Glow Plugs Appear Not to be Functioning

The glow plug light on my TD4 doesn't come on when I turn on the ignition, even when the engine is stone cold. Its also difficult to start sometimes

I've connected up my black box and can get the light to turn on, so its not the lamp. I can also switch the relay with the black box but the lamp doesn't come on. Relay checks out ok with a multimeter and it can be heard clicking.

Thought it might be the engine temperature sensor but the balck box gives sensible readings.

Has anybody come across this before? Any ideas?

Do your glow plugs draw correct amps? ....... my Hippo had all sorts of strange things happening ..... unitl I changed all 4 glow plugs with NGK jobbies..... otherwise I'm scratching my head...hmmmm