I am trying to fix my rear defrost that has no power at the defrost grid. When I press the rear defrost button, the light does turn on. The 40A fuse is still good, I did a visual check and a continuity check on the fuse. I have power to the fuse at the fuse box. I checked this by grounding my test light on a bolt behind the passengers kick panel check checking the fuse box with no fuse in it.

I pulled the relay and tested the continuity of the relay, and applied 12v to the relay and it did 'click' and I have continuity.

I have continuity from the center of the grid to both the left and right side contacts at the hatch struts.

When I turn the car on, and turn the rear defrost on, i have no power to the grid. I am checking for power at the center of the grid near the targa top. Is this the 12v power feed in?

With the car and and the rear defrost turned on I ran a wire direct from battery ground to the three connectors on the defrost grid (Center, left at the strut and right at the strut) and have no power.

Next I ran a wire from the battery power (+) to the three connectors on the defrost grid (Center, left at the strut and right at the strut) and all three lit up my test light. I didn't understand this, expecting it to only work on the struts due to the ground, and the center connector to not work, thinking this was power into the grid.

I have not found any other posts on rear defrost or rear defogger that would be helpful with this.

Any help is very much appreciated.

 

What year, Manual or Dual HVAC control?

Also pull your dash DIC codes.


Mr. Sam

 

I see you mentioned that you've done testing with the car on which is required. What is sticking in the back of my mind is that the rear grid requires the outside air to be below a certain temp to activate. Hopefully someone can either confirm or deny my thought. I know temperature applies to the AC compressor operation during front defrost.

 

Sorry, my bad - I know better.

2000 coupe, 6 speed manual, single control HVAC.

DIC codes
40-BCM -Body Control Module
B0432 Rear Defogger Relay Circuit

B0-RFA - Remote Function Actuation
U1016 Loss of Communication with PCM
U1064 Loss of Communication with BCM
U1096 Loss of Communication with IPC

 

If someone has a wiring diagram that they can post that would be great.

 

This should have all you need. If not let me know.

Schematic, DTC, and troubleshooting info.

https://www.dropbox.com/sh/3i23sn1yt...G2NIVfSWa?dl=0


Mr. Sam

 

This is great - Thank you.
I will post my progress and hopefully a resolution.

 

Back at this diagnosis and repair now that I have brake lights fixed… Spent an hour or so last night, still gathering information and testing power and grounds.

 

Looks like I am not getting a ground from the BCM to the defogger relay, which would switch the relay and send voltage to the defogger grid. I have hot (ignition) power to the relay (fuse #18) with the ignition 'ON', and I have constant Battery power to the relay

If I use my test light to ground the relay at the Relay Coil ground there is an audible click in the RR DeFog relay and I get 12v at the defogger grid.

any ideas?

 

I am sorry, I just tried to look at the schematic I provided and I must have copied it when in enlarged view for my old eyesight. Here is the full schematic.

https://www.dropbox.com/s/yius6qi5vt...matic.pdf?dl=0

As you can see the HVAC unit has the button and light and sends the request to the BCM to have it ground the relay coil circuit which then closes the relay contacts and powers the defogger.

If you get the HVAC unit to light the button then it is probably good and most likely it is the BCM. It could be the wiring from the HVAC unit to the BCM or from the BCM to the relay but that is less likely unless you were doing something in the dash. You know the relay is good by what you did.


Mr. Sam

 

I thought it might be missing some information. Haha

The HVAC unit does light up when I press the button. I have a second HVAC unit from the parts car I just bought, and that light turned on too.

Do you have a wiring diagram for the BCM? I can try to check the signal from HVAC to BCM and from BCM to relay.

My parts car has a BCM that I could try (It's a 2000 coupe, auto transmission, manual HVAC control.)

This may be one luxury I live without nad I go buy a new ice scrapper :-)

 

No, I know of no one who has the internal BCM design. You could try the other BCM, just would require a PCM to BCM relearn. I have that info at home. IF the BCM does not have your specific RPOs it would require Tech 2 programing. Automatic versus manual is something done in the PCM. I don't think I am wrong on that in having a Tech 2 and being in the RPO listing for changes.

Alternatively you could just put in a switch to ground the relay coil wire. If I did that I would put in a relay that was only powered when the ignition was ON. With that design it would close contacts that would be in the switch circuit so whenever you turned OFF the car the defogger/switch would be inactive. That way if you forgot to turn it OFF you would be protected.

If you want details on the design and where to get the relay power control let me know. It would be super simple.


Mr. Sam

 

Sorry, I didn't mean a diagram for the BCM, I meant a diagram for the pins to and from the BCM. I found an older post that you posted the BCM connector views for a 1999 that I downloaded. Do you know if 1999 and 2000 would be the same?

The 99 points me in the right direction...

 

No problem, here are both.

1999

https://www.dropbox.com/s/jg8zhlm8ph...Views.pdf?dl=0

2000

https://www.dropbox.com/s/lg80xymzc7...Views.pdf?dl=0


Here is the Manual HVAC connector views. You want to look at C1.

https://www.dropbox.com/s/v5wndcthbg...Views.pdf?dl=0


Mr. Sam

 

Yes, both my driver and parts car are 2000. I was looking for the BCM pin out and could only find a 1998 and 1999 posted on the forum, so I downloaded the 1999 to review it.

If I need relay power control, would I get it from the GM auxiliary you told about when I was fixing my brake lights?

Thank you for the PDF's. I will continue to update on my progress.

 

Yes I would power a control relay from:

Yellow wire taped off. Powered from ignition switch when in "ACC or On". Powered from Fuse #11 (20 Amp). Not shared with anything.

The other two are ALWAYS HOT and not what you want to use.


If you go that way and have any questions let me know. I could even draw up a circuit for you.


Mr. Sam

 

Definitely something in the BCM. I get a signal from the HVAC control to the BCM (White). When I ground the Purple (From BCM to relay) the relay coil activates.

I am wondering if I can use the White from HVAC (to BMC) and Purple (from BMC) to relay with a new control relay? This would let me have stock look, and use stock RR defogger button. Maybe use a relay with a build in timer to shut off after a set time?

If you could draw up a circuit that would be great. I am a mechanical engineer and am pretty good with electrical, but this I need help with.

 

HOLD THE PRESSES!

So I started looking into my column lock on the car, while sitting in the drivers seat I decided to start the car - just for laughs I press the rear defrost button and I hear a faint 'click' . So I set up my power probe and grab an assistant. Start the car again and press the rear defrost button, and I now have voltage at the defrost grid. Apparently the engine needs to be running, not just the ignition in the ON position to supply voltage to the RR defogger.

It definitely didn't work before all of this diagnosis. I would stat the car and let it run while I had the front defrost and rear defogger on, and the rear did not work. Maybe cleaning the contact while doing the diagnosis fixed a connection in the circuit.

anyway FIXED IT!

 

Well, that is great. If you have any issues in the future we can revisit this.


Mr. Sam

PS: Are you looking for a CL problem or prevention fix?

 

My column doesn't lock, so I was looking to see what was done to the car. At some point in the cars life it had a GM recall for the CL. I ordered a LCM5 last night to prevent the fuel shut off and DIC error messages. From aha tI read on the forum, this is something I definitely should do.

And thank you again for all your help with the electrical.