I have finally achieved the badge of honor of 2nd-Gen owners: Code P1870! I just got my '98 Jimmy on the road over the weekend and did some gigs out on the peninsula and on the way home the SES light came on. I pulled the code and—voilà—P1870. I couldn't be prouder.

Thanks to the collected experiences of other owners I found out what that means and why the transmission started having a very rough 1-2 upshift. I looked at the Sonnax kit and reamer and that's a little under $200 and would likely fix the problem. I haven't pulled the pan on the transmission yet to see what's on the magnet. The fluid doesn't smell burned but it does look a little dark. Since this is a high-mileage transmission (195,000) I'm leaning toward a replacement. I found a low-mileage 4L60E on Craigslist out of a 2000 GMC Sierra 1500 with only 88,000 miles on it. According to the seller it didn't slip or have any other problems when he pulled it. He took it out along with the motor because he put a built-up 454 in the truck along with a heavier-duty transmission. I can get that tranny for $300.

I'd like some input from the wisdom of the masses:

The newer transmission is a 2WD version. Can I just pull the tailpiece and replace it with the transfer case adapter off my current transmission?

Would the newer transmission benefit from a shift kit and separator plate? I prefer a firmer shift than the stock unit and I'd like to do anything to prolong the life of the newer transmission.

If I do get a shift kit for it is there a particular one that you would recommend?

Any other upgrades I should do while I'm swapping transmissions?

I'm getting a new roof on my garage this week so I'm thinking this would be a good time to be under the truck where I'm safe from falling debris.

 

As has been asked and answered MANY times in the past, NO. The tail shaft must be swapped and to do so, the transmission must be gutted as the tail shaft comes out the front of the 700R4/4L60E. While it's apart, it would be a good time to assess the health of the clutch packs and R&R as needed.

IMO, yes. I would recommend what I installed in my Tahoe. I posted details so you should be able to find the info with a search or two.

If you already have a it apart, I would recommend replacing the sun shell with The Beast replacement sun shell for cheap insurance against failure.

 

I had my 4L60E rebuilt with a TransGo Mr Shift shift kit complete with the Beast sun shell, kevlar belts, corvette servo, stringer springs etc. The kit was I think around $300 but the servo, sun shell were more $. In total the shop charged me $1800 & said that trans will be good for upwards of 350HP or so.

Well worth it & it shifts clean & crisp, very little belt burning going on.

 

Thanks for all the info! This sounds like a good opportunity to tear the transmission apart and check out what's going on inside. You guys are great!

 

Here's what I've decided to do:

It seems like a lot of work to tear apart the transmission just to swap the tail shaft so I decided against that particular swap. If I was going to gut a transmission I would just rebuild it. Instead I've ordered the Fitzall TCC valve, a TransGo shift kit, a Corvette servo, and a TransGo separator plate with new gaskets. This is my busiest time of year for performing and I don't have the time right now to drop a transmission and rebuild it. If I can fix the P1870 code and resultant slippage and shifting problems then I'll be happy. I hope to get some more miles out of this transmission. I would rather spend money on new Sennheiser pickups for my new (to me) Crucianelli accordion than on a full transmission rebuild.

Many thanks to those who answered my questions and also documented their previous transmission work. It's a huge help.

I'll let you know how it goes. I hope to tear into it next week.