I got my new voltmeter gauge installed and the first thing I noticed is that when the truck is COLD and turn the key to on, the volt reads 11.2 volts then slowly greeps up to 11.6 volts. About 15 seconds after the glow plug light goes out it will finally jump to 12.5 volts... Is this normal? Once the truck is warmed up the glow plug light goes out instantly and the volts start out at about 12.3 volts. Once running volts jump to about 14.1 volts.

 

Sounds like it's in the normal range.

 

The glow plugs take about one volt when operating. The light and the actual glow plug on time is not connected. The light just goes on while the glow plugs will go for about a minute and a half when cold. When hot the don't come on even though the light comes on. The PCM seems to protect the alternator by timing when it comes on to when the glow plugs are off.

 

Here's some GPR/GPCM info from the service manual.

The 7.3L diesel engine uses a glow plug system that preheats air in the combustion chamber to improve cold engine starting.

The system consists of:
<DIR><DIR>glow plugs.
glow plug relay.
glow plug module (GPCM on CA Trucks and Excursions)
powertrain control module (PCM).
engine oil temperature (EOT) sensor.
barometric pressure sensor (BARO sensor).
</DIR></DIR>The glow plug system is electronically controlled by the PCM. If the oil temperature is below 131°F the PCM will energize the glow plugs immediately after the key is placed in the ON position. Then, depending on the readings from the engine oil temperature (EOT) sensor and the barometric pressure (BARO) sensor, the PCM determines how long the glow plugs will be on.

Glow Plugs — PCM-Controlled
The glow plugs are located in the cylinder heads, under the valve covers.
The glow plugs are self-regulating. If the engine oil temperature is above 131°F, the PCM will bulb-check the WAIT TO START lamp but not energize the glow plug relay or control module.

The PCM system uses the glow plug module (if equipped) to conduct current to the left and right bank glow plugs.

The PCM protects the glow plugs by energizing them for short durations if the battery voltage is abnormally high.

The glow plug ON time varies from 0-120 seconds depending on battery voltage, engine oil temperature and barometric pressure.

On California vehicles the glow plug monitor (GPM) system is part of the On-Board Diagnostics II (OBD II) System.

The GPM system is designed to find failed glow plugs or failed wiring in the glow plug system.

A failure in the glow plug system will set a diagnostic trouble code (DTC).

 

Might be worth checking your battery cable connections and grounds. Mine reads around 11.X volts when the glow plugs are on, but as soon as the GPR shuts off it starts working it's way up to 14 or so volts in a little less than 5 seconds.