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1990 GMT400 TH400: Slipping when cold.

Road_Case

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Hey everyone. Pearl is starting to act up lately!
There seems to be a F***ton of trans issues/questions threads on this site. So I'll make mine short and sweet.

'90 8-lug K2500. Odometer reads 146,XXX. 5.7/TH400/floor-shift-TC(NP241?). 14-Bolt semi-float, 3.73, G80-optioned. I think that covers drivetrain. Now symptoms.

First thing was slow to engage reverse. Problem worsened gradually, rectified in-the-moment by increasing engine RPMs. Results in reverse engaging after a light jerk rearward. Problem has worsened very slightly over time and is still present.

Recently developed problem is 2-3 shift/slip while cold (like, sitting overnight cold). 1-2 shift while cold is normal, usually around 10mph vehicle speed. 2-3 shift NORMALLY occurs/occurred roughly around 40mph prior to abnormal operation.

Lately, once that 40mph-ish mark is reached, I'm feeding her 1/8-to-1/4 throttle, she continues revving up very high in 2nd, forward pull seems to plataeu and will not complete the shift to 3rd. Once I let off throttle, shift to 3rd occurs. Once I give it anything more than 1/8 throttle, seemingly, transmission sort of slips back into 2nd and revs real high with very little pull feeling in the seat of my pants. Just a big noise-maker lol.

After 10-15 minutes of driving this all goes away and transmission shifts normal and tight.

No lights on dash on account of these shift points being vacuum-modulated.

Open to all suggestions, including any information available concerning what's needed for swapping to a 4l80e from a TH400. I hope I made this easy to read.

I have an alldataDIY subscription for this truck, just to throw that out there. Thanks!

EDIT: I just want to add that reverse gear engagement does NOT improve with a warmed up drivetrain.
 
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FRANKENBURBAN
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Sounds like a plugged filter, either the main filter, or one of the screens in the valve body. As to the 4l80e swap, its quite a bit of work. You will either need a complete donor truck, or get a stand alone trans controller. You'll need a trans, I would reccomend a later model like a 94+, but I would also reccomend installing a reluctor wheel on the output shaft for your stand alone controller(just my opinion, many use the opti 4 and splice into the speed sensor wires at the transfer case). Then you will need to find a shift cable and probably want to swap out your gear indicator to an OD style. You will have to move your trans cross member back and drill new holes for it, get a new longer front shaft, and have your rear shaft shortened. Since you have a th-400, your transfer case will bolt right up, you'll just have to lengthen the shift rod.

Its a doable swap, but alot more work and money than mpst think. If you could find a 91-94 donor, it can be done semi cheaply, but its still alot of work swapping harnesses around.
 

Road_Case

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Thanks for the replies guys.

Ferm, can the valve body screens be removed/inspected with the trans in place?

As for the 80e swap, I honestly figured if I could get quite a few things from a 91-93 truck, then that would be the easiest way to run an 80e. I know the later years made improvements but I know things are different like cooler lines, but I don't know what else.

If I was going to run a 91-93, I thought I would need an ECM from a 5.7l TBI/4l80e truck, maybe repin the connectors if anything was different, swap the trans assembly, possibly front or rear driveshafts, or both. And plug the vacuum line port on my intake manifold. If I have to drill new holes for the crossmember, I will. Also the cluster, and I guess the shift cable is different?

Thanks again
 

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From what I've found the switchover was 95 on the cable style. Cooler lines changed in 97, but your going to have to buy lines regardless as I doubt yours will work. As to wiring, there is ALOT more to it than just repinning the ecm plugs. Your 90 truck uses a mooney cluster with a built in drac. 91+ with a 4l80e uses an external drac module and different cluster. You will be adding ALOT of wires to make the 7427(the preferred model from what I've read) or older 7060 work. You will have to add all the trans wiring, swap the cluster and add in a drac, and the wiring for it. Its all doable, but its alot of wiring to go through.
 

Road_Case

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Thanks for all the helpful info. I think if I stay auto I'm definitely going with a 4l80. Even my girl agreed and she hates spending money lol.

So hardware-side I'd need new cooler lines, driveshafts, shift cable, gear indicator, and move the crossmember. Also you mentioned a output-side reluctor wheel. Is that needed on any 4l80e/ECM?

As far the wiring goes, I know it's a real time-consumer but it doesn't scare me. I've seen past the black magic stigma of it all lol. I'm an electrician/lighting guy at a local concert venue so wiring is nothing new to me. I'll just try to find the proper diagrams.

