how do you know when pre glow time relay is bad, light on dash doesnt come on, all glow plugs have been replaced

Go to DieselGiants page and follow his GP instructions to determine where the problem lies.

http://dieselgiant.com/glowplugrepair.htm

Add your vehicle to your Signature (User CP tab) so we know what type of vehicle you've got.

Finally, post on the Diesel Forum for better help.

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Terry Allison
N. Calif. & Boca Chica, Panama

09' E320 Bluetec 77k (USA)
09' Hyundai Santa Fe Diesel 48k (S.A.)