Issues

8. If you measure 0 volts on both sides of the grid, the 12-volt connection (left side) may be faulty. Proceed to check the electrical connector on the left side.
9. If you measure 12 volts on both sides, the ground connection (right side) is likely faulty. Proceed to check the electrical connector on the right side.
10. Inspect both electrical connectors for corrosion, looseness, or damage. Clean or secure them as needed.

Step 4: Check Fuse

11. If you did not measure 12 volts on the left side of the grid, check the defroster fuse:

    For Pre-85.5 models: Locate the Auxiliary Fuse Panel under the driver’s side dashboard (left-hand drive). Check Fuse #4 (25A, N 017 131 12). Replace if blown.

    For 85.5 Onwards models: Locate the Central Electric Panel under the hood. Check Fuse #13 (30A, N 017 131 14). Replace if blown.

Step 5: Test the Defroster Relay

12. If the fuse is good but the defroster still doesn’t work, test the defroster relay:

    Locate the relay panel (Central Electric Panel under the hood for 85.5 onwards, or under the dashboard for pre-85.5). Remove the defroster relay (944 615 117 00). Now turn the ignition switch ON. Use a spade connector jumper to connect relay panel contacts 30 and 87. Check if the defroster operates. If it works, replace the faulty relay with a new one (944 615 117 00).

Step 6: Repair Disconnected Grid Tabs

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

    Clean the damaged lug and window glass thoroughly with rubbing alcohol. Use an electrically conductive adhesive kit (e.g., Permatex, 21351 or Loctite, 21351) and follow the kit’s instructions to reattach the tab securely.

Step 7: Repair Damaged Grid Lines

14. If the defroster grid lines are damaged (e.g., scratched or broken):

    Use a rear window defogger repair kit (e.g., Permatex, 15067 or Loctite, 15067) and follow the kit’s instructions to apply conductive paint or material to repair the damaged grid lines.

Post Procedure Checklist

• Turn on the defroster and verify it warms the rear hatch glass evenly.
• Recheck voltage on the left side (12 volts) and right side (0 volts) of the grid.
• Ensure all electrical connectors are secure and free of corrosion.
• Confirm the fuse and relay (if replaced) are functioning correctly.
• Check repaired grid tabs or lines for secure attachment and proper operation.

Sources

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