8.1 workhorse 4L85-E Downshifting problem
Hi, anyone have a problem with auto downshift from overdrive? My rig will over rev to between 4-5 grand before shifting. Any ideas???
Forum Discussion
Is it just slow in shifting? Meaning when you step on it, does TC unlock, engine increase RPM and then downshift?
The 4L85e has a continuously variable lock up TC. The pressure valve is pulse width modulated, so it allows the TC to slip, depending on the need for power. At normal cruise, the clutch pressure varies from about 50% to about 70% depending on the load. As you lean into the gas, it will allow clutch slip to raise the engine RPM, so you gain some power. GM had 2 goals in mind when they designed it. The first was to completely eliminate any feel of the TC locking up. The second was to be able to allow slip instead of downshifting. The TC clutch is patented, as it is porous so allow the fluid to keep it cool so it doesn't burn up, as it slips. (replacement TC must be bought from GM, otherwise they will burn up)
So now you know how it is supposed to work, you will understand it is difficult for us to judge from our armchairs as to what is bad, compared to how it is designed. I don't know the ECM well enough on that year to know at what point does one determine it is broke. I do know the trans control is really sophisticated, in that it monitors engine RPM, input shaft speed, and output shaft speed. Pump pressure is variable throughout the whole trans, as they monitor slip in every gear to determine wear. They then compensate for wear with pump pressure. So when the slip it too great, it increases pump pressure. This info is stored in the ECM, and is continually updated. My thinking is that with the level of monitoring that the ECM is doing, it should throw a code if something is wrong - if there is too much slip anywhere in the trans. Some codes are silent codes, meaning they don't turn on the check engine light, but the dealer would see it, if things are getting out of hand. Were the codes checked by a dealer or were they checked by John Doe's Auto Parts?
The 4L85e has a continuously variable lock up TC. The pressure valve is pulse width modulated, so it allows the TC to slip, depending on the need for power. At normal cruise, the clutch pressure varies from about 50% to about 70% depending on the load. As you lean into the gas, it will allow clutch slip to raise the engine RPM, so you gain some power. GM had 2 goals in mind when they designed it. The first was to completely eliminate any feel of the TC locking up. The second was to be able to allow slip instead of downshifting. The TC clutch is patented, as it is porous so allow the fluid to keep it cool so it doesn't burn up, as it slips. (replacement TC must be bought from GM, otherwise they will burn up)
So now you know how it is supposed to work, you will understand it is difficult for us to judge from our armchairs as to what is bad, compared to how it is designed. I don't know the ECM well enough on that year to know at what point does one determine it is broke. I do know the trans control is really sophisticated, in that it monitors engine RPM, input shaft speed, and output shaft speed. Pump pressure is variable throughout the whole trans, as they monitor slip in every gear to determine wear. They then compensate for wear with pump pressure. So when the slip it too great, it increases pump pressure. This info is stored in the ECM, and is continually updated. My thinking is that with the level of monitoring that the ECM is doing, it should throw a code if something is wrong - if there is too much slip anywhere in the trans. Some codes are silent codes, meaning they don't turn on the check engine light, but the dealer would see it, if things are getting out of hand. Were the codes checked by a dealer or were they checked by John Doe's Auto Parts?
