Wiring diagram explain please

[01 monte fan]

I am trying to figure out the issue with my rear defrost.  I have no voltage back at the grid or the plug to the defrost/antenna module. I press the button and the light turns on but I don't hear a click for the relay.  I have tested for voltage at the relay and have 12v on the 30 pin but no voltage on any other pin when the button is pushed on.

I am wondering if it's possibly the hvac control panel. I don't know how power is routed into the relay to know if it's the panel or bcm that supplies the power for the relay. 

I sure hope someone can direct me on where to look next. Thanks for any guidance here!!

Edited March 17 by 01 monte fan

[GnatGoSplat]

You should be seeing +12V when measuring across pins #85 and #86 on the relay socket (N3 and L2 in the diagram).  L2 should be grounded and N3 should be +12V with the ignition turned on.  If you don't see +12V on N3, try checking with positive lead of your voltmeter in N3 and negative lead on any grounded metal screw.

 

[01 monte fan]

Ok, so I have tested voltage previously with relay removed. I have 12v on L3, but I ground out the negative terminal of my volt meter and have tested n3/L2 seperate. I have .5v on n3, 0v on L2 with button on.

I will try a jumper for n3/L2 and see if any voltage is there and report back. 

I was thinking that I should have 12v coming from the defrost button (L2) and not the bcm (n3)...

Edited March 24 by 01 monte fan

[GnatGoSplat]

Connect your meter positive lead to the 12V on L3 and negative lead to the relay socket L2.  With button on, do you see +12V?

L2 should short to ground when the button is on.  On the diagram, the "Rear Defog Relay Control" is an electronic switch and that tapered 3-line symbol is ground.

[01 monte fan]

Yes I have 12v between L2 & L3 with button on. But I also have 12v with the button off....

There's one thing I forgot to say, but the temp control panel was swapped out at one point from another monte... I didn't have my window terminal fixed before the change (its fixed now)... don't know what yr it was but they were identical at least on hookups. Could it be possible that the panel is different from the original that would cause this?

Edited March 24 by 01 monte fan

[GnatGoSplat]

Interesting that it's 12V with the button on or off.  It may be a transistorized output with its "off" state floating, so we won't be able to see the voltage change without a small load across it.  With the relay out, can you use jumper wires to connect L2 and L3 to the relay's #85 and #86 pins (the coil) and see if it clicks on and off with the button on the HVAC panel?  It's possible the panel is different, but if it looks identical and has an identical connector, I doubt it would be different.

[01 monte fan]

Ya I can feel and hear the relay switching with the button on and off. 

[GnatGoSplat]

Ah, so the relay is good and the HVAC panel is good.  Looks like the BCM is bad or there's a bad connection between the ORN wire going from BCM to N3.  Now I would disconnect the BCM and check for continuity between B8 on connector C2 of the BCM and N3 pin on the socket of the relay.

 

[01 monte fan]

Sounds good,  I will check that this weekend and see what I get. So I just want to make sure I am understanding all this... but am I to understand that the 12v should be coming from the bcm to the relay and over to the switch to close the circuit? On the bcm terminal there's only .5v

[GnatGoSplat]

1 minute ago, 01 monte fan said:

Sounds good,  I will check that this weekend and see what I get. So I just want to make sure I am understanding all this... but am I to understand that the 12v should be coming from the bcm to the relay and over to the switch to close the circuit? On the bcm terminal there's only .5v

Yep, there should be +12V from the BCM on relay pin N3, I'm thinking when the ignition is on.  The panel then grounds L2 when rear defog is on, which should close the relay.  Looks like the +12V is missing.

The BCM supplies the positive to the relay coil while the panel supplies the negative.

The BCM also powers other things, according to the little note "Starting and Charging Schematics in Engine Electrical".  I wonder if any of those things are not working?

[01 monte fan]

Honestly I have no other issues with the car. Once in a while my oil psi gauge won’t read but if I turn the key off and back on it resumes functioning normal. The only other problem I have is with a/c pump not turning off and staying engaged continuously. That I figured is probably a pressure switch issue. I replaced the pressure switch but I am waiting till it gets warmer to charge the freon back up.

When I pull the bcm down I will check for voltage out of the orange wire and see if or where it's loosing power to the relay fuse block.

Are there any images of how it's routed and loomed from the bcm to the relay fuse block on the passenger side?

Edited March 25 by 01 monte fan

[GnatGoSplat]

Do your heated seats work?  Looks like heated seat relays are powered by the same wire from the BCM.

I couldn't find how the wire is routed, just a view of the BCM.

I did come across an interesting passage in the Rear Window Defogger Description text: "If battery voltage is low, the BCM will disable the rear window defogger by removing the battery positive voltage supplied to the REAR DEFOG relay coil."

How's your battery voltage?

[01 monte fan]

Don't have heated seats, but do have heated mirrors. Voltage is 12.3 at the battery. I am gunna try putting the battery charger on and see if I get voltage on the bcm pin. Also I will see if the mirrors get warm.

Edited March 26 by 01 monte fan

[GnatGoSplat]

I bet your mirrors won't get warm because they're controlled by the same relay as the rear window defogger.

What's the voltage with the car running?

