[johnpimentel72]

hi just bought a 2k 4x4 1500 silverado.
first issue was the service 4wd. searched the internet: cleaned the 4x4 switch. later found out that the atc fuse was blown. replaced it and still blows. serched the internet again: checked the wiring harness (transmission digest june 2004) and cleaned all grounds and sensor contacts and the fuse is now ok.
funny thing is now when i drive it sometimes the rpm suddenly goes down and sometimes there's this loud clunk that you hear and feel when the tranny shifts gears on 2wd (haven't tried it on 4hi) and when i remove the fuse the power is a little bit ok but not at par with my friends truck of the same model. cleaned the filter and changed the transmission oil, changed the air cleaner, differentiial fluid, engine oil and filter, replaced the fuel fiter, brake pads and cleaned the throttle body but its still the same. gm cant find any codes.
hope anyone could help...
thanks in advance

 

[johnpimentel72]

update: mine only has 33k kms on it. the previous owner havn't driven the vehicle for around a year before i bought it he just starts it a few minutes every morning.

 

[johnpimentel72]

checked with gm using tech2 scanner.everythings fine except that we noticed that when the transmission seems to 'surge' OBD detects that it shifts directly from 1st to 3rd or fourth and when theress low power OBD indicates that the tranny doesnt downshift and maintains at 4th gear even when you slow down.
could it be the solenoids?

 

[MrShorty]

Yes, the solenoids are one possibility. I'm sure there are other possibilities as well.

I thought the tech2 had the ability to manually command those solenoids? If so, did you try to see if you could manually command a downshift when it wouldn't downshift?

I'm no expert on electronic transmission diagnosis, but whenever I see a problem like this, the first thing I would look at is this: Is the computer not commanding the shift, or is the transmission on responding to the PCM's commands. I would hope that your technician would have been able to maybe identify which scenario while he had it hooked up to the scanner. Did he give you any indication as to the nature of the problem?

 

[johnpimentel72]

thanks for the reply. we haven't tried using the tech2 to command shifts. the technician even didn't suggested it. im really not sure if its the pcm or anything. we cleaned the solenoids and valves though and found that is was dirty and one of the o-ring/gasket was worn out and replaced it. now even if i floor the pedal the speed is slow. seems like the shifting got even worse. are we on the right track? i wouldn't want to follow gm's advise to replace this and that. seems that they're just guessing and it would be very expensive for me replacing all the sensors (speed, brake etc.) they even suggested replacing the transmission.
one possibility could be the module behind the driver's side aircon vent which is encased in metal. tccm or tcscm whatever they call it. they said its related to my atc fuse which kept on blowing before. but after we fixed the atc fuse problem the shifting is still the same. is the atc related to the transmission?

 

[MrShorty]

It is kind of frustrating when a technician starts in on the guess and check method. Did you have a reason for choosing this technician or were you randomly assigned? It might be time to consider a different technician.

Clarify terms: TCCM=transfer case control module? ATC=automatic transfer case? Assuming this is what you meant, yes, there is a connection between the transfer case and the PCM. I'm not certain of all of it, but the PCM at least needs to know if the t-case is in high or low range so it can adjust the shift points for the 2.7 gear reduction of low range. The Haynes wiring diagram for my '98 shows both a 4wd input and 4low input. If the PCM erroneously thinks it's in low range, then I would expect it to wait longer to downshift. Again, though, I would expect your guy with the tech2 to be able to see that fairly quickly on the scanner. Or, with the proper knowledge of what those two signals are supposed to look like, one should be able tos see them with a DVOM.

Can you manually downshift with the lever?

 

[johnpimentel72]

thanks MrShorty checked it before, the 4x4 and 4lo doesn't engage when the problem shows up it stays on 2hi. we only found the transmission shifts to be erroneuos. ah so it could be the tccm. could it be damaged after only 33k kms?
I'm gonna transfer shop and its a good thing that my friends silverado is here and he's not using it for a couple of days. were planning to swap parts just to see what could be the problem. any inputs on where to start and what to do?

 

[johnpimentel72]

yes i can manually downshift and upshift with the lever

 

[MrShorty]

I wouldn't expect the TCCM to be bad, but I also won't say that it is impossible.

Will you have access to a scanner? If so, I think the first thing I would do would be to drive yours with the scanner hooked up, and do a "freeze frame" when the problem occurs. Then hook it up to the other truck and drive it, and see if you can see anything different.

If you don't have access to a scanner, I might backprobe the shift solenoid circuits (maybe the shift solenoid circuit for the 4-3 downshift since that seems to be a fairly consistent failure) and see what his truck is doing different from yours.

As far as parts swapping, I don't know. Would it be too much work to swap transmissions and see if the problem follows your transmission? I expect so.

Is your buddies truck exactly the same (year, engine, transmission, etc.)? You could try swapping PCM's if he'll trust you to do that and see if the problem follows the PCM.

I wish I knew what the lever did inside the valve body to effect the downshifts: is it electronic or not? If you can manually downshift the transmission, then what is happening differently than when the PCM should downshift?

 

[johnpimentel72]

i had the tccm checked with three different technicians and they found no problems.
our trucks are exactly the same. i'll try to see if i can hook it up again with tech2. if not i'll do your second suggestion and also ask him if we could swap pcms.
thanks

 

[johnpimentel72]

hi, where can i find the solenoid for 4-3? pulled out the solenoids but dont know which is which. i think we'll pull out the whole valve body and clean it also just to make sure

 

[MrShorty]

I'm sorry, but I haven't ever needed to know that much detail about the 4L60E, yet, so I don't know which solenoid is responsible for the 4-3 downshift. Does anybody know? Do you have access to any manuals (maybe at the library)?

 

[johnpimentel72]

my friend has a manual. i'll try to check. we'll be polling out the valve body and clean it too just to make sure.

 

[johnpimentel72]

cleaned the valve body. found that its still dirty even after cleaning the filter. the shifting improved a lot but there's still some shifting lags. i will try to use it for a while maybe it will go back to normal