Well, thank your cousin for me
- and thank you for passing it on, as it wasn’t really on my radar.It’s a potential issue I can cross off the list, and that is definitely a good thing!
96 GMC K2500 Suburban Restoration (with a couple of mods)
- Thread starter South VA
- Start date
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- and thank you for passing it on, as it wasn’t really on my radar.It’s a potential issue I can cross off the list, and that is definitely a good thing!
Drove 60 or so miles to the shop this morning to leave the Subdivision for the transmission guy to figure out what’s going on with the hard shifting. Being only Tuesday, it’s possible that he‘ll have it figured out and fixed this week.
Next week’s camping trip, of course, is still up in the air; whether we go or not will depend on what the transmission guy says. It’s tempting to just cancel it, but we have until Sunday to make a decision without an additional penalty, and so have decided to wait.
Oddly enough, on the way up there the car alarm chirped randomly, and seemed to do it more when I hit a bump. Pushing the alarm disable button didn’t affect it, but the door unlock button on the remote stopped it. I’m thinking there just might be yet another loose/frayed/chewed wire or two to hunt down.
Pack Rats: the gift that keeps on giving.
Next week’s camping trip, of course, is still up in the air; whether we go or not will depend on what the transmission guy says. It’s tempting to just cancel it, but we have until Sunday to make a decision without an additional penalty, and so have decided to wait.
Oddly enough, on the way up there the car alarm chirped randomly, and seemed to do it more when I hit a bump. Pushing the alarm disable button didn’t affect it, but the door unlock button on the remote stopped it. I’m thinking there just might be yet another loose/frayed/chewed wire or two to hunt down.
Pack Rats: the gift that keeps on giving.
Oddly enough, on the way up there the car alarm chirped randomly, and seemed to do it more when I hit a bump. Pushing the alarm disable button didn’t affect it, but the door unlock button on the remote stopped it. I’m thinking there just might be yet another loose/frayed/chewed wire or two to hunt down.
Pack Rats: the gift that keeps on giving.
Sorry to hear about the bump-stimulated alarm chirp. But really like the fact that you verified that of the 2 input controls,
1 didn't work, but the other did. (!) That's valuable troubleshooting info. And if the failure repeats & you can confirm this
then the collected talent in this forum will be able to take your 'test drive' observation, and using SDD (Symptom Directed Diagnosis)
they will turn this into specific areas to look in the electronics/wiring harness. It will take some sleuthing, but very fixable.
To me, it sounds like your project is still in debugging mode.
****
So I throw out that last sentence, and to me it means a specific situation requiring a matching mindset. And for anyone
I worked with for years would immediately know exactly what I meant without my having to elaborate.
But thinking about that whole Perception vs Reality thing, this is where *I* would fail by making the assumption that a 'normal'
person would still be on the same page that I was. And we all know what miscommunication leads to. Been there, done that.
Here's the thing. Thanks to a lifetime of marketing, we as a people have been conditioned to think of a purchase as a 3-step
process: Choose > Buy > Enjoy. Not just the folks reading this, but our significant others. It's pervasive. And for simple
stuff like a beach ball, or flatware, it's true. And with 100% quality control, more complicated items also follow this model.
And for the really complicated stuff? That's where 'the warranty' was added, to give the manufacturer the ability to give
the consumer a 'virtually perfect' product (eventually) in those cases where a customer ended up with an imperfect one off
of the assembly line.
****
But there are a bunch of people reading this who's career involves take something complex, install it, verify
the proper functionality, and then release it to the customer as ready for use. Sometimes it's that easy. Other times,
the 'verify for proper functionality' becomes test > fail > fix > retest > different fail > fix > retest > loop until done.
For this small subset of our society, they think of a purchase as a 4-step process. Choose > Buy > Debug > Enjoy.
I've done this with brand new large computer installations, and learned a lot. I also got to do this with previously-enjoyed
military fighters when our wing would transition from an older block F-16 to a newer one. I learned a lot more, for
despite the fact that all jets involved were supposed to be 'Code 1', there would be some deep-seated intermittents
included with the 'new to us' jets at no extra charge.
****
Pulling this all together, buying a new tow vehicle = Choose > Buy > Enjoy. Or, given the difference in the price of admission,
buy a used tow vehicle. Choose > Buy > Debug > Enjoy. Given the price of new vehicles, it would have to take *a lot* of
debugging for the value proposition to not work out. (Or, worse, you decide to buy new & skip the debug phase, but you end
up with something like those early 8-speed autos that the dealership couldn't ever fully sort out?)
Now where I always got discombobulated was when someone involved would substitute 'blah blah blah' for 'debug'. It's not their fault.
If you look at the sum total of what we are all bombarded with every day, there is no need for debugging. If it doesn't work right,
throw it away, and buy another.
But thanks to the sum total of your life experience, you choose to read the GMT400 forum, and you get all this. We just have
to educate those in our sphere of influence how stuff really works in the real world.
Especially when rodents are part of the equation. :-( But eventually, you will exit debug mode. And feel pretty good about outlasting
the problem(s). And you will have picked up a lot of new knowledge along the way.
