It's been my suspicion that the earlier bipolar NPN transistor technology was susceptible to alternator load dumps and starter motor voltage spikes. A power transistor damaged in this manner will short emitter to collector...permanently on.
Where the later MOSFET technology usually avalanches the device and the worse that happens is a brief current pulse to the contactor coil.
Anyway, just my guess as to why the earlier controllers were failing ON. I suspect they redesigned them to use the later tech.
My controller seems to be working, but the contactor contacts eroded to failure. That is, I get a click, but no DC at the load terminal. I have DC at the other terminal. The contactor doesn't make contact anymore. Open.
Shorting the white wire terminal to ground engages the coil, clicks the contactor, but provides no voltage out.
Therefore, I replace the contactor. About a $20 part via NAPA. Eichlen brand.
I have known sometimes poorly designed contactors weld themselves closed. And wondered if others had experienced this kind of disappointment with these failing in this manner.
If the controller makes a pain in the butt out of itself, I'll replace it with a pushbutton... Connecting the white wire contactor terminal to ground. Disconnecting the white wire from the controller removes the ability of the controller to engage the contactor coil. But once you do this, you don't want anyone messing with your truck...they may not know glow plug usage, and blow em all up for you.
