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Discussion Starter #1
I have a 2005 Cobalt. Rear window defogger doesn't work.

When I press the button the light comes on so I am assuming the switch is working. Maybe its just that part of the switch, I dont know.

The only fuse I found was in the engine department. Large 40 amp fuse. Appears to be fine. Switched the relay out with a different one. Still no power on the rear window grid.

Any suggestions as to where to look?
 

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time to get the DMM (digital multimeter) out (if you have one) and start checking for current. fastest way to find out. Start at the rear window plug and work your way from there or vise versa.
 

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Here is what I know.

I am pretty sure relay is fine. If I put my hand on the relay and have someone press the button inside, I can feel it click.

Now, does the relay provide direct power to the rear window defogger? I am guessing it does but I do not know for sure. According to the Haynes manual, there should be a light blue and white wires going to the rear window. Do these wires connect right to the relay? I guess one of those 2 wires is probably a ground.

How in the world do you find the wire going to the rear? where does it run. My car has the pioneer amp installed in the trunk, looks like I will have to pull that out of the way and maybe I can find the wire that comes down from the glass. Where it goes from there might be a different story.

Also, which side of the defogger is the positive side? My Silverado has power at the drivers side. Is the Cobalt the same way?

Thanks for the help.
 

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I dug out the DMM to check voltage. With all the wires plugged in. It shows 0 volts at the drivers side connector. I pulled that connector off and checked the connector and got 12 volts. Soon as I plug it back into the defogger I get nothing. I removed the passenger side wire which I am assuming is the ground. Then as you can expect, I had 12v (Meter grounded to chassis) at the drivers side. Soon as I plug in passenger side it goes back to 0.

Does this mean that the defogger grid is bad? Thats kind of what it sounds like to me. It looks fine. I dont see any breaks or anything. If that is the case, its just something I have to live with. Not sure its worth spending a few hundred $$ to replace the rear glass.

Let me know if you have any suggestions.
 

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honestly I think it's a bad connection/wiring somewhere. Sounds like there is no continuity in the circuit once you plug it in. Are you sure you're using your DMM correctly? It's hard for me to tell what's going on since I'm not right there to see it..
 

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Discussion Starter #6
Yes, I know how to use a meter.

What it acts like is that the defogger is shorting straight to ground. Like I said, if I ground the meter to the chassis and pull the wire, I have 12.5v at the plug. As soon as I plug it in and check, I have 0 volts.
 

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Discussion Starter #8
No sorry. I am thinking there is an issue with the grid and the only way to fix it is to replace the rear window which I am not going to do. I really dont know what else to try.
 

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My guess is a poor (high resistance) connection between the relay output and the rear window terminal. Voltage passes through the dirty contact, but as soon as any real load is applied, voltage drops to nothing. Think of a pinched garden hose. If left with no load (no water flow), you can build pressure (voltage) at the nozzle. As soon as the nozzle is opened, pressure drops to nothing as little water can flow past the pinch point.
 

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Have you guys tried replacing the relay? Kinda looks like you have burnt relay contacts.
 

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Discussion Starter #11
My guess is a poor (high resistance) connection between the relay output and the rear window terminal. Voltage passes through the dirty contact, but as soon as any real load is applied, voltage drops to nothing. Think of a pinched garden hose. If left with no load (no water flow), you can build pressure (voltage) at the nozzle. As soon as the nozzle is opened, pressure drops to nothing as little water can flow past the pinch point.
I think you are onto something there. I ran a jumper wire (with fuse) from the battery to the positive side of the defroster grid and sure enough, the defroster worked just fine.

Any suggestions as to what the next step would be to repair this? I am guessing have to run a new wire but not sure where? There are after market setups that come with a new switch and timer but I really don't want to add another defroster switch in the car unless that is a last resort.

Yes, I did replace the relay. Of course my luck, its not that easy of a fix.
 

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Keep trouble shooting backwards you will find where the bad connection is. You now know it's before the window, just keep moving back.
 

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Discussion Starter #14
Ok I fixed it.

The wire from the defroster goes under the rear deck near the rear drivers side speaker and has a connector. Well the wire to the defroster was melted right at the connector. I cut the wire and spliced in a new piece and it is now working.

What worries me is that the rear defroster has a 40 amp fuse in the fuse box under the hood. Well its only 14 gauge wire going to the defroster which handles 15-20 amps max. Any ideas why that fuse is so high? I would think if the wire got hot enough to melt part of the plug it should have blown the fuse.
 
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