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Rear Window Defrost


aspevacek

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Ok my second vehicle is a 2004 Chevrolet Impala (picked it up for 2500 a few years back) so I have an inoperative rear window defogger. Going thru and doing testing According to numbers I have it should be working but it won't. My Brother bough the car new in 2004 and said it was some time in 2005 is when the defogger stopped working and he just never cared as to why and went on with it not working.

There is a 4 pin relay and circuit breaker in the passanger side fuse box that control the defogger.

Pin out for the relay:

N3: Load Management from BCM / Pin 86 on relay 14.35 volts with engine running /.56v engine off
N2: Relay out to circuit breaker / Pin 87 on relay no voltage. Wire is good from Box to the rear window no resistance.
L3: Hot at all times 14.35 volts engine running / Battery voltage with engine off
L2: Ground signal from HVAC control unit Open circuit with switch off / completes ground ciruit when switch is pushed and also tested usign it as a ground ot test voltages on N3 and L3.

Looking at the numbers the defogger should work and points to a bad relay, I swapped the relay with 2 different relays from the cooling fans and it acts the same way but the fans work with the relays as they are supposed to. I took the cover off of one of the relays and if I manually push the contact closed it will energize the rear defooger as long as I hold it.

Does anyone have any insight on what I am over looking on this set up.

Ohh I have also tried 2 different HVAC control unit both yeilding the same results.
 


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martin

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Don't know if it's any help in this case but some of the earlier vehicles had a relay for the de-fogger with a built in thermal timer. If that's the case here it will need the correct relay to operate.
 

aspevacek

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GM uses the same relay for the cooling fans and the Defrost that is why I went with using them to test it. I have power for the command off and on, but the relay will not automatically pull the contact closed. It has had me baffled for a few days and I am on a quest to see if the issue can be fixed. I have recently put a new Body Control module in the car thinking with otehr issue the car had it would all be fix. Every other issue was fixed with the BCM except the rear deffoger. The HVAC control has logic built into it and seems next to impossible to isolate any internal issues on it.
 

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Check the grid over carefully for any scratches or spots that the grid might not be connected anymore.
 

Mark_88

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Check the grid over carefully for any scratches or spots that the grid might not be connected anymore.
This was what happened to my Sidekick...a small scratch in one of the wires where they attach...bought some of that repair stuff and it worked fine thereafter...
 

aspevacek

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every line on the grid is fine. That was one of the first things I had suspected and looked for and check resistance across each line ot see if that was the issue. Tommorow I am going to do a seperate relay outside of the fuse box and see if I can get that one to atleast turn on and of fwith the factory controls. I am stumped since i have power where I am supposed to and grounds at all the correct locations.
 

Mark_88

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Could be a relay...were you able to test the line going to the defroster? Should be exposed in the trunk or the back window ledge. If you have power there then it is the grid...if not it is somewhere in between.

My Zuki had a hatchback and a little spade connector that was also dirty and I thought that was the problem at first...cleaned that up and still no joy...

I made the mistake of tinting my rear window on the Tempo and in the process of removing the tint I scraped a few of the lines of the grid...now that was a PITA to fix...
 

aspevacek

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I have continuity from the fuse box all the way to the end of the line where it connects to the grid. The fact the relays will not pull over can close the contact is the main thing that has me baffeled. I will see if I can force a relay closed and check for voltage across the wire to the defogger. I know it does not have a short to ground along the wire as it shows as an open circuit if I disconnect it from the defogger and test for continuity to ground. GM is stating there is a antenna module located in the rear pillar that has wiring for the antena and the defogger but I am unable to locate a module as I know it. This whole thing is just very confusing in the way they over engineered the thing.

With the ground being behind a logic module inside the HVAC control unit I can only assume it is workign properly because it is open until I turn on the Defogger switch then it becomes the ground. I think it is safe to say that is how General Motors intended for it to work. The referrence voltage on BCM looked really low to me at .56v witht he key on engine off but once you start the engine it jumps up to 14.35v with is the output of the alt. Everything tells me with a ground and power the switch should close in the relay and I should get power across the contacts from the constant hot to the out put to the rear window.

There have been alot of complaints when you search online about the impala rear window defoggers being inop but most are fixed by a simple bad relay or circuit breaker replacement. Neither of which are working in this case.
 


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