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300 Tdi glow plugs - how long to activate?

Troll Hunter

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As in the title.  I'm trying to develop the new wiring I need for my rebuild, and studying the various wiring diagrams that I have I cannot see any device that indicates that the glow plugs have reached design temperature and that the ignition key should be advanced to "START".  I note that the "Low temperature " warning light comes off the same supply as the glow plugs.  What am I missing?  Does the system rely upon the increase in glow plug temperature increasing their resistance so that the warning light eventually comes on when the glow plugs have reached their design temperature?

Many thanks in advance for throwing any light on this problem.


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The light indicates when the plugs are warming.  When the light goes out, the plugs have reached full temperature and you can start the engine.  However, a Tdi shouldn’t need the plugs except in very cold conditions (your location noted), and the 2.8TGV version made in Brazil (and quite likely their licence built 300s) didn’t even have them.  The lamp serves more as a confidence check that the plugs are getting power, and that you don’t have a blown fuse or circuit failure.  You don’t need to wait for maximum temperature to start the engine.

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As far as I recall the plugs actually stay on for some time after the light goes out - if there is no other noise you can clearly hear the 'clunk' of the relay dropping out usually after about 10 seconds. If you want maximum heat, wait for the clunk and then turn the key. The plugs themselves start glowing red on the end around 3 or 4 seconds after getting power, IIRC. It is a while since I tested a Tdi one but I had plugs out of a Ford Ranger the other day and that's about how long they took (or rather it took, as the other 3 were f*ed!)

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