Cruise Control – General Information and Repair – 2.5L NA

Overview

The cruise control system in the Porsche 944 (8-valve engine configuration, 1983–1988) allows the driver to maintain a consistent speed without using the accelerator pedal. Common issues include failure to engage, inconsistent speed maintenance, or unintended acceleration after engagement (typically 5–10 mph (8–16 km)). This guide provides steps to diagnose and repair the cruise control servo unit and computer, tailored for DIY enthusiasts working on Pre-85.5 (1983–1985.5) and 85.5 Onwards (1985.5–1988) models. Most repairs can be performed with basic tools and minimal cost if done yourself.

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Tools Required

• Metric socket and wrench set (sizes 10mm (3/8 inch), 13mm (1/2 inch))
• Phillips screwdriver
• Small flat-tip screwdriver
• Spray contact cleaner
• Cotton swabs
• Grease (suitable for automotive electrical components)
• Magnifying glass or lens
• Soldering iron and flux-core solder
• Solder removal tool (solder sucker)
• Small wire brush
• Alcohol (for cleaning)

Step-by-Step Procedure

Step 1: Diagnose the Cruise Control Issue

1. Test the cruise control system by driving the vehicle at a steady speed above 30 mph (48 km) on a safe, open road.
2. Attempt to engage the cruise control and note the symptoms:• If the system accelerates 5–10 mph (8–16 km) above the set speed, the issue is likely with the servo unit.

   • If the system operates intermittently, the issue could be with either the cruise control computer or the servo unit.

   • If the system fails to engage or operate, the issue is likely with the cruise control computer.

3. Based on the symptoms, proceed to clean the servo unit first (Steps 2–4), as it’s a common source of issues and easier to address.

Step 2: Remove the Cruise Control Servo Unit

4. Locate the cruise control servo unit on the firewall in the engine bay, just in front of the battery tray.
5. Disconnect the throttle cable from the servo:• Remove the metal retainer from the clip holding the cable.

   • Slide the clip off the ball on the servo’s operating arm.

6. Disconnect the electrical connector from the servo unit.
7. Remove the servo unit from the vehicle by loosening the mounting bolts using a 10mm (3/8 inch) socket or wrench.
8. Sketch or photograph the servo’s components and connections before disassembly to aid reassembly.

Step 3: Clean the Cruise Control Servo Unit

9. Remove the operating lever arm from the servo by loosening the nut with a 10mm (3/8 inch) wrench.
10. Using a Phillips screwdriver, carefully remove the servo cover. Be cautious, as a small spring inside may pop out. If the cover is stuck, gently pry it off with a small flat-tip screwdriver.
11. Locate the circuit board inside the servo, which has two conducting strips contacted by a wiper arm.
12. Clean the conducting strips using spray contact cleaner and a cotton swab. Apply gentle pressure to avoid damaging the strips.
13. Check the wiper arm’s motion to ensure it makes solid contact with the circuit board.
14. Apply a small amount of automotive grease to the moving parts inside the servo, ensuring no grease contacts the conducting strips or wiper arm.
15. Reassemble the servo by securing the cover with the Phillips screwdriver and reattaching the operating lever arm with the 10mm (3/8 inch) nut.

Step 4: Reinstall and Test the Servo Unit

16. Reinstall the servo unit onto the firewall using a 10mm (3/8 inch) socket or wrench to tighten the mounting bolts.
17. Reconnect the electrical connector to the servo.
18. Reattach the throttle cable by sliding the clip onto the ball of the servo’s operating arm and securing the metal retainer.
19. Test the cruise control system by driving at a steady speed above 30 mph (48 km) and engaging the system. If the issue persists, proceed to repair the cruise control computer (Steps 5–7).

Step 5: Remove the Cruise Control Computer

20. Locate the cruise control computer in the driver’s footwell (left-hand drive vehicles), above the hood release lever. It appears as a silver box.
21. Disconnect the electrical connector from the cruise control computer.
22. Remove the mounting screws securing the computer using a Phillips screwdriver, then remove the unit from the vehicle.

Step 6: Repair the Cruise Control Computer

23. Open the cruise control computer by bending back the cover tabs with a small flat-tip screwdriver.
24. Remove the circuit board from the computer housing.
25. Inspect all solder joints on the circuit board using a magnifying glass. Good solder joints are shiny and bright; poor joints appear dull or have a ring around the component leg.
26. Repair any poor solder joints:• Heat the solder joint with a soldering iron and remove old solder and protective coating using a solder removal tool (solder sucker).

    • Clean the joint area with a small wire brush.

    • Re-solder the joint with flux-core solder, ensuring the solder flows evenly. After cooling, clean residual flux with alcohol and a cotton swab.

27. Reassemble the cruise control computer by placing the circuit board back in the housing and securing the cover tabs.

Step 7: Reinstall and Test the Cruise Control Computer

28. Reinstall the cruise control computer in the driver’s footwell using a Phillips screwdriver to tighten the mounting screws.
29. Reconnect the electrical connector to the computer.
30. Road test the vehicle at a steady speed above 30 mph (48 km) to verify that the cruise control engages and maintains speed correctly.

Post Procedure Checklist

• Confirm the cruise control engages without delay at speeds above 30 mph (48 km).
• Verify the system maintains the set speed without unintended acceleration of 5–10 mph (8–16 km).
• Check that the throttle cable and electrical connectors are securely attached to the servo unit.
• Ensure the cruise control computer is firmly mounted and its electrical connector is secure.
• Inspect for any loose bolts or components in the engine bay and driver’s footwell.

Sources

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