the reason the rear defrost relay burns up with a perm fix

[Newfie_dan]

I found the reason why it goes bad if anyone is interested in the actual source of the issue as opposed to temporary fixes. The relays under the dash are all mounted on the plate. the one which controls the rear defrost has a heavy blue wire/white tracer and a heavy black wire with yellow tracer and if yours does not work then its likely burnt on the plug. Well being that I hate just replacing a relay without knowing what caused it I dug a little deeper, printed off the wiring schematics and what not looking for the source of the excess ampage causing the burning. Here is what I found, the fused source from the main engine bay fuse box has a 40Amp fuse in it this supplies the constant 12v to the relay (the blue/yellow wire) which is going to be sending it to the rear window defrost element. The relays itself however is only rated at 20 amps. Now do the math here 40 amp source, 20 amp relay. Guess what burns up the relay of course. First rule of running a relay in a circuit is to NEVER fuse it for more than the relay can handle. So using logic here the relay burns up since the contacts are not meant to handle the extra ampage. So the correction? 1 of 2 options use a heavier 40amp relay than the 20amp factory one or option 2 replace the 40amp factory maxifuse with a 20amp one therefore matching the output of the fuse to the relay. You might want to sticky this as a solution to the burnt up rear defrost relay problem guys. As for the cpu timer of course it goes bad since to much juice is running through the circuit.

[msd-supermx3]

man this is very usefull! thank you for the info..... Good tip!
ill change mine for sure!!!!
Thx

[Daninski]

Good find. Damn Mazda did this on purpose.

[mitmaks]

Did anyone try this? Would hate to do this and have my car catch on fire.

[Ryan]

I think this would make it less likely that your car catch on fire. Its like running a hot thin wire versus a colder fat wire... I suppose to make it real safe we should get a bigger plug too with higher capacity terminals.

[Newfie_dan]

I ended up wiring in an inline 20 amp fuse to the constant power wire on the relay soldering and shrink tubing the connections to protect it, worked great for me and no more burnt relay, fuse burns out before relay does if there are any issues with the rear defrost. I also fixed a friends mx3 that had the same issue, ended up finding a wire for the defrost rubbed through where the grommet is touching bare metal causing a high current draw for his mx3.

[King58]

This was SUPER useful !