Tranny shift problem???

mikendeb1202 · October 31, 2005

I will try to explain this the best I can.

I have a 1999 S10 Blazer 4.3 W 4WD 4door.

There is a long list of stuff I have tried and nothing has worked yet.

It all started with my wife telling me it felt like it would hold back and go, but I never felt a problem. Then about a week later she had no heat, turns out water pump was leaking, so fixed that.

a few days later she said it's starting to get worse(holding back).

So I took it for a ride. When goig from stop upon accel. it would all of a sudden drop rpm then jump back up, sometimes almost stalling.

Now, I put in a fuel filter & air filter. Still same problem.

I take it to work on my day off to investigate further(auto repair shop).

I find a small antifreeze leak around right front corner of intake manifold.

Once I got it all apart while cleaning the surface of the manifold I find a good size crack!!! I'm thinking this must be it, the gasket was all malformed and cracked too.

I get a used manifold, clean and inspect, all is good. While doing this I also put in new ac delco plat. plugs, wires, cap , and rotor. I finally get done and low and behold still have the problem.

Now I have a couple extra bald spots at this point. LOL !!

I call the "Tech Line" our company provides us, and now the suspect is th fuel pump? I don't have access to much more than a cheap fuel pressure guage, so I figure I have tried just about every thing else. I put a new Delphi fuel pump assy in. Still does it !!

Now I start looking up every website I can find, I see info from TPS to transmission, but nothing quite like my problem.

I had seen one article that mentioned TCC solenoid problem.

I figured it's not a real tough job to change one, so I grabbed a filter kit and a TCC solenoid and hit the lift. When done I also flushed the trans.

But to my dissapointment I still have the same problem.

A friend of mine owns a shop here and I talked to him, we hooked up the OTC Genesis and watche the shift pattern.

Here is where it gets interesting now I can see the issue, but have no clue where to starty to find the problem.

What it does is goes from 1st to 4th by the time you hit 30 mph!!!

So now I see it's acting like when you are driving a standard and miss a gear, if you get my meaning. Plus when you get up to 30 or 35 and punch it , it will not downshift at all until you slow down. If you manually shift through all the gears it works fine, but won't downshift on it's own, at least until almost to a stop.

I have no DTC's and everything else is in working order. I have been a technician for almost 20 years and have never had such a problem trying to diagnose.

I hope you have some ideas or even might have seen or had a problem like this one.

Thanks

Mike n Deb

GM-Tech · October 31, 2005

What it does is goes from 1st to 4th by the time you hit 30 mph!!!

I wouldn't call that abnormal on light throttle. So, we need to discuss throttle percentage vs. shifts to get a real idea if it's shifting too soon. At 100% throttle, yeah, 30 mