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Weird tranny

matuva

matuva

Tropical 6.5er
Messages
1,996
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21
Location
New Caledonia. An island in south west pacific, cl
Hi all,

this post is a duplicate of the one I did on DP. The guys are answering me, and show some path I will explore.
That said, I miss some of you, so I post here too ;)

here's the story :

thursday, a friend with a 1995 K1500 took 3 hours to come back home after an 80 kms trip : his tranny was stuck on 1st gear... At the end of the trip, he was a bit "nervous" and rev the engine up to red zone and felt a chock : the tranny accepted to engage 2nd, but no more

Today, we changed his Trans, swaping from a NP 241C to a NP 243C. (Hope they are interchangeable, aren't they?)

Still stuck.

We hooked a laptop for DTCs reading and found these :

  • 24 vehicule speed sensor circuit low (Trans output speed signal)
    38 TCC brake switch stuck OFF
    72 vehicule speed sensor circuit loss (Trans output speed signal)
So, we changed the speed sensor on the T-case, I think these codes mean the sensor is kaputt.

Still stuck

Though, only following code remains :

Stuck tranny step1.jpg

38 TCC brake switch stuck OFF

So, we don't know what is this TCC brake switch. We don't know what TCC means (any?) but we saw the brake lights were not working anyways and so pulled out the brake stop switch, cleaned it out, made the stop lights working again and went for a test drive : Still stuck, code 38 disappeared but new codes show up! :

Stuck tranny step2.jpg


  • 18 pump cam reference pulse error
    36 injection pump width error (response time long)
    54 PCM fuel circuit error


With the software (GMTDscan), I was able to force the shifting. The tranny shifts that way, but stays set on the gear I forced e.g, if I force up to 3rd, it stays on 3rd, even if you stop.

We are a bit lost and discouraged. Any of you get an idea? Another ground story? or PCM giving up.
My thought at the moment is that we are facing an electrical gremlin rather than a mechanical one...
 
Looks like electrical problems.

Start with the basics - clean ALL grounds, unplug all connectors and spray them with contact cleaner.

Try all over again and tell us what codes come back.
 
matuva said:
Hi all,

this post is a duplicate of the one I did on DP. The guys are answering me, and show some path I will explore.
That said, I miss some of you, so I post here too ;)

here's the story :

thursday, a friend with a 1995 K1500 took 3 hours to come back home after an 80 kms trip : his tranny was stuck on 1st gear... At the end of the trip, he was a bit "nervous" and rev the engine up to red zone and felt a chock : the tranny accepted to engage 2nd, but no more

Today, we changed his Trans, swaping from a NP 241C to a NP 243C. (Hope they are interchangeable, aren't they?)

Still stuck.

We hooked a laptop for DTCs reading and found these :

  • 24 vehicule speed sensor circuit low (Trans output speed signal)
    38 TCC brake switch stuck OFF
    72 vehicule speed sensor circuit loss (Trans output speed signal)
So, we changed the speed sensor on the T-case, I think these codes mean the sensor is kaputt.

Still stuck

Though, only following code remains :

View attachment 1187

38 TCC brake switch stuck OFF

So, we don't know what is this TCC brake switch. We don't know what TCC means (any?) but we saw the brake lights were not working anyways and so pulled out the brake stop switch, cleaned it out, made the stop lights working again and went for a test drive : Still stuck, code 38 disappeared but new codes show up! :

View attachment 1188


  • 18 pump cam reference pulse error
    36 injection pump width error (response time long)
    54 PCM fuel circuit error


With the software (GMTDscan), I was able to force the shifting. The tranny shifts that way, but stays set on the gear I forced e.g, if I force up to 3rd, it stays on 3rd, even if you stop.

We are a bit lost and discouraged. Any of you get an idea? Another ground story? or PCM giving up.
My thought at the moment is that we are facing an electrical gremlin rather than a mechanical one...
Click to expand...

Record your codes for future reference, clean the grounds. Then reset your PCM : disconnect battery for 60 seconds and this will initiate a relearn process in the PCM. If this still doesn't clear it let us know, you may have a PCM that is starting to have an internal fault.

The current codes are all fuel related. Optical sensor and FSD reference signal codes PCM recognizes it has a fuel delivery problem (54). The 54 code has no diagnostics other than replace PCM if code 17 is not stored with it. I think you ARE CHASING CODES NOW. Stop, clear the codes, and do the relearn: and then take a fresh look at the problem.
 
Last edited:
matuva said:
Will try.
By the way, what is that TCC brake switch?
Click to expand...

The brake switch has a TCC switch that disengages the TCC when you hit the brake. This is in effect a coast feature for the trans or otherwise it would be like a manual trans the engine would start to brake the vehicle with engine braking on a one to one lockup.
 
