I just got done building the glow plug timer circuit for a client's 1984 Blazer. It works like a dream. I employed the original relays and glow plug lights, but the actual controller someone by-passed, which led to the quick destruction of the glow plugs. With the use of one general purpose transistor (ECG 210) two resistors and one capacitor used as the discharge controller, the relay stays on for 9 seconds precisely, after pressing the momentary contact button.
I modified this circuit to make it hold the relay on for 9 seconds. I ditched the diode, and the Darlington pair transistors in favor of a single NPN ECG 210. The button is in between the 12volt supply and (I switched the 47k ohm with a 33k ohm resistor.) I used a 10k resistor is instead a 47k
This is Jerry's diagram of how the Isuzu can be modified
This diagram was found on a Mitsubishi Pajero forum, Shows very well how the glow plug circuit is supposed to work. However this uses a 555 timer chip and I wanted a simpler circuit. Mine has 4 parts.
I hope you find this helpful.
Thanks go out to Jerry Lemond for his input.