Advice Please: Planning 4L80 Conversion

FlameRed

Active member
148
225
43
Location
Florida
I would like to ask the collective advice on collecting parts to replace the 3 speed TH350 with a 4 speed 4L80(E?). I have a 1990 M998 with a 6.2, and TH350. I have done a lot of reading and actually I think I am more confused than before.

I decided I want to buy the transmission and other stuff rebuild and "tested" so I don't have to install something twice or more times! I am not a drive train expert so I have lots of questions.

From what I read, and the people I have talked to, I am more confused and have the following questions:
  • A 4L80 or 4L80E. Are they one in the same?
  • I am not sure what "spec" torque converter I need for a 6.2 GM diesel in a 4L80?
  • What should I do regarding the 4x4 transfer case that will fit on the back of the 4l80E? Can I just reuse the NP242 I have already? Will the NP242 direct fit a 4L80E? Or do I need some other transfer case that will bolt right up to the 4L80E? Is there a good source for a rebuilt one?
  • I know I need a different transmission mount for the 4L80E. See them available all over the place.
  • A different shifter for the 4-speed. I bought one for a "reasonable" price already.
  • A stand-alone transmission controller/harness. I think I see what I need.
  • I was told I can use the existing shift linkage from the TH350, and gear box to the new 4 speed shifter I have.
  • I was told I would have to get one driveshaft shortened and the other lengthen because the 4L80 is longer than the TH350. I think I can find a driveline shop in the area to handle this. Does anyone know how much the one needs to be shortened and other lengthened or is there a source for new driveshafts that are direct fit that are reasonably priced?
  • Are there any connections needed from the 4L80 to any engine sensors?
Thanks in advance!
 

Mogman

Moderator
Staff member
Moderator
Steel Soldiers Supporter
3,428
4,474
113
Location
Papalote, TX
All 4L80s are 4L80Es
Most likely the stand alone controller is going to need RPM and TPS input from the engine
EDIT, if you actually have the 242 TC then you can use stock A2 drive shafts, I think you will have to replace the rear diff, yoke
But your M998 came with a 218 TC, with the correct output shaft and trans adapter ether will bolt up to the trans, you will need a cross member also
 
Last edited:

FlameRed

Active member
148
225
43
Location
Florida
I got a quote from a local drive train shop in Florida. Here is what they stated:

  • They would supply a rebuilt 4L80e that is fully tested (y)
  • They would supply a 1300-1600 Stall HD Torque Converter (y)
  • They stated I should be able to use my current gear box.
  • They stated I should buy the transmission cross member from somewhere like Black Dog Customs myself.
  • A different shifter for the 4-speed. I bought one for a "reasonable" price already.
  • They will supply the transmission controller.
  • I was told I can use the existing shift linkage from the TH350, and gear box to the new 4 speed shifter I have.
  • I would have to get one driveshaft shortened and the other lengthen locally myself.
  • They recommend getting a GPS Style speedometer elsewhere.
Was over $6200 shipped with taxes. :eek:

Any better ideas on how to handle the speedo?
 

Mogman

Moderator
Staff member
Moderator
Steel Soldiers Supporter
3,428
4,474
113
Location
Papalote, TX
That is a very high price for a 4L80 transmission, why are you changing the speedo, comes off of the transfercase so there is no need to change.
My 6L80 cost $3000 with the diesel TC
 

FlameRed

Active member
148
225
43
Location
Florida
That is a very high price for a 4L80 transmission, why are you changing the speedo, comes off of the transfercase so there is no need to change.
My 6L80 cost $3000 with the diesel TC
Thanks for the info on the speedo - I did not know. ;)

A lot of that is because I do not have a like-for-like core - that is like $900 alone. Then the controller is $1600. Shipping and taxes are like another $600.

The tranny itself is like $2300 which is not terrible for a tested tranny.
 

