How are you monitoring the voltage? From the dash gauge or from a device reading the data from the data bus?
I saw the spike like 15.3V a couple of time while the grid heater kicks off on a real cold morning. I think the target charging voltage was 15V.
Reading 16.5V from the bus would be an issue.
I would check the alternator.

 

The Electronic Voltage Regulator (EVR) is part of ECM.
The EVR monitors driver side battery voltage and battery temp to determine a target charging voltage.
If the sensed battery voltage is 0.5V or lower than the target voltage, the ECM grounds the field winding until sensed battery voltage is 0.5V above target voltage.

If you saw 16.5V from the driver side battery/data bus and the alternator was good, then for some reasons the target voltage was 16V.
I would check the battery temp sensor too.

 

I'm using a digital voltmeter that plugs into the 12v power port (cigarette lighter). The power is coming from the PDC.

I'll see 15-15.5v with the grids on which is normal. But for an instant it will shoot to 16.5v before dropping back down to 14.5v.

 

I'll see around 12.5-13V when the grid heater kicks in and 15V when the heater kick off.
I think the target charging voltage for trucks before 05 was around 14.5V. The voltage would around two volts when grid heater kick in.

 

So the lights would be brighter when the grid heater came on?

 

Sorry for the confusion, I had wording incorrect. 15-15.5v in-between the grid cycling. 11.5-12v with the grids on.

 

Checked the system over yesterday. Found both batteries slightly undercharged with the driver side being 100mv lower than the passenger. Found the driver side negative terminal slightly loose. Trickle charged the batteries back to 12.7v and all is well now.

Guess between the loose terminal and a week of extra cold weather with very short driving distance didn't help.

 

I checked out my truck today. When started the system voltage would cycle between 12.5 and 14.8 V as the grid heater cycled. The cycle rate was about 1 second on and then 1 second off. Ambient was 6 degrees C. As the engine ran and I drove away after 3 minutes, the grid continued to cycle for another 3 minutes. The cycling remained in 1 second intervals, but the voltmeter in the dash reflected a steady 12 volts, but the high voltage slowly reduced from the 14.8 to 14.4 V. When the grid cycling stopped, the system voltage gradually dropped from the 14.4 V to 14.3. After 30 minutes of driving, I stopped. Battery voltage was 12.8 V. after 10 minutes of rest, the battery voltage was 12.5 V.

 

This was an indication that the batteries were NOT fully charged (11.5-12V only)
All the alternator output would be used to operate the grid heater when grid heater kicks in and the batteries are not charged.

 

I know. I drive 3 miles to work and with the extra cold weather we had a few weeks ago the grids stayed on longer than usual which didn't help the situation. Now with the warmer temps all is fine now.