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ZZDaryl, Chevy Technician
Category: Chevy
Satisfied Customers: 12549
Experience:  28 year experience, Master Mechanic
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2007 Impala LS: defogger..doesnt work..the voltage at the window

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2007 Impala LS, rear defogger doesnt work. I checked the fuse- it works. Checked the voltage at the window - NONE. Is there a relay somewhere to check, where is it - how do I get to it. Is this a common question?
The first thing you will need to do is check to see if there is power at the BATT3 fuse in the under hood fuse box with the key on and the defogger on.
Customer: replied 8 years ago.
I checked the Batt3 and I get 7.5 volts
Whats next?
There is a rear defog relay in that same fuse panel, remove it and see if there is battery voltage where the terminals plug in (not all the terminals will have power but the ones that do should have battery voltage). If there is battery voltage replace the relay.
Customer: replied 8 years ago.
In my original question I had thought it was a fuse, I said I checked it. That is a little gray box 3604 in there and it says Rear Defog. I thought that was a fuse and I switched it with the Starter one before. The starter does work too. Any other ideas? I did not do any of the battery voltage checks you asked above. I thought switching it with another would disprove the replacement of the relay. Please advise

You did not check the power at the fuse or the relay?

Customer: replied 8 years ago.
I checked the batt3 fuse you told me to. I did not check any voltages under the 3604 box or the Rear Defog designated thing. I figured that switching it with the starter would have determined that it was good. Right???
That is possible but I wouold still like to know the voltage at the relay. And are you sure you do not have 7.5 volts at the rear window grid?
Customer: replied 8 years ago.


I did my multimeter on each side right and left tab and got 0 voltage. I tried it twice.

I guess you are saying the 3604 is a relay. How do you want me to stick the probes down in there and check it. I think there are 4 slots. what orientation to test it.

You may need to use a small piece of wire to get it in the slot, you use you meter just like you did checking the fuse, black lead on ground and the red lead on the power set for volts
Customer: replied 8 years ago.


You said key on, not ignition on. Just for kicks I started it up, got 12 volts in the back window, I guess it is fixed. I did stick pins in the relay slots and got 12 volts and some other voltages when I crossed over. I dont know what I fixed. When I put the relay back in - it clicked so I thought that it might work. Who knows. thanks

When someone says key on that typically means ignition on (sorry if there was a miscommunication) if it still does not work the problem is likely a ground or the grid is broken.


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Customer: replied 8 years ago.
The key was in the on position, the lights were on and the defog light was on. When I started the engine is when I finally got the 12 volts. No miscomm. I felt warm on the window. thanks
Thanks for using JA
Customer: replied 8 years ago.


The rear defogger never did work. I just checked the batt 3 and get no voltage. What should I do?

Customer: replied 8 years ago.
The rear defog is still not working. No voltage at the window. I started the car, turned on the rear defog and no voltage on the Batt 3 fuse. I do not get a click when I push the button.
Does the light still come on for the rear defogger?
Customer: replied 8 years ago.
Yes the light does come on the defogger button. Tricky one huh???
Customer: replied 8 years ago.
Yes the light comes on when you push the button.
Customer: replied 8 years ago.
any more thoughts on this
Ok, on the defog relay you need to pull the relay out and check the terminals to see if there is power to the relay, let me know what you have.
Customer: replied 8 years ago.

there are 4 contacts under that 3604 grey relay

Which 2 contacts, wont I short out something if I do?


If you use a test light or a multi meter you will not short anything out. Do you have one of these?
Customer: replied 8 years ago.


Yes I have a multimeter.

I stuck it in 2 of the holes across from each other

and got over 12volts

I stuck it in others and got like 5 or 7 volts.

Key was not in the ignition

Remember i said in the past

I switched out the relay with the starter which was the same number.

What next

Ok, test it the terminals this way, hook the black lead to a good ground (like the battery) and then probe the terminals again and see what you have, do this with the key turned on so that the defog switch will work and be sure the defog is on.
Customer: replied 8 years ago.

Hi, I got 12 volts in the top 2 and nothing in the bottom 2.

I then plugged back in the relay and it clicked as I put it in.

I went back and got 11 volts at the defogger.

This happened before and then later it wont work.

What is up do you think?

The problem may be a connection problem where the relay plugs in or possibly a loose wire on the back side of the fuse panel. I would look closely at the terminals where relay plug in and see if they look corroded or discolored if they do that is likely the problem, sometimes a little dielectric grease will help with the connection, it can be purchased at any auto parts store.
Customer: replied 8 years ago.
Thanks, XXXXX XXXXX be why it is intermittent
Yes thats probably why, good luck with the repair.