Curious if swapping out the bad G80 for a good GT4 will hurt anything?

Discussion in 'Chevy Truck Forum' started by BC BADBOY, Apr 17, 2013.

  1. BC BADBOY

    BC BADBOY Full Member

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    I bought a 1990 K1500 with a stock 5.7 and 700r4 trans and the G80 rear-end which turned out that all were bad. I replaced the engine with another 5.7 and been juggaling the idea of throwing in a 400 turbo out of an 89 K2500 along with a spare GT4 rear-end i have laying around bout need to knw if thats a headache waiting to happen....... PLEASE HELP I NEED MY TRUCK BACK!!!!!!!
     
  2. Boudin

    Boudin Forum Moderator

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    I'm confused here as the RPO codes you mentioned of G80 is (I believe) the rear end type an (a limited slip) and the GT4 is a separate code indicating the gear ratio (3.42).

    Do you have the harnesses of both?

    The TH400 is a non-overdrive transmission. The 700R4 is an overdrive version of the TH350.

    You mentioned "all bad". Surely you didn't mean that as it reads.
     
  3. BC BADBOY

    BC BADBOY Full Member

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    well see thats where im not sure because the rear-end rpo tag isnt very readable on the one from the 94 so i went off the rpo codes in the glovebox and the rear-end that came with the 90 the guy told me he had researced it and was told it was the limited slip. I know the 400 is a non over drive but not worried about that other then if i can hook up the detent solenoid to an aftermarket switch to make it work. When i said all bad i meant the engine, 700r4, rear-end and i believe the transfer case. I used the front diff from my old 94 so i knew the gears would match up in the front diff and the rear-end but after install i noticed my front diff was damaged and cant find the rpo from original front diff for the 90.
     
  4. BC BADBOY

    BC BADBOY Full Member

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    Oh and its a 3.73 not a 3.42
     
  5. Boudin

    Boudin Forum Moderator

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    Actually, the TH400 is a simpler setup as far as the detent switch is concerned. It is a simple electrical circuit that actually is engaged when you push the accelerator pedal to the floor. The switch itself is a simple unit that is physically located under the dash attached to the accelerator pedal. Truth be known, I had mine go out on me awhile back and rather than purchase a new one....I just installed a momentary toggle on the dash. When I want it to drop from 3 to 2 or 2 to 1, I just would need to reach over and flip the switch. It is spring loaded and will just go back to "off" when I take my finger off of it. Similar to the way a push button start would work.

    One of the unique properties of the TH400 over say the TH350. As you probably already know....the TH350 operates via a solenoid attached to the carburetor itself.

    I don't use my "rigged" detent very often. Honestly....I like it better this way.
     
  6. Boudin

    Boudin Forum Moderator

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    If you want to be sure on the ratios....just make you some chalk marks on the yoke and count the number of times it rotates for every wheel rotation. If you watch it closely, you should be able to determine exactly what it is. I say that meaning you can tell if it rotates a full 3 times and can easily tell the difference between almost halfway around for the 4th time (3.42) and almost 3/4 the way around for the 4th time (3.73).

    I see now I was incorrectly assuming you had a 2wd and not a 4wd, but I see now that I missed that "K" in your initial posting. I'm no expert on 4WD axle swaps....but don't you have to match the ratio between the front and rear?

    I would definitely check the ratios on those axles "manually". RPO codes and axle ratios in my experience are sometimes wrong. My truck has RPO codes indicating a locking differential and axle ratio of 4.10. However, it definitely has a 3.42 rear end on it. Strange since this truck was bought new in 84 by my uncle and has been passed to my uncle and then to me. Bottom line is that nobody ever changed gears on it....but it certainly doesn't have what it's supposed to. My uncle always speculated that the dealership switched them out thinking my great uncle wouldn't notice. He claimed that the one it was supposed to have was more valuable. Obviously, my great uncle didn't notice since he was not a gear head.....but his nephews certainly did. To be honest, I'm fine with the 3.42 since it is still a locking differential.

    You can also look and see what is stamped on the outside of the ring gear. It will be in this format...."##/##". Divide the two numbers and you have your ratio. A 4.10 for example would have "41/10".
     
