Thetford fridges have used 3 wiring colour schemes over the years, as per the table below:


Terminal

R1G Manual

R1G LED / Auto

R2G Automatic

R2G Manual

R3G/R4G

Source (Caravan)Source (Motorhome)

HC+

Red w/ Yellow Trace

N/A

Red or Red w/ Yellow Trace

Red or Red w/ Yellow Trace

Red

Tow socket Fridge +
Starter battery +

HC-

White w/ Black Trace

N/A

White or White w/ Black Trace

White or White w/ Black Trace

White

Tow socket EarthChassis -

LC+

Green w /Yellow Trace

Red

Purple or Green w/ Yellow Trace

Purple or Green w/ Yellow Trace

Purple

Leisure battery +Leisure battery +

LC-

White w /Orange Trace

White

Black or White w/ Orange Trace

Black or White w/Orange Trace 

Black

Chassis -
Chassis -

D+

N/A

Black

Orange or Brown

N/A, looped to HC+ at PB

Orange

Looped to HC+ at PB
Alternator / Ignition +


The majority of older caravans only had 4 wires in the fridge supply loom (a permanent 12V supply from the onboard leisure battery (LC +/-) and a supply from the tow socket (HC +/-). In these cases, the D+ and HC+ are linked together at the power board to provide a signal on the D+ terminal to enable 12V mode.


To convert an 4 wire supply loom to suit an R3G fridge, connect the D+ and HC+ together and connect both to the HC+ supply. This way, the fridge will sense the 12V supply whenever the tow car is connected and 12V mode will be enabled. If the D+ terminal is left disconnected, the fridge will show a fault code 7 when 12V mode is selected, regardless of whether or not there is a supply at the HC + / - terminals. The rest of the wires can be connected as per the diagram above.


Some older installations did not have the permanent LC +/- supply and instead, only had the 12V HC +/-. This is typically the case for the older manual fridges which did not have electronic ignition (N112 Piezo etc) and did not need this supply. In these cases, a supply from the leisure battery would need fitting to provide the LC +/-.


It is worth noting that the R1 LED fridges with the early power board (N145AD as an example) only had 3 wires on the 12V connections, these being 12V, GND and D+. In this case, do the following:

Join the new RED and PURPLE wires together and connect to the old RED wire.
Join the new WHITE and BLACK wires together and connect to the old WHITE wire.
Connect the new ORANGE wire to the old BLACK wire.


Lastly, it is worth noting that the wiring for some older fridges (particularly R1G models) may not be able to cope with the additional current draw of an R3G/R4G fridge and fault code 6 will show, or the 12V performance will be lacklustre. In these cases, the HC wiring must be upgraded as per the recommendations in the fridge installation manual.


There is a diagram attached showing how the 5 wire system works overall and also some information on the wiring options on older PCBs.


Thanks

Rich