Which transmission for 1984 GMC pu?

[RayP(MI)]

Pushed one or two many snowbanks last spring and the transmission on my old farm truck gave up. Only first and reverse. Finally have collected enough (hopefully) to cover replacement or rebuild. Checked with my NAPA dealer, and he says it is a 4L60 transmission. Shop manual shows only 350, 400, and 700. Inspection shows definitely not a 350. I think it is a 400. Can anyone give me a difinitive answer as to what it is? 1984 GMC Sierra Custom 1500, 6.2L diesel, 4WD. Serial 2GTEK14C9E150xxxx. Any place on the internet where this can be confirmed? Would this oddball transmission be due to short tailpiece for 4wd transfer case?

 

[bradk]

Almost guarantee that it's a 700R4(early version ot 4L60-E.And I'm positive that you broke the sunshell.A hardened stronger one is available at GM dealers and are pretty cheap & easy to install.
~brad

 

[D17 man]

ok, 700r4 has rectangular pan, t-350 is square with the corner cut off, and the t-400 is a kidney like shaped pan. i think the 4l60 wasnt out til later. ask allan in ne he'll have the answer most likely

 

[Gene Davis (Ga.)]

The TH 400 transmissions I have seen had an electric downshift instead of a cable like the 350 and the 700-R-4s. The 1984 6.2 diesel pickup I had came with a 700R4 transmission and it didn't hold up too well under a strain. I changed to a TH-400 and used a manual down shift detent cable instead of the elecric down shift switch and the transmission was tough and never gave any more transmission trouble and the fuel milage was actually better considering it didn't shift up and down all the time under a load. The 700R4 had to be run in drive under a load so the overdrive was pretty much useless. Of the choices you listed the 400 is probably the heavier duty of them.

 

[Allan in NE]

Ray,

You DO have first and reverse? Believe me, that's a good thing! :>)

Crawl under there and see what shape pan it has; gotta know what you're dealin' with 'cause they all could be configured with the short shaft for 4x4 operation.

Second, pull the dipstick and see what kind of condition the fluid is in. Either gonna be black mud or a resemblance of pink tranny fluid.

This could be a really, really simple fix or an old fashioned tear-jerker.

Allan

 

[Anonymous-0]

Gene can you switch an electric kickdown trans to a manual kickdown or did you find a 400 with a manual kickdown. I changed over an '84 K10 to a 400 electric and miss the manual kickdown I had with a different truck that had a 350 trans. chris

 

[roger roland]

can tell you this much .. the 700 is a over drive trany so if u dont have a over drive spot on shifter u dont have that one

 

[RayP(MI)]

Wll, first of all, tranney fluid is clear, but a little burned smell, and darker than the nice pink. It had been hesitant to shift 1-2 for some time, especially when cold, but while plowing, it just quit shifting at all. Not the govenror - already checked that. Probably tranney is pretty well shot, anyway.

Square pan. Most probably a 700R4. Am told that 4L60 is a new designation for the 700. Right?

 

[Allan in NE]

Yes Sir,

The 700 and the 4L60 mean the same thing. Just a newer designation.

Here's the order in which I used to run the dianostic tree on those old buggers for a no upshift condition:

1. Fuel filter
2. Throttle cable
3. Governor
4. Finally, pull the pan to check for debris and/or a plugged filter. Rip the filter open to see what's inside.

If the pan has signs of black "soot" in the bottom, she's toast. If its clean as a whistle in there, chances are it has a stuck TV valve or a broken band.

Just for what it's worth, a stuck valve condition or a broken band can be verified by having the truck up in the air and idling in drive. Shove in on the 2-4 servo cap with a large pry bar and the transmission should shift up.

I'm like you tho. Since you've been using it for snow plow work and given the fact that it has that "clutchy" smell, I'd be leaning towards the notion that I've got some major work ahead of me.

Hope ya get 'er,

Allan

 

[Allan in NE]

Mornin' Brad,

Yeah, those input sun gears and/or shells were a bugger for sure. However, when they broke they left you with only 3rd and 4th gears.

"Almost always" no drive in forward or reverse from a stop. :>(

Allan

 

[JDemaris]

GM didn't make any 1/2 trucks with 6.2 diesel with TH350 or TH400 trans. The half-ton 6.2s only came with the 700R4s if automatic - and if standard usually the New Process four-speed overdrive.

Many early half-ton trucks, Suburbans, and K5 Blazers with 700R4s got converted to TH400s. Often, the conversions use mechancial modulators in the TH400 to avoid vacuum issues. Otherwise, a VRV (vacuum regulator valve) has to be hooked to the injection pump to make the TH400 work correctly.

The other issue is - since your truck is 4WD with a NP 208 transfer-case - the transfer-case that was originally used with the 700R4 will not fit the TH400 - the splines are different. The original transfer-case will fit a TH350.

By the way - the TH400 is also called the 3L80, and the 700R4 is the 4L60.

I've got over 20 trucks with 6.2s in just about every variation you could think of.

