my 85 300 sd has to be dieseled with the starter when cold,but when block heater is pluged in and engine is warm starts right up.Replaced glow plugs,no change. HELP!

 

Are you sure they are working???

 

i got voltage to the plugs,i purchased beru plugs, whe warm runs fine.

 

Did you test them with an amp meter? The glow plugs are PTC's(positive temperature coefficient) which means as temperature goes up so does resistance. If you use an inductive amp clamp to check them each glow plug should start at about 20 amps and taper off as they heat up, cause if resistance goes up and voltage stays constant, current flow will decrease(Ohm's law). The amp test is more of a dynamic test. Also where did you check for voltage, at the relay or the individual glow plugs?

 

How cold? I recently solved this problem when advised to count to thirty after GP light goes off. Starts every time at -10C with out block heater plugged in.