First problem with the crew cab

[User_name]

2000 GMC c3500 cclb dual rear wheels. 107,000 miles. 4l80e/7.4L.

When I start the truck and for about the first 10 minutes of driving, the truck shifts smooth as butter, through all four forward speeds.

Then out of nowhere, the truck will kick its self out of overdrive and shift hard, staying in third and not going back into overdrive until I cycle the key.

When I cycle the key, it’ll do the same thing, hot or cold. Shifts like butter then locks out overdrive and I assume it also commands max line pressure which is why my shifts get hard.

-trans is full and clean
-trans does not seem to be overheating
-engine operating temp hangs out around 190-200*f
-shift linkage seems good
-no codes, pending or stored

I’m thinking this has to be a tcc solenoid issue, wiring issue, or the brake light switch. Truck was very well taken care of until the guy bought a diesel and it has sat since 2018.
I just want some input to point me in the right direction before I jump on it this Friday.

Thanks to all who respond.

 

[NickTransmissions]

2000 GMC c3500 cclb dual rear wheels. 107,000 miles. 4l80e/7.4L.

When I start the truck and for about the first 10 minutes of driving, the truck shifts smooth as butter, through all four forward speeds.

Then out of nowhere, the truck will kick its self out of overdrive and shift hard, staying in third and not going back into overdrive until I cycle the key.

When I cycle the key, it’ll do the same thing, hot or cold. Shifts like butter then locks out overdrive and I assume it also commands max line pressure which is why my shifts get hard.

-trans is full and clean
-trans does not seem to be overheating
-engine operating temp hangs out around 190-200*f
-shift linkage seems good
-no codes, pending or stored

I’m thinking this has to be a tcc solenoid issue, wiring issue, or the brake light switch. Truck was very well taken care of until the guy bought a diesel and it has sat since 2018.
I just want some input to point me in the right direction before I jump on it this Friday.

Thanks to all who respond.

Scan again for codes (sometimes a few drive cycles with the same issue are required for a DTC to set) then take for a ride with your scan tool - display live TCC and trans data streams to se what's being commanded...Post a picture of the trans data if you can (may be best to have a helper monitoring the scanner while you drive). This could be an ISS problem but could also be many other things as well.

ETA: check/monitor MAF, TPS, Ignition Switch, Brake Light switch as well as battery health, battery terminals and main cables...These all can cause odd drivability symtoms such as the TCC unlocking when it shouldnt...

The main thing you want to determine is if the Converter clutch is being commanded off for some reason or if it's dropping out of lock up despite being commanded to apply.

 

[Orpedcrow]

I’ve had some weird transmission stuff happen on vehicles that have sat. The fluid oxidizes and you get some surface rust on some of the hard parts.

 

[User_name]

Ya know, I only have one good battery in the truck with dual batteries…. I swear if the fix ends up being a $70 battery I’m gonna laugh my butt off. That’s the first thing I’m gonna do!

Scan again for codes (sometimes a few drive cycles with the same issue are required for a DTC to set) then take for a ride with your scan tool - display live TCC and trans data streams to se what's being commanded...Post a picture of the trans data if you can (may be best to have a helper monitoring the scanner while you drive). This could be an ISS problem but could also be many other things as well.

ETA: check/monitor MAF, TPS, Ignition Switch, Brake Light switch as well as battery health, battery terminals and main cables...These all can cause odd drivability symtoms such as the TCC unlocking when it shouldnt...

The main thing you want to determine is if the Converter clutch is being commanded off for some reason or if it's dropping out of lock up despite being commanded to apply.

 

[HotWheelsBurban]

Ya know, I only have one good battery in the truck with dual batteries…. I swear if the fix ends up being a $70 battery I’m gonna laugh my butt off. That’s the first thing I’m gonna do!

Be nice if it was that simple....

 

[Supercharged111]

MAF failure can drive a full line pressure condition, got any codes?

 

[User_name]

Be nice if it was that simple....

Indeed it would be!

MAF failure can drive a full line pressure condition, got any codes?

It reads 1.58 at idle. I forget what the unit of measurement is. I think that’s kinda low isn’t it?

 

[NickTransmissions]

Indeed it would be!

It reads 1.58 at idle. I forget what the unit of measurement is. I think that’s kinda low isn’t it?

If the MAF is underreporting relative to reality, line pressure will be lower than it should; slipping is the usual symptom...If it's overreporting, pcm will increase line prrssure when it shouldnt...Im not sure what the reference values should be but perhaps someone else can provide or maybe check the FSM, Alldata or another resource...

 

[Supercharged111]

Indeed it would be!

It reads 1.58 at idle. I forget what the unit of measurement is. I think that’s kinda low isn’t it?

Ought to be 6ish if it's the units I'm used to. But it won't kick you out of OD or even unlock the TCC on second thought.

 

[1998_K1500_Sub]

2000 GMC c3500 cclb dual rear wheels. 107,000 miles. 4l80e/7.4L.

When I start the truck and for about the first 10 minutes of driving, the truck shifts smooth as butter, through all four forward speeds.

Then out of nowhere, the truck will kick its self out of overdrive and shift hard, staying in third and not going back into overdrive until I cycle the key.

When I cycle the key, it’ll do the same thing, hot or cold. Shifts like butter then locks out overdrive and I assume it also commands max line pressure which is why my shifts get hard.

-trans is full and clean
-trans does not seem to be overheating
-engine operating temp hangs out around 190-200*f
-shift linkage seems good
-no codes, pending or stored

I’m thinking this has to be a tcc solenoid issue, wiring issue, or the brake light switch. Truck was very well taken care of until the guy bought a diesel and it has sat since 2018.
I just want some input to point me in the right direction before I jump on it this Friday.

Thanks to all who respond.

I’ve seen symptoms like this on a ~2001 Blazer that were traced to a bad TPS.