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Vortec 7.4L / 4L80E / doubler issues: Handling double 4 low

Russell

Russell

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A local buddy of mine just SAS'd his 96 OBS Suburban. It has a Vortec 454 / 4L80e in it. He also installed a 241/205 doubler at the same time and now he is having issues with it not shifting correctly in double low. The transmission shifts fine in single low with the 2.73:1 ratio of the NP241 planetary reduction, but as soon as you pull in the 1.98:1 ratio of the 205 then it stops shifting correctly.

Is there any way anyone knows of that a guy can make the Vortec PCM handle the double low gearing? I know on the newer Vortec LS stuff you can program the tow/haul mode to handle the double low gearing, but the older Vortec does not have a tow/haul mode switch.

I am kind of thinking he may be best to swap to a manual transmission instead or put a manual valve body into the 4L80e and eliminate the electric shifting entirely. This is what I do with Penny and she is happy in 4 low without any input from the transfer case.
 
Russell said:
A local buddy of mine just SAS'd his 96 OBS Suburban. It has a Vortec 454 / 4L80e in it. He also installed a 241/205 doubler at the same time and now he is having issues with it not shifting correctly in double low. The transmission shifts fine in single low with the 2.73:1 ratio of the NP241 planetary reduction, but as soon as you pull in the 1.98:1 ratio of the 205 then it stops shifting correctly.

Is there any way anyone knows of that a guy can make the Vortec PCM handle the double low gearing? I know on the newer Vortec LS stuff you can program the tow/haul mode to handle the double low gearing, but the older Vortec does not have a tow/haul mode switch.

I am kind of thinking he may be best to swap to a manual transmission instead or put a manual valve body into the 4L80e and eliminate the electric shifting entirely. This is what I do with Penny and she is happy in 4 low without any input from the transfer case.
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I would assume that what needs to happen then is double the output from the VSS. I'm assuming that's possible without too much difficulty, just need to know how to do it.

What I don't understand is why that is an issue here, when people run VSS off the trans in some 4WD applications, and claim to have no issues with low range, which should not be the case....the doubling+ of low range alone, if not "known" by the ECM, should screw up shifting.

In a bit of a hurry, but what about something along the lines of reprogramming the PCM to alter the calculation for the low range calculation? Probably a bad idea, but just a thought.
 
dyeager535 said:
I would assume that what needs to happen then is double the output from the VSS. I'm assuming that's possible without too much difficulty, just need to know how to do it.

What I don't understand is why that is an issue here, when people run VSS off the trans in some 4WD applications, and claim to have no issues with low range, which should not be the case....the doubling+ of low range alone, if not "known" by the ECM, should screw up shifting.

In a bit of a hurry, but what about something along the lines of reprogramming the PCM to alter the calculation for the low range calculation? Probably a bad idea, but just a thought.
Click to expand...

Usually when people say they have no issues it is because they are running a 4L60e. Those transmissions do not have an input speed sensor like the 4L80E does and they don't go into protection / limp mode with the PCM thinking the transmission is slipping. They'll just continue to shift at the same speeds as if they were in high range or until they hit the RPM limit for that gear and the computer up-shifts by force. They are happy with this and it won't set the MIL but it isn't particularly "right". Telling the PCM they are in low range changes the speeds and RPMs when the transmission shifts to what is right for the ratio of the transfer case.

I'd love to just re-program the PCM for the double low ratio, but then it'll work there but think something is slipping at the higher ratio and limp there instead. If a guy had a tow/haul or performance mode then you could program a different set of tables for shifting in that range and even specify a different final ratio as well, but none of that is an option for the earlier PCMs. Infact, 96/97 cannot even be programmed by EFIlive at all.
 
Ahh, now I see. because there is an input sensor, any variation is limited to whatever programming there is for the low range of the trans. Simply doubling the VSS signal won't fix the issue of the difference between input and output.

Would doubling the input AND output sensors serve to "trick" the PCM properly, or would that introduce a whole host of new problems with the PCM seeing an engine RPM that doesn't jive with the trans input RPM?
 
Russell said:
A local buddy of mine just SAS'd his 96 OBS Suburban. It has a Vortec 454 / 4L80e in it. He also installed a 241/205 doubler at the same time and now he is having issues with it not shifting correctly in double low. The transmission shifts fine in single low with the 2.73:1 ratio of the NP241 planetary reduction, but as soon as you pull in the 1.98:1 ratio of the 205 then it stops shifting correctly.

Is there any way anyone knows of that a guy can make the Vortec PCM handle the double low gearing? I know on the newer Vortec LS stuff you can program the tow/haul mode to handle the double low gearing, but the older Vortec does not have a tow/haul mode switch.

I am kind of thinking he may be best to swap to a manual transmission instead or put a manual valve body into the 4L80e and eliminate the electric shifting entirely. This is what I do with Penny and she is happy in 4 low without any input from the transfer case.
Click to expand...


As far as I can see there are a few options but all of them have a downside or two.

1. Have someone disable all of the incorrect gear ratio codes. I think team 208 motorsports could give you more info on your PCM

2. Move the speed sensor to the transmission output shaft by making something like this in the existing adapter and machine a tone wheel to press over the transfer case input shaft. http://www.advanceadapters.com/prod...5e-4-speed-automatic-transmissionadapter-kit/

3. Convert to a later LS style PCM

4. Install manual trans.
 
VSS off the trans will work. Only thing then is the Speedo will be off when in low. There are ways around that though.

OR - Split the low range difference in the tune. EFI Live can't do it but I have something that can.
 
I'm doing a 4L80-293/205 doubler and my question is this: How much shifting should the transmission need to do in low/low?
 
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