I've never needed to measure -- The one time when my Prodigy failed, I just traded it in for a new one (free, lifetime warranty) at an RV dealer, per Tekonsha's instructions.
When you get the answer from Tekonsha's excellent tech support people (888-785-5832), post it here
OK, disregard calling Tech Service because the answer, actually why there is no direct answer to your question, is in the second of two Tech Bulletins available for PDF download from the Tekonsha site:
Tekonsha Technical Bulletin 2020020062, 3/2003
which sez, in part:
When a boost setting is used, the Prodigy applies a minimum output to the trailer at the onset of braking to initiate deceleration. The level at which the power knob is set, determines what that initial output will be. Boost level 1 generates about 13% of power set for the first 2 seconds and boost level 2 and 3 generate about 25% of power set for the first 2 seconds. All levels reduce by 50% after 2 seconds when the braking system is generating enough deceleration of itsí own accord.
So it looks to me like the voltage will be a percentage of the voltage YOU set with the power knob, so what you see at Full Power will be reduced to the percentages above if the B1/2/3 circuitry is working correctly, but what that FP voltage should be is likely dependent on your tow vehicle's voltage coming into the controller...
The other Tech Bulletin also dates from 3/2003 and sez in part:
Written in Conjunction with Sales Bulletin 20020063
Tekonsha has released new software (Version 2.4) to correct false ďo.gĒ (open ground)
displays on Prodigy. The new software will be installed on Prodigys with serial numbers
greater than 0130000.
On Prodigys with serial numbers less than 0130001, if a False Open Ground is displayed, the
Prodigy will still continue to provide stopping power.