Rear defogger mystery.... When does it activate?
Thread Starter
Racer
Joined: Mar 2005
Posts: 286
Likes: 36
From: Scituate, MA
Rear defogger mystery.... When does it activate?
In order for the rear window defogger to activate must the temperature be below a certain number? I'm guessing about ten years ago I just couldn't get the small LED to light up after replacing all of the bulbs in the HVAC head. Later, on a cool damp night when I needed the thing to actually work it did and so did the LED on the HVAC control.
I just finished replacing the bulbs again in that dual HVAC unit and I can't get the defogger LED to light up, but then it's 70 degrees. I did an extensive internet search on the subject and found a vague recollection like mine about temperature being a factor in order for the thing to work. Am I right about this or should I be shopping for a new head? Any way to test it?
Yours,
Bluemill
2000 coupe, 6 speed, 124,000 miles
Thread Starter
Racer
Joined: Mar 2005
Posts: 286
Likes: 36
From: Scituate, MA
I've got a 2000 coupe I've had for 17 years and over a hundred thousand miles. For the third time I recently had to replace the bulbs in the HVAC head unit. Not easy, and afterwards one has to check that all still work. The one thing that didn't light up was the rear window defroster- LED. The last time I did this, like 10 years ago I think I remember having the same issue, but later in colder weather when I needed the defogger to work it did- and the LED on the HVAC console did light up. After a long internet search and reaching out on 3 Corvette forums, I found only one thread where another C5 owner thought in the back of his mind that the light would not go on in too warm a temperature, or when it's not needed.
Could some of my fellow C5 owners simply go to their cars, turn them on, and try to put on your rear defroster? Also, when you do it, what was the approximate temperature in your neck of the woods? That's all I'm asking, and post back your results. Thanks a million!
Yours,
Bluemill
2000 Navy Blue coupe, black interior, 124K miles, Z06 exhaust, 6 speed, Zip tie air cleaner Mod, smoothed intake-throttle body throat, cold air cut out panels in fog light bulkhead, HUD, C6 short throw shifter.
Could some of my fellow C5 owners simply go to their cars, turn them on, and try to put on your rear defroster? Also, when you do it, what was the approximate temperature in your neck of the woods? That's all I'm asking, and post back your results. Thanks a million!
Yours,
Bluemill
2000 Navy Blue coupe, black interior, 124K miles, Z06 exhaust, 6 speed, Zip tie air cleaner Mod, smoothed intake-throttle body throat, cold air cut out panels in fog light bulkhead, HUD, C6 short throw shifter.
Moderator, Tech Contributor
Joined: Sep 2013
Posts: 15,626
Likes: 4,069
From: Cape Coral, Florida
Key on engine off my LED comes on when my rear window defogger switch is pushed in …Southwest Florida and it’s HOT !!…it only operates when the engine is running.I looked in the FSM, Mitchell, and Alldata and could not find anything about a temp requirement for operation !!…at 70 degrees is yours working ??…it should !!…you should feel the bottom of the window become warm after 5 minutes or so.
Last edited by C5 Diag; Sep 27, 2021 at 09:38 PM.
Thread Starter
Racer
Joined: Mar 2005
Posts: 286
Likes: 36
From: Scituate, MA
Thanks for great tech help and research......
The LED is not lighting up so I don't know if it's working or not. I could try one click and wait to see if there is any heat in the grid, and if not after a few minutes click it again, etc. I will look at the grid too to see if it is in good shape, and has continuity. The LED was replaced ten years ago and not the same as OEM, so that could be an issue, and the solder connection may be poor. If the grid looks good I'm likely going to shop for a a redone HVAC control head. My original has been through a lot of bulb changes and stress on the board. Thank you so much for the help!
Thread Starter
Racer
Joined: Mar 2005
Posts: 286
Likes: 36
From: Scituate, MA
Awesome PDF.....
OK I've got the three volume factory service manual for the 2000 "Y" body car and I could not find anything like this regarding the rear defogger- so thank you Purrvert. Great handle BTW, too funny! I'm not too ashamed though as I know it's not super easy to find anything in those 1,500 pages. Yet conversely I am thinking I might be a little dumb in that not until reading this today did I know the car had heated outside mirrors! Where did you find the PDF?
