Fuse keeps blowing for running/tail lights

SAZ50

MEMBER
2000 MAZDA PROTEGE
1.6L
4 CYL
FWD
AUTOMATIC
90,000 MILES

I can't diagnose where the short could be. Fuse blows as soon as running lights are turned on. Headlights, brake lights and signals all work. Interior dash instrument panel does not work either. ( I think they are on same circuit).
I have found one blown bulb and pulled it from system. I have checked all exposed wiring for direct shorts to frame and found nothing. All other bulbs are good and bulb sockets are clean and undamaged.

Any ideas of where to start a more thorough exam? Any short cuts you can think of that are prone to this model?

2 Replies

CARADIODOC

MECHANIC
33,873 POSTS

A simple trick to finding a short is to replace the blown fuse with a pair of spade terminals, then use small jumper wires to connect them to a 12 volt light bulb. A brake light bulb works well. When the circuit is live and the short is present, the bulb will be full brightness and hot so be sure it's not laying on the carpet or against a plastic door panel. Now you can unplug electrical connectors and move things around to see what makes the short go away. When it does, the bulb will get dim or go out.

It sounds like the tail light relay can be shorted out but to be sure here is a guide and the tail light wiring diagrams so you can see how the system works and which wires to test.

https://www.2carpros.com/articles/how-to-check-an-electrical-relay-and-wiring-control-circuit

and

https://www.2carpros.com/articles/how-to-check-wiring

Check out the diagrams (Below). Please let us know what you find. We are interested to see what it is.

Images (Click to make bigger)