Mike
On the early systems you will something Aqua Hot switch in the kitchen this switch turns on the diesel boiler powering up the control board two things have to happen to make the boiler burner operate first the thermostat has to be set at a temperature requiring heat second the boiler temperature has to be below the thermostat cutting switch on the boiler this won't happen if the boiler electric element is turned on or working as the thermostat for the electric element is set at 190° whereas the cutout for the diesel boiler is set at 180°. Therefore the system will work on electric with the diesel turned on the diesel will not work until the temperature drops below approximately 160° and then cuts off at 180 when the boiler has brought it up . The theory here is that with both the electric and the diesel turned on the diesel only operate when the electric cannot keep up.
To test your diesel operation with a cold boiler set the thermostat above the temperature it is inside the coach and turn on the kitchen switch the switch should light up telling you that the board is live at this point the temperature in the boiler should be below the cutting point of the diesel thermostat on the boiler once this is achieved the control board will try to run the boiler diesel burner. By first going through a purge cycle and then an ignition cycle with fuel if the photo cell does not see light from an ignition or burn the controller will shut down the fuel and go back through a pericycle before shutting off.
The pump that pumps the diesel fuel through the boiler is driven by the main motor in the burner it is the same motor that drives the blower so if you hear the scream of the blower the diesel gear pump should be pumping , in order to do the pressure test the nozzle must be removed and the gauge installed but without the specialty electric controller tool to allow you to run the boiler without the ignition you will have to disconnect the wires going to the ignition coil located under the ignition coil they are the smaller wires not the high tension leads ,now to do the test install the gauge , disconnect the ignition coil primary wires and activated the kitchen boiler switch the burner will start to run however there should be little or no pressure showing on the gauge until the controller has completed the purge cycle approx 2 minutes after that time the controller will activate the diesel solenoid which allows high pressure fuel to be delivered to the nozzle or in this case the gauge then watch the gauge there will be a small flat headed screw on the side of the gear pump below one of the fuel line banjo housings that is the adjustment for the fuel pressure. If you look up the manual for any of the Aqua Hot 100 series units you will find they are using a DBW 2010 wabasto burner any one of the manuals will walk you through the adjustment of the fuel pressure 145 PSI . the pump is not electric the pump that you are looking at that is electric is the circulation pump that circulates the hot boiler fluid to each of the radiators in the coach.
https://acrobat.adobe.com/id/urn:aaid:sc:VA6C2:181eaee0-3cf9-480b-ba36-2d917bc6ec56Hope this helps