Why Does Multimeter Show Low Voltage Fluctuation When Testing Brake Controller Output Circuit

Question:

I can’t get an accurate voltage reading on my truck’s 7 way plug using a multimeter in the 5 and 7 position because the truck is outputting PWM which causes the voltage to fluctuate rapidly at .1 to .2v in the display instead staying steady at 12 or 13 volts. Is there a method to overcome this so that I can see the output voltage as a steady number? I have the engine running and a friend pressing the manual brake controller with the gain on 10.

asked by: Don D

Expert Reply:

On a 7-way trailer connector the pin in the 5:00 position is the brake output circuit and the 7:00 position is the ground. Sounds like you have the positive pin of the multimeter on the 5:00 position and the ground on the 7:00. The intermittent low volts you are seeing is actually the brake controller testing the circuit to see if a trailer is connected. It puts a low volt load on the circuit to see if it can detect trailer brake magnets which if it finds none it won't show a trailer connection but once it does it will show a connection.

So a circuit tester like the part # PTW2993 might be the easier tester to use as it's light does draw enough amperage for the circuit to acknowledge that a trailer is connected.

expert reply by: Jameson C