1994 K1500 stalls only when brakes applied

[FM9694]

The problem is that the truck stalls "only" when the brake is applied. Not when put into gear.

If the TCC was the problem, the truck would stall any time the vehicle was in-gear, and the speed got low enough--brakes applied, or not applied.

As described so far, this is not a TCC problem. It is "probably" a vacuum-booster problem.

Only stalls when In gear and brakes applied. Doesn’t stall In park or neutral

 

[Schurkey]

Does it stall in gear with no service brake, (and therefore no vacuum booster involvement) when stopped or very low speed?

For example, find a SAFE PLACE, drive fast enough to engage the torque converter clutch, stop the car with the PARK BRAKE (carefully, probably with the park brake release handle pulled, so it doesn't lock-on and skid) and see if the engine stalls.

 

[Schurkey]

I originally thought it was when it was put in gear, but it’s only when brakes are applied that it stalls out.

Only stalls when In gear and brakes applied. Doesn’t stall In park or neutral

Is this new? A change in operation? Or just an update to the original post?

 

[Road Trip]

Thank you so much for this. I will update when the hunt is complete to let you know what it was exactly. I did pull code 69 the first day this happened, but read the wrong chart online and mistook it for an A/C related code and thought it had nothing to do with it. Could you please explain the difference between ECM and PCM for this truck. Are they terms that are used interchangeably? Or are they separate. Also where is it located is it the wires behind the glove box ?

Hello FM9694,

That's a real good question about the ECM & PCM terms. The way the terms are bandied about it can
get confusing. Based upon what I found in different year GMT400 FSMs, here's what I was able to piece
together:

1988 FSM: Only the term ECM is found. No PCM. And according to the book, E stands for either Electronic or Engine:

You must be registered for see images attach

1990 FSM: Still only ECM mentioned.

1991 FSM: PCM (Powertrain Control Module) added to manual. Looks like it was designed/used to handle
the new Electronically-controlled transmissions. (-E versions) Note: Manual trans vehicles continued to use
the ECM.

1994 FSM: ECM still referenced 35 times. PCM is now referenced 235 times. (!) And the physical location
illustrations show both ECM and PCM in seperate pics:

You must be registered for see images attach

(source: '94 FSM)

1996 FSM: A new term was added: VCM (Vehicle Control Module) was referenced 6,747 times in the
Volume 2 of the Service Manual. PCM was referenced 2,532 times, and seemed to be used only on
Diesels. And ECM was only referenced 48 times, and that always seemed to be the old fuse name
showing up in the prints? By the way, the new VCM was relocated to the engine bay.

****

So there you have it. I know that I've used the term ECM as a generic reference to the computer.
Sometimes when talking to non GMT400 owners they will refer to the computer as the 'ECU'.

Hopefully the above answered your question. It will be interesting to see which of the 2 illustrations
above that your computer will match. And somewhere in the '94 FSM there should be connector
pin-outs for both computers.

Cheers --

 

[JDGMC]

Some history on the truck would be helpful - why you replaced the components listed, the problem occurred immediately after the replacements, etc.

1. Conformation- it only stalls in R and D when you apply the brake with the engine at operation temp and stone cold.
2. Have you made any timing adjustments? Does the engine run smooth or rough? check the distributor advance wire connector on the firewall (make sure it’s connected and functional)
3. Have you started the truck in N and it did the same?
4. Does the truck move at all before it stalls?
5. Does it stall if you lightly press the brake for the position(s) stated in #1
6. What exactly is the stall speed and brand/PN of your TC?
7. Is this the stock trans for the truck (4L60E?)?
8. Can you keep the engine running with one foot on the brake while using the other foot to apply gas? (EBrake on for safety and nothing in front of the vehicle and wheels blocked).
9. Just to confirm - you have no lights illuminated on the dash, trans fluid is at level, and no modified tune on the PCM?

 

[FM9694]

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:

You must be registered for see images attach

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 solenoid,
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 --

have confirmed that it is an uncontrolled grounding of circuit as stated in #2. Chafe of wire occurring somewhere. Now this issue is finding where

 

[FM9694]

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:

You must be registered for see images attach

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 solenoid,
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 --

I am having difficulties tracing the path of the pink circuit 1020 that gives power from ignition switch to trans

 

[Road Trip]

Is this the stock trans for the truck (4L60E?)?

I am having difficulties tracing the path of the pink circuit 1020 that gives power from ignition switch to trans

Hello FM9694,

I'm in the '94 Electrical FSM, and I need to know the answer to @JDGMC's question in order to make progress.
Actually, I don't see any mention of engine displacement in this thread. (4.3 / 5.0 / 5.7 / 7.4?) Stock appearing, or possible engine
swap by a PO?

The FSM makes it look like there may be some wiring differences based upon engine option, and I don't want to infer/guess with my answer.

Help us help you. :0) Thanks in advance --

 

[FM9694]

Hello FM9694,

I'm in the '94 Electrical FSM, and I need to know the answer to @JDGMC's question in order to make progress.
Actually, I don't see any mention of engine displacement in this thread. (4.3 / 5.0 / 5.7 / 7.4?) Stock appearing, or possible engine
swap by a PO?

The FSM makes it look like there may be some wiring differences based upon engine option, and I don't want to infer/guess with my answer.

Help us help you. :0) Thanks in advance --

Update: narrowed down the short to ground to within transmission. New wiring harness on the way

 

[Road Trip]

Update: narrowed down the short to ground to within transmission. New wiring harness on the way

Outstanding! Sincerely appreciate your continued status updates. FWIW here's an interesting
illustration of the wiring harness to the internal transmission solenoids for those reading
this thread and trying to picture the physical implementation of this circuit path:

You must be registered for see images attach

Looking forward to you getting a good new harness installed and solving this
difficult stalling symptom.

Nice job staying with it. (!)