If you are running 12 gauge wire to the starter that is what is used in house wiring. and is good for 20 amps. Not a couple hundred that the starter wants. Even the glows are fused at 70 or 80 amps.

12 gauge is thinner than 10 gauge (30 amp). The wire you use should be at least 5/16" thick plus the thickness of the insulation.

Do your glow plugs actually glow. They can short internally and have low resistance and not work effectively.

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85 Merc 300D - Unwinding 31 years of wear
86 VW TD
Mahindra Diesel
Iseki Diesel

In 2007 I didn't own a diesel.