RMS Diagnostics: Troubleshooting a 1998 Ford Puma A/C System

Air conditioning faults can be deceptively tricky, even on vehicles that appear simple. In a recent video from RMS DIAGNOSTICS, the team provides a detailed walkthrough of diagnosing and troubleshooting the A/C system on a 1998 Ford Puma with very low mileage, demonstrating how systematic electrical diagnostics can pinpoint elusive faults.

Robert Shervill, founder and managing director at RMS Diagnostics is joined by Barney, as they emphasise the complexity of the Puma’s A/C system, which combines relays, pressure switches, and ECU control of the compressor.

To tackle this, they simplify the factory wiring diagram, isolating only the essential components power, ground, relays, switches, and ECU connections.

This approach makes it easier to test each element logically, rather than being overwhelmed by the full factory schematics.

Using basic yet effective tools like power probes and back-probing techniques, the RMS DIAGNOSTICS team systematically tested:

• Compressor Relay: Confirmed functional by checking coil and output voltages.
• Pressure Switches: Inspected the low-pressure and triple-pressure switches, which control compressor operation and protect the system.
• Wiring & Connectors: Traced voltage through the circuit to identify any interruptions.

The root cause was traced to a loose pin in a multiplug connector between the fuse box and the low-pressure switch.

This prevented voltage from reaching the switch, stopping the compressor from activating.

Securing the connection restored continuity and highlighted a key lesson: minor connector issues can have major system effects.

Even after restoring electrical continuity, the A/C system did not engage.

Further inspection revealed the system was empty of refrigerant gas, preventing proper compressor operation.

Recharging the system completed the repair, illustrating the importance of considering both electrical and mechanical factors during diagnostics.