Seen the original from Monaco RC I must say it is a bit overpriced. The idea of using an ammeter to measure force is not new to me as I have used that idea from the start of my heated oil bath break in bench for nitro engines so this is nothing more based on an electric motor and an ammeter.

I promised some guys to make some so I went online shopping in Asia, all it needs:
- a smal 6 volt 300 rpm geard motor
- 5 digit ammeter (is also used by the Monaco RC version)
- a case
- a switch
- a small 5v regulator
- cheap 7mm hex wrech to modify
- some wires

The electronics is simpel: wire -> switch -> 5v regulator -> ammeter -> motor
Between the switch and the regulator I mounted a diode so wrong polarity will not harm

I thought to be smart by using cheap Chinese steel 7mm hex wrenches to cut of the 1/4" hex part and drill an hole through it but the guys now how to hard metals, so no go....
Lucky I also had ordered the Tamiya style wrenches which was easy to modify for the 4mm motor shaft mount so that can be used on the wheelnut of car.

The adapter for the diff itself was made on the lathe from 10mm aluminum

Without the work it is about 20~25 dollar worth of parts and some time to put it all together.