Truck just out of the transmission repair & now the glow plugs don't work. I have power to the relay, but when I turn on the key, what looks like the relay control wires (orange & red /w stripe) both go to 12v. Fuse is good. Anyone know about '96 glow plug wiring?

 

Both sides should measure 12V, when the relay activates, the PCM pulls one side to ground. Also, if you have an ohm meter, check each GP by removing the connector and the valve cover, check the outer two on each side of the connector, this will tell you if the GPs themselves are bad. Resistance should be around 1Ohm or so. anything higher is starting to go bad or already there.

 

Here's what I have. With key off, 1 large (incoming) post is at +12v, all others are at 0v. When I turn the key on, the incoming post stays at +12v, both control posts go to +12v, and the outgoing post stays at 0v. Normally, when the truck is started, you can see a huge drop in the in-dash voltmeter. That drop no longer occurs, indicating to me that the failure isn't the plugs or relay. I've never heard of losing all 8 plugs at once. I'm going to disconnect the batteries in hopes of resetting the PCM.

 

Bingo!!! The Diablo flipchip failed!!! In stock position, the glowplugs work; in the 54hp position, they don't.

 

First I have heard of that happening, but glad that you found it.