Any Chevy transmission guys here?

[Scootertrash]

Truck is a 99 Silverado 2500 6.0. I believe the transmission is the 4L80E

Just recently (like the other day) it has started giving me trouble with shifting.:
When I take off from a stop it wants to shift into 2nd right away, at like 10-15mph.
If I get a little heavier on the pedal it won't do it most of the time
When it shifts like that and I step on the gas to get it to downshift it will kick down and shift OK as long as I don't let off the gas too much. If I give it a medium pedal it seems to want to search for the right gear.

There is no slipping just shifts a little hard when it does this

If I shift manually it shifts fine no slipping or searching. ETA: No codes on my low buck code reader


Thoughts?

[DasUberKraut]

Fluid flush and filter change.

[RIDE-RED 250r]

I had an 01 Silverado that would not rev out and shift aggressively no patter how much I mashed on the throttle. It would shift right at 1500 or so every gear no matter what. Had a relative who owns/operates an auto repair shop plug in and it gave him a throttle position sensor code. Chaged the sensor and problem still persisted. Ended up being a shorted wire in the power supply circuit for the TPS.

Not exactly what you are experiencing, but if the fluid and filter change doesn't correct the problem, you may have a TPS/TPS circuit issue. That transmission is totally elecetronically controlled. If anything is electrically wrong with the components the system reads to tell the transmission what you want it to do, it won't shift right.

[G-Ram]

http://www.454ss.com/Articles_new/red/4L80E_Transmission_Problems.asp

hope this helps you out

[Mosh]

If it is a 4WD, you need to know if your TCCM (T case module) isn't sending a false request or input for 4WD lo. Generally it will upshift fast in all gears.
If it is a 4wd model with those symptoms, you could try pulling fuses for the 4wd system and see if it stops..If it does you will need a scanner that can communicate with the TCCM and find the issue. The only bad thing about pulling fuses is you erase any codes that may be in the TCCM that may help with diag...

And yes, any number of speed senors, TPS sensor, MAF sensor or MAP sensor can be causing the issue. You really need a full function scanner to diagnose these newer trucks. The sensor can be working, and not throwing a code, but skewed. Just because there are no codes does'nt mean a thing. The tow haul switch can be shorted causing the PCM to boost the shift points and line PSI..(That wire in the shifter for towing).. Then un shorted..That freaks the PCM out...

Like I said a scanner will make this alot easier and save money. This one of those case where you find a good tech that will check it out for a fair price before you waist money.

Simply changing the fluid and filter is an Old Timers gimmic from the 60's and 70's. That is only done for maintennace

[just ben]

I had the same issues with a 98 blazer,I believe it's the same tranny. while researching the issue I found it was a common issue,apparently there is an issue with the first to second gear valve that was corrected in 2000. For whatever reason that valve wears the valvebody cuasing it to not work properly. The fix was a replacement valve body from 2000 and newer. when I did mine I found a million replacements on ebay I think it was around 125.00 maybe it was a 4l60e in the blazer, I just remember a 4LE lol.

[G-Ram]

^ yep, the Blazer would be a 4L60e

[fabiodriven]

Fluid flush and filter change.

Believe it or not, that's actually a good way to blow the transmission depending on how regularly it's been changed in the past.

[Scootertrash]

If it is a 4WD, you need to know if your TCCM (T case module) isn't sending a false request or input for 4WD lo. Generally it will upshift fast in all gears.
If it is a 4wd model with those symptoms, you could try pulling fuses for the 4wd system and see if it stops..If it does you will need a scanner that can communicate with the TCCM and find the issue. The only bad thing about pulling fuses is you erase any codes that may be in the TCCM that may help with diag...

And yes, any number of speed senors, TPS sensor, MAF sensor or MAP sensor can be causing the issue. You really need a full function scanner to diagnose these newer trucks. The sensor can be working, and not throwing a code, but skewed. Just because there are no codes does'nt mean a thing. The tow haul switch can be shorted causing the PCM to boost the shift points and line PSI..(That wire in the shifter for towing).. Then un shorted..That freaks the PCM out...

Like I said a scanner will make this alot easier and save money. This one of those case where you find a good tech that will check it out for a fair price before you waist money.

Simply changing the fluid and filter is an Old Timers gimmic from the 60's and 70's. That is only done for maintennace

Thanks Mosh

It seems to be only first to second that shifts too soon like that, the rest of the time it shifts OK.

I threw the no codes thing in there in case anybody would ask, I know you can have an issue without throwing a code.

After my research I've found that it could be a mulititude of things like you suggested, Throttle position sensor, 1-2 shift solenoid. I do have a mechanic I trust specifically for issues I have no knowledge in, I'm fine with motor work, but auto transmissions are out of my league. Maybe I'll take it to him for a diagnostic and a flush and filter, then I'll be good all the way around.

Thanks again.

No offense to the other guys, but yea, I don't accept the fluid and filter as a fix for an issue like this. Although I'll take the fluid and filter as a maintenance item during the repair.

[tri again]

Believe it or not, that's actually a good way to blow the transmission depending on how regularly it's been changed in the past.

I was warned by a transmission guy that sold me a great trike (so I trust him)
to never allow the backflush method.

I guess it's when they just hook up to your lines and flush fluid backwards through the trans
because it's an easy way to make 100 bucks and get a customer for life.

[Scootertrash]

Thanks for the help guys.

I took another drive yesterday and noticed that when I throttled down to get it to downshift it wouldn't respond to my mashing of the pedal all the time. Today I swapped out the throttle position sensor from our 99 Tahoe in to the Silverado and viola! No shifting problems. 35.00 part and I'm good to go.

[timex69]

Changing the filter and fluid when having trans trouble is a bad idea especially if slipping is a symptom. All the "grit" thats in the old fluid is what is making a worn out transmission not slip. Then when it gets drained and new fluid gets put in its 10 times worse then before. Changing the fluid is a preventative maintainence thing, not a fix. Glad to hear a tps sensor fixed it scootertrash.