Transmission/Transfer Case Adapter Questions

[Evil SS 2006]

The new tow rig is an '87 V3500 crew cab (currently a dually, soon to be SRW) with a TBI454/THM400/NP205. I am in the process of procuring all the parts to swap a built 4L80E in place of the THM400. I already have the trans core being built, the engine/transmission harness, new ECM programmed for the TBI454/4L80E combo. The 4L80E is being built with the reluctor wheel in the normally "dead" Output Speed Sensor location so the ECM will have the ISS/OSS sensors which are both required. My next can of worms is now figuring out how to mate the 4L80E to the NP205 (32-spline/6-bolt Round Late Model Pattern)

I have read a bunch of different threads on various sites talking about machining the THM400/NP205 adapter currently in my truck, but nothing of any real detail or substance. Can anyone chime in on exactly what I need to do? Or even how to find the "ultra rare" 1991 V3500 4L80E/NP205 adapter that seems to be a mythical unicorn-type part?!?!?!

Steve

 

[nvrenuf]

First, you'd probably never find a factory adapter by itself - it was only available one year and only in 4 door trucks (no Subs or K5's).

Search the Garage forum for info on machining the adapter. Iirc, you'll need to machine an o-ring groove on the trans side and grind down an area on the on the bottom of the trans side mounting flange because it contacts the trans case.

 

[Evil SS 2006]

Holy crap! The big old pile of threads I have found make my head hurt! I have figured it out, but I really don't want to use the 5/8" spacer if I don't have to! I am going to search and search and hopefully find the 1991 80/205 adapter, but I feel it will be a fruitless search!

I am also considering having these made of billet or even casting reproductions IF I can find one that someone is willing to send me for mock-up and modeling...

 

[sweetk30]

nos or used plan on around 500 bucks for the 91 only adaptor.

the 4l80 is longer than the 400 anyways and that 5/8 spacer wont hurt a thing.

if that tranny core was 2wd dont forget the oil feed plug has to be swaped out to solid plug in the tranny tail at the 3 oclock position . or oil pressure can drop off and cause tranny to burn up.

 

[Evil SS 2006]

nos or used plan on around 500 bucks for the 91 only adaptor.

the 4l80 is longer than the 400 anyways and that 5/8 spacer wont hurt a thing.

if that tranny core was 2wd dont forget the oil feed plug has to be swaped out to solid plug in the tranny tail at the 3 oclock position . or oil pressure can drop off and cause tranny to burn up.

I'd gladly pay the price for one...but i'm having no luck even finding even used ones!

 

[Evil SS 2006]

If the 241 or 208C weren't so weak, i'd totally just run one of them to make the 4L80E swap easier!

 

[sweetk30]

241 good case. lots of guys here run that one.

you could do this tho and be done with it. http://coloradok5.com/forums/showthread.php?t=253000

 

[Evil SS 2006]

241 good case. lots of guys here run that one.

you could do this tho and be done with it. http://coloradok5.com/forums/showthread.php?t=253000

Your project in the link is confusing because you are doubling and using a 203 mated first to the 4L80E with an NP241 adapter...right?

So if I get the later model adapter for 4L80E to NP241, which is still available, I can bolt everything right in and build new drive shafts, correct?

 

[sweetk30]

4l80 uses flush tail shaft on the tranny. unlike 350/400 use recesed tail that gives all the room for the parts.

so i had to use the 241 adaptor and redrill the front ofthe 203 to bolt up correct.

but if you dont need to build a crawler then its a big waste of cash for you then.

but if you did it would let you run highway gears and get low off road with 4:1 crawl .

 

[Evil SS 2006]

I definitely don't want to double case it, and I definitely don't need a 4:1 low range...unless I literally want to pull a house off it's foundation!

I have a friend that does all my fab work and as long as I can find the dimensions of what I need, we can built anything to make this work!

