4L85E No Engine Breaking 2nd 1st Gear
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Teching In
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4L85E No Engine Breaking 2nd 1st Gear Hello
I have just upgraded to a 4L85E transmission and am using a TCI controller (377500), the one built for them by (PSC) Powertrain Control Systems. I load TCI’s Base Calibration and adjust the part throttle shift points a bit (Throttle Position Sensor (TPS) to vehicle speed). All seemed OK but to my surprise I did not get engine breaking when the transmission is manually shifted into 2nd or 1st gear. Not a good or acceptable situation for towing a trailer downhill. PSC tech support suggested I move the Idle Line Pressure Duty Cycle down at the idle point only from the Base Calibration, as you already know this increases line pressure but will only at the point the throttle is closed, TPS = 0.0. Basically we moved the Duty Cycle at throttle closed from around 60 to 40 with no change but when moved to 20, engine breaking appeared in 2nd and 1st. Good? Now I am running what appears to be rather high line pressure at idle, vehicle jumps much more harshly into gear when moved P to R or N to Drive. PSC and TCI tech support don’t seem to be really concerned at why it took so much line pressure to work. This all leaves me wondering why such an extreme change was necessary and the possible risk of premature transmission failure years down the road when I am the only one at risk. I have been looking un-successfully for the sequences that take place when a Factory OEM Equipped Vehicle’s Transmission is shifted manually from Drive to 3rd to 2nd and 1st. That would be any electrical commands and what is actually happening in the transmission. Any help would be appreciated and I don’t mind buying a book that would describe the process.
Thanks in Advance
I have just upgraded to a 4L85E transmission and am using a TCI controller (377500), the one built for them by (PSC) Powertrain Control Systems. I load TCI’s Base Calibration and adjust the part throttle shift points a bit (Throttle Position Sensor (TPS) to vehicle speed). All seemed OK but to my surprise I did not get engine breaking when the transmission is manually shifted into 2nd or 1st gear. Not a good or acceptable situation for towing a trailer downhill. PSC tech support suggested I move the Idle Line Pressure Duty Cycle down at the idle point only from the Base Calibration, as you already know this increases line pressure but will only at the point the throttle is closed, TPS = 0.0. Basically we moved the Duty Cycle at throttle closed from around 60 to 40 with no change but when moved to 20, engine breaking appeared in 2nd and 1st. Good? Now I am running what appears to be rather high line pressure at idle, vehicle jumps much more harshly into gear when moved P to R or N to Drive. PSC and TCI tech support don’t seem to be really concerned at why it took so much line pressure to work. This all leaves me wondering why such an extreme change was necessary and the possible risk of premature transmission failure years down the road when I am the only one at risk. I have been looking un-successfully for the sequences that take place when a Factory OEM Equipped Vehicle’s Transmission is shifted manually from Drive to 3rd to 2nd and 1st. That would be any electrical commands and what is actually happening in the transmission. Any help would be appreciated and I don’t mind buying a book that would describe the process.
Thanks in Advance
I'm not familiar with the controller but I have a couple questions. Is this a new/fresh unit, or a used unit? Does your controller use a manual lever position sensor for data or a manifold pressure switch? Have you had a pressure gauge on this trans to see where the line pressure is at?
For the most part in both manual second and first a brake band is applied. Also the over run clutch and the OD sprag must be applied to accompany this engine brake function in the 2nd and low positions.
Depending on vehicle speed and also the way the trans is setup, it might not have manual ranges until vehicle speed and tps is low enough. The shift solenoids are identified as the "A" solenoid (1-2 shift solenoid) and "B" solenoid. (2-3 shift solenoid) Below is the solenoid strategy.
1st a on b off
2nd a off b off
3rd a off b on
4th a on b on
It almost sounds like an internal issue because you increased the line pressure and your engine braking started to work. No doubt that running high line pressure will probably accelerate an issue however at this point that is what you had to do to make it work right. I guess there is a couple places you will need to look to see if the trans or the tune is the problem. Hope this helped. Vince
For the most part in both manual second and first a brake band is applied. Also the over run clutch and the OD sprag must be applied to accompany this engine brake function in the 2nd and low positions.
