Hello! Need help!

Hi there,

We are in Sydney Australia.
Just bought a second hand refrigerated truck for our small food delivery business and noticed this problem.
All was fine during inspection several days so you are imagine why we are pretty upset.

Brand of the refrigeration unit - Thermal Master 1400 FS

It is fitted with an additional compressor run from an external power socket for main power when not being driven.
This compressor was very recently serviced and the gas topped up. There are no problems when the truck is driven under engine power, however when engine is off and it is plugged in to the mains power, it will go down to the set temperature and then switch off (presumably with the thermostat).

However even though the compressor starts up again when the temperature has risen a few degrees, there is no cooling and the temperature keeps rising back to the ambient air temperature.
If I power off and on the mains supply it will start cooling again down to the set temperature and the cycle starts again. Any ideas?

I have food spoiling overnight which needs to be delivered fresh in the morning and I don't want to toggle the power off/on every half an hour overnight.

Any help would be appreciated. Thanks, Tim