Actually, research shows that my suspicion was incorrect and the TD does have servotronic.
I still think it's the pump, though.
Thks Kevin, I'm inclined to agree. However, is it possible to check the relay on the bench with limited test equipment and a 12v supply?
It's not actually just a relay, although the appearance might make one assume that it is. It's actually an ECU which takes as an input the vehicle speed signal that's used to drive the speedometer, and outputs a pulsed signal that's used to modulate a valve in the steering box that alters the level of assistance of the steering.
A mapping within the ECU relates vehicle speed to level of assistance.
If the system is completely unpowered, the valve is open, allowing fluid to flow through a bypass passage in the box, and this results in a drop in hydraulic pressure at the steering box, giving the minimum level of assistance, so this is the worst case scenario. The minimum level of assistance is, I would say, roughly what you used to get in a car without PAS. I.e. noticeably more weighty, but you wouldn't worry at all about driving home.
At low road speeds, the servotronic ECU will close the valve progressively by pulsing its' actuator, increasing the available hydraulic pressure and the assistance provided.
So, it's not that easy to test on the bench, although it can be interrogated by Tech 2. I don't believe its' failure would give the symptoms you describe, however.