Transmission slip or clunk between first and second

[Iahawkeye]

I have been looking for some information on the matter and have come up empty.

I seem to be having a slip or a clunk when coasting around corners and accelerating again. When I take a corner slowly, 5-10mph, and then begin to accelerate again, the transmission feels like it is in second gear. The transmission then downshifts back into first with a slip or slight hesitation and then it feels like it jams back where it needs to be. It seems like there is some slop in the drive line. Is this making any sense?

From a dead stop, there is no hesitation or any slack in the line, between the shifts. Transmission fluid has been regularly changed by dropping th pan every 30,000 miles, and the envoy now has 150,000 miles. Do I need to be talking with a transmission shop? Would a dealer be worth the extra money? Does the gmtnation have any suggestions? I appreciate the help!

 

[RayVoy]

Well, not sure I'm following what is happening, but try this: if you are taking a corner slowly and then accelerating, I'm assuming you are accelerating somewhat slowly (by that, I'm saying that you are not jamming it to the floor as you accelerate). Under that scenario, I don't think it will downshift into 1st (I think it will under hard acceleration).

Your not giving me a lot of info, so I'm guessing you were driving at normal street speed as you approached the corner (which would have you in 4th gear), you slow as you enter the corner and the speed drops to 10mph. At this point, the tranny may still be in 4th gear and the torque converter clutch may be starting to release. As you accelerate, the tranny downshifts into 2nd probably and the TCC will be released (maybe it falls all the way to 1st and pops back to 2nd, maybe it expects you to pushing the go pedal harder than you are, maybe you are on the balance of do I go into 1st, or do I stay in 2nd)

I guess, where I'm going with this, is that, unlike older transmissions, this one is shifted electrically and the shifting is commanded by the PCM based upon a whole bunch of inputs; some of which, are engine condition. If the engine has fuel delivery problems, spark problems, etc, that make the PCM think the engine is working hard (ie: trying to pull a heavy load up a hill), it will downshift the transmission.

You may be having a small engine problem that appears to be a transmission problem.

 

[Iahawkeye]

Well, not sure I'm following what is happening, but try this: if you are taking a corner slowly and then accelerating, I'm assuming you are accelerating somewhat slowly (by that, I'm saying that you are not jamming it to the floor as you accelerate). Under that scenario, I don't think it will downshift into 1st (I think it will under hard acceleration).

Your not giving me a lot of info,

Often times, my wife tells me the same thing. I really do appreciate your response! I tapped out the first post in a hurry and may not have given all of the needed information. Let me try again.

I have noticed the problem when I have been driving on residential areas. I will be driving 25-30mph and slow down to take the corner. The transmission should be in 2nd or 3rd at this point. As I come out of the turn, I will be going about 5-10mph and will slowly hit the gas pedal. RPMs will be around 1200-1500 as I make the turn, and I assume the transmission is now in 2nd gear. As I try to accelerate faster, the transmission will downshift into first (I think), and it feels like there is some slack in the driveline. It feels like a clunk when it finds 1st gear and then the engine and transmission pulls hard like it should. RPMs go back to 2000-2500 and then it will shift back to 2nd with no problems.

At this point, I should also mention that I got a pcmforless tune with a level 1 firmness. I switched out the PCM two weeks ago and love the power (I have found where all of the horses have been hiding!!). However, the shift from 1st to 2nd seems a little stiff. It feels like the shift when the envoy is cold and the shift is hard, but it seems to happen all of the time, even after it has warmed up, if I am being easy on the throttle. Now, when I put the pedal down and get the RPMs up to about 3000 or higher, the shift is quick and much smoother.

I got the tune because I was looking for better mileage, and not necessarily looking for the power. Better mileage is acheived when the RPMs stay down, so I drive like a grandpa. However, if the shift is so hard, should I be driving differently? Is the hard shift normal after the tune? Or did the tune just accentuate the pre-existing issue?

