1990 Suburban 4x4, TH400 to 4L80e swap

[rack]

Hello all,

First of all, I am in Europe/Bulgaria and I do not have easy access to junk yards and spare parts for USA cars.

I am planning to swap TH400 with 4L80e in my 1990 Suburban 4x4.
Engine is SBC 350, TBI, which I will also inspect and rebuild as necessary. Transfer case is NP 241C.

What I know so far is that I will need at least the following (random order):
1. TH 4L80e to NP241C transfer case adapter (GM Part 15724745).
2. Stand-alone transmission controller (I may go with US Shift Quick 4 Gen2).
3. New cooler lines and bigger radiator.
4. New transmission mount and to relocate cross member.
5. New transmission dipstick.
6. Adjust driveshafts, front and rear.
7. Adjust shift linkages for transmission and transfer case.

What I do not know and need info and help about are:

1. Do I need new flax plate for this engine and torque converter combo?

2. I have read somewhere that 4WD 4L80e have only one speed sensor, but in this swap to fully work, it needs two speed sensors. Is that true?

3. TH 4L80e transmission itself. Here I have two options:

3.1 Buy rebuild TH 4L80e from USA (I have some places in mind, will discus this later).

3.2 Buy “rebuild” TH 4L80e from an individual here. As far as know the story, he order it for his H1 6,2 Diesel but never install it.(He is reach guy, changing vehicle often, so, I would believe this story). It sits abandoned for 10 years in his garage. It looks clean. The code on transmission is 8KYP on front label, and KYP on yellow rear label. Unfortunately, there might be some water penetrated in it. There was leaking rain pipe near it and there was water in the transmission transport box. Isn’t it too risky?


Thanks a lot for any info, advises and help,
Ivo

[kwmech]

You should not have to replace the flexplate. I'm pretty sure I didn't when I did my swap. You will only use 3 of the torque convertor bolts anyways. I do remember having to replace the starter and bolts though. Transmission lines are up for grabs depending on what year the 4L80 you have is. I got an early one where it had both cooler lines in about the same place as the 700r4 and is the same as the 400 also. The later model 4L80s had a front and a rear line. If you have the later one where one of the lines goes to the back of the trans case, yeah they are quite different with the fittings and will have to be adapted to the radiator. Cross member will have to be moved back and possible re-drilling of the frame may be necessary. T-case shifter rod will have to be lengthened as well as the brace that bolts to the side of the case. I think I used the bell housing dust cover braces over without modification. The 4L80 should have both tone rings in the transmission, you just have to purchase another sensor and can plug into that with the stand alone harness and not use the sensor on the T-case. This is a good time to take the T-case apart, inspect, reseal and maybe a good time to put a slip yoke eliminator kit in the case. I mounted my controller in my glove box on the side with the peel and stick velcro cut to fit kit. My modifications were more involved as my 'burb had a 700r4 that I replaced. For your filler tube and gauge I would look for a matching used pair. I don't know what happens to these at the wrecking yards---they just vanish never to be seen again, just like the 1 sock that goes missing

[Hawkynz]

Hello. I am currently doing this exact swap right now. I will try to answer some of your questions and tell you what I did. I have an 89 K5 blazer though but its pretty close. I am also running a 5.3l and swapping the 700R4 out to a 4l80e with NP241 transfer case.


What I know so far is that I will need at least the following (random order):
1. TH 4L80e to NP241C transfer case adapter (GM Part 15724745).
I got this adapter off of Amazon for $36 Dorman part 926-891. Uses the same transmission mount

2. Stand-alone transmission controller (I may go with US Shift Quick 4 Gen2).
I am using a holley terminator x. Uses both front and rear input output sensors on the two wheel drive 4l80e.

3. New cooler lines and bigger radiator.
I am using the stock lines and stock radiator. I might add a cooler later.
The front line on the transmission will retain the stock trans line. The rear line will be too short. I used AN to 5/16 compression fitting and rubber hose to extend the line. (Evil energy 6an male to 5/16 hardline compression fitting off of Amazon).

4. New transmission mount and to relocate cross member.
Use the stock mount and crossmember. Just slide it back and drill new hole.

5. New transmission dipstick. I used the 4l80e stock one.

6. Adjust driveshafts, front and rear. The new set up will be about two inches longer. My front shaft extends enough to use and my rear I think will work, but I also have a blazer.

7. Adjust shift linkages for transmission and transfer case. You will also need to extend the long bar brace that connects the transfer case to the transmission.

You will need to get a new input shaft for the transfer case. The 4l80e is 32 spline. My 700R4 is 27 so I have to change the input on the transfer case.

I suggest getting a two wheel drive transmission because it will have both input and outputs on the transmission and you can use mechanical speedo output on the transfer case tail housing. I was told you will need to change the mechanical to VSS or something in order for what ever system you are using to recognize the correct speed while in four wheel mode.

If you get a two wheel transmission you will need to shorten the output shaft and plug up the freeze plug that lubricates the tailhousing on the two wheel drive transmission. There are videos on youtube.

What I do not know and need info and help about are:

1. Do I need new flax plate for this engine and torque converter combo?
You will need 4l80e flex plate. Some people call them 6.0L flexplates.

2. I have read somewhere that 4WD 4L80e have only one speed sensor, but in this swap to fully work, it needs two speed sensors. Is that true?
This is true because the output sensor is on the tailhousing of the transfer case. Thats why most people get a two wheel drive.

3. TH 4L80e transmission itself. Here I have two options:

3.1 Buy rebuild TH 4L80e from USA (I have some places in mind, will discus this later).

3.2 Buy “rebuild” TH 4L80e from an individual here. As far as know the story, he order it for his H1 6,2 Diesel but never install it.(He is reach guy, changing vehicle often, so, I would believe this story). It sits abandoned for 10 years in his garage. It looks clean. The code on transmission is 8KYP on front label, and KYP on yellow rear label. Unfortunately, there might be some water penetrated in it. There was leaking rain pipe near it and there was water in the transmission transport box. Isn’t it too risky?

I would buy the used one and change all the fluid. Hope this helps.

[rack]

Hi Hawkynz,

Thanks a lot for the useful info. If you have thread about your swap, let me know to follow it.

I might follow your advice and next week I will go to inspect the transmission that is here in Bulgaria.

I have some more questions.

Do I need neutral safety switch at the trans, or I would use the one in the steering column (I guess it is there)?

Since my truck originally came with three speed auto, do I need to change anything in the steering column or it will shift 4 speed auto as well?

I am not native English speaker, so, please point me what is the name of the part that shows which gear is engaged on the dash?
The one with the red arrow and PRND21. I have seen this with D in circle and 321. I would like to buy and install one of these when I do the swap.

Thanks a lot for the help!

Ivo

[kwmech]

Steering column will not be needed unless you plan on pulling the transmission all the way down into first gear and hold it there. You're right, neutral switch on column will suffice

[rack]

Hi kwmech,

Thanks for replies.

I guess I will hardly need first gear and use it for long time. I am not planning to tow any heavy trailers.

May be, if my life goes well, ;-), some speed boat, but no more than 1,5 metric tons.

Best regards,
Ivo

[Hawkynz]

Hello. You do not the need the neutral safety switch on the transmission. They were used on later model trucks with digital guages. You stock one is fine.I believe your steering column will shift through all the gears but they will not line up on the shift indicator. You can replace your three speed indicator with a 4 speed shift indicator. It will read over drive, drive, 2,1. I sent some pics for reference from the LMC truck magazine.

[Hawkynz]

Here is a pic from my truck.