For the last couple of years I have had this issue and hope someone knows what is causing it.
After I average 60mph + for about an hour or so my check engine light comes on( or when towing my boat at any speed for more than 30 minutes), after I stop and re-start, the transmission SLAPS (jerks) into 2nd. This continues until I have started and restarted the engine 10-20 times, or disconnect the battery for about 10 seconds. After that the check engine light goes out and the tranny works properly again.
To avoid the slapping I just manually start the vehicle in 2nd gear and then bump it back up to 3 and then OD.
It's more of an annoyance than anything. I talked to a transmission shop and he has some theories, but nothing I am willing to spend a Grand on....... it seems to me it's more of a computer problem than a transmission problem, since disconnecting the battery ALWAYS makes my problem go away. I read somewhere before that this vehicle has a computer regulated pressure system in the transmission, one theory is that there is a computer problem for some reason, telling the computer that it needs "more pressure" when shifting to 2nd, thus causing the slap.
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It sounds like when you accelerate it downshifts and acts like it is in neutral, when you let off, it shifts back to coast like it is in drive. I would question shift solenoid first. I know this is what 1-2 shift solenoid can act like if you have no low 1 and starts in low 2 or 2nd. There are two solenoids, 1-2nd and 2-3rd. You can drop pan and change them easy.
I don't know about a 1996 modle, but older chevys had a vacume modulator right side of transmission ,above pan some where between frount corner, and middle of transmission. if the vacume line came of motor would rev extreamly high before a shift,(if it ever did) my 1988 GMC with 700R4 trans does't use the modulator valve. yours may not either, but it's worth a look. I hope this is helpfull
It's not too hard at all. You'll need a decent size drain pan. You'll want to remove all but a couple of the transmission pan bolts on the bottom of the transmission. Let the fluid drain out as you loosen the last couple bolts. Once most of the fluid has drained out, remove the last couple bolts and let the pan done and pour out the remaining fluid, discard the old pan gasket. I would also replace the transmission filter, the new filter should have a new pan gasket with it. Remove the old filter, install the new filter, use some gasket cement to glue down the new gasket to the pan and reinstall and fill with new fluid. I would also check your transfer case fluid level, the transfer case is behind the transmission.
you should have this scanned at a at good transmission shop . its can be tough situation to solve these issues when concerning trasmission issues. even tougher when you cant afford mechanical issues even worse transmission issues like differentals ,shift solenoids ,pressure solenoids ,etc. i just noticed rereading your situation you dont mention any trouble with 3rd or 4th gear i assume no problems 3or4th gear and take off in 1 st i assume is no issue either. just to let you know w/transmissions there can be simular issues with different solutions, couple of things first check for a air modulator valve, driver side between transmission and radiator it will have vacuum line on it going to your intake make sure all the way connected or need replaced. also right below your modultor is a transmission mount have someone help you for this. have someone put the car in drive hold brake firm and give it gas see if the trans mount is sloppy or broke if it is fast acceleration would cause your shifting problem 4 sure esp going to 2nd gear. tras mount $65 modulator $50 . 1 nut holds air mod. on easy do it your self fix. dont forget o ring if replace the air mod. . cheaper then $1800 to $2200 good luck hope this helps you..
Your going to have to get the ECM scanned, Auto Zone scan's for free and most garage now will scan for $10.00 to $15.00 since they all have the scanners now and need the business.
You could have a Vehicle Velocity Sensor (V V S) going bad or the wire harness it self. The V.V.S tells the ECM how fast the drive shaft is rotating and calculate's what gear it should be in relative with the engine speed and throttle position, and transmission fluid pressure.
Regardless you need the Blazer scanned to pin point why your transmission is shifting the way it is. Could be as simple as time for transmission oil and filter change for $125.00 or a loose wire harness.
Check your transmission fluid and try not to run the blazer to hard, to replace the transmission start at $1,900.00 for a rebuilt.
Good luck and hope this helps. Keep me posted
If you open the hood on top of engine in front you will see what looks like a tempature sending unit it screws into the intake with a 2 wire plug on it, i cant remember what its called, but i know when truck gets to tempature it sends a signal down to trans. so it will shift into overdrive, when it goes bad it won't send the signal hense no overdrive. I have chg. a couple of them with a simmler problem you got and it fixed them. $10.00 at autozone.
Ahhhh I see from the next post why. Have you changed the transmission fluid and filter?
Let me know, and is this the only problem with the transmission you are aware of?