Bextreme04
Full Access Member
- Joined
- May 13, 2019
- Posts
- 4,464
- Reaction score
- 5,632
- Location
- Oregon
- First Name
- Eric
- Truck Year
- 1980
- Truck Model
- K25
- Engine Size
- 350-4bbl
You have to think about what the PCM is seeing compared to what the vehicle is actually doing. If you have the VSS at the transfer case, then the VSS will always show the actual vehicle speed. The engine RPM will be MUCH higher for a given vehicle speed when in 4lo than it is in 4hi, which is why you need to have a separate 4lo shift table(the shift table gives a mph at the vss that an upshift or downshift is started).Can you dumb this down for me? Are you saying 4 high (and 2 high) will behave like 4 low? Or are you saying 4 low will not shift correctly? I thought with the use of a switch, i could tell the pcm “hey i am in 4 low” and it would use 4 low shift tables?
If you are reading VSS at the transmission output, then it doesn't matter what the ACTUAL vehicle speed is or what the transfer case position is. The transmission will still shift at the right engine RPM depending on throttle position, regardless what the transfer case position is. If you want your shift RPM to be different in 4-lo, then you can still use the 4lo tables and wire up a switch, but it should drive just fine in 4lo without that.
A VSS on the transfer case output with no 4lo switch will drive TERRIBLY in 4lo as it will want to rev to the moon before shifting even at idle throttle position. In most cases it will just never shift in 4lo because the first shift is commanded at ~11mph at 0%TPS, which is going to be ~3500RPM if you have 4.10's and 35's. If you go up to ~25% TPS reading, then your shift point moves to ~20mph... which would put your engine at redline(6500RPM) before it would try to initiate the shift. Here's what the shift tables look like for a stockish 4L60 in an 0411 ECU.
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