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  #1  
Old 02-23-2009, 05:13 PM
 
Join Date: Feb 2009
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Default Please help diagnose overdrive to downshift surging

Hello! This is my first post so I apologize if its in the wrong spot. I have a 2000 Blazer 2dr. 4.3 with a stock 4L60E trans. and 138k mi. From 0-45mph in gears 1-3 it works near fine except under heavier load (uphill) it wont change gears until 2800rpm with little "pull" during 2500-2800. However the problem occurs when I hit 50-55 into overdrive. I can cruise normally in OD without the tach going over 2k, however if I need to speed up or start moving uphill, I can press the gas pedal down considerably and only get a little "pull" with the tach still stuck around 2k. I then press the pedal even harder, it switches to third, the tach jumps to 3-3200k, and the truck surges or "jumps" torque like its stalling out. It then downshifts normally if I floor it out of 3rd or slow to a stop. Very rarely it will do this surge in 1st gear as well but not nearly as bad.
Ok as far as troubleshooting: The battery and alternator are brand new. I cleaned my MAF sensor, the throttle body, and my EGR valve (which did WONDERS for my power), all in good condition. I had the trans. fluid and filter changed and the trans. shop said nothing was in the pan and the comp. read that I have all gears working. My oil, filter, and air filter all new. SO......... my next guesses are...... TCC solenoid, VSS, and TPS from what research has shown me. Anyone have any ideas or the same problem? My other question is if I replace the TCC solenoid, is it wiser to just replace them all while the pan is dropped to save time in the shop down the line? Thank you for your time
  #2  
Old 02-23-2009, 09:22 PM
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Default RE: Please help diagnose overdrive to downshift surging

It does not make sense that it takes almost WOT to get it to down shift. My old 2000 Bravada downshifted quite readily. This may indicate a failing or partially failed TPS. You can check it with a volt meter by back probing the wires with the wiring still connected. You need ~0.5V at idle (throttle closed) and ~4.5V at WOT (throttle fully opened). It needs to be a smooth transition between these two approximate voltages.
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Old 03-01-2009, 05:34 PM
 
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Default RE: Please help diagnose overdrive to downshift surging

Thank you for the fast reply, during testing the TPS I couldnt really tell how "smooth" the voltage needed to glide so I bit the bullet and replaced it anyways. The problem with the "searching" between OD and 3rd and the bucking is still there. The only change is now the throttle doesn't have to open as far or as fast to downshift. Through more test driving it I found its almost exclusive to inclines, as if its not reading the engine load and downshifting to compensate.
Like if I'm coasting at 65 in OD with the rpm at 2k and I approach a hill the truck will begin slowing down staying at 2k rpm until I open the throttle considerably to get it to jump to 3rd. Then it bucks and jumps in rpm like it cant decide which gear to be in until its back up to speed and the shift into OD again.
Although I don't really understand the TCC, I was told a way to see if the solenoids bad is to tap the brakes in OD and watch to see if the tach jumps, it did not. Is there a solenoid that controls OD > 3rd? Also is it possible for a VSS to be sending bad signals but correct speed, or does the speedometer have to be bad for the sensor to be? And lastly is there a sensor that monitors engine load for hills and towing and such? I appreciate any help ^^ Thank you!
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Old 03-01-2009, 06:02 PM
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Default RE: Please help diagnose overdrive to downshift surging

To really diagnose further, you need a full function scan tool so that you can monitor the states of the solenoids as well as the other inputs used to control shifting.

There is definitely a problem if the torque converter does not unlock if you tap the brakes while holding the throttle steady. It is possible that it really is not locking up in the first place and the surging that you experience is due to it attempting to lock up.

The TCC (torque converter clutch) was an addition that made for better fuel mileage and less heat generation by the transmission. It would lock up the torque converter while cruising down the road, but would still unlock to allow for a slight change in RPM for minor speed corrections prior to a full gear change. It typically drops RPM by 200-300RPM when it locks up. Lockup typically happens in OD at speeds above ~40MPH.
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Old 03-01-2009, 06:33 PM
 
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Default RE: Please help diagnose overdrive to downshift surging

Thank you swartikk I didnt know the throttle had to be open when tapping the brakes to test the TCC. Is there anything to look for to tell that its unlocking properly? If that solenoid is bad is it wiser to replace them all at once to avoid # of times its opened?
Is a scan tool something I can rent from an autozone or would I need to take it to a shop?
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Old 03-01-2009, 07:08 PM
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Default RE: Please help diagnose overdrive to downshift surging

I do not know if the scan tools that are available from most autoparts stores are capable of bidirectional control which may be required to run through certain diagnostic steps. Problems with an automatic transmission are generally best dealt with by a professional. These systems are very complex and it requires indepth knowledge just to decipher what a scan tool is telling you.

Now, there is a problem with the TCC solenoid or rather the bore that the valve rides in on many of these 4L60E. An update is available to cure this problem and is typically called the P1860 Upgrade Kit as P1860 is the code which most prominently pops up when this is the problem. However, if the problem is really the clutch in the torque converter, the update will not help you.
Old 03-01-2009, 07:08 PM
 
 
 
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01, 2000, automatic, blazer, chevy, downshift, egr, overdrive, sensor, surging, tach, tcc, test, valve, working


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