Rear Fog, wiring using existing factory harness

spearfish25

Alright gentlemen,

This post could go in a number of places but we need a bit of high profile traffic to get this figured out. Our car has all the hardware to get the rear fog working except for the control yolks with the switch. Essentially, the rear fog is wired all the way up to the BCM which takes input from the Combination Switch. The BCM is located in the passenger footwell (outer side). The Combination Switch location is a mystery to me, but may be incorporated into the BCM.

Here is what I'd like to do but need help. The combination switch reads combinations of inputs and outputs which alters a signal sent to the BCM. Based on the signal changes, the BCM activates/deactivates various systems. To get the rear fog turned on, we have to close the circuit between Output 3 and Input 5. This corresponds to terminals 5 and 13 on the M33 connector.

Can someone with some know-how confirm this and comment on whether splicing a switch between the terminal 5 and 13 wires would give rear fog light control? If we make this work, we can move the thread to the DIY to give people a wiring option that uses the factory wiring and avoids wire runs through the back of the car.

Attached Images

	Rear Fog IO combination.jpg (188.2 KB, 24 views)
	Combo Switch Schematic.jpg (207.6 KB, 21 views)
	M33 Combination Switch Connector.jpg (75.6 KB, 10 views)

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2013 Cadillac V-Wagon, RIP Z

spearfish25

Ok, how about this...

My wife's 2008 Altima has the exact style yolks that we need for the rear fog to work. I'm very tempted to take hers out and see if I can install it on my Z.

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2013 Cadillac V-Wagon, RIP Z

kenchan

before going into details, just wanted to let you know my car (09 base sports package) did NOT have any wires going to the rear bumper for the fog lamp... so best you check that first.

seems like some cars have it pre-wired, some dont. i just made my own harness and used a rocker switch.

spearfish25

When I swapped the true light in place of the butthole, there is a wiring harness right there which I plugged in. Looks like my car has the wiring there and ready.

__________________
2013 Cadillac V-Wagon, RIP Z

wilsonp

I think you have the wrong terminal highlighted in your connector diagram - you need terminal 5, but have highlighted terminal 3.

I believe you are correct, if you connected output 3 to input 5, the BCM would read the fog light switch as on and turn on the rear fog light.

The circuit shows diodes for the switches (except one) in each column to prevent cross-talk, and you would need a diode for the fog switch. Without a diode, when you had the fog switch on and another function on input 5 enabled, the BCM could read the wrong switch settings and activate a third device.

Of course, this assumes the firmware isn't different for cars without factory fog lamps and the BCM code still has the routine for enabling the fog lamp. Update: Since the BCM configuration indicates WITH is set for both FR and RR Fog Lamps, I think it is probably included.

Interesting design - it uses 5 output and 5 input pins to read a 25 switch matrix, but only needing one switch per function. This is the way old calculators read their keyboards (and new, cheap ones today), by scanning for closed switches. I would have expected a binary encoder to minimize BCM lines, but that would have required more wire lines to the combination switch and I guess software is cheaper than hardware in cars, like the old days.

Funny that it says reads 20 switches but actually has 21.

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2014 Cayman S PDK Sport Chrono White/Black
Was 2009 370Z Touring Sport+Nav 7AT Metallic Black/Persimmon
Was 1991 NSX Auto Black/Black

spearfish25

Nice catch on the diagram...I've updated it now.

So I need to look into diode switches. I'm also going to pursue the Altima yolks this weekend. If they measure up the same and I can remove them, I'm going to try the swap.

__________________
2013 Cadillac V-Wagon, RIP Z

wilsonp

Hmm looks like grounding Pin 67 on the BCM would add an inside Trunk opening switch!

Yolks threw me (airplane pilot?), assume you mean combination switch?

__________________
2014 Cayman S PDK Sport Chrono White/Black
Was 2009 370Z Touring Sport+Nav 7AT Metallic Black/Persimmon
Was 1991 NSX Auto Black/Black

spearfish25

Ah, I've sparked your interest in the BCM! Alright, now we all desperately need you to show us how to implement some of this stuff.

Yolks, stalks, combination switches...you know what I mean

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2013 Cadillac V-Wagon, RIP Z

spearfish25

Well, I got some quotes for the Canadian combination switch (turn signal and wiper stalks) that include the rear fog light switch. $400!

Doesn't make much sense as CourtesyParts lists the combination switch for the US version at $36. Typo?

__________________
2013 Cadillac V-Wagon, RIP Z

kenchan

wow... $400 just to turn on the fog. i still think it's more convenient to run new wires and get a rocker switch like the 3 i have in the center console.

wilsonp

I would hope the most convenient is to use the existing wiring and run a new switch to the appropriate terminals on the BCM.

This will get you any additional protection features the BCM offers with the stock light.

__________________
2014 Cayman S PDK Sport Chrono White/Black
Was 2009 370Z Touring Sport+Nav 7AT Metallic Black/Persimmon
Was 1991 NSX Auto Black/Black

elijah

This is quite an interesting topic you have there. Well, I have to admit that I'm not really quite familiar with the 370z but my brother would be delighted if he read this because he owns one. As you know, my brother is currently experimenting things on his car and I'm quite sure he would be interested to try this out one too. Anyway, I'm wondering if anyone have successfully done it on the rear fog. I'll appreciate if anyone could share.