Transmission swap from TH400 to 4L80E (WEIRD!)

I have a 1985 GM P30 Van chassis (aluminum Grumman ex-Mail truck) with a 2001 5.0L GM "crate" engine conversion complete with appropriate engine and trans. wiring harnesses, computer etc. Canada Post in their
wisdom left the original TH400 with 4.10 rear gears, goes like stink to the end of the block, but useless on the freeway. I need it to be efficient about the city and on the freeway, so I have picked up a 4L80E from a 1995 G3500 pickup, crossmember relocation, driveshaft mods are no problem. But I am getting concerned about compatibility issues with the computer and maybe wiring also. The trans wiring harness is in place with dummy components plugged into some cables and some kind of a VSS connected to the speedo cable for the benefit of the computer. Here is what concerns me so far... 1) Will the 2001 computer talk to a '95 transmission at all? 2) Are the wiring plugs compatible? If not can they be changed? 3) Can the computer be flashed to accomodate changes needed if there are differences above and beyond wheel sizes and diff. ratios? 4) I have heard that there may be "soft shifting clutches??" in the TC that the computer may be looking for solenoids that may or may not be in the '95. True / False?? 5) Can the existing '85 steering column with its linkage shift this baby thru all positions? Or do I need to change the column to the newer style with the cable shift linkage right away. ( will do it eventually cuz I need tilt) Sorry about the long list of ??, but any help or redirection would be greatly appreciated, Thanks!
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Coilman wrote:

.http://www.powertraincontrolsolutions.com/content-4.html
pricey....but it'll solve your problem.
~:~ MarshMonster ~:~
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it will have plugs for a 4L60E tranny, unless it is for a bigger truck with the 4L80E tranny. I'm betting that the 4L80E has the same fittings/wiring as the 4L60E trans. These plugs consist of one 2-wire plug to connect to left(driver's side) rear of tranny where the VSS is situated(provides signals for factory speedo & cruise control), and another 4(+?) wire plug that plugs down onto an upward-facing connector about 2/3 towards the front of right(passenger) side of tranny, which make up the connectors that tell the solenoids when to actuate shifting--both up and downshifting. Good luck & if you have problems, print out these 4 wiring diagrams(for the '95 truck) from like Mitchell 1 On Demand--you'll need the transmission-, the cruise control-, the PCM-, and the instrument cluster-diagrams. If you know what harness "Canada Post" used, it'd be nice to have those same 4 diagrams for that application. At the worst, you may need the '95 truck PCM and harness. BTW, a well-built 700R4 trans, same overdrive characteristics, would be a heck of a lot easier to wire up--virtually none; just install a $75-100-kit from Summit which makes that trans shift from vacuum. HTH & good luck, s
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Thanks for your input, I really need to get in there again and match up connectors on the harness and tranny. I have two speed sensors, and a round ten pin connector, two additional connectors on the shift lever. I will try to find the scematics that you suggest, thats a good idea.
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Coilman wrote:

Then.....comments to follow... whether you want them or not.
1) The computer doesn't know what year the tranny is......... and the tranny is'nt going to confess it. 2) Who knows.... I mean....you haven't stated what harness was sent to you. If it's a 4L80E harness....then yes. If it's not...then no. 3) Again.......who knows. You haven't stated if the computer is for a 4L60E, or a 4L80E. If it's for a 4L80E..then yes. If it's for a 4L60E..then no. 4) I have NEVER, EVER, NEVER...even once..heard of a "soft shifting clutch". and...95 to present trannies all have the same amount of solenoids in them on each model number. Be it 4L60E, or 4L80e, or....th125c. 5) Your original shifter will be fine....if.....you're willing to modify the linkage set up. And note.....I mean fairly extensive modification. . . . Now..... some ~Random Thoughts~
this dudes done went and bought a crate eng. with a computer for a 4L60E that IS NOT going to work with a 4L80E tranny...NEVER!! EVER!! never,never,never,never,.......ever. and I don't care what Tom says.
this dudes harness is for a 4L60E. the above ever,nevers apply. so does the "Tom says" statement.
this dude ain't got a clue who Tom is.
this dude reeeeeeeeeellly ain't got a clue....or...... he's got deep pockets........ in which case.... he should have bought a 3500 series Parcel truck with the setup he wanted. and......he'd be frustration ahead.....and at about the same price.
Tom ain't got a clue either.....that's why he ain't posted yet on this thread, cuz......well all know he's read it.......cuz....he's jest that way.
~~End......Random Thoughts~
Suggestions to the OP.......
check out the link I sent you.....it's yer easiest option with the setup you've got.
I'm fairly convinced, but you have'nt stated, that the harness and computer that were shipped to you are compatible with a 4L60E tranny.
The computer will think it's commanding a 4L60E, which has different gear ratios than the 4L80E tranny!!!!!! Not to mention the fact that the two units have different "case" connectors that CAN NOT be interchanged.
You asked about "soft shifting clutches". The is no such thing. What you "may" have heard, or "may" be thinking of, of "may" be associating this terminology with.....is the fact that newer trannies have what is called....."Pulse Width Modulated" torque convertor apply. Now.....you've got a 2001 computer...it's logic "should have" PWM programming. I say should......because as I stated earlier, you have yet to state for fact that the computer is from GM. I'm jest Ass U Meing that it is. Now.....here's the deal.....that pwm computer isn't..........
fk it.
do a google.
or.......
jest stick the damn 400 back in it and drive the damn thing for what it is.... a piece of sheet.
sorry so frank. sorry so cold. sorry .......but you've done this thing to yerself.
and... you've either got more money than sense....... or... you're too dang ignorant to understand what yer attempting to do.
okay..... so i lied........ i'm not sorry.
but honestly...... you asked for it.
~:~ MarshMonster ~sips his crownroyal.......and ponders ignorance.....is it reeeely bliss, or is it.........well.......jest ignorance~
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