We had this 1989 Ford Ranger XLT with a 2.9 engine come into the shop with a customer concern of no brake lights.
I decided to start my diagnostic at the stoplight switch. I could have started at the fuse, but if I have voltage at the switch my fuse is good. (I already knew the bulbs were good, we had replaced one, so I wasn't worried about them.)
When I checked the stoplight switch without the brake pedal depressed, I found voltage on one wire and ground on the other. This is normal and what we would expect to see. Below, you can see me testing the two circuits with a PowerProbe.
First, the circuit with voltage. (Notice the red light illuminated in the top right of the Probe and the reading on the face at 12.6 v.)
Next, the ground circuit. (Notice the green light on the top left of the Probe illuminated, along with the reading of 0.0 v.)
So far, so good. This is as it should be. Again, this shows my fuse to be good (because we have voltage) and we have a good ground circuit as well). So, at this point, I am not concerned about my fuse or the wiring from the fuse to the switch, all that is good.
The next step in the diagnostic is to load the circuit by depressing the brake pedal. When this is done the switch should activate and allow voltage to flow. So then, when testing it with the Probe, we should see voltage on both circuits.
However, when we depressed the pedal (applied the brakes) the circuits remained the same as above: that is, one circuit showed voltage and the other still showed ground.
Below you can see that the brake light is not illuminated.
When I unplugged the switch and applied voltage to the ground circuit, the brake lights illuminated.
This proved that the stoplight switch was faulty and needed replacing.
After I replaced the switch, I once again depressed the pedal, unlike before, now both circuits showed voltage.
Below is the first circuit, clearly showing voltage.
Next, is the second circuit, which before had remained ground, but now shows voltage (as it should when the switch is working properly). (Sorry it is a bit blurry.)
Now, with the switch working properly, the brake lights were illuminated.
We put everything back together and another job was done!