[01 monte fan]

So no change in voltage at the bcm pin with charger going.  but I located the bcm and I guess when I had a security setup and then removed cause it sucked, I was able to pull it down.  I don't know if there are more than 1 orange wire on that connection but I did read I think 12v with poking it with the voltmeter if it was the right orange wire in the middle connector on the bcm. Is there a diagram of what pin that orange wire is in on the connector? I have 14.75v running. 

Edited March 26 by 01 monte fan

[GnatGoSplat]

Yep, it's the middle connector C2, pin B8.

If you have a multimeter, you can check and make sure there's continuity or low ohms between that pin and N3 of the relay socket.

[01 monte fan]

Ok, so I located the orange wire off the bcm and only got .49v with key on.

[GnatGoSplat]

Sounds like the BCM might be defective.

Not 100% sure if the HVAC panel works with BCM disconnected, but you could try unplugging connector C2 from the BCM and use a piece of wire to jumper B8 to A1, see if the rear defogger relay will switch on.  If it does, that will be 100% confirmation the BCM is defective.

 

[01 monte fan]

Ooooh! Great idea, I will try that and see.  Any suggestions on how to do this? I have very little play in moving that connection around to really be able to see and get a jumper wire stuck into each point...

[GnatGoSplat]

Are you able to unplug it at least?  If so, possibly an easier way is to take a small piece of wire and wrap it around the terminals of the relay corresponding to L3 and N3, then plug it in.  That will keep N3 powered at all times.  I suggest unplugging connector C2 of the BCM just in case backfeeding +12V into the Load Management pin is bad for it.

[01 monte fan]

Yes I can disconnect it from the bcm. That method I believe is totally doable. 

I don't think that there would be any issue with leaving the bcm plugged in... If it's to supply 12v then it shouldn't matter if it's producing it via another method... as long as we aren't putting voltage where it shouldn't be right?

I might have time tonight to try that, otherwise I will definitely do it tomorrow. If it does turn out that its the bcm, how best and cheapest method to get it re-flashed?

Thank you for helping me with this issue!! I really can't thank you enough for all of your knowledge and assistance here.

Edited March 27 by 01 monte fan

[01 monte fan]

Yes I can disconnect it from the bcm. That method I believe is totally doable. 

I don't think that there would be any issue with leaving the bcm plugged in... If it's to supply 12v then it shouldn't matter if it's producing it via another method... as long as we aren't putting voltage where it shouldn't be right?

I might have time tonight to try that, otherwise I will definitely do it tomorrow. If it does turn out that its the bcm, how best and cheapest method to get it flashed?

I have been thinking of investing into a new scan tool that can flash and read deeper into the cars systems. I also have a 2014 traverse and it would definitely be nice to be able to deep dive into any issues it might arise.

Thank you for helping me with this issue!! I really can't thank you enough for all of your knowledge and assistance here.

[01 monte fan]

Sooo..... I ran a jumper wire from N3 to L3, plugged the relay back in.  Pressed the defrost button and heard the relay click. Checked at the rear window for power and I had 12v at the glass.... 

[GnatGoSplat]

Yep, sounds like BCM is the culprit.

35 minutes ago, 01 monte fan said:

I don't think that there would be any issue with leaving the bcm plugged in... If it's to supply 12v then it shouldn't matter if it's producing it via another method... as long as we aren't putting voltage where it shouldn't be right?

Did you end up trying with the BCM plugged in?  I was hesitant to suggest that because there's a slim possibility backfeeding power into one of the BCM's outputs could damage it worse, and a completely dead BCM will be an even bigger problem.  If it were my car, I might be willing to take the gamble, but I have other cars I can drive.  There are no schematics of the inside of the BCM, but my guess is that output is transistorized (open-collector output) so backfeeding power wouldn't hurt it, but again I don't want to suggest something that could make things worse since it's not my car.  If you did try it, and everything else is still working, you could just leave that jumper wire on the relay and call it fixed, so long as the relay shuts off when you turn off the ignition.  I honestly can't think of too much of a benefit for the relay to be BCM-powered.

Flashing the BCM needs a Tech 2 tool.  Just a guess but it may need a connection to GM TechLine which requires a very costly subscription.  Easiest way might be to grab a BCM from a similarly equipped car which should already be programmed correctly.  I'm not sure if the BCM needs to be married to the car, if so, swapping in a junkyard BCM might not work.  Having to program in the VIN and Theft Deterrent Re-learn procedure point towards the BCM might need to be married to the car.

Even more advanced electronics work might be to crack the BCM open and diagnose it down to a faulty component.  Probably not easy to replace as it likely has SMD components.

 

[01 monte fan]

I did test it with the bcm still plugged in. I have been thinking of investing into a higher quality scan tool. I just haven't pulled the trigger due to the costs of some these tools.

I have been looking at TOPDON ArtiDiag Pro Bidirectional Scan Tool. Would you be able to tell me if this tool would be able to program the bcm? Everything I have read on these bcm's is that they are married to the car via vin and such. Also read that using a wrecking yard one usually won't work. 

I'm leaning towards just replacing the bcm since it's already starting to have issues.  I wouldn't want to be stranded somewhere if it completely dies on me.