Porsche 928 S4 Service

John Pirtle, February 2, 2001

Cruise Control

The Automatic Cruise Control (Tempostat) often fails for several reasons. The electrical connection at the vacuum servo corrodes, the vacuum system leaks, or the control brain fails. The workshop manual includes a good troubleshooting procedure in section 27.

  1. Check fuse #4 (S4). The same fuse runs the stoplights.
  2. Test the vacuum to the reservoir by connecting a vacuum pump to the line running from the check valve (connected to the brake booster) into the fender. It will take quite a number of pumps to bring up the vacuum in the reservoir.
  3. Remove the left front wheel and inner panel.
  4. In the rear of the fender is the vacuum servo. Check that the electrical connection is clean and tight. The servo is removed by removing the bolt at the bottom. Also check the vacuum line for twists or kinks.
  5. If everything checks out, replace the fender trim and wheel.
  6. Remove the two side carpet panels of center console.
  7. The control is a silver metal box mounted on top of the tunnel next to the firewall. Remove the bolt on the right side.
  8. Remove the multiple pin plug.
  9. The workshop manuals include a check of the servo by bridging terminals 3 and 5 on the plug. "Connect a push button switch on terminals 7 and 12. Turn on ignition and press the push button switch with engine stopped. The vacuum servo should move the throttle uniformly to full load position. This requires vacuum."
  10. Check the switch controls with a volt/ohmmeter. Turn on ignition (do not start engine).
  11. Connect the voltmeter between terminals 5 and 12 (ground). Terminal 5 should have battery voltage.
  12. Connect the voltmeter between terminals 6 and 12. Terminal 6 should have voltage when the brake is pressed.
  13. Connect terminals 8 and 12. There should be voltage. Pulling back on the cruise control lever (cancel) should go to 0 volts.
  14. Terminals 9 and 12 should have 0 volts, but pushing the lever forward (set) should show voltage.
  15. Terminals 10 and 12 should have 0 volts, and voltage when pushing the lever down (repeat).
  16. An ohmmeter should read 0 ohm between terminal 12 and car ground.
  17. Terminals 3 and 7 should read @ 14 ohms.
  18. Terminals 11 and 12 should alternate from 0 to infinite ohms while turning one rear wheel and holding the other.
  19. If everything checks out thus far, bend open the metal tabs of the control and pull the circuit board out.
  20. Carefully use a soldering iron and heat each joint.
  21. Replace the control making sure to correctly orient the multiple pin plug.
  22. Hopefully your cruise control now works!

928 S4 Page

John Pirtle
February 2, 2001