1989GTA

I thought I had a handle on this but now I am not so sure. I am looking at the E38 ECM pin X1-9 wiring to the brake switch. My diagram shows a NO switch wired to 12 volts that feeds the X1-9 pin. From what I have read with other ECMs it is a NC switch hooked to 12V. When the pedal is depressed the power goes away.

Who has done this and what do I need at X1-9 with the blue/white wire?

LS1MCSS

iTrader: (18)

What's the wire for? If it's for convertor lockup, then it will have to be a NC side of the brake switch.

71403

I've tried it n/c and n/o on mine.

Brake status on the scan tool never changes it's always unapplied. TCC engages but won't disengage if the brake pedal is tapped.

I've triple-checked wiring and everything is 100% correct as per '08 Express van wiring diagrams. Wondering if brake signal is coming over the CANbus from the BCM.

62n62

iTrader: (17)

71403

This diagram shows where circuit 6311, "TCC Brake Signal", splits out to the BCM also.

Pins C&D of the brake switch are circuit 7062 "Brake Switch Signal" BCM to circuit 812 "IP dimming voltage ref" on the BCM. 812 appears to be a 12 volt ref from the BCMs

Can anyone with a E38/auto swap and no BCM confirm tcc/brake operation both scan tool and driver's seat?

Jones'n

What you are missing isn't that it should have power when you press the pedal, but that it should get a ground when you are not pushing the pedal.
This ground is not seen on that schematic, but it is gotten through the bulb itself in the brake lights.
This is often a problem when people run LED brake lights.

62n62

iTrader: (17)

jonesn got it out first but that is one of the poblem i had if you have leds in the brakes lite put a 1157 bulb in thrunk and it will take care of it

1989GTA

I just got back from working on my L92 motor and have not been on the board yet and the reason for my late response.

That is what it looked like to me also and the reason for the post. So basically all I need to do is tie the light blue/white wire from X1-9 into the NO side of the brake light switch. Then when the brake pedal is applied there will be 12V going to X1-9 and of course the brake light bulbs. When the pedal is not applied there is a ground through the tail light bulbs. No need for the dual switch that I have purchased. Thanks guys. I think this will help a lot of people.

71403

The tail lights aren't powered from the brake switch, the BCM performs that function so using them to pull the circuit low isn't really the answer. But that gave me a shove in the right direction - real answer is circuit 6311 / TCC Brake Switch isn't held at a factory state when only using ECM and TCM. By the schematics

The ECM pulls 6311 high
The TCM pulls 6311 low
The BCM pulls 6311 low

The schematics are slightly wrong --- or my TCM is bad.

With the TCC switch open and only a ECM/TCM installed circuit 6311 on mine was at 9.2 volts KOEO. Connecting the BCM the voltage on 6311 dropped to 0 and the scan tool brake status showed "released". Closing the TCC switch, pulling 6311 high, brake status changes to "applied"

It appears the BCM keeps 6311 at a low (released) state not the TCM.

Instead of connecting 6311 to the brake lamps a 1k resistor pulling 6311 to ground will simulate the BCM and allow the circuit 6311 to work as factory intended.

With a late 80's GM dual output brake light switch (DR495) and a 1k resistor to ground on 6311 it's working as intended on mine.

1989GTA

No BCM in my application as you surmised. Blue/white wire runs from transmission(6L80E) to where the fuse block was and then continues on to X1-9(ECM38). I have added a blue/white wire to make a Y connection with the third wire going to somewhere on the brakeswitch.

So does the wire go to a brake switch common with the other side a NC circuit with a 1K resistor to ground? Then when the brake peddle is depressed the 1K resistor is no longer in the circuit?

1989GTA

No BCM in my application as you surmised. Blue/white wire runs from transmission(6L80E) to where the fuse block was and then continues on to X1-9(ECM38). I have added a blue/white wire to make a Y connection with the third wire going to somewhere on the brakeswitch.

So does the wire go to a brake switch common with the other side a NC circuit with a 1K resistor to ground? Then when the brake peddle is depressed the 1K resistor is no longer in the circuit?

Regarding the ECM supplying the voltage I see in the circuit at X1-9 an internal resistor going to ground, not a pull-up resistor going to 12V. I would think if the ECM is supplying the voltage it would be a pull-up resistor.

71403

Attached is the Alldata diagram for '08 Express van 4l80e, shows a pull up in the ECM on circuit 6311 / X1-9.

The 1k resistor would stay connected to the TCC line. So when the Brake/TCC switch is open it is pulled low. When Brake/TCC switch closes it pulls the line high.

1989GTA

Mine is like the one shown in the diagrammatic in post 4. I think I now know what to do. The blue/white wire will go to the brake light switch on the NO side that goes to the bulbs. I will branch(T) off of that with the same blue/white wire and go to the other side of the dual brake switch and the N/C side of the second switch set. The C "common" side of the second set of contacts will have a 1K resistor that goes to ground.

So when the brake pedal is depressed I will have 12V on the blue/white wire that also feeds the brake light bulbs. When the brake pedal is released the continuation of the blue/white wire will go through a 1K resistor to ground through the second set of contacts on the brake switch. So I do need the brake switch with two sets of contacts.

Thanks to everyone for this discussion as it has been enlightening. So to sum things up instead of relying on the brake light bulbs for the ground when the brake pedal is not depressed it is better to install a 1K resistor for the ground through a second set of contacts on the brake switch.

Hart_Rod

iTrader: (3)

My apologies for bringing up and old thread but I have several questions. 89GTA - how did this work out for you? Does this wiring setup appy to all years of A6's? What do you log on HPTuners in order to verify that the TCC is working correctly? I found some 1K resistors but they have different wattages, does it matter which one you use? Here's a diagram of how I planned to wire mine, let me know what you think.

1989GTA

Mine seems to be working just fine. From what I can tell that circuit should work.

Hart_Rod

iTrader: (3)

What wattage of 1K resistor did you use?

1989GTA

I think a 1/4 watt. There really is not much current flow to the ECM.

Hart_Rod

iTrader: (3)

LOL, figures. All my radio shack had was a 1K ohm, 1 watt. Do you think that will work? Also, were you able to log the activation of the TCC on HPTuners?

1989GTA

That should work. I have not logged the TCC on HPTuners yet. Car is still in the paint shop. Then on to the upholstery shop. So some time around the 1st of the year I should be able to have the car back.

rascalking

Sorry to bring this up...again. 1989GTA, I have pretty much the same setup as you. I am wrapping up my harness and have a few loose ends. One of them is the light blue/white wire from the trans. I get that x1-9 on the ECM needs 12 volts when the brake is applied. What about the same wire going to the trans? Is it spliced into the same side of the pedal that gets 12 volts when the pedal is applied? Thanks