Also best way to deal with the transfer case shift lever? The 4l80 is a tad longer in total than the th400 isn't it?

Also any info anyone has on swapping to an NV4500 or what particular model to look for, I'm all ears. Considering all options currently.

Thank you!
 

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Output reluctor wheel depends on which ecm you run. The transfer case is simply lengthening the shift rod for it. If you undertake doing this with oem wiring and ecm, only do it if you buy a donor truck with the same engine as yours. You will be modifying the engine harness, ecm harness, and redoing the whole cluster harness.
 

JayTheCPA

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Also any info anyone has on swapping to an NV4500 . . .
Did the 4L80E to NV4500 swap in the Burb a couple years ago.

Here is a T-56 swap in a '96 for reference: http://www.gmtruckcentral.com/articles/t56.html

For the actual parts list, consider researching as some parts are different across the GMT-400 series. IIRC, the different parts are bellhousing and clutch hydraulics. Basically, '95 was an odd-ball year. '94 and prior should interchange. '96+ should interchange.

The only part I could not get either new or reman was the brake pedal (got it from a bone yard). Everything else was readily available via RockAuto, other big-box-online stores, or Midwest Transmission. Only down-side to buying the reman tranny was that I did not have a core to return, so it cost a bit more than the advertised price.
 

Road_Case

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Well, if I can help it, I'd like to keep her 3l80 running right. I don't think the vacuum modulator is causing any issue, seeing as the shifting is normal once the trans is up to a good temp.

So it's maintenance time. I'm gonna do fresh 80w-90 in front and rear diffs, and fresh Dexron VI in the trans and transfer case. Wish me luck getting her set straight! These could all be original fluids for all I know.

 

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Dexron vi is a HORRIBLE fluid imo, and is NOT recommended for use in your transfer case. GM even tells you NOT to use it in your transfer case(they still sell dex 3 labled as manual trabs and transfer case lubricant). dex vi is KNOWN to cause seal failures in old seals. If you want to finish it off, go ahead and put dex vi in it, otherwise get some dex/merc fluid for it.
 

Road_Case

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I appreciate you keeping up with me. I figured it would work as it claims to be appropriate for use in systems calling for ii and iii, if I'm not mistaken. I wouldn't have assumed they were all that different. I will try to find the proper fluid then.

Of course I don't want to "finish it off", I simply didn't know any better. I understand where you're coming from though. Thanks again.
 

Road_Case

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..... GM even tells you NOT to use it in your transfer case(they still sell dex 3 labled as manual trabs and transfer case lubricant).....
Did a little research, would you recommend GM part # 88861800?

I presume that's what you were referring to. And I would intend on purchasing ~8 quarts of the stuff to have enough to do the trans filter change, and transfer case drain/refill. Would that be a better idea than Dex VI? Thanks
 

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I always use castrol or valvoline dex/merc fluid in applications calling for dex3. GM says to use dex vi in all auto trans now, but they could care less if the fluid takes out your viton seals, and leaves you with fluid leaking. They just say its an old vehicle and finslly failed. And to my knowledge it has never been recommended for transfer cases. You can use the Gm fluid, but I wouldnt spend the extra money for what is probably the same exact fluid as the valvoline dex/merc fluid(valvoline seems to manufacture alot of oils sold under other labels). Walmart normally has the best prices on oil as they sell them at cost to try and get you in to buy other things.
 

Road_Case

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I've got two gallons of the good stuff on the way, should arrive Monday, alongside my filter kit, rear diff gasket, and gear oil. I'll probably get to it Tuesday. Let you know how it goes.

Also, what can I do to check the valve body while I'm in there? Can the valve body be removed?

 

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The valve body can be removed, but its task best left to somebody with all the tools to work on one, and has done them before. Lose just 1 checkball and you'll be in a world of hurt.
 

Road_Case

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I have a lead
The valve body can be removed, but its task best left to somebody with all the tools to work on one, and has done them before. Lose just 1 checkball and you'll be in a world of hurt.
I see. I'm not afraid of buying specialty tools if need be, even if I rarely use them. However I would hate to mess things up, given my inexperience working on transmissions, so I won't try to remove it myself.

Is there any way I can clear the valve body screens while I'm in there, with everything still in place, after the filter and related parts are removed? Thanks for the hand-holding lol
 
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