cummins block heater
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could be your relays.. i thought all mine was bad too and i did some experimenting and it turned out to be my exciter wire on the relays so i cut the ends off of it.. put the ends on new wire.. and got me a heavy duty push button switch and now they are manuel, the timer on the wait to start still works. the colder it is. the longer the light stays on and i keep them pressed in. you could take your intake horn off put a jumper between them and the battery one at a time and look at the grid.
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