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4l80e speed buffer help.

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white90burb

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Ok so I have a 75 k5 that I originally swapped a tbi350 into. The motor and computer came out of a 95 g30 van with a 4l80e. I blew up my original t350 and decided since u have an ECM to support a 4 l80e why not. So I nabbed all the other parts I needed from the van ie speed buffer and other small things. I bought a 4l80 from a 94 3/4 ton 4x4 suburban and a 241 from a 91 suburban. Bolt everything up wire everything up and the trans isn't shifting right. It won't shift into second unless I take my foot off the accelerator. Same for third gear but when I try to accelerate in third it downshifts to second.
I hooked up a scanner to see if I could spot something not right. On the vehicle speed reading its showing about half the actual speed I'm driving. I'm thinking that's why it's not shifting right but I could be wrong. Any ideas suggestions or anything?
 
Completely unfamiliar with the van setup, but on the DRAC modules, there are multiple different PPM outputs...wiring to the wrong one can cause issues. I had my cruise wired up to the 4K output and could engage at about 12MPH. :)

I'm thinking you might be using the DRAC from the '91?
 
No no drac unit. It's the speed buffer from the van. I pulled a pin out for it I know it's all wired right.
 
Huh. Only familiar with the 2K/4K PPM difference, which would account for your problem, but if that's not it, sorry, I can't be of more help.
 
did you drive the suburban you got the tranny out of first ? if the tranny wasnt getting a signal, it wouldnt shift at all, so leting your foot off the gas to let it shift wouldnt do any good if the tranny wasnt getting a signal for the vss, is there a problem with the tranny itself ?
 
also ,,if the vss wasnt sending a signal, the tranny should start in limp mode,, at second gear, so it would be real doggy to take off
 
No I did not drive the suburban. I can watch on my scanner the tranny shifts through the gears and I can feel it shift and see a drop in rpms. I know the speed sensors are sending signal to the computer. I'm more curious as to why the computer is showing the vehicle speed to be so much less than actual speed. I think that is part of the reason it is shifting weird and won't stay in third while accelerating. For instance the computer will show 20mph and ill be driving actual speed of 40 in second gear I let up on the gas and it shifts to third. But when I try to accelerate it down shifts. To me it seems the computer thinks I'm going to slow to be in third gear?
 
what does the speed sensor on the input side of the trans read? Is it possible you got the two switched?
 
No I've made sure that I have the speed sensors going where they need. The input straight to the ECM and the out put runs through the buffer to the ECM. I have ohms tested the wires and the sensors multiple times making sure I got it right.
 
So I'm clear, the "overall" ECM speed is coming from the t-case VSS, right? And that is what is reading double true vehicle speed?
 
No the transfer case speed sensor is feeding my autometer speedo. I only have the transmission speed sensors going to the computer. The output speed sensor sends signal to the speed buffer and the speed buffer send trans output signal and vehicle speed to the computer.
 
Im not super familiar with 4l80 stuff, but isnt there supposed to be some kinda something for low range? So the ecm doesnt freak out (high input speed, low out put speed)
 
No idea I've read that but can't find any info on it. The computer is a 2wd computer so I don't have anything from the tcase hooked too it. It does say on the scanner 4wd low yes.
 
white90burb said:
No idea I've read that but can't find any info on it. The computer is a 2wd computer so I don't have anything from the tcase hooked too it. It does say on the scanner 4wd low yes.
Click to expand...

so it thinks it is in low range then? From my understanding, one wire to the ecm is grounded to know the t case is in 4 low.

does it shift fine in low range?
 
I dunno if it thinks that or not. I saw it thought it was odd but figured since it was a 2 wd computer it wouldn't read it. Maybe ill try to throw a ground to that pin and see what happens.
 
white90burb said:
I dunno if it thinks that or not. I saw it thought it was odd but figured since it was a 2 wd computer it wouldn't read it. Maybe ill try to throw a ground to that pin and see what happens.
Click to expand...

might want to verify if it gets power or ground, just looked at a diagram, and it shows it gets power when in low. It is a diesel diagram though??
 
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