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Old 06-16-2020, 09:05 PM   #1
Chunfin
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2002 SL2
Question Reverse Slam and Hard Shifting

Hello all,

So I've recently been having problems with the transmission hard shifting into gears and having the notorious reverse slam too. I've done some reading into replacing the Valvebody and possibly re-torquing the Input/Output shaft nut on the transmission to prevent further slippage or damage to it.

My main question is how to reset the adaptive parameters? Do I need a certain Scan-tool to reset them or do I have to do the Relearn procedure? And if I have to do the relearn procedures; how do I do it correctly?

Thanks y'all!
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Old 06-16-2020, 10:09 PM   #2
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2001 SL1
Default Re: Reverse Slam and Hard Shifting

Hello Chunfin, welcome to the forum.

Reverse slam. Have you read up and followed Wolfman's reverse slam method?

In a nutshell, drain the transmission, replace the spin on filter and refill with Dexron III compatible fluid (not any other Dex type).
Find a solid object, like a nearby large tree, back the car up to the tree and leave the car in reverse, pull up on the handbrake.
Leave engine idling in reverse up against the tree for ~30 minutes.
Usually, if the tranny has just started the slamming stuff, usually this method will fix the issue.
I did this twice in about 100K miles and never needed to touch the valve body or the input shaft nut.

It worked so well, that I found it helped my '92 Dakota's transmission too.

What vehicle are you working on? Adding it to your Avatar makes it easier for us and will cause fewer repeat questions as to which car you are referring to. Thanks.

Last edited by toggenburg; 06-16-2020 at 10:16 PM.
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Old 06-16-2020, 10:22 PM   #3
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Default Re: Reverse Slam and Hard Shifting

If you do change the VB, the adaptive parameters will be re-learned automatically with just a short bit of normal driving. No special tool or procedure is needed. Yes, using "the procedure" may do it quicker, but normal driving will have it shifting acceptably (but probably not fully learned) in just 10 miles/30 minutes; and it will finish the learn without you even noticing.
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Old 06-17-2020, 10:37 AM   #4
Chunfin
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Default Re: Reverse Slam and Hard Shifting

@toggenburg I'm working on a Saturn SL2 2002. I've also tried Wolfman's procedure on the Reverse Slam cure that you've mentioned but unfortunately it wasn't successful so I'm probably going to replace the Valvebody and hope for the best that'll fix the issue.
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Old 06-24-2020, 09:07 PM   #5
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1994 SL2
Default Re: Reverse Slam and Hard Shifting

After further research:

The loose nut on the input shaft causes slack in the system, resulting in a leak of the fluid used to apply the 2nd clutch pack. This leak causes a slow and sluggish application of 2nd gear, which can be noticed as a flair of the engine rpm (transmission slippage) as 2nd gear is applied.

It also results in a hard application (slamming) of reverse gear, because the fluid that doesn't leak past the nut is enough to compress the reverse accumulator piston, but not enough to actually apply the clutch before the accumulator is completely compressed. Thus the clutch is applied after the accumulator is compressed, resulting in a SLAM. (The accumulator is a cushion for reverse application).

Once in awhile the accumulator piston will jam in it's bore and cause reverse to slam because the lack of accumulator movement stops the cushioning effect on reverse application. This is the case where some kind of flush or a valve body change might repair the problem. But this is also a symptom of high mileage, so the loose input shaft nut is just as likely to be a problem as well.

If the input shaft nut has loosened, changing the valve body will NOT stop a reverse slam condition. Continued driving under this condition will result in loss of 2nd gear and/or reverse gear.

Considering the cost to tighten a nut versus the cost to replace the valve body, you should start with torquing the input shaft to 111 lbs/ft. Then do a relearn procedure on the engine computer to see the new results for 2nd and reverse gear application. Actually, you should replace the nut with a new one, rather than reuse the old one. BUT? Where does one get a new nut to replace the old one?

I've heard stories of removing the nut, cleaning it and the input shaft, and reinstalling it with loctite while torquing it to 111 lbs/ft.


