A little backstory details before getting to the symptoms I've been experiencing.. The truck is a 2000 RCSB GMC Sierra(listed in the signature). It has a 414 cu in with a Circle D 3200 stall and a 4L80e that I built about 8 years ago. I've put about 40k hard miles on this setup. The transmission has never skipped at beat up until recently...

About a month ago after a cold startup, I shifted the transmission into Drive, and noticed the dash indicator had about 2 second delay. The truck felt like it went into gear and still shifted great. So I didn't think anything of it. A few weeks go by and the delay on the dash started to get a little longer. No signs of abnormal shifting. I also noticed that after the truck warmed up, shifting back into Park would show up on the dash indicator when shifting from D to P. Sometimes it would finally appear after 5 seconds. If I shift back to D, it would indicate D sometimes right away, but then back to P it wouldn't indicate that it was in P on the dash... A few days later, I was driving the truck and felt a slip in 2nd gear for the first time. A day later, I began to notice a delayed engagement into D when it was warm. This followed by eventual excessive slipping. At this time, I began to notice a whine from the bell housing area. I has always had a low pitched whine after it warmed up, but began to get louder. I am suspect that the pump or TC bearing is the culprit of the noise.

Now due to the fact that none of these issues came about until after I noticed the issues with the shift indicator delay on the dash, I want to make sure that I can correct or rule out the underlying issue before I pull the transmission to rebuild it(Yes, I am aware that it should be rebuilt). I am trying to avoid having any issues persist after I rebuild it. I logged the transmission Slip and TCC slip with HPT to verify that it is indeed slipping. It still usually drives just fine(with slow shifting, but not slipping) when its cold. But after it warms up, it'll get to the point where there is hardly any forward movement. The PRNDL indicator on the dash simultaneously starts acting up when I notice the transmission slipping badly. My questions is: is one usually the root cause of the other? meaning is this an electronics issue that caused the transmission failure or are the electronic issues a by product of the faulty transmission itself?

 

When you shift into P does it ever get stuck on R or there is flat out no indication of what gear you are in?
Or perhaps you shift into P and it shows no gear indication but then the truck settles slightly and it pops into P?

 

I’ll do my best to answer your question based o what I think you’re asking: There isn’t much of a pop in/out of any gear. Its sluggish to engage and slow to shift between gears(manually or in D)… after it gets warm and i shift between park and D, it usually goes through neutral and reverse, but doesn’t illuminate anything when the shifter pawl gets to park. Sometimes if I let it sit there after shifting back into park, it’ll illuminate P on the dash after like 5 seconds. I’ve been using a B&M hammer shifter on the truck ever since the 80e swap. So I don’t think that has anything to do with it unless the cable got stretched. But When this happens, it’ll just start slipping like its in neutral. I took a data log with HPT to see what was going on. It appears the computer doesn’t know what gear its in. The calculated gear ratio in the log was somewhere in between gears when the slip was taking place. But I think its just using the input and output speed sensors to calculate it. I can post a data log if you think it might help.

could it just be out of adjustment or a bad ground? Its strange that it happens usually after it gets warmed up.

If its just out of adjustment, I mean, its not like I won’t have to set the adjustment again when I put it back in again. I’m starting to think I just need to pull it out

 

Could you take a video of your shifting into park when this occurs?

This sounds incredibly like the symptoms I have been having since rebuilding my trans minus all that slipping and what not.

 

I don't think the the PRNDL switch would cause a transmission to fail. When you shift into a gear with gear selector that causes the hydraulic circuits to be engaged via the shift shaft. I think the switch works from the mechanical position of the shift shaft and converts the position into electrical feed for the ECU/BCM for reverse lights, shift interlock solenoid, etc.

The cluster delayment sounds weird too. Have the solder joints on the cluster been reflowed? The joints can fail, but usually that causes the gear indicator and odometer to become dim. Maybe it's related to that? Can you swap another cluster?

You said there's an incorrect ratio code? I thought that means the transmission is slipping. I think it throws that code if gear and input/output shaft speeds don't match, based on speed sensor data?🤔

Did you do the AFL valve when you rebuilt the transmission? Are you by any chance using the Transgo separator plate?

 

Thanks for the response. There was no code for incorrect ratio. The data log was not registering the correct ratio for the gear the computer thought it was in when it was slipping. The cable on the shifter does feel like it has slightly more slack than usual.
yes, I did do the AFL valve when I built the transmission. It has the Transgo separator plate but I did not do that over pressure plate modification. It has sonnax BV and billet alum th400 pistons machined for the 80e. Dual feed center support and drilled some extra holes in the reaction gears for the sprags. It has BW high energy frictions, rollerized. Its been so long since I built it or even thought about it. I think it lasted a good while, all things considered. My suspicion is worn out pump or the Bearing in the TC failed causing return pressure issues. It was starting to whine about the same time all the gear indicator stuff started happening.

If I unplug the transmission connector, would it just default to max line pressure and ignore the PRDNL switch? Or are the circuits separate from eachother?

like I said, its definitely getting pulled out for a rebuild. I just want to make sure I know why so I don’t have to do it twice.

 

i’ll see if I can get one for you tomorrow. I think this may have to do with the cable adjustment. So If I have time to get up under the truck, I’ll inspect the linkage to see if there’s any slip in the cable. 4l80e shifter pawl also has a slightly longer detent to get back into park than the 60e does. But the pawls are all sold as interchangeable?. I’m running a ratchet shifter and the better quality B&M cable(there are two) for this reason. The cables that come with the b&m shifters tend to have more slop In them and getting back into park can be tedious with an 80e. Once I changed to the better cable it’d been good for almost 8 years.

 

The PRNDL switch only sends signals to let the truck crank in P or N. And for reverse lights. And also for the dash indicator. has nothing to do with inside the transmission. Now if your shifter cable isn't adjusted correctly you can have the spool valve inside the valvebody centered up and cause a cross leak from passage to passage.

 

I guess I should have mentioned to start with the basics and check fluid level and condition if you haven't already done so. Maybe consider dropping the pan?

If you suspect the pump, get a line pressure and check it against specs. The plug is a pain to get to though, when I checked it on mine I wound up removing and the PRNDL switch so I could thread it on. Whine sound is usually not a good sign.

Removing the big electrical case connector puts it in limp mode and commands high line pressure. I think you only will have second gear (?) and reverse.

I've read the Transgo separator plate can have cross leaks in the hydraulics causing weird issues. I'm not knowledgeable enough to know if that's something that would develop over time. Also, I believe dual feeding also cause a slight delay when put in gear, although you mentioned this is a new symptom, so I don't think that matters.

Are you racing your truck at the track? Or just daily driving it? Was the fluid ever replaced? If there is excessive clutch material in the pan, it can get sucked up into the filter, causing fluid starvation. Usually the transmission will work on cold start-up only. I would think you'd have a tremendous amount of clutch material for that to happen though.

 

that's sort of what's been happening. I can take the truck out around the block and it seems to drive ok. Sluggish shifts. But once it warms up, I can barely get it home.