 

[RayP(MI)]

Talked with my son's friend who runs an independent auto repair facility, and is an instructor at the local community college. His bid for a rebuild of present transmission was several hundred less than a fatory rebuild. So we'll see what happens. I'm sure the transmission is crapped, and a rebuild is in order. Think the clutch pack is done in. From what I can gather, it is (was) a 700 - and I had it rebuilt about ten years ago, with whatever updates they had then. So who knows what it is now! Apparently the newer designation is 4L60.

 

[ADoble]

This is a very old post but I'm new to the forum and this swap. I have an 84 k10 6.2. It currently has a th350 and I want to put a freshly rebuilt th400 in it. The one I have is a 2wd trans (i know they make adapters) but the price was right. Has anyone here done this swap? What other parts should I be looking to get for the th400 swap?
I got the trans just rebuilt w/shift kit for free so even being a 2wd trans I think it's worth making it work.

 

[Steven Allen]

Yes Sir,

The 700 and the 4L60 mean the same thing. Just a newer designation.

Here's the order in which I used to run the dianostic tree on those old buggers for a no upshift condition:

1. Fuel filter
2. Throttle cable
3. Governor
4. Finally, pull the pan to check for debris and/or a plugged filter. Rip the filter open to see what's inside.

If the pan has signs of black "soot" in the bottom, she's toast. If its clean as a whistle in there, chances are it has a stuck TV valve or a broken band.

Just for what it's worth, a stuck valve condition or a broken band can be verified by having the truck up in the air and idling in drive. Shove in on the 2-4 servo cap with a large pry bar and the transmission should shift up.

I'm like you tho. Since you've been using it for snow plow work and given the fact that it has that "clutchy" smell, I'd be leaning towards the notion that I've got some major work ahead of me.

Hope ya get 'er,

Allan

Can anyone give me a hint as to why the fuel filter might be part of this diagnostic tree?

 

[scootergmc]

This is a very old post but I'm new to the forum and this swap. I have an 84 k10 6.2. It currently has a th350 and I want to put a freshly rebuilt th400 in it. The one I have is a 2wd trans (i know they make adapters) but the price was right. Has anyone here done this swap? What other parts should I be looking to get for the th400 swap?
I got the trans just rebuilt w/shift kit for free so even being a 2wd trans I think it's worth making it work.

Longer tailshaft on the TH400? Different splines? Shorter driveshaft different yoke? Crossmember? I don't recall...

 

[used red MN]

Can anyone give me a hint as to why the fuel filter might be part of this diagnostic tree?

Just a note on waking up and smelling the coffee this is a 2007 thread! However, as is proven by the new user ADoble in reply 13 who revived the thread there still are some 1984 GM trucks still on the road so adding info to the OP question is still relevant. If the transmission in question simply is slipping and not moving then the ability of the engine to rev up and make power is probably not relevant. It sounds like the Ray’s transmission was still moving just not shifting out of 1st gear the way I read it. Changing the fuel filter would assure the engine is revving and making power so it can shift the transmission and move the vehicle at higher speeds. Much cheaper to change a fuel filter instead of a transmission. The fuel filter change would probably be a greater chance at solving jerky shifting or shuttering such as often lock up torque converters do. I can’t remember if Allen in NE, was a service technician or not. Sounds like he has some experience with AT diagnosis.

 

[used red MN]

This is a very old post but I'm new to the forum and this swap. I have an 84 k10 6.2. It currently has a th350 and I want to put a freshly rebuilt th400 in it. The one I have is a 2wd trans (i know they make adapters) but the price was right. Has anyone here done this swap? What other parts should I be looking to get for the th400 swap?
I got the trans just rebuilt w/shift kit for free so even being a 2wd trans I think it's worth making it work.

Any idea of the original application of the TH400? If it had been an earlier cast iron transfer case model I would be fairly certain the your 400 would be a bust with the long tail shaft. I know they made changes when the went to the aluminum transfer case in the early 80’s but I don’t remember the specifics.

 

[UncleTom]

I learned the fuel filter trick many many years ago form Allen of Neb. by hanging out here. I worked with a kid that had a truck with an issue of missing a gear. I think 2nd but its been so long ago. I remember what Allen said about the fuel filter. I told the kid to change the fuel filter and his missing a gear went away and had all the gears shifting again. I didnt know Allen was still around on here. Welcome back Allen.

 

[used red MN]

. I didnt know Allen was still around on here. Welcome back Allen.

Apparently you drive right past this in my reply 16.

Just a note on waking up and smelling the coffee this is a 2007 thread!

Allen last posted on the forums Sep. 19, 2023.

 

[ADoble]

Any idea of the original application of the TH400? If it had been an earlier cast iron transfer case model I would be fairly certain the your 400 would be a bust with the long tail shaft. I know they made changes when the went to the aluminum transfer case in the early 80’s but I don’t remember the specifics.

I believe it came from a '78 1ton puckup. However the adapter I've found online comes with a new tail shaft for the 32 spline adaptation.