defrost in 2000 grand prix
Printed From: the12volt.com
Forum Name: Car Security and Convenience
Forum Discription: Car Alarms, Keyless Entries, Remote Starters, Immobilizer Bypasses, Sensors, Door Locks, Window Modules, Heated Mirrors, Heated Seats, etc.
URL: https://www.the12volt.com/installbay/forum_posts.asp?tid=107763
Printed Date: July 05, 2025 at 5:02 PM
Topic: defrost in 2000 grand prix
Posted By: bigbuffs
Subject: defrost in 2000 grand prix
Date Posted: September 28, 2008 at 1:09 AM
OK I've been searching the forums for a while. I'm trying to locate the rear defrost wire on my 2000 Grand Prix. From what I've found it should be a white wire hooked up to a relay. I can't seem to find it under the driver side dash or steering column.
Does anyone know where the wire is? Also, once I find it, am I going to need another relay when wiring to my car alarm? I have a Python 872 that will automatically turn the defrost on when the car is remote started.
Replies:
Posted By: KarTuneMan
Date Posted: September 28, 2008 at 1:18 AM
It's at the switch.
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Posted By: bigbuffs
Date Posted: September 28, 2008 at 1:35 AM
Are you sure? Is it coming off the wires running out of the back of the HVAC? I tried pressing the button and listening for the relay to click and I thought I heard it over on the passenger side somewhere.
Posted By: i am an idiot
Date Posted: September 28, 2008 at 2:21 AM
There is a switch to turn the rear defrost on manually. That is the switch he is referring to. It may be mounted in the panel with the A/C controls. You are not going to need another relay if you find the wire that controls the factory relay.
Posted By: bigbuffs
Date Posted: September 29, 2008 at 2:16 PM
well thats what I figured he was saying but im not going into the HVAC unit itself. There are plenty of wires running out the back of the unit but I'm not sure which one runs the defrost. How would I test them to find out?
Posted By: i am an idiot
Date Posted: September 30, 2008 at 10:11 PM
Have you removed the panel with the switch mounted to it, to see if you can follow the wires or traces from the switch to the plug? There should be only 4 or 5 wires going to the switch. If you can find those wires, we should be able to help you.
Posted By: KarTuneMan
Date Posted: September 30, 2008 at 11:03 PM
As the idiot stated..... pull the panal and look at the plugs on the back. The wire you are needing is there!
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Posted By: bigbuffs
Date Posted: October 01, 2008 at 3:02 PM
well i took the HVAC unit out and there are about 20 wires coming off the back of it. So I used my multimeter and tested some wires. One of the white wires tested at like 0.3 volts when idle and 12v when i pressed the rear def button so I assumed this is the wire I need. I spliced into it and hooked it up to the remote start but when I hit the button for it the interior lights flash on and off about four times. Nothing happens to the rear def. Any ideas as to what is happening?
Posted By: tall technician
Date Posted: October 01, 2008 at 3:14 PM
ok the signal u got on the white wire is 12 positive, the signal from ur alarm will be negative, yo'll have to use a relay like this. wire pins 30 annd 86 to a constant 12 volts, pin 85 to the defrost wire from the alarm, and hook pin 87 to the wire on the defrost switch. hope it helps.
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installin's easy. its da customers who make it hard.
Posted By: bigbuffs
Date Posted: October 01, 2008 at 5:31 PM
ok i wired up a relay like you said but i get the same result. Also when I wired in the relay the defrost switch does not work at all. Before the relay the switch still worked when pressed manually.
Posted By: i am an idiot
Date Posted: October 01, 2008 at 6:26 PM
It sounds as though you may have tapped into the illumination wire.
Posted By: bigbuffs
Date Posted: October 01, 2008 at 8:14 PM
well i probed all the wires coming out the back of the HVAC and the white wire i tapped into is the only wire I get activity from when I press the button. I can hear the relay clicking on and off when i press the button on the HVAC. It's coming from behind the fuse box. I think i might see if I can find the wire back there if I can't figure anything else out.
Posted By: tall technician
Date Posted: October 01, 2008 at 8:26 PM
ok if it's the wrong wire then u may need to look for a wire with a negtive reading when the switch is active, the wire must be behind there, because it's where the signal for the relay is generated. find a constant 12 volts wire and stick the red probe of ur meter on it and use the negative probe to look for the wire. the dmm will read 12vots when the switch is active and o when it's off. had a hunch u had the wrong wire at the beggining lol. hope it helps.
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installin's easy. its da customers who make it hard.
Posted By: bigbuffs
Date Posted: October 02, 2008 at 2:42 PM
well I actually got it wrong when I wrote it earlier. This wire test at 12v when the defrost is off and 0v when its on. Wouldn't this have to be the wire?
Posted By: i am an idiot
Date Posted: October 02, 2008 at 3:09 PM
Have you tried grounding said wire to see if it turns the defroster on?
Posted By: bigbuffs
Date Posted: October 02, 2008 at 7:12 PM
OK i just tried grounding it and it and I can hear the relay click when I ground it. I'm not sure if this is the relay for the defrost but the sound is coming from the same area. But, the defrost button has a light on it that comes on when you turn it on and it doesn't come on when I ground the wire. I'm not sure what this means or how to tell for sure if the rear defrost is on.
Posted By: i am an idiot
Date Posted: October 02, 2008 at 8:29 PM
Leave it on for a couple minutes to see if the rear window is warming up a bit.
Posted By: bigbuffs
Date Posted: October 28, 2008 at 4:59 PM
OK sorry for not responding I was gone for couple weeks and didnt get a chance to work in this. I did find out that grounding the wire does turn on the rear defrost so I think all I need to do know is hook up the relay to provide a ground right.
Posted By: joch1314
Date Posted: October 28, 2008 at 7:30 PM
no you shouldn't need a relay. The alarm should send a negative pulse. If the output you are using sends out a positive 12V,(unlikely) then you will need a relay to invert the polarity....but ONLY if you're getting a positive output from the alarm. If so, wire it like this....Terminals 85 & 30 are connected to ground, 86 is connected to your + trigger (alarm), and 87 (negative output) goes to your defrost wire! ------------- ...half of the truth can be worse than a lie. <----Roger Russell said that..
Posted By: i am an idiot
Date Posted: October 28, 2008 at 7:36 PM
If you ground the wire and then remove it from ground, do you hear the relay turn off? If so you will have to program your remote start unit to LATCH that output. Some are not programmable for a latched output. If it is not capable of providing a latched ground output, you may have to do something of this nature. | Latched On/Off Output Using a Single Momentary Pulse | Similar to the momentary to constant configuration above, we can engage and disengage the latched output with a single pulse from a switch or an output from an alarm or remote keyless entry. The first pulse from the switch will engage the latch. The next pulse from the switch will disengage the latch.
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You will have to use a 5th relay triggered by the output of this setup. The 5th relay is wired as follows. 12V output from this setup to terminal 86. 85 and 30 need to be grounded. 87 goes to the wire you found in the car.
Posted By: bigbuffs
Date Posted: October 29, 2008 at 2:49 PM
yes is does turn off when I remove the ground, luckily it does have a latch setting so I will use that.
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