During a trailer electrical brake inspection, what should the ammeter read with the controller in the OFF position?

When conducting an inspection on trailer electrical brakes, the ammeter should indeed read zero with the controller in the OFF position. This is because, in the OFF position, there is no electrical current flowing to the brake system, indicating that the brakes are not being engaged or activated.

A positive reading would suggest that there is some power being supplied to the brakes, which should not occur when the system is turned off. Similarly, a high negative reading would indicate an issue, as it could imply that there is an excess current flowing in the opposite direction, which is not a normal or safe operating condition. A fluctuating low reading would also indicate that there might be some variable electrical activity in the system, contrary to the expected behavior of a system that is meant to be inactive when turned off. Thus, a zero reading confirms that the electrical brake system is not receiving any power, which is the desired and appropriate outcome when the controller is OFF.