A few glow plugs operate on 4-5 volts on modern motors which is why a 12v feed shouldn`t be used to test them as they can be burned out,an ohmmeter would be the best bet to test them and if there is zero resistance there then I would say that plug should be renewed.
cdc
August 2013
They are earthed through the body into the cylinder head,no wires.
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix vans.