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Old 06-24-2020, 11:57 PM   #6
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Default Re: Reverse Slam and Hard Shifting

I'm going to question that analysis. My understanding is that the slam causes the nut to loosen, then other shifting problems (like no reverse) happen. The slam is because of a bad VB, and can be there without the nut loosening. That is why it is wise to fix the slam soon, so you may be able to avoid having to go in after the ISN.

I have re-used a nut, put back with Loctite 620 (high strength, high heat grade)
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Old 06-27-2020, 11:07 PM   #7
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2002 L-Series 2.2L Sedan
Default Re: Reverse Slam and Hard Shifting

Niether job is very hard, but they both need knowledge , and patience. Read everything you can get your hands on.
if you are rebuilding VB s, I use any solenoids that ohm out well except the pressure one. That's the one that does all the work. I throw that away from the getgo. Make sure to check the pin clearance of the connector plate. That could easily be all that is wrong. For my money, I'd look at the VB first. Lot's less work. Then attack the nut. Make sure you pull the cover off straight. If the tubes get bent you are just going to make more trouble for yourself. I have some long 1/2 threaded rod to hang the tranny on so it can be stable, and drop it down below the body frame. Good Luck
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Old 06-28-2020, 01:07 PM   #8
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Default Re: Reverse Slam and Hard Shifting

If it's banging and has harsh shifts, it does not mean you have low line pressure, it's high line pressure, so for now you can forget about the input shaft nut. For the record, don't ever worry about the output shaft nut, I've never seen one of those loosen up. If it were me, the first thing I would do is check the resistance of all the valvebody solenoids. The main line pressure one in the later years of the S-series were known to be problematic and cause harsh shifts in forward and reverse gears. With the old Tech 2 we could even buzz the solenoids to see which ones were working or not. The input shaft nut loosens when the vehicle is driven with low line pressure and cause slamming when shifting into reverse. The input shaft nut when loose actually causes low line pressure to the clutches and causes either no gear engagement or slipping between gears.
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Old 06-28-2020, 08:36 PM   #9
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Default Re: Reverse Slam and Hard Shifting

All of you guys need to read the transmission sites relative to the TAAT transmission.

The slam is because the cushioning accumulator isn't cushioning. It's either jammed in it's bore or already compressed to the bottom of it's bore. Simple as that.

IF the accumulator piston is cocked over and jammed in it's bore, then there's no cushioning. Then you get a SLAM.

A fluid leak in the apply line to the reverse clutch causes the accumulator to compress (because it has lighter springs in it) before the clutch pack gets applied. Thus there's no cushioning when the clutch engages. Then you get a SLAM.

The fluid leak is because the loose Input Nut allows the shaft to move laterally inside its bore misdirecting the the reverse apply pressure to dump inside the 1st gear clutch pack. The loose nut CAUSES the reverse slam, though in the beginning the internal fluid leak isn't severe enough to manifest in the slaming. By the time it's actually slamming, the nut is extremely loose.

So, a repair is either to stop the lateral movement of the input shaft in it's bore, or to get the accumulator piston to move in it's bore as it should. One requires to tighten the input shaft nut. The other is to replace the valve body.

The input shaft nut is much more often the culprit for reverse slam.

All of this information comes directly from various transmission rebuild sites online.

Check it out! Stop guessing.


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Old 06-28-2020, 11:57 PM   #10
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Default Re: Reverse Slam and Hard Shifting

Some of that is correct but not all of it. Apparently you have not changed enough valvebodies on the S-series to know a lot of the time they are the cause of 90-95% of the issues with these transaxles. Been there, done that. By the way, engineering has stated, don't tighten the input shaft nut, replace it. It has loctite or something similar on the threads so when you start to tighten the nut you keep tightening until correct torque (111 ft. lbs.) is reached. Don't go replacing the input shaft nut or the valvebody yet until you check the solenoid resistance, especially the main line pressure one. Ohms being off by just 2 or 3 ohms can be enough to totally screw up shifts between gears. I've seen them fail on 2000-2002 S-series but not earlier years for some reason. If there is no solenoid issue, hook up a pressure gauge, start the car and pull the mlp fuse out and pressure should go up to around 245psi. If it does not get to at least 175 psi you will have trouble with reverse gear engaging because it needs 175psi minimum to apply. That's why when you have low pressure, you usually notice a problem with reverse gear first, before any other gears.
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