Good luck with the ongoing debug of the transmission!
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Re: Debug Mode
When I first joined the computer industry back in '81 as a entry-level Field Service engineer,
there was a highly respected elder that shared a great story about the first formally documented
'debugging' session in computerland:
(credit: From really interesting article over in JSTOR)
By the way, her name was Rear Admiral Grace Hopper. What an amazing person well worth a googling. And especially if you are a girl-dad,
definitely someone to share as an example of what one can accomplish if they choose to make things happen instead of just accepting the victim
mentality.
****
And I've added this to my previous post to @South VA , because knowing that when it comes to failing machinery due to the
uncontrolled forces of nature, he is in very good company.
When I first joined the computer industry back in '81 as a entry-level Field Service engineer,
there was a highly respected elder that shared a great story about the first formally documented
'debugging' session in computerland:
You must be registered for see images attach
(credit: From really interesting article over in JSTOR)
By the way, her name was Rear Admiral Grace Hopper. What an amazing person well worth a googling. And especially if you are a girl-dad,
definitely someone to share as an example of what one can accomplish if they choose to make things happen instead of just accepting the victim
mentality.
****
And I've added this to my previous post to @South VA , because knowing that when it comes to failing machinery due to the
uncontrolled forces of nature, he is in very good company.
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This project is most definitely in debugging mode. With respect to the alarm, the transmission, and the ABS.
Speaking of warranties, I’m glad there’s one on this transmission, as it going to take expertise beyond anything I could ever acquire to sort it out.
****
But there are a bunch of people reading this who's career involves take something complex, install it, verify
the proper functionality, and then release it to the customer as ready for use. Sometimes it's that easy. Other times,
the 'verify for proper functionality' becomes test > fail > fix > retest > different fail > fix > retest > loop until done.
For this small subset of our society, they think of a purchase as a 4-step process. Choose > Buy > Debug > Enjoy.
I've done this with brand new large computer installations, and learned a lot. I also got to do this with previously-enjoyed
military fighters when our wing would transition from an older block F-16 to a newer one. I learned a lot more, for
despite the fact that all jets involved were supposed to be 'Code 1', there would be some deep-seated intermittents
included with the 'new to us' jets at no extra charge.
****
Pulling this all together, buying a new tow vehicle = Choose > Buy > Enjoy. Or, given the difference in the price of admission,
buy a used tow vehicle. Choose > Buy > Debug > Enjoy. Given the price of new vehicles, it would have to take *a lot* of
debugging for the value proposition to not work out. (Or, worse, you decide to buy new & skip the debug phase, but you end
up with something like those early 8-speed autos that the dealership couldn't ever sort out?)
Now where I always got discombobulated was when someone involved would substitute 'blah blah blah' for 'debug'. It's not their fault.
If you look at the sum total of what we are all bombarded with every day, there is no need for debugging. If it doesn't work right,
throw it away, and buy another.
But thanks to the sum total of your life experience, you choose to read the GMT400 forum, and you get all this. We just have
to educate those in our sphere of influence how stuff really works in the real world.
Especially when rodents are part of the equation. :-( But eventually, you will exit debug mode. And feel pretty good about outlasting
the problem(s). And you will have picked up a lot of new knowledge along the way.
Good luck with the ongoing debug of the transmission!
Thanks. And, as it turns out, the transmission guy just called, and said that the Subdivision is shifting normally for him.
Isn’t that some sort of cliche?
Ah well, of course I had to laugh out loud, and explained why. He understood.
He asked about it being stuck in 4WD, whether it was 4HI or 4LO. I told him the former. He then mentioned that it’s a good idea to put it into 4HI and 4LO once per month. I’ll add that to my regular maintenance list.
He said that it also showed an intermittent low voltage TPS code, which surprised me since I replaced it a week ago. So maybe that one is bad. Or not.
It also has ABS codes, which I knew, but he’s going to print those out for me so that I can compare them with what my scan tool reads. I kick myself for not running a scan last night, while I was at it replacing the IACV.
He also mentioned that if he recalled correctly, ’96 was the year that GM combined the ECU with the TCU into a single unit. Those early units apparently had internal shorting issues, and GM came up with a fix involving some jumper wires. He’d done a bunch of them, maybe 15 years ago, but those were all 1500s with 350 engines and 4L60E transmissions. He doesn’t recall seeing a 454/4L80E that was so affected. So he wasn’t sure if that was a thing for this particular vehicle or not. But he’s considering it.
Anyway, he wants to keep it overnight and drive it in the morning, when it’s cold, to see what happens with the shifting.
He is clearly in debug mode.
I expect more will be revealed.
Wait a minute, that might be an important clue. According to other threads/SMEs in here, the TPS output
plays a big role in how the transmission is shifting? In other words, the transmission (victim) is properly responding to
erroneous commands from the misled ECU/PCM (victim) ...due to the flagged flaky/bad low-voltage inputs from the TPS
circuit? (perpetrator)
Here I am going to step aside, and hope that some of the folks that have found/fixed this TPS<>trans shifting issue
firsthand will chime in.
