Which transmission do I have?

[Graham Piccinini]

Hello, need help.
A few hours ago bought a 1990 GMC Sierra SL1500. 2WD, 4.3L TBI, 8ft bed, 34gal fuel tank.
Perfect for my needs, but I'm having trouble finding out which transmission it has. My small research brings lots of conflicting information: 4L60, 700r4, 3L80, THM400, etc... Basically - school me. On a pre-purchase glance underneath I did see "Hydromatic" along other words stamped on the trans pan. Couldn't get under it to read everything stamped on it though, just remember "Hydromatic". I'll check it again tomorrow, but is there an easy way to see which transmission I have for sure?
ATF looks original, 132k miles and 32 years old... Shifts great, but fluid smells worn/burnt, if that makes sense... Correct level and pan is dry though. It seems like I can only feel 3 shifts, but I don't know if the forth one is just so smooth that I can't catch it, or if it's missing due to a malfunction, or if I truly do have the 3-speed automatic with that 4.3L V6. (Some websites say only 454SS came with the 3-speed, so more confusion for me.) Tried a couple GM VIN decoders, and none tell me which transmission it has.
Truck was on school district duty it's whole life. Officially I'm the 2nd owner, but definitely not the 2nd driver...

 

[Trashman]

Hello, need help.
A few hours ago bought a 1990 GMC Sierra SL1500. 2WD, 4.3L TBI, 8ft bed, 34gal fuel tank.
Perfect for my needs, but I'm having trouble finding out which transmission it has. My small research brings lots of conflicting information: 4L60, 700r4, 3L80, THM400, etc... Basically - school me. On a pre-purchase glance underneath I did see "Hydromatic" along other words stamped on the trans pan. Couldn't get under it to read everything stamped on it though, just remember "Hydromatic". I'll check it again tomorrow, but is there an easy way to see which transmission I have for sure?
ATF looks original, 132k miles and 32 years old... Shifts great, but fluid smells worn/burnt, if that makes sense... Correct level and pan is dry though. It seems like I can only feel 3 shifts, but I don't know if the forth one is just so smooth that I can't catch it, or if it's missing due to a malfunction, or if I truly do have the 3-speed automatic with that 4.3L V6. (Some websites say only 454SS came with the 3-speed, so more confusion for me.) Tried a couple GM VIN decoders, and none tell me which transmission it has.
Truck was on school district duty it's whole life. Officially I'm the 2nd owner, but definitely not the 2nd driver...

Should be the 700R4. Open the glove box and there will be a white sticker on the inside of the door with all of the RPO codes, that will tell you which tranny you have. Look for a code that starts with "M", those are for the tranny. 700R4 is MD8. Hope this helps.

 

[clinebarger]

I would guess a 4L60 which is just a updated & renamed 700R4.

There are only a few RWD GM transmissions with Hydra-Matic stamped in the pan.....The TH400 & the light duty version TH375, TH200-4R, & the TH200.

There's a outside chance it has a TH375 from the factory, The SPID sticker in the glove compartment will list the transmission the truck was assembled with. MD8 is the 4L60 code.

 

[supton]

I’d look at rhe RPO codes like clines says, but then crawl underneath and look at the pan. 30 years is long enough for someone to swap.

The Novak Guide to the GM TH400 Automatic Transmission

The Novak Guide to the GM TH400 Automatic Transmission

www.novak-adapt.com

I can’t find a good site but look at the pan and identify shape, then look to see if 350, 400 (doubtful!) or 700.

 

[skyactiv]

See if the truck puffs smoke on a cold morning start. The valve stem seals wear out in those engines but are very simple to replace in a few hours.

 

[Graham Piccinini]

Should be the 700R4. Open the glove box and there will be a white sticker on the inside of the door with all of the RPO codes, that will tell you which tranny you have. Look for a code that starts with "M", those are for the tranny. 700R4 is MD8. Hope this helps.

I would guess a 4L60 which is just a updated & renamed 700R4.

There are only a few RWD GM transmissions with Hydra-Matic stamped in the pan.....The TH400 & the light duty version TH375, TH200-4R, & the TH200.

There's a outside chance it has a TH375 from the factory, The SPID sticker in the glove compartment will list the transmission the truck was assembled with. MD8 is the 4L60 code.

I’d look at rhe RPO codes like clines says, but then crawl underneath and look at the pan. 30 years is long enough for someone to swap.

The Novak Guide to the GM TH400 Automatic Transmission

The Novak Guide to the GM TH400 Automatic Transmission

www.novak-adapt.com

I can’t find a good site but look at the pan and identify shape, then look to see if 350, 400 (doubtful!) or 700.

Thank you fellas! Will look in the glove compartment and update.

See if the truck puffs smoke on a cold morning start. The valve stem seals wear out in those engines but are very simple to replace in a few hours.

As far as valve stem seals - she's a clean breather so far. No smoke on cold start, or anytime really. Ran it all the way to Redline and didn't see a puff of smoke behind me, no smoke on idling either. Must've got lucky, for now at least.

I will say it sounds great for a V6. In low RPM range sound is identical to my previously owned 1995 5.7L TBI V8, nice and throaty. Hopefully MPG will be better than that 5.7 was.