There's something weird in the site :
I have no notification of your replies, though I asked for, and, when I posted my last reply, it was for answering Crisk1500, the only follow up I can see, and now I see the other posts.
The server is a bit lazy?
Slim shady, unfortunately for us, I'm thinking like you. I think PCM is giving up. At least there's a short somewhere wich makes it turning crazy.
Oh, sorry, I forgot to mention that the speedometer is dead too. It died at the same time the tranny get stuck. Does that means something to you?
We will try tomorrow to reset the PCM..
 
matuva said:
Slim shady, unfortunately for us, I'm thinking like you. I think PCM is giving up. At least there's a short somewhere wich makes it turning crazy.
Oh, sorry, I forgot to mention that the speedometer is dead too. It died at the same time the tranny get stuck. Does that means something to you?
We will try tomorrow to reset the PCM..
Click to expand...


Yes there may be an electrical problem from the speed sensor buffer to the PCM. I need to look at the wiring schematic. I can do that later tonight, I might not be able to post until later tomorrow
 
Thanks Slim shady ;)

One thing I'm wondering about.
The T-case we found as a spare is a NP243.
The stock one was a NP241.
I wasn't there when they replaced the T-case, so I didn't have the opportunity to compare them side by side, but I do believe they are interchangeable? Am I wrong?

One thing I'm wondering about too is if we don't have to make adjustments on electric wiring to make things working. I haven't got a precise enough wiring diagram, but it seems that the wiring to speed sensor are different from a 1995 and a 1996 truck.
Anybody can tell me more about that?
 
Last edited:
Quick update.
yesterday night, we found a spare PCM.
We connected it to the truck. She started at first crank and iddle as a charm.
Previously, we had to cranck a loooong time before she starts.
We heard a noise inside the old PCM and opened it. We were almost surprised to find a burst capacitor unsoldered inside.
Looking carefully on the PCB, we found the place where it was supposed to be and realized that it has been unsoldered by a short!!! Strong one I believe as it has produced enough heat to burst the capacitor un unsolder it.
I don't know what did happen to that truck, but sure there might be troubles in the harnesses now. the game is to find where...

It was night time and the truck was on a hoist, we didn't have time to do a road test. We'll check that today...
 
Glad to hear you found a place to start. To get a big enough charge to burst the capacitor and remove it from the board would take quite a bit of current.

There may have been other circuits on the board that were damaged as well. Electromagnetic pulses do weird things to electronic components.

Let me know what happens, I have a wiring diagram for the 1995 trucks.
 
We fixed it!
After changing PCM, speedometer, visual inspection of harness failed, we hooked up again the laptop and checked again for codes.
No codes except one relative to EGR (the vacuum pump is dead).
Then I checked the input and output speed sensor signals.
At iddle, input was 650 rpms, and output 0!
We changed the input sensor, still 0 at output.
we check again the harness and finally found that the wire was broken, inside! Nothing can be seen just by visual.
Cut, splice, solder and voila! Back on the road. The young owner (a young dad) says that's a perfect gift for father's day ;)
 
'way to stick with it, Matuva... nice determination!

re: the burst capacitor... we've had motherboard issues in about 300 computers, all with burst capacitors. It was a bad batch of capacitors, from a replica manufacturer, that ASUS bought.

They swell, and POP! No predicting when they are gonna go, and there isn't any warranty util they do, so one at a time, we swap 'em out when they crap out.

Could be similar thing... doesn't necessarily have to be a short or ton of current.
 
Holly cow!!! I'm pretty sure now this truck is cursed!

I just received a phone call today from the young owner. He is a bit discouraged. Yesterday evening the truck was working perfect and strong.
This morning, it stalls. Took 2 hours rest for the truck to start again, with low power.
He had to bleed fuel filter as it looks like it was starving fuel.
When he 1st open the bleed valve on the top of fuel filter, he heard a kind of vacuum, and then with a long cranking, the fuel finally flood out with a lot of air bubbles, and then she starts...

We had the same trouble few times ago and thought we solved the problem after cleaning the tank and inside sock-filter.
The only thing I can suspect for now is the O-ring under the fuel filter, where the fuel heater is plugged.
If it is not a perfect match, could it let some air go inside and stall? But why vacuum inside the fuel circuit.
I told him to find a piece of hose to replace the part from tank to feed pump.
Any suggestion?
 
This is a 95?

For a temporary fix, run a diode from the 'hot' side of the GP fuse to the LP relay in the under hood fuse box.

This will allow power to run to the LP during the glow cycle, but will stop the power from backfeeding into the GP circuit once the truck is running.
 
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