Melted Defog Relay Plug?!

[Shades]

Anyone else have this problem of the defog relay plug melting all the time, this is the second time it's done this and it pisses me off. It just melts the plug so that you can't plug in a new relay... now I'm gonna have to go to the junkyard and grab another plug and relay.

Is there a solution to fix this?

[PATDIESEL]

If it is melting then there is something wrong. You need to find the problem and fix it first. The defog runs off of heat which is created by resistance. That resistance is too high for some reason though and the extra resisitance is heating up the whole wire. I'd bet you have a ground fault or a poor ground in the system. Is it the front or rear defog?
If rear, then unplug the defog from the window (the little black plug on the windows edge). Then test for ohms at the "wire" just after relay. If you have any ohm reeading then there is a ground fault somewhere. You'll need to find a pinch or cut in the power wire, or a ground that is corroded, loose or cut.

[Red Rocket]

in my experiences, being an electrician, usually where it melts is where the problem is. Probably a bad connetction right at the relay, if the wire was pinched the insulation most likely melt from there back. So I'd make sure you have a good connection right there.

[PATDIESEL]

In my experience it will melt at the lowest melting point spot in the circuit. Since if a ground fault, the whole wire will heat up and the plastic will melt before the wire then the connection will be the first to go... (well acutally the wire sheathing will go first, but that will be harder to see unless it is somewhere visible)

[Cy]

I just picked up all the relays and the fuse blocks from my junkyard...

I'd burnt out that relay to...I have YET to find out How or why...but it works now

[mx3optidrive]

What usually happens is that Salt (from winter driving) or water gets into the relay connections and corrodes the connections which causes more resistance inside the relay or shorts it our internally and causes it to melt.

[pbuoye]

where does the defog relay live

thanks

[Newfie_dan]

The reason it burns out is that there is a 40 amp fuse that feeds the constant power side of the relay and its a 20 amp relay, draw over 20 amps and it melts plain and simple, the plug melts due to the high heat generated by the contacts during the exchange. So long as it does not draw over 40 amps the fuse is good.The relay is the weak part of the rear defrost circuit. Now with that said that also means that you are drawing over 20 amps of current and that means you need to do some troubleshooting on that circuit. So that means you are going to have to find out where the problem lies usually the first place to start is where the contacts are located are the rear defrost make sure that they are in good contact with the solder points and not touching off bare metal, next place is where the wiring for the hatch runs through that rubber grommet see if it has chafed through exposing some wire. So check that and let us know what you find. If you want to prevent the problem wire in a 20 amp fuse holder on the constant power side of that relay it will pop before the plug melts.

[Nd4SpdSe]

I've never touched that relay before, but most standard automotive relays are 40amp that I've worked with...guess that one isn't the same.

On that note, I've never melted a relay. Normally they just don't want to stay engaged and keep clicking

[Newfie_dan]

yeah I discovered this lil mazda engineering snafu whilst troubleshooting my own melted plug, I traced the schematic and removed the relay to discover that the factory one is a 20 amp. My problem occured when the defrost switch decided to stick closed making the relay stay on for an excessive amount of time melting the plug on me. I bench tested the relay to find that it was starting to melt internally as well but with the metal surface area of the male plug end it heated up first causing some electrical issues.

[Ryan]

I've had mine melt on me too.

I just tried to make a habit of not letting it run too long. Its supposed to be on a timer, but who knows...