Depending on vehicle speed and also the way the trans is setup, it might not have manual ranges until vehicle speed and tps is low enough. The shift solenoids are identified as the "A" solenoid (1-2 shift solenoid) and "B" solenoid. (2-3 shift solenoid) Below is the solenoid strategy.
1st a on b off
2nd a off b off
3rd a off b on
4th a on b on
It almost sounds like an internal issue because you increased the line pressure and your engine braking started to work. No doubt that running high line pressure will probably accelerate an issue however at this point that is what you had to do to make it work right. I guess there is a couple places you will need to look to see if the trans or the tune is the problem. Hope this helped. Vince
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Teching In
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Thanks Vince
Trans is new, I had already measured line pressures and they are now 90 lbs when in gear at idle and 175 wide open throttle. 90 is high but probably not damaging high for the moment, none the less, I will not be driving my vehicle until I have a complete understanding of the situation. I really appreciate your suggestions and really need to see exactly what should happen when an OEM Factory equipped vehicle is manually shifted down via the gear shift lever. My semi-uneducated guess is the Safety Neutral switch on the side of the OEM transmission may play into the picture on the OEM system.
I might be way off. My system does not have or use that switch for its Safety Neutral protection and does not understandably have any computer connection to a switch like that. There are internal transmission indications of gear selections. The vehicle is safe and cannot be started in gear. I really don’t mind purchasing a book or something that covers the shifting conditions in detail so I can see where and what’s really missing.
Thanks again and I appreciate your suggestions
Trans is new, I had already measured line pressures and they are now 90 lbs when in gear at idle and 175 wide open throttle. 90 is high but probably not damaging high for the moment, none the less, I will not be driving my vehicle until I have a complete understanding of the situation. I really appreciate your suggestions and really need to see exactly what should happen when an OEM Factory equipped vehicle is manually shifted down via the gear shift lever. My semi-uneducated guess is the Safety Neutral switch on the side of the OEM transmission may play into the picture on the OEM system.
I might be way off. My system does not have or use that switch for its Safety Neutral protection and does not understandably have any computer connection to a switch like that. There are internal transmission indications of gear selections. The vehicle is safe and cannot be started in gear. I really don’t mind purchasing a book or something that covers the shifting conditions in detail so I can see where and what’s really missing.
Thanks again and I appreciate your suggestions
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The $1200 TCI TCU from GMPP for my 4L85E "Supermatic" comes with "standard" shift profiles. The line pressures in shift schedule A were too low to keep the clutches engaged on the freeway and so burnt up my brand new tranny due to TC clutch slipping and all the other clutches not having enough line pressure to keep them engaged. Dropped a new tranny in, mounted a permanent electrical tranny line pressure gage and went in and reset all the duty cycle vs. load tables to yield higher line pressures.
Now GMPP's solution for my no engine braking was to crank up the line pressure at the zero TPS position. So I did this on my shift schedule B, and can toggle instantly between profiles with a dash mounted switch. It seems to work, but I think it is a pretty Mickey Mouse solution for such and expensive set up. Also, I'm not convinced that the increased line pressure at zero TPS is enough to keep the clutches firmly engaged and not slip. I guess time will tell. I'm not going to use it unless I have to while towing in the mountains.
Anyway, that is the follow up to my story.
Matt
Now GMPP's solution for my no engine braking was to crank up the line pressure at the zero TPS position. So I did this on my shift schedule B, and can toggle instantly between profiles with a dash mounted switch. It seems to work, but I think it is a pretty Mickey Mouse solution for such and expensive set up. Also, I'm not convinced that the increased line pressure at zero TPS is enough to keep the clutches firmly engaged and not slip. I guess time will tell. I'm not going to use it unless I have to while towing in the mountains.
Anyway, that is the follow up to my story.
Matt
