Backup Light Fuse Blows When 1990 Mazda B Series Pickup Shifts Into Reverse

Question:

I have a 1990 Mazda B2600i 4x4 Automatic that I installed the Curt T 4 wire connector in. The installation went well and when I connect the trailer lights to the vehicle all the lights work. However, when I put the vehicle in reverse, the backup light fuse which also happens to be the engine electrical fuse blows and completely disables the vehicle. I have tried the same trailer on my Honda Pilot and the lights work fine and have no issues with the back up lights.

asked by: Robert T

Expert Reply:

A 4-pole wiring harness carries stop, turn and tail light signals only, and does not connect at all to a vehicle's reverse light circuit. Since this custom harness uses connectors specific to your truck and simply plugs in, it sounds like there could be a short caused perhaps by a damaged connector on the vehicle. This could account for the harness interacting with the truck's reverse light circuit.

Given the truck's 26-year age it is quite likely that the issue is simply corrosion in the vehicle connector. Corrosion can cause adjacent circuits to become bridged together, which would cause the short that blows your fuse. You can try spraying an electrical contact cleaner (such as Caig De-Oxit or a similar product) into the connector to loosen any corrosive build up. If this does not remedy the problem, you can just hardwire the harness by removing its connectors and splicing the wires together with butt connectors # DW05744-5. You'll need a circuit tester like # PTW2993 to test the vehicle wires to confirm each one's function.

expert reply by: Adam R