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Discussion Starter · #1 ·
It seems that the rear defroster is very weak and takes a long time to clear the rear glass. The side mirror defrosters work normally. The rear glass gets warm but it takes a while. The voltage readings on all the lines of the grid read correctly. Has any one else experienced this?
 

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OK, you have voltage. Check the resistance of the defroster grid (with the defroster OFF). I cannot find the spec, but usually it is around 1 ohm. I bet your resistance is higher (meaning less current flows, meaning less power: P=I*I*R, where I is current and R is resistance... OR alternately P = V*V/R , where V is voltage).
 

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Discussion Starter · #3 ·
I couldn't find the resistance spec in the Mazda repair manual either. I am guessing it should be similar to my other cars. I have never seen a case of high resistance in the grid unless there was a poor grid repair (didn't see any on this car) or there was corrosion on the connector (haven't taken the C-pillar trim down to look yet).

The grid seemed to work ok yesterday. I will know for sure once their is frost on the back glass.
 

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One way you might be able to test it is to measure the amp draw to see if it is up to par, "if" you can find what the par should be, that is.

One easy way to do this is to use an external amp meter over a battery cable and see what the amp draw is with and without the defroster on, with all other things being equal.
 

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It seems that the rear defroster is very weak and takes a long time to clear the rear glass. The side mirror defrosters work normally. The rear glass gets warm but it takes a while. The voltage readings on all the lines of the grid read correctly. Has any one else experienced this?
 

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If you have a weak rear defogger almost for sure you will have a corroded ground lug. Shine-up said connection with sand paper. Also, consider corr. that could occur with the cad. plating of the open-barrel crimp. I think you can use DeOxit (?) or for sure "Stabilant 22a". This last product is also sold thru NAPA Autoparts. Expensive but it works really well.
 

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If you have a weak rear defogger almost for sure you will have a corroded ground lug. Shine-up said connection with sand paper. Also, consider corr. that could occur with the cad. plating of the open-barrel crimp. I think you can use DeOxit (?) or for sure "Stabilant 22a". This last product is also sold thru NAPA Autoparts. Expensive but it works really well.
Where is the ground lug? Inside the rear door, or inside the body somewhere?
 

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I suspect it is behind the trim-panel... i.e. the C Pillar panel.
Any idea which side? I've managed to take off the trim panel at the inside top of the hatch, and there's a grounding lug with two black wires connected located there. This seems to presume that ground to the body is made thru the hinges, which doesn't seem to me to be a good idea.
 

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Any idea which side? I've managed to take off the trim panel at the inside top of the hatch, and there's a grounding lug with two black wires connected located there. This seems to presume that ground to the body is made thru the hinges, which doesn't seem to me to be a good idea.
Quite right. Was thinking sedan. There s/b a power + grd wire in the split conduit feeding the hatch. I wd guess grd is 'real close to where split conduit feed pops out from trim panel or the sheet metal of the body.
 
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