These truck did come with the option of dual alternators which would have enough amps to get the charging voltage up to 14v and ford would of tested this scenario and figured 16v is the limit these plugs can handle.
My truck indeed has that dual alternator option, and Ford did indeed test that very scenario. And this must be why Ford commands the second alternator to remain
off until the glow plugs are commanded off. So having two alternators is the same as having one alternator, as long as the glow plugs are commanded on. The one alternator that Ford brings online (a stock 110a alternator) only generates 50 amps at idle, and as such there is typically a system wide voltage drop to about 12v while the glow plugs are still commanded on, regardless of whether the truck is optioned with single or dual alternators, in the stock configuration.
Keep in mind also that your pcm is monitoring the current the plugs use to determine if one burns out, adding in extra resistance could mess up the pcm operation.
I'm not sure that this is the case in Federal 49 state trucks that do not have the California Glow Plug Monitor (GPM) Relay. Even in those trucks that have the GPM, the PCM is not able to determine individual glow plug failure, only a fault with the left bank or the right bank, as whole banks, not as individual plugs. Later California emissions equipped trucks came equipped with a solid state Glow Plug Control Module (GPCM). This device is able to provide diagnostic feedback on the status of individual glow plugs, but requires a PCM calibration, a pin populated PCM connector, and the engine wiring harness outfitted with all the wires required to connect a GPCM. I have none of these.
With just a simple Federal Glow Plug Relay (GPR), I don't know how the PCM can know the status of individual glow plugs, or even the status of the left or right banks of glow plugs. There doesn't seem to be the physical means present to communicate that information to the PCM.
While the language of the service manual suggests that the PCM "controls" the glow plugs based on barometric pressure (BARO, to infer altitude), engine oil temperature (EOT), and B+ voltage, the level of actual "control" does not appear to be as sophisticated as the language suggests, even though the language is truthful.
For EOT, the PCM refers to a look up table that assigns a certain number of seconds for on time, based on EOT, from 0 seconds at any temperature at or above 131°F, to the full 120 seconds at any temperature at or below 20°F, with a more or less linear rate of change between these two extremes, with only two changes of time/temp rates when graphed.
For BARO, if the absolute atmospheric pressure is 11.34 PSIA (equivalent to 7,000 feet elevation at 34°F) or below, the PCM will simply add 5 seconds to the time chart under EOT, still not to exceed 120 seconds total. While this simple additive measure can still be technically considered "control", it is not that sophisticated, and is not going to save glow plugs from idiots like me who feed them too much voltage with aftermarket parts.
We've already discussed the PCM's monitoring of voltage. If above 16 volts, throttle back. Else, let er' rip.
So this is why I became interested in looking back a decade, to the Glow Plug Controller (GPC), which at first glance to me appeared to have stand alone, autonomously functioning features to protect glow plugs. That's why I had to come to an IDI forum, to learn more about it.
Have you measured the voltage actually going to the glowplugs after the relay and metal shunt resistor? You mite be only getting 12-13v after all the connection and wire losses.
My truck, with a Federal relay, does not have any type of shunt resistor. The voltage at the power pin on the relay that feeds the glow plug is the same voltage as the B+ terminal behind the alternator. Some type of shunt resistor is what I was fantasizing about adding to my relay, to get the exact effect that you describe. This may or may not work as I imagined though, but I haven't learned enough one way or the other to discontinue pursuit of the idea.
I really appreciate all efforts to help. Even if my answers may seem to shoot down each question or idea presented, my intent is only to supply the actual facts of the situation, so that you can better help me with ideas and solutions.
In general, the folks who know how to keep older vehicles alive, tend to have a lot more can do that the folks who trade in their vehicles before the warranty expires, so I really do value the participation and help that I am receiving here.