Mechanics

CHEVY REAR WINDOW DEFROSTER

2004 Chevrolet Impala

Electrical problem
2004 Chevy Impala 6 cyl Front Wheel Drive Automatic 42000 miles

Rear window defoster does not work.
Light comes on when button pushed. No noise at relay. Have power to relay on upper right terminal. Left upper right terminal when engine is started.
right lower terminal changes in resistance when activating defrost switch. No change in left lower.
Relay has been replaced and breaker was changed out with power seat breaker with no change. No power to grid on rear window when switch activated.
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Wayne Boren
October 29, 2010.




Hi Wayne Boren, Welcome to 2carpros and TY for the donation

Check the HVAC control module rear defog relay control circuit and the radio antenna control module-see diagram below


http://www.2carpros.com/forum/automotive_pictures/12900_w1_18.jpg




http://www.2carpros.com/forum/automotive_pictures/12900_w2_14.jpg




http://www.2carpros.com/forum/automotive_pictures/12900_w3_11.jpg


Rasmataz
Oct 29, 2010.
I mentioned in my email that I already have the diagram. Your diagram is unreadable an is too small to see detail because it is blurry. I was hoping that you could give me some pointers based on what I have check. The antena module does not affect reception as other people have mentioned. If I have power to the relay (and it has been replaced) then why doesn't it click when the button is activated?

Tiny
Wayne Boren
Oct 29, 2010.
I'm not aware of any email you have sent-I was asked to assist here-If you have the diagram-

Check the body control module it powers the coil side of the relay and the HVAC module grounds it -also the defog circuit goes thru the antenna module before it hits the grid-inspect the grid for open

Rasmataz
Oct 29, 2010.
Do you need special equipment to check the body control module? I mentioned earlier that their is pwr to both top pins on the relay. Left side is for the coil so that tells me there is power. Does the body control module act as a ground to complete the circuit?

Tiny
Wayne Boren
Oct 29, 2010.
Its the HVAC module that grounds it-that's if you got power on the coil side of the relay

Yes you do -to check the BCM and the dealer has it

Rasmataz
Oct 29, 2010.
I think it is the HVAC control module because a jumped the circuit across from the always hot to the rear window and it worked. Athough I would not recommend a paper clip because it get red hot. The relay is misleading because it is not set up the same a the diagram. It is in an X pattern. I will take it from here.
AD

Tiny
Wayne Boren
Oct 29, 2010.
Very wel, l you got the basic idea on what's going on

Rasmataz
Oct 29, 2010.

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