Top Tips: Wet weather calls Jaguar tools into action

Wet weather calls Jaguar tools to action

The sight of a fault warning message at 70mph can be very concerning, and an owner embarking on a 5 hour motorway trip in a 1999 Jaguar S-Type had exactly that problem recently - about an hour into the trip. Accompanying the warning message was an uncomfortable 'jolt' from the engine and jump in fuel consumption.

The fault message was displayed for a while, then would disappear for a few minutes, then reappear - accompanied by another jolt. This continued for most of the journey, until the fault was being displayed permanently.

Fault diagnosis

Whilst the dashboard was reporting a 'Failsafe Engine Mode' message, the Check Engine light wasn't on - indicating that this wasn't a 'major' engine fault. Scanning with a generic EOBD scanner showed no fault codes (understandable given the engine fault light wasn't on).

But once the Jaguar Enhanced AutoEnginuity tool was connected, the fault became immediately apparent.

The fault code report was showing 'Pedal Position Sensor Disagreement Between Sensors' - pointing to a problem with the sensor unit on the throttle pedal. This is a 'black-box' unit on the throttle pedal assembly, which houses 3 separate sensors to monitor the position of the pedal (pictured on the right).

More investigation

The fault was confirmed by watching the graphs from the individual sensors. Their outputs were all changing as the pedal was depressed, but there were inconsistencies appearing almost at random. A repair didn't look possible, so a replacement was probably the only solution.

Before spending any money, the first job was to check the wiring to the pedal sensor unit. Connections seemed firm, but on disconnecting the wiring and removing the pedal, part of the problem became clear. The contacts within the connector appeared slightly corroded.

We began to suspect what might have caused the issue: the British wet weather!

The next step

The car was returning from a very wet week in the Lake District - having had people jumping in all week in wet clothes.

As the connector to the pedal assembly didn't have a firm seal, moisture in the car seemed to have condensed on the connectors, causing the corrosion.

After cleaning the connectors, the pedal was now working much more reliably, but was still intermittently triggering the same fault. Checking the graphs, the readings from the 3 individual sensors seemed better, but we were still seeing a few random inconsistencies.

And finally...

Before spending the £150 on a new pedal sensor unit, it was decided to try drying the sensor on a radiator overnight to make sure the moisture wasn't causing any further problems. The following day, the pedal was refitted and was back to perfect working order. The fault code was again cleared and hasn't returned since.

Cost of repair: a little time investigating and £0 on parts. Owner very happy!

Amazing that a wet British week was probably the cause of this fault. With all this rain, I'm sure this won't be the last weather-related fault we'll hear this Winter.

More details on the AutoEnginuity Jaguar package can be found here.