OK....popped a P1870 the other day

After I cleared it here's what I have found

Only get it when going over 50MPH after driving a while
If I stay on surface streets and under 45 or so I can go days with no codes

When it does set I get hard shift 1 to 2 but no other times
After it's cleared it goes back to smooth shifts all the way through.

I'm aware this is pretty common. Will Lucas trans stuff help?
Am I doing more damage driving it under 45?
Is swapping the valve body with an upgraded my best solution?

This is my only vehicle and really don't have $300 to throw at it right now...probably will need a couple weeks

 

You're probably OK driving with the situation for a short time, but I wouldn't recommend leaving it for a long time. The code sets once the transmission oil reaches a set temperature and the (can't remember the solenoid name) in the valve body is leaking. This is why when you do short trips at low speeds you don't set the code because the oil stays cooler, but a longer trip at highway speeds will set it because the transmission oil will warm up enough.

I had the same problem quite a while ago and ended up swaping the valve body with a rebuilt one with the upgraded solenoid. I've now been driving the Blazer daily for 5 years without any transmission problems.

 

this is all the fixes identifix.com has posted



25 - Transmission Valve Body
4 - Torque Converter Clutch (TCC) Solenoid
3 - Automatic Transmission Service
2 - Transmission Wiring Harness
2 - Transmission Fluid Level
2 - Automatic Transmission Fluid, Transmission Cooler Hose/Line
2 - Powertrain Control Module (PCM)

 

2 different problems going on. The P1870 is the TCC solenoid has a problem, and the hard 1-2 shift is a problem with the 1-2 shift valve bore in the valve body is worn. Both are common problems.

 

Thanks for the reply.... Correct me if I'm wrong. I was under the impression that the hard shift after the code sets was due to the trans going into max line pressure after the code sets

TCC is part of the valve body yes? If so swapping the valve body will correct both issues. Other than dropping the pan and being careful with the check ***** anything else to be aware of while doing the swap? Will the separator plate come down with the valve body? Are there any check ***** above the separator plate? Is there a torque spec on the valve body bolts? Is there a specific tightening pattern?

 

Captain Hook knows a lot more about this than I do, so if he posts information that contradicts anything I say please take his advice over mine. That being said from what I remember, the code P1870 puts the transmission into max line pressure like you said, which causes the hard 1-2 shifting. When the conditions that set the code clear and the transmission cooled down the shifting returned to normal for me.

When I had the problem I did a ton of research on how to do the valve body replacement. The kit I bought came with good instructions, as well as replacement gaskets. If I recall correctly the thin metal plate that sits between the valve body does have 2 check ***** that sit on top of it that you have to be careful with if you take this plate down. I'd also recommend you replace the transmission filter while you're in there. It's a $10 part and takes no extra time to replace. I don't remember what the bolt torque value is, but I tightened in a criss-cross pattern.

 

Capt knows a lot more than me as well. I'm an engine/suspension kinda guy
To be honest transmissions scare the crap outa me

I'm just trying to gather all the info I can so I go into the repair informed and ready for what I might get into.

Deciding what to do is tough....swap the valve body or swap the trans
Found a rebuilt/sonnax updated one for $770 or do the valve body for $250 Swapping the trans id my gut, but that extra $500 that I don't have would go a long way towards other things

 

Thanks for the votes of confidence guys, I do my best for ya

P1870 only sets when the TCC should be engaged, (Overdrive) and the PCM detects a difference in trans output RPM and engine RPM, (should be very close to each other in OD). I've had several P1870's on various 4L60E's and 90% of the time fluid leaks past the TCC solenoid seal, (which sets the DTC). Replacing the solenoid solved the problem. Best to follow the flow chart though: https://blazerforum.com/forum/diagno...plained-18227/ I'm not 100% certain, but I don't think line pressure is affected, and you just end up with a mushy or nonexistant OD upshift.

The harsh 1-2 shift happens mainly when the trans is at operating temp, (~30 minutes of driving). When/if the bore in the valve body wears, the 1-2 spool valve can bind in the bore. Pressure continues to increase until it forces the valve to move, that's when you get the harsh 1-2 shift. The more it binds, the harder the 1-2 shift. There are "repair kits" available, or you can replace the valve body. Make sure you get one that specifies that it has had the modification done to it, junkyard valve bodies are a huge risk.

 

My 99 blazer has been throwing that code since april and been driving it since(+30k miles) Im just waiting till it goes completely.

I also installed a small trans cooler which seemed to help a bit but still throws it

 

The longer you wait, the more it's costing you in fuel!