Mogman

Moderator
Staff member
Moderator
Steel Soldiers Supporter
3,428
4,474
113
Location
Papalote, TX
You don't need a controller that expensive, here is one for 599, I will bet you could find one even cheaper.
Go down to your local pick a part and you should be able to buy a core, Craigs list is another good place to look for a core, I found my 6L core for $25
Find a local shop to build your trans. that way if you have problems you will not be faced with what one member just did, having to pay to have a trans. fixed locally that he bought mail order and hoping the original seller gives him some compensation.
 

Maxjeep1

Well-known member
Steel Soldiers Supporter
1,267
1,695
113
Location
Las Vegas Nevada
Sell it and start over with a newer truck… probably cheaper in the long run and you will probably have heavier duty stuff on the newer truck. Swap whatever stuff you like on yours before you sell it. I decided that I would like to have a turbo and a hardtop with some ac. I put a pencil to it and it adds up more than a new truck. Tuff decision and less effort to just get the transmission upgrade. Seems like you have a few suggestions on how to make it more affordable and I think it would be more tempting at a few thousand less
 

springer1981

Well-known member
783
1,002
93
Location
Maine
I got a quote from a local drive train shop in Florida. Here is what they stated:

  • They would supply a rebuilt 4L80e that is fully tested (y)
  • They would supply a 1300-1600 Stall HD Torque Converter (y)
  • They stated I should be able to use my current gear box.
  • They stated I should buy the transmission cross member from somewhere like Black Dog Customs myself.
  • A different shifter for the 4-speed. I bought one for a "reasonable" price already.
  • They will supply the transmission controller.
  • I was told I can use the existing shift linkage from the TH350, and gear box to the new 4 speed shifter I have.
  • I would have to get one driveshaft shortened and the other lengthen locally myself.
  • They recommend getting a GPS Style speedometer elsewhere.
Was over $6200 shipped with taxes. :eek:

Any better ideas on how to handle the speedo?
I bought a 4L80 and a 242 low mileage take out for $1200 with TQ converter. A transmission crossmember $75, new factory 4 speed shifter $450, shifter rods $50, new rear drive shaft $120, front drive shaft and rebuild it $150, rear yoke $25, speedo $100. I used the Quick Shift 4 and couldn't be happier $750. https://www.steelsoldiers.com/threads/my-new-1992-m998-avenger.184818/post-2408610 The only additional parts were a TPS $80 and fan kick down switch relocation $80. DEX VI tranny fluid $120.

I did the install myself https://www.steelsoldiers.com/threads/my-new-1992-m998-avenger.184818/post-2406637 and at the same time I upgraded the fuel tank, fuel sending unit, and everything associated with it about $500.

I put a 24 to 12 vdc converter in https://www.steelsoldiers.com/threads/my-new-1992-m998-avenger.184818/post-2411292 $50 and used it to power the TCU and other 12v items.

Quick total is $3750 with new fuel tank and everything. Add in shipping and a bunch of miscellaneous stuff probably $4500 to do the entire job.
 

FlameRed

Active member
148
225
43
Location
Florida
I wanted to thank all the forum member that replied here and that kindly spent their time talking to me on the phone. Much good advice. The consensus was to only buy a 4L80E with the 242 gearbox out of a HMMWV and not one from some other GM application.

I did a bit of searching and I found one that looks pretty clean a couple hours from me. It is out of a M1123 which had a blown engine. It does not have the driveshafts, or crossmember, so I would have to deal with those plus the tranny controller & harness, and rigging up the cooling and all the long list of little, but costly things. I hope to go see it this weekend.

s-l1600.jpg


s-l1602.jpgs-l200.jpg
 
Last edited:

springer1981

Well-known member
783
1,002
93
Location
Maine
That looks like a really good start. Later style transmission, has both shift linkage's, dipstick tube and probably the dipstick, trans fluid lines intact, muffler bracket, mechanical speedometer drive, both sensors and still has the wiring connector to protect the plug. Looks clean, no tell tale signs of abuse or high miles.
 