  7. Boudin

    Boudin Forum Moderator

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    Btw....the axle codes based on RPO code depend on what they were originally put on (1/2 ton or 3/4 ton). A certain RPO code could represent two different ratios depending on this.

    I would make your marks and count and/or look for the ##/## stamped on the ring gear to be absolutely sure.
     
  8. BC BADBOY

    BC BADBOY Full Member

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    I've had the rear cover off on the 1 from the 94 which was a half ton and it was stamped 3.73. Now so im correct the computer dont run the detent?..... What front diffs usually came stock in the half tons? And did engine size matter?
     
  9. BC BADBOY

    BC BADBOY Full Member

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    Also did the front diff for the half ton with the 3.42/3.73 rear-end change with a positrac rear-end?
     
  10. Boudin

    Boudin Forum Moderator

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    #10 Boudin, Apr 20, 2013
    Last edited: Apr 20, 2013
    I'm having trouble understanding what you are asking on some of what you said in your last two posts. The computer does control the detent function in some setups....but not with anything you have mentioned here. The 4L60E or 4L80E transmission detent is controlled by the PCM. I didn't think you mentioned either of these in your possible options here. The TH400 is controlled by the switch on the accelerator pedal like I mentioned. The wire plugs in on the driver side of the transmission and is probably orange in color (the wire). The TH350 or its overdrive version, the 700R4 is operated by either the detent cable (TH350) or the TV cable (700R4). Is that what you were asking?

    As far as you asking about the positrac rear end changing the front gear ratio....I'm not sure what you are asking. Actually, to be correct here.....it's not a positrac, but actually a locking differential. Doesn't really matter with what you are asking....but I just wanted to clear that up. The RPO code of G80 tells you that you have a locking rear differential. There is a separate RPO code for the gear ratio. About the only relation between the two is that there might be certain gear ratios that were only available with either the locking differential or not. For example....I doubt you will find a 3.08 rear end ratio in anything but a standard (non-locking) rear end (usually called a peg-leg rear end).

    The ratio that was "stock" or the "standard" or maybe also called the "base model" was something that varied by year. Within those years, there were certain options like 2WD or 4WD....packages called a "towing package"....or even the infamous "Z71" package that would usually include only certain rear end ratios. Around the years that you are mentioning here (early 90s)....if the truck was a 4WD....had a "trailiering" or "towing" package....or the Z71 suspension package....you won't find a gear ratio higher than a 3.73. Remember, that a higher gear ratio actually is a lower number.

    Where did you see the 3.73 stamped exactly? If it was on the ring gear, I would expect it to read "41/11". The format of actually writing the gear ratio out like you mentioned (ex....3.42, 3.73. 4.11) was usually only something found on things like the window sticker or the RPO code sticker. Stamped on parts, you would usually see it in the format of ##/## like I mentioned (ex...41/12, 41/11, or 41/10 for the same three ratios). Of course this varied depending on model year....and certainly varied if the gears were aftermarket.

    I am wondering if I am missing something or misunderstanding with what you have described as far as details of your situation. I was under the impression that you are basically piecing a drivetrain together from a junkyard for your 1990 K1500 truck. Your entire drivetrain (engine, tranny, and both front and rear differentials) on the original truck are as you called "bad". Meaning....you are needing to replace all of them. You are looking for a replacement engine, transmission, and new differentials both front and rear.....right? You mentioned you already threw in another 5.7 liter motor in there. Was it also from a TBI truck? So, that leaves the transmission.....and front and rear ends. I'm still confused on the "GT4" rear end you mentioned. I'm assuming that you have a 10 bolt rear end originally on your truck....right? I would expect any 2500 model rear end to be a 14-bolt. Is there any particular reason you are wanting to change from the OEM setup and gears? I would stick with the 700R4 transmission and pick the gear ratio you want keeping in mind that you probably had a 3.73 gear to begin with. The TH400 transmission would work with some modifications.

    You mentioned something about a 94 model truck. Is it a complete truck or are you just saying that the parts came from what you think was a 94 truck? You need to check the actual ratios of the parts themselves to make sure they match. It's the only way to be sure here.
     

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