2.9TD Glow Plug Relay Fault (UK Sprinter)

[Neversaydie]

Background.

1. The heater plug warning light does not come on, and my 1998 2.9TD Sprinter Van does not even try to start. Extra heavy battery does crank engine well. Well above freezing test conditions.
2. All 5 glow plugs test out OK on resistance.
3. Relay box is 019 545 89 32 and there is power to red supply post.
4. There are 6 outlet spade terminals and 5 cables run from block connector to glow plugs.
5. There are three control pins fed with two wires. Pin 31 is apparently grounded. Pin DL apparently requires live and pin TX is not used.
6. By grounding pin 31 and connecting pin DL to live, power is supplied to the glow plugs. Confirmed by low wattage bulb connected to glow plugs.
7. However the relay switches "off" after only 2 to 4 seconds, which for my thinking is far too fast even for modern fast heater plugs.
8. The relay will not operate the glow plugs again for between 30 and 90 seconds.
9. Fuel is also arriving at all injectors under pressure.

My Questions....Does the very short "on time" suggest that my relay box is defective? Any ideas where I should look to get the Indicator light to come on and also how to get voltage down to the DL pin? The head temperature sensor is open circuit when cold but I understand this may also control relay operation. Is this all managed by the ECU?

Many thanks, Alex......Neversaydie !!!