Now if this is a computer issue, or engine problem, do you have a course of action to take or any recommendations? The plugs are new (ac delco), and the fuel pressure was tested a couple of months ago at about 52psi (if I remember correctly) in the line. I have read that the fan clutch can cause some problems and that has been replaced within the last year or so.

Am I worrying too much? I have a tendancy to do just that. We have some big trips planned and would hate to get stranded between Baker and Barstow Ca or across southern Wyoming. Thanks for your help.

 

[seanpooh]

I don't think that clunking is the transmission. I too have a clunk but I feel it's the driveshaft engaging and then "clunking" in the differential.

I'll try to explain... I would slow down at a stop sign but not come to a full stop. Then ease down on the gas pedal to go and the clunk would occur before the actual truck starts moving.

It's as-if the slowing down is causing some space somewhere between the driveshaft and the rear axle then when speeding up again all the slack/space is taken away hence the clunk.

So I think you and I both think it's somewhere in the driveline but I don't think it's the tranny.

For me I think it's not a tranny problem because mine was rebuilt 50K miles ago. I also have a level 3 shift firmness from PCM4Less. I know when the tranny shifts and it doesn't shift when I hear the clunk.

Also the new PCM should adapt to your driving style. I saw you have level 1 firmness, does it shift faster and less noticeable than before? If it is then that's how it should be and will adjust slightly.

 

[RayVoy]

At this point, I should also mention that I got a pcmforless tune with a level 1 firmness.

I'm sitting here with my hands turned palm up, a shocked expression on my face, saying to myself "why would he think that info was not important?"

You need to talk to PCM4less, not me.

I don't think that clunking is the transmission. I too have a clunk but I feel it's the driveshaft engaging and then "clunking" in the differential.

Sean, your problem may be a problem that has caused a lot of GM owners a lot of frustration. It seems, that the rear diff yoke material binds with the seal on the diff. It sticks and then releases, causing a clunk at the rear. The only fix is to change the yoke. The new yokes are made from a steel that does not stick to the seal.

 

[seanpooh]

Oh... another project for the books But it's alright, it's been going on for years. No frustration. Trips to NYC and back with no problems (knock on wood). I'll have it done when I change the differential seal where the driveshaft meets.

I'm just assuming that this is what the OP is experiencing right now. But just to make sure, there are tranny shops that would take a spin with you for a diagnosis.

If it all started after changing to a tuned PCM, the problem may have been there all along but now more noticable because of the change of shift firmness. Try disconnecting the battery for 30 minutes to an hour and reconnect. The computer will be reset to PCM4Less stock. Take it for a spin and see.

 

[Iahawkeye]

I'm sitting here with my hands turned palm up, a shocked expression on my face, saying to myself "why would he think that info was not important?"

You need to talk to PCM4less, not me.

I was thinking that the problem was in the transmission only. I will be emailing them tonight.

If it all started after changing to a tuned PCM, the problem may have been there all along but now more noticable because of the change of shift firmness. Try disconnecting the battery for 30 minutes to an hour and reconnect. The computer will be reset to PCM4Less stock. Take it for a spin and see.

This is what i think. I have felt this for awhile, but it seems to be more pronounced since I switched out the original pcm for the tuned one. I will disconnect the battery and give it a spin tomorrow. But just so I understand, Rayvoy seems to think the problem may be electrical (in the pcm or other control modules) and you still think it may be mechanical - in the differential.

I do appreciate the help. I love the tune and i have seen an improvement in the gas mileage, but it is starting to appear that I may be over my head. I was hoping for a simple plug-in and play. Could the tune have been done incorrectly? Just asking. I am just glad I kept the orginal pcm. thanks again.

 

[seanpooh]

The disconnecting the battery and resetting the PCM is one step. The next is to swap it out entirely to the stock one (if you have it). Then from there you can figure out if it's a mechanical problem or electrical.

I'm not saying that it's the differential. You may have the same problem I have, a weird yoke. It's just more pronounced since you have faster and less firm shifting.

Pretty much trial and error and elimination. Just keep us updated.