springer1981

Well-known member
783
1,002
93
Location
Maine
I put together a list of some of the parts I used in my conversion. Flamered I send you a PM

2520-01-432-3492 5937525 rear yoke
Dana Spicer 3-70-28X Strap Kit 1350/1410
2520-01-413-0080 Front Drive Shaft
Rear drive shaft 12K 919800-2109
5340-01-474-2298 12460455 backup light brackets QTY 2
Backup light switch for shifter with 3 wire. Not needed with shifter with 5 wire.
3040-01-409-2501 shift linkage
GM TPS Connector Pigtail 1995-2005 3.8 3800 V6 Engines Throttle Position Sensor
Throttle Position Sensor Standard TH160
 

FlameRed

Active member
148
225
43
Location
Florida
Would you mind sharing the information you got about not using one out of a GM vehicle?
Two very kind forum members spoke to me at length about this and shared a wealth of information. Both agreed that getting a 4L80 out of a HMMWV is best and get the 242 gear box mated to the transmission with all the cooling lines installed already. The way it was explained to me is there are some internal differences between a 4L80 out of a HMMWV and a civilian truck. But I am not a transmission guys that understands the magic.

Here is what I will try to summarize of what they taught me.

  • The conversion is not as popular to do as it once was as more later model are being sold at auction.
    • A good reason to do the conversion is because you live in a state, like mine, that is run by Kommunist bureaucrats that will NO LONGER let you road register a HMMWV anymore!
  • If you don't have much work into yours, and it is not "rare", it might be cheaper and easier to sell yours and buy a late model HMMWV.
  • Look for the cleanest "used" transmission you can find. Ones that are all rusty, caked in mud, or look like crap probably had a hard life. Or just be prepared to have it locally rebuilt! Don't have it rebuilt by far away re-builders as if there are problems shipping is going to kill you.
  • Buy the transmission first - you may find one that comes with all the needed stuff and you don't want to buy extra stuff you get with the tranny.
  • If you can get a package deal with as many of the necessary parts as possible
  • You are going to want a "late" style 4L80 which has the two cooling outlets separated. They are more robust than the early style 4L80 which had design flaws.
  • You are going to have to convert your cooling plumbing to match the new location of the late style 4L80 which are separated.
  • You are possibly going to need:
    • 242 gearbox
    • a 4L80E cross-member - but you can convert your old cross-member if you can fab a little and weld.
    • A new rear driveshaft with the correct yoke that is shorter, and the front one that is longer.
    • Possibly a new gas tank if yours does not have the "X" embossed on it, or you possibly can modify your old tank to prevent rear driveshaft interference but I am not sure what exactly this modification entails.
    • A torque converter to match a diesel.
    • The four speed shifter.
    • Independant tranny controller, and harness.
I am sure I missed some things or did not state things correctly, so don't be surprised when I am corrected by others that know far more than me!
 
Last edited:

Retiredwarhorses

Well-known member
Steel Soldiers Supporter
3,697
2,193
113
Location
Brentwood, Calif
Springer1981, please tell me about the fan kick down switch relocation parts, where you got them etc. That's one thing I still need to do from my 4l80e conversion.
you Would have to move the switch to how its installed on a turbo truck…moved to throttle cable bracket and uses a cam bracket
 

Vapor Trail

Active member
88
125
33
Location
Kentucky
I have an M1123 so I think it comes with a 4L80? Looks to be in rough shape. Would a 6L80 bolt right into the same spot and also if I put a 6.2 LS in there can the transmission stay where it is and I mount the motor and build motor mounts to it or does the whole combo need to move around?
 
Top
AdBlock Detected

We get it, advertisements are annoying!

Sure, ad-blocking software does a great job at blocking ads, but it also blocks useful features of our website like our supporting vendors. Their ads help keep Steel Soldiers going. Please consider disabling your ad blockers for the site. Thanks!

I've Disabled AdBlock
No Thanks