Hi, I was not sure where to post this exactly, because it has to do with electronics and transmission…

The conditions are as follows: Ignition “on”, motor “off”

I have 12 volts at pin “f” of the aldl connector. To my understanding, if I press the brake, the circuit becomes open, and the voltage at “f” should go down to 0 volts.

But it stays at 12 volts. I disconnected the plug from the brake pedal switch and on both cables (pink/black and purple) the voltage is the same.

As a next step I removed CKT422 (tan/black cable) from the ecm. The voltage on “f” now went to 0 volts, and on the connector “A7” at the ecm I was having 12 volts.



As far as I can see it in the manual the pin “A7” goes to an electronic switch inside the ecm, which switches the tcc apply solenoid to ground and engages the TCC.

With 12 volt at “A7” coming out of the ecm, can I say my ecm is faulty???

Jens

 

Well first of all, are you doing your diagnostic checks according to the second page of that chart or are you just pulling connectors and checking voltage?

 

Trickster,

to be honest, I do both

At the moment I'm at the point "Brake Switch out of adjustment or faulty, or ckt 422 shorted to voltage"

The brake switch is not out of adjustment or faulty, CKT 422 is shorted to voltage coming out of the ecm..

 

Do you have a test light that you can use? If you do, it will make things a little easier to follow when doing those tests. I do believe that chart you have posted is one I sent to another board member, could be wrong though. Another thing to look at is the TCC solenoid in the tranny case.

 

On the diagnostics of this situation, what is the vehicle doing? Does it unlock at low speed? Does it just not unlock when you press the brake? I'm not sure in your case, although TCC solenoids do go bad, which is why I'm asking.

 

gta88 flamered,

Just for sh*ts & giggles and to test a theory. I went out to my car and tested it with a multimeter set to volts like you have described in your first post. With the key on & engine off, I put the positive probe in pin "F" of the ALDL and the negative probe to ground. I got 12.3 volts showing on the meter, I still had this while pressing the brake pedal. When using the test light as described in the chart, the light came on and went out when I pressed the brake pedal. Use the proper test tool when doing these tests or you will have nothing but headaches and an empty wallet.

 

trickster,

I've made the test again, this time with a test light. And by pressing the brake pedal the light went out!

THANKS FOR YOUR HELP!

The reason for my post was that I'm getting "3rd gear/OD switch ON" in Diacom all the time as soon the I switch the ignition on. If I push the brake pedal or not the message stays the same. I've double checked it with a snap-on scanner - same result...

Am I making another mistake???

Jens

 

That switch is always on until the transmission shifts into 4th.

 

Apeiron,

thanks for your help, too!

Jens (feeling a little bit stupid)

 

Jens,

No need to feel a little bit stupid. If you don't know, ask. That is what the board is here for, to help answer questions.

 

I know this topic is old, but I'll post for searching purposes.

The TCC solenoid is powered by the gage (ign 1) circuit. For power to get to the solenoid, the brake switch must be closed - which is when the brake is NOT depressed. Getting power to the solenoid is half the battle. The circuit now needs a ground path to operate, and that is handled entirely by the ECM. The ECM takes imputs from the VSS, 4th gear signal, coolant temperature sensor, and the TPS, and decides whether or not to engage the TCC solenoid. The only other "sensor imput" is the brake switch which is on the supply side of the circuit and has nothing to do with the ECM. When that switch opens (stops the current flow), there's nothing the ECM can do to activate the solenoid.

One thing to note is the 4th gear signal. This SIGNAL circuit is counter-intuitive at first because the flow of electricity is from the ECM to the transmission. This leads one to believe that the ECM is telling the transmission something, but in reality, it's the other way around. This is just like many other sensors (coolant temp, oxygen, oil pressure), where the ECM supplies a voltage down a single wire to the sensor. It may be 12 volts, but it can't really run anything because the current is clipped at a few hundred miliamps (mA). Notice the resistor inside the ECM at the C8 junction. Depending on the resistance of the sensor (they're mostly variable resistors going to ground), the current and voltage will vary which the ECM can measure on its way out to the sensor.