Given the above, he sounds like the real deal. I'll bet a dollar that an SME like @tayto or @NickTransmissions can shed light
on the above. The good news is that the 4L80-E has proven itself over the years to be really trustworthy/something
you can work, and it will stand up to the usage. You aren't trying to fix something that could never hold up, long term,
with what you want it to do in the first place.
Of course, the motive is not to armchair quarterback your mechanic. Instead, the more knowledge you have to work with, the
better you will be able to communicate with your mechanic.
Here's to getting the debugging phase of this subsystem all wrapped up.
Cheers --
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A low (or otherwise anomalous) TPS voltage signal to the ECM will impact transmission performance/function as the ECM relies on TPS data, among other sensor data flows, to control the transmission. If the TPS was replaced with a non-OEM part, there's always higher risk of it being DOA. If the new part is working correctly, then there's likely a short somewhere between TPS and ECM (including the ECM). I'm not super-informed as to how TPS actually work but my understanding is that they will send a variable voltage signal to the ECM based on throttle position. If the ECM thinks it's not seeing enough voltage at any given point it will store a DTC (usually P0122). Late, harsh or otherwise erratic shifting patterns are most commonly reported and this is when the transmission was previously functioning normally with no previous drivability symptoms.
Hi Nick,
The Throttle Position Sensor is a potentiometer. (So is the volume knob on an old radio - more on this in a minute.)
Roughly speaking, the resistance varies linearly from one end of the travel to the other.
The way the potentiometer is implemented in the TPS function, is that a +5V reference is connected to one side,
and Ground is connected to the other. Meanwhile, a wiper arm (connected to the throttle shaft) rubs an electrical contact
across the length of this resistor while traveling from one end to the other.
When parked at the Idle position, the voltage derived from the wiper arm is sent through a third wire back to the
Black Box. (ECM/PCM) With the throttle(s) closed, the voltage sent up to the PCM is ~0.5 volt.
As the driver depresses the gas pedal, the arm connected to the throttle shaft travels up the resistor, and the voltage
reported to the black box steadily rises until we reach WOT. At this point, the voltage reported should be ~4.5 volts.
NOTE: If you look at post #3 in this thread (by @east302 ) you can see the wiring diagram for the TPS. It's only 3 wires.
(Gray for +5, Black for Ground, Dark Blue for the actual tattle-tail signal). By the way, these schematics are organized
with the positive voltage always on top, and the ground always on the bottom, so this would explain why it looks like at first
glance that the TPS is connected to 2 different boxes. Actually the TPS is only connected to the 'VCM'. (Black box)
****
For troubleshooting purposes, I'd like to show my age for a moment and refer back to the analog volume knob on
old radios. If you had a good volume knob, then the listener could turn the volume knob from very quiet to as
loud as the radio could sing & back, and all that would be heard was the varying amount of music.
But after many thousands of twists, the arm wiping that resistor would create wear. Or dirt would ingress &
mess up the path. And how would we know this? Because there would be all kinds of static superimposed upon
the music when the volume knob was moved. Or you would have a favorite (worn out) spot where the volume would
drop out suddenly right as it got to the sweet spot...and then return suddenly upon further cranking.
What does this have to do with TPS sensors? Because when you assume that measuring good Idle & WOT
voltages = a perfectly good TPS, it's not always the case. It's been documented where a car driven mostly
on the same highway, always using the cruise control, that after some large number of miles accumulate,
a worn spot develops in the travel that creates a duff input signal = intermittent engine stumbling/misbehavior.
So, a good mechanic will sweep the throttle shaft (key on, engine not running) from Idle to WOT while
monitoring the voltmeter, and look for a steadily increasing voltage, no dropouts, no erratic readings.
And if you have a scope, even better for catching any 'crackles' in the signal as it travels. Personally,
if I could feed that voltage into an amplifier and actually listen to it, I would want to hear exactly
nothing. (ie: Pure varying DC voltage.) But if I heard crackling just like on an old radio with a bad volume
potentiometer, then I wouldn't hesitate to swap out the part. And also test the new
replacement for the proper signal before returning the truck to service. (!)
****
So, the previous 5 paragraphs are written from the perspective of the Throttle Position Sensor
as being the perpetrator of the error codes being thrown. As a good troubleshooter, I need to
consider the alternate universe, where the TPS sensor is a victim of a flaky/intermittent +5V reference,
Ground, or the To/From wiring interconnecting the TPS to the engine ground and also the Black Box.
One last thing. The thread that I referenced was kind of a bust because the OP never replied back
with what the final outcome was. But given South VA's track record, we will turn this from
an informed guessing game to good, solid, empirical testing/results in clearing up the 'low voltage
TPS code'.
Apologies for the length, but just wanted to remove some of the mystery of exactly how the TPS
tattle-tails on the human by translating throttle opening angle into a varying DC voltage to the
ECU/PCM/VCM/Black Box/whatchamacallit. :0)
Cheers --
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Solid, man! Great explanation. Thank you for taking the time and providing the level of detail you did.
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Never feel bad about a long explanation, those of us who know can just skip to the next post but those young uns and less experienced are getting free education out of it. So keep up the great replies.
