Also, the controller is well known to fail. It has very fine wires between the circuit board and the output connectors. These wires are "welded" to the board terminals and these welds fail (vibration?). As the unit is glued shut AND the guts is "potted" (filled with sealant) it's not really serviceable by mere mortals, so it should be replaced too.
Testing the glow plugs may well show up a few dead ones, but due to their age, it's better to replace them all. And many folk have replaced all the plugs (or even just the faulty ones) and found they are still "dead" as the controller has failed welds. Back in they go...
I'm not normally a "replace everything on spec." kind of guy - I try to keep stuff going as long as possible - but in this case, nothing else makes sense.
