I put in the wrong transmission, yes I know I am dumb.

[Quincy McBride]

Hi everyone, I have a 1989 gmc k3500. It still has the original 454 but the original 3 speed quit on me a few days ago. So myself and a friend look around our yard (scrapyard we work at) and see a nice looking trans that looks like it will fit. So we yank the old trans out and plop this one in--not much else to do right now so why not? Turns out it was a freshly rebuilt 4 speed out of a 2000 chevy 3500. It fit just fine but there are electrical connectors on the side and the 89 obviously doesn't have the wiring. Truck moves again but it doesn't shift because there is nothing telling it when to shift!

My question is, do we have to pull this one out and get the correct trans in or is there a way to bypass the wiring or wire the 89 to work with this trans?

I know I will take a lot of crap for this, but I feel like it could also be helpful for others to know if there is anyway to swap these different year transmissions. And yes next time I will make sure I swap in the correct parts.

 

[Eveready]

Sounds like you will need a computer to handle the trans. That one is above my level of knowledge but I expect someone on here will be able to help. It would seem that you could add the computer from a later truck (maybe a similar 3500 ) to handle the transmission . I would sure try before I changed it out.

 

[Supercharged111]

Computer and harness from a later truck to run the 4L80E. I say go through with it if the later trans will run on an older PCM. I think you'll be looking for a 92-95 harness from a 454/4L80 truck.

 

[Madscientist]

Best option for you is to get a transmission controller. https://www.ebay.com/itm/333607239608

There ya go!

 

[Supercharged111]

Best option for you is to get a transmission controller. https://www.ebay.com/itm/333607239608

There ya go!

That's a smoking deal, I thought they were way more than that.

 

[Madscientist]

That's a smoking deal, I thought they were way more than that.

They are normally $650-$1000 depending on what ya get. This one is usef but yes fantastic deal. Only 6 days to decide on it lol.

 

[someotherguy]

Computer and harness from a later truck to run the 4L80E. I say go through with it if the later trans will run on an older PCM. I think you'll be looking for a 92-95 harness from a 454/4L80 truck.

Agreed, that's the "easiest" way to a factory-style setup, though he'll have more than a few hurdles putting a 1991-1994 harness (1991 first year for 4L80E) and PCM into his 1989 truck. There will be firewall differences for the pass-thru block, some sensor wiring differences, later years (depending on GVWR) may expect a heated O2 sensor, 1992-1994 use a DRAC module whereas his 1989 handles the speed signal internally on the cluster (great opportunity to swap to needle gauges IMO), hell there's even power wiring differences to the HVAC stuff including the fan relay setup. It's one of those "they ain't all the same, and different enough to make you crazy" situations. It can be done, if you're persistent.. I'd skip 1995 as there will be way too many differences in the harness to easily overcome.

Once he's got all that done, then he discovers the 1991-1994 PCM doesn't like the 2000 4L80E. I believe there's a wiring harness change for the newer 4L80E's as well, I forget the year break though.

Or as someone else mentioned, get the aftermarket transmission controller. Easier, unless he wants the other benefits of going the harness swap route (mostly the needle gauges, and ease of re-calibrating the speedometer via DRAC modification.)

Richard

 

[stutaeng]

Didn't that generation come with Turbo 400?

Why not swap that in it? Plug and play. Probably not easier to find anymore. 4L80e would better with overdrive gears, no?

It really depends how much time and effort you are willing to put into it, and how you intend to use your truck.

 

[someotherguy]

Didn't that generation come with Turbo 400?

Why not swap that in it? Plug and play. Probably not easier to find anymore. 4L80e would better with overdrive gears, no?

It really depends how much time and effort you are willing to put into it, and how you intend to use your truck.

That's his original transmission that went out, a TH400, or by then was probably referred to as a 3L80HD (truck version of the 3L80) to conform with the newer naming convention.

Richard

 

[stutaeng]

That's his original transmission that went out, a TH400, or by then was probably referred to as a 3L80HD (truck version of the 3L80) to conform with the newer naming convention.

Richard

Ok, gotcha. I somehow miss understood he had a manual originally. TH400 = 3L80HD ('87 to '90) to match new naming convention; learned something new. Wikipedia says these transmissions were also used in some models of Ferrari, Jaguar/Daimler, Rolls-Royce, Bentleys, among others. That's pretty insane.

I do LOVE the 4L80e transmissions in my trucks...