 

[Jimbo*]

Those adapters are not unicorns, I have one in my possession; not for sale.
If I did not have the correct adapter, I would not hesitate to use a spacer, as the adapter is a hen's tooth, and a spacer allows you to get on with your project, with complete reliability.
I like the way you are having your 4L80 built with the reluctor ring, sort of eliminates the need for the vss in the 205 tailhousing. I say sort of because everything will work great until you put your t/c in 4LO, then the engine will probably go into default mode as the vss will not be reading true vehicle speed; I still don't see a problem, as you most likely won't have your foot on the floor in 4LO

 

[Evil SS 2006]

Those adapters are not unicorns, I have one in my possession; not for sale.
If I did not have the correct adapter, I would not hesitate to use a spacer, as the adapter is a hen's tooth, and a spacer allows you to get on with your project, with complete reliability.
I like the way you are having your 4L80 built with the reluctor ring, sort of eliminates the need for the vss in the 205 tailhousing. I say sort of because everything will work great until you put your t/c in 4LO, then the engine will probably go into default mode as the vss will not be reading true vehicle speed; I still don't see a problem, as you most likely won't have your foot on the floor in 4LO

Ya, I doubt the truck will ever see 4LO, but I hadn't considered the issue with the VSS accuracy...

Is there anyway I might be able to get you to send me the adapter for mock-up so I can set up a jig to either fabricate our own adapter or cut one out of billet on the CNC and use yours for specs? I'd even be willing to send you Paypal money as a deposit on it...I will continue to search for one, but help from a fellow member would be awesome!

No worries if you don't feel comfortable doing this, worst case, maybe a full spec sheet on the one you have would help...

 

[Jimbo*]

PM sent.

 

[Evil SS 2006]

PM replied!

 

[Evil SS 2006]

So, in doing the abundance of research I have, a few things strike me as odd...

1. In the '91 V3500 4L80E/205 combo, the 15629600 adapter housing was long, from what I can tell, 4.0" or so. Since we can assume the NP205 in the V3500 had the 32spl long input shaft. This is totally evident in why GM used such a long adapter, right? I have the original GM part number of the output shaft used in the V3500 and I will edit this post once I am back home and have it. Am I correct in what I've said so far?

2. I pulled the schematic for the transfer case shift linkage assembly AND the same schematic for the '91 V3500. Everything in the shift linkage is the same, so what my question is, since the 4L80E was physically longer than the THM400, how much did the shift lever move in the cab? Was there a different hole cut in the floor in the '91 or was there enough space to compensate for the longer 4L80E in the existing hole?

 

[Jimbo*]

Actually, the adapter is not very long at all; about 4.50 inches if I recall correctly. I include for your viewing pleasure; 4L80e/NP205 adapter, TH400/NP205 adapter, and comparison of the two side by side.

As for your second question, I sould be able to answer that in a couple months.

 

[sweetk30]

also notice the ears for the mount . thay are back on the 4l80-e adaptor to clear the tranny pan.

 

[Jimbo*]

also notice the ears for the mount . thay are back on the 4l80-e adaptor to clear the tranny pan.

Precisely SweetK30. This is what allows many of us to simply use a 5/8" or 3/4" spacer on the 4L80 side and be happy.

 

[Evil SS 2006]

What I was getting at is that in comparison to other 4L80E adapters, this one is long, which clearly means one of two things...

1. the 4L80E used in the '91 V3500 did not use your typical 4WD 4L80E output shaft...

OR

2. the transmission was virtually the same and the adapter was different simply because the leftover NP205's used in the V3500 had the long 32spl input hub. Support of this would be if the NP241 input hub was nearly flush on how far it protruded from the mating surface of the adapter...this seems to be the case since even the THM400 to NP208C adapterosaur in my Dad's truck is short like tge 4L80E to NP241 adapter...

 

[Evil SS 2006]

Also, what needs to change on the crossmember when switching from the THM400/NP205 to the 4L80E/NP205? It seems to me that you would have to do somethingwith the longer case AND the difference in the mounting feet...