E38 ECM Brake Switch Wiring
Thread Starter
TECH Junkie
Joined: Dec 2005
Posts: 3,092
Likes: 11
Hmmmm, I can't find my schematic at the moment. However the resistor simulates the brake light bulb when the pedal is released. I would look at the car but it is in the shop getting the A/C charged for the first time and a 1/2" spacer under the front coil springs to raise the front 1". By the way I have no problems with my transmission shifting.
With the dual switch when the brake is applied there is twelve volts applied to the blue/white wires. When the brakes are off there is a ground through the resistor to ground simulating the bulb. From my GM schematics, there is a built in resistor in the ECM. Same with the TCM. So they can take a straight shot of 12V from an ignition on source. Fuse 44 on my schematic.
With the dual switch when the brake is applied there is twelve volts applied to the blue/white wires. When the brakes are off there is a ground through the resistor to ground simulating the bulb. From my GM schematics, there is a built in resistor in the ECM. Same with the TCM. So they can take a straight shot of 12V from an ignition on source. Fuse 44 on my schematic.
Last edited by 1989GTA; Mar 18, 2014 at 12:56 PM.
Staging Lane
Joined: Feb 2010
Posts: 59
Likes: 0
That helps a lot, thanks. If you don't mind, I have a couple of other questions I've been struggling to find answers to, and since you have the same setup I thought you would know. As far as the cooling fan and fuel pump wires coming from the E38, do they have enough voltage to trigger regular 12 volt relays? I don't have access to all of the wiring schematics and I want to wire it correct the first time.
Thread Starter
TECH Junkie
Joined: Dec 2005
Posts: 3,092
Likes: 11
That helps a lot, thanks. If you don't mind, I have a couple of other questions I've been struggling to find answers to, and since you have the same setup I thought you would know. As far as the cooling fan and fuel pump wires coming from the E38, do they have enough voltage to trigger regular 12 volt relays? I don't have access to all of the wiring schematics and I want to wire it correct the first time.
On my car I have A/C. I have provided a relay so that anytime the A/C is on the high speed fan also comes on. What this A/C relay does is provide a 2nd ground for the high speed fan relay. EMC X1-17 provides the other ground for normal circumstances.
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Verdad Gallardo Thread Starter
TECH Junkie
Joined: Dec 2005
Posts: 3,092
Likes: 11
Yes it is. In this case the power to the power train relay coil comes from B+ or the battery. Then the ECM through pin X1-59 provides the ground to the relay coil when it is ready and will turn it on.
Teching In
Joined: Mar 2014
Posts: 35
Likes: 0
From: Galveston, TX
GTA,
Is the Powertrain Control Relay even needed?
Everything I have read has people deleting this circuit and just tying all of the ignition wires into a key-on 12v+ source.
I'm just finishing up my harness as well...and I removed this pin (didn't cut anything, just unpinned it).
Thanks.
Is the Powertrain Control Relay even needed?
Everything I have read has people deleting this circuit and just tying all of the ignition wires into a key-on 12v+ source.
I'm just finishing up my harness as well...and I removed this pin (didn't cut anything, just unpinned it).
Thanks.
Thread Starter
TECH Junkie
Joined: Dec 2005
Posts: 3,092
Likes: 11
GTA,
Is the Powertrain Control Relay even needed?
Everything I have read has people deleting this circuit and just tying all of the ignition wires into a key-on 12v+ source.
I'm just finishing up my harness as well...and I removed this pin (didn't cut anything, just unpinned it).
Thanks.
Is the Powertrain Control Relay even needed?
Everything I have read has people deleting this circuit and just tying all of the ignition wires into a key-on 12v+ source.
I'm just finishing up my harness as well...and I removed this pin (didn't cut anything, just unpinned it).
Thanks.
If others have done it differently with no problems then I don't know.
Teching In
Joined: Mar 2014
Posts: 35
Likes: 0
From: Galveston, TX
I read somewhere that you might get a code or a problem with the ECM if that was done. Basically the ECM is looking for power from one source first than the 2nd source shortly there after. I just went along with the way the factory did it so there would be no problems.
If others have done it differently with no problems then I don't know.
If others have done it differently with no problems then I don't know.
Thread Starter
TECH Junkie
Joined: Dec 2005
Posts: 3,092
Likes: 11
That could be true, I just don't know. To be safe I just followed along as to what the factory did. Basically I just followed the GM schematics regarding the wiring as I modified the factory wiring harness. I figured I would have enough problems to sort out without adding any potential problems.