 

[supton]

Hopefully MPG will be better than that 5.7 was.

Good luck—but don’t hold your breath. I’ve seen few reports of the 4.3 being more economical. The old tbi motors, in a big truck, prefer low rpm, but under any load the smaller mills have to spin up to make the necessary power. Where they might not do so well. Uncorking the exhaust may help, and if you are driving in town “a lot” a deeper rear gear may help mpg.

fwiw the rpo codes will also have rear end ratio. Look for something like gu4 or the like. Good info to have, even if you don’t plan to change. also look for g80, that is the locker option.

 

[SwampSurvivor]

I have heard of some 4.3 trucks in the GMT400 coming with a TH400. Take a picture of the transmission for better identification.

 

[Graham Piccinini]

Anyone able to decipher this?

 

[Trashman]

Anyone able to decipher this?

View attachment 102284View attachment 102285

M40 is the TH400, no overdive, but it's about the toughest tranny you could ask for in a GM at that time. It also says MX1, usually RPO codes that start with "M" are for the tranny, I'm not sure what MX1 is, hopefully someone else can chime in on that one. But M40 is TH400, which is a great tranny.

 

[supton]

Pan looks like a 400 to me. Which is 3 speed with no lockup.

gu2 comes up as 2.73 rear gears. Highway gearing. Seems rather tall, especially with the 2.48:1 TH400 first gear. Might turn ok rpm on the highway, but I’d think sluggish around town? Edit: looks like gq1 means open diff.

 

[clinebarger]

M40 is the TH400, no overdive, but it's about the toughest tranny you could ask for in a GM at that time. It also says MX1, usually RPO codes that start with "M" are for the tranny, I'm not sure what MX1 is, hopefully someone else can chime in on that one. But M40 is TH400, which is a great tranny.

I've heard a few different explanations for MX1, One it's just a "place holder" for the actual transmission installed (M40 in this case).
The other is because the trans is bought from a different division (Hydramatic) or bought outside the company umbrella like a Borg Warner.

700R4 & TH350 units for example are not Hydra-Matic built.

 

[clinebarger]

M40 can be a TH400 or a TH375. TH400's installed in trucks have a 4" Tailhousing, TH375's have a much longer 9.5" Tailhousing.

This is a Jaguar TH400 with a 4" Tailhousing.

 

[Graham Piccinini]

I've heard great things about the durability of a TH400 and TH375. Hopefully it is as durable as it is claimed to be. Seems to be digesting all the torque of the mighty 4.3L quite easily.

Did a 200+ mile drive today. Glad to report no obvious issues. It happily uses all of the speedometer, but seems to be the happiest at 65MPH. At that speed it requires very little throttle to stay there, and feels very effortless.
- Below 60MPH temp gauge stays 1/4" cooler than 210°.
- 60MPH to 70MPH temp gauge sits at 210° mark.
- 70MPH to 85MPH it sits 1/4" towards the Hot zone from 210° mark.
Used about half tank. Will calculate MPG at next fill-up.
Oil pressure never dropped below 30psi mark. It currently has a frankenbrew of 50% Idemitsu 0w20 and 50% Redline Racing 50WT (15w50). Just a short OCI to hopefully clean it up just a little. Whatever oil was in there before looked and smelled pretty rough.

Yes, that's a dead insect inside the gauge cluster. I'll get it out eventually. Gear selector indicator doesn't work. Trip meter doesn't work either. Odometer and the rest of the gauges operate like they should.

 

[Bottom_Feeder]

Was it common to put the TH400 behind a V6 in truck applications? Seems like an odd combination.

 

[Graham Piccinini]

Was it common to put the TH400 behind a V6 in truck applications? Seems like an odd combination.

I wonder if it was a custom order by that school district that I bought it from. They got this truck when it had less than 200 miles, according to the title.

 

[Graham Piccinini]

@supton Going to keep a fuel log on this thing. Almost 19MPG on a very mixed driving day. I'd say it was 1/3 City traffic, 1/3 Hills in Boone NC area, and 1/3 Highway between 60-80MPH. Not bad.

 

[clinebarger]

Was it common to put the TH400 behind a V6 in truck applications? Seems like an odd combination.

Usually base model stripper 4.3L trucks have a manual transmission, The base auto option was a TH400/TH375 through 1993.

I can say I've never seen a GMT400 1500 V8 truck have a factory TH400 other than 454SS trucks.

 

[nascarnation]

Impressive mpg. My 99 S-10 4.3 with the 4L60 (overdrive 4th) and a 3.73 axle rarely gets over 17.

 

[paulo57509]

M40 is the TH400, no overdive, but it's about the toughest tranny you could ask for in a GM at that time. It also says MX1, usually RPO codes that start with "M" are for the tranny, I'm not sure what MX1 is, hopefully someone else can chime in on that one. But M40 is TH400, which is a great tranny.

M40 = TRANSMISSION, AUTO 3 SPD, TORQUE CONV VAR 1
MX1 = MERCHANDISED, TRANS, AUTO PROVISIONS

I have a list of over 3500 RPO codes. I'd attach it but Excel files are not allowed. It is no where complete and in some cases, GM reused RPO numbers and over time, they can denote something else all together.

Mothra inside the instruments.