944 Hub by FormFactor Performance

Electrical

Rear Hatch Defroster Troubleshooting and Repair

CG Sourced from Clark's Garage Intermediate ELECT-05
CG

This guide is preserved from Clark's Garage, a foundational 944 resource. View the original →

Tools you'll need

See the full tools list for where to buy each one and budget alternatives.

Introduction

The rear hatch defroster grid is basically a big resistor. So, when the defroster is operating properly, with a voltmeter you will be able to measure +12 VDC on one side of the grid and 0 VDC (or ground).

Tools

  • Multimeter
  • Spade Connector Jumper

Procedure

  1. Turn on the ignition and the defroster switch.
  2. Using the voltage measuring function on the Multimeter, connect the negative lead to a ground point on the car and use the positive lead to check for voltage on both sides of the grid. You should measure +12 VDC on the left side of the grid and 0 VDC on the right side.
  3. On the right side of the grid, you should be able to measure 0 resistance from the grid to ground using the ohmmeter function of the Multimeter.
  4. If you read 0 VDC on both sides of the grid, there may be a bad connection on the 12 VDC side (left) of the grid. If you read 12 VDC on both sides, the grid’s ground connection (right) may probably bad.
  5. If the readings did not check properly, check the electrical connectors on either side of the grid to make sure they are making good contact.
  6. If you did not read +12 VDC on the left side of the grid, another possible cause is a bad fuse or bad defroster relay. On early 944s (pre-1985.5), the defroster is powered via Fuse #4 (25A) on the Auxiliary Fuse Panel (under dash driver’s side - LHD). On later 944s, the defroster is powered from Fuse #13 (30A) on the Central Electric Panel (under hood).
  7. If it is suspected that the relay may be the cause of the problem, perform the following:
    1. Remove the relay from the relay panel.
    2. Turn the ignition switch ON.
    3. On the relay panel, jumper defroster relay contacts 30 and 87. Check for proper operation of the defroster.
  8. If the electrical connector tab has become disconnected from the rear window defroster grid, there are several adhesive kits made specifically for reattaching the grid:
    • Permatex 21351, Electrically Conductive Rear Window Defogger Tab Adhesive
    • Loctite 21351, Rear Window Defogger Tab AdhesiveMake sure that you clean the damaged lug and window glass thoroughly before attempting the repair.
  9. If you discover that the defroster grid lines are damaged, there are kits to repair the grid lines:
    • Permatex 15067, Quick Grid Rear Window Defogger Repair Kit
    • Loctite 15067, Rear Window Defogger Repair Kit