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Hello, so when you ground the small post on the glow plug relay it energizes and the vehicle will start?
Each glow plug tested with an ohm meter to ground should be between .5-1.9ohms. If grounding the glow plug relay energized it and it started this is a bad ground inside the PCM itself. The repair for that will be to replace the PCM or run a manual switch to control the glow plug relay. If you energized it you should keep it energized for at least 45sec before attempting to start.
Correct if the ground driver in the PCM is bad. Replacing the PCM is the correct repair. If you wanted an inexpensive repair you could wire the small post to a switch inside the vehicle. Run the switch to ground you can manually control the glow plugs.
Hello, can I still offer you assistance? If not please remember to rate my answer so I receive credit for assisting you.
Wiring a switch is the route I have gone for many customers when this happens.