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Transmission shifts out of O/D


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#1 Mosseybige

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Posted 25 February 2020 - 02:00 PM

I have a 99 v6 Firebird with 141k miles auto trans. I was driving down the road today and the transmission kept coming out of overdrive. Here is the kicker tho. If you are just trying to maintain speed 65/70/75 mph O/D comes out, BUT if you give it just enough throttle for the car to gain speed it will shift back into O/D, but as soon as you try to let off throttle enough to maintain current speed, it shifts back out of O/D. I have checked fluid level and it is just fine, with NO leaks... Is the Transmission out/going out?

#2 JoePeek

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Posted 25 February 2020 - 04:09 PM

Are we talking the transmission shifts out of 4th down to 3rd, or is it the torque converter is falling in and out of lock up?



#3 Mosseybige

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Posted 25 February 2020 - 08:08 PM

Well that's what I'm curious of. When it was doing that I DID switch it manually from D to 3rd, and you could here the RPMS come up just a little. Sooo I'm not 100% sure. I have little Knowledge on the powertrains on these cars. I did have a 24v Cummins that the PCM had grounding issues and I had to put a Manual switch to for the Torque Converter lockup. Otherwise it would basically do the same thing go in, and out, in, and out.

#4 Injuneer

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Posted 25 February 2020 - 08:31 PM

The F-Body 4L60E changed lockup strategy from “switched” to “pulse width modulated” starting in 1995, so its behavior is not going to be comparable to a switched system.

 

I would recommend you download (free, thanks to GaryDoug) the 1999 factory service manual.  The transmission section has a lot of troubleshooting and diagnostic charts.

 

https://www.mediafir.../?40mfgeoe4ctti


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381ci all-forged stroker (Callies Stealth, Oliver billet rods, BME 2618-T61 nitrous pistons) - 10.8:1 - CNC LT4 heads/intake - Comp Cams solid roller - MoTeC M48 Pro engine management - 8 LS1 coils - 58mm TB - 74#/HR injectors - 300-shot dry nitrous - TH400 - Gear Vendors 0.78:1 O/D - Strange 12-bolt - 4.11 Pro-Street gears - AS&M headers - true duals - Corbeau seat - AutoMeter gauges - roll bar - Spohn suspension - QA1 shocks - a few other odds 'n ends. 800HP / 800lb-ft at the flywheel, on a 300-shot. 11.5 @ 117 MPH straight motor
 
https://www.firebird...9_698_32777.jpg

 





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