Sir,
I pulled up the following page from the '94 Driveability, Emissions, & Electrical Diagnosis FSM.
This is a concise layout of the electrical control circuit for the TCC:
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When looking at this circuit, realize that there is *
always* 12v being applied to the TCC solenoid. The circuit is controlled
not by the application or removal of +12 volts. Instead, the control is done by the application or removal of the
circuit ground. (!)
It's important to note that it's actual
current flow that is going to create the electromagnetic field that will close the solenoid.
However, UNTIL the PCM
grounds the wire connected to the far side of the electromagnetic coil inside the TCC, there
will be no current flow = no electromagnetic field = TCC solenoid off.
So as a troubleshooter, you really want to know the state of the TAN/BLK wire. If it's floating high, then the
PCM isn't providing a ground, and is not commanding the TCC on.
Off the top of my head here's the failure scenarios that I think you have to choose from, in no specific order:
* Mechanically malfunctioning (new) TCC solenoid. (Good Torque Converter victim of sticky TCC solenoid.)
* Good TCC solenoid, malfunctioning Torque Converter.
* Chafe to ground affecting BLK/TAN wire for circuit #422 between TCC & PCM. (Causing continuous current
across TCC solenoid, leading to engine stall when vehicle put in anything but Park or Neutral.)
Note: Good TCC solenoid, PCM, and Brake Switch, constant TCC solenoid operation due to bad wiring harness.
* PCM is good, being misled by bad Brake Switch? (But doesn't explain not in 4th gear not preventing activation.)
* PCM itself is malfunctioning, getting good inputs but is incorrectly grounding TCC circuit when it shouldn't.
Throwing this out there, for me I would want to figure if this is a control failure vs a wiring harness failure vs a
TCC or torque converter failure? Let's employ the half-split method.
1) I'd want to carefully depin E10 on the PCM. Reinstall the connector with the BLK/TAN wire in free air & test.
Problem disappears? Now we have to figure out *why* the PCM is grounding this circuit? Is it the PCM failure,
or Brake Switch/wiring to/from PCM? (Brake switch is drawn elsewhere as 'normally closed', so continuity check back
to new brake switch would be the next step, followed by checking operation of the new switch.)
2)
Problem remains? Remember that BLK/TAN wire in free air? Check this wire's resistance to ground. If
there is 12v on it, that's expected electrical behavior. Again, this circuit is designed to have +12v at all times,
and doesn't operate until there is current flow. Our troubleshooting will eventually lead down the path towards
a malfunctioning TCC and/or torque converter.
On the other hand, IF you discover the disconnected BLK/TAN wire is at 0 volts, shut off the vehicle and
perform a resistance check between this wire and ground. If there is continuity, then this tells us that
there's a chafe, and this uncontrolled grounding of this wire = TCC engagement/engine stalling.
(ie: Good torque converter, good TCC solenoid, good VCM, good brake switch, all victims of a wiring harness chafe
providing an uncontrolled grounding/enabling of this circuit. (!)
****
I was going to wall paper this response with other diagrams from the FSM, but I decided that just reasoning
through this single big picture would give you a better overall troubleshooting approach. But you *will need*
the '94 Factory Service Manuals in order to look up the brake switch wiring, transmission control strategies, etc.
Therefore, if you don't already have a set of '94 FSMs in hand, then follow this link to where you can download
the documentation that you need to succeed: (
'88+ GMT400 FSMs)
****
To summarize, the circuit description above shows that a constant 12v is actually a feature, not a fault. By the
same token, IF we have a continuously grounded condition on the far side of the TCC solenoid, then indeed
we have a problem in the
control circuit to figure out. (wiring harness, PCM, brake switch, etc)
But if we discover that the ground is being applied *and* removed from the BLK/TAN wire properly, then
we have to take a closer look at the proper electromechanical operation of the TCC solenoid, and finally
proper mechanical operation of the torque converter itself.
By the way, I am hoping that you haven't disposed of all the original parts yet. It wouldn't be the first time
that the original problem was made even harder to diagnose after replacing one or more parts with new/bad
ones.
One last thing. Erik's suggestion to unplug the harness to the TCC solenoid is equally valid. Some mechanics
would prefer to do that instead of of depinning a wire at the PCM, while others would prefer to depin at the PCM
instead of dropping the pan. Either method will give us the same troubleshooting info, so whatever you are
more comfortable with.
My hope is that this description of how the control circuit operates will shed a little light on your situation.
I am looking forward to your next status report.
Best of luck with the hunt --