Today I'll check the grid connections, and check for continuity. Then I'll pull the head again and resolder the non-OEM 3 mm LED that I put in ten years ago. While installing new bulbs then I used a 9 volt battery to test the bulbs after putting them in. I hit the defogger LED in error and it exploded with a pop as loud as a .22. It was impossible to get the same LEDs that were in the control head, so I got a few LEDs from Radio Shack and put one in. It has only worked sporadically. If anybody has a source for those stock LEDs I'd like to know as my jury rig repair is suspect at best.
Yours,
Bluemill
The car is all original except the water pump and catalytic converter section of the exhaust.
Today I'll check the grid connections, and check for continuity. Then I'll pull the head again and resolder the non-OEM 3 mm LED that I put in ten years ago. While installing new bulbs then I used a 9 volt battery to test the bulbs after putting them in. I hit the defogger LED in error and it exploded with a pop as loud as a .22. It was impossible to get the same LEDs that were in the control head, so I got a few LEDs from Radio Shack and put one in. It has only worked sporadically. If anybody has a source for those stock LEDs I'd like to know as my jury rig repair is suspect at best.
Yours,
Bluemill
The car is all original except the water pump and catalytic converter section of the exhaust.
Last edited by Bluemill; Sep 28, 2021 at 09:06 AM. Reason: sporadically
Racer
Joined: Aug 2004
Posts: 438
Likes: 15
From: satellite beach fl
At a cozy 87 deg in my garage my rear window defogger comes on with key on engine running or not running, a/c blower on or off, anytime the key is on.
Have you considered that the button might be faulty?
There is a schematic towards bottom of PDF.
Have you considered that the button might be faulty?
There is a schematic towards bottom of PDF.
Last edited by purrvert; Sep 28, 2021 at 01:31 PM.
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Thread Starter
Racer
Joined: Mar 2005
Posts: 286
Likes: 36
From: Scituate, MA
Got it fixed!
I took a look at the solder connections on the main circuit board with a 10 x power jewelers loop and saw that there was NOT a lot of solder on one lead of the non-OEM 3mm LED I put in 10 years ago. When it did work that one time in the cold it was likely tighter due to the cold-perhaps the outside mating hole had contracted. I resoldered today and the LED lit up like a Christmas Tree. Put it back together then I got a case of the DIM main display, likely a result of handling the boards so much. Went back in and resoldered the 8 capacitors on the inner board. That worked and reassembled all. Started up the car and hit the button and waited 2 minutes, and the side mirrors were positively HOT- which i didn't know about until today. Then I felt the bottom of the grid, and the heat was coming thru that big hunk of glass.
Thanks for the help on this. When I saw the prices on refurbished head units I knew I had to go back to the bench!
Thanks for the help on this. When I saw the prices on refurbished head units I knew I had to go back to the bench!
Drifting
Joined: Jul 2017
Posts: 1,365
Likes: 471
From: S.E. Pennsylvania
Key on engine off my LED comes on when my rear window defogger switch is pushed in …Southwest Florida and it’s HOT !!…it only operates when the engine is running.I looked in the FSM, Mitchell, and Alldata and could not find anything about a temp requirement for operation !!…at 70 degrees is yours working ??…it should !!…you should feel the bottom of the window become warm after 5 minutes or so.
Moderator, Tech Contributor
Joined: Sep 2013
Posts: 15,626
Likes: 4,069
From: Cape Coral, Florida
After reading your post I went out to the garage and tried using mine. Up to this point I had never used the defogger once. I turned the key on - no start - and pressed the button. The lights immediately dimmed so I'm thinking it started immediately. The top was down so I quickly turned it off. You stated that it only works with the engine running?? What do you make of that?
Burning Brakes
Joined: Oct 2014
Posts: 870
Likes: 76
From: The Rainy Side of Washington State
My 99 vert has a glass rear window with the electrical grids on it.
I'm not sure if you guys call it a defogger or a defroster.
I'm thinking a defogger blows air and a defroster uses the grids.
Idk, do the hardtops have an air duct setup running off of the front blower?
I'm not sure if you guys call it a defogger or a defroster.
I'm thinking a defogger blows air and a defroster uses the grids.
Idk, do the hardtops have an air duct setup running off of the front blower?
Drifting
Joined: Jul 2017
Posts: 1,365
Likes: 471
From: S.E. Pennsylvania
Ahhh. Maybe that's the confusion. Thanks for the head's up... Sorry C5...
Moderator, Tech Contributor
Joined: Sep 2013
Posts: 15,626
Likes: 4,069
From: Cape Coral, Florida
The factory manual refers to it as a “defogger” !!…the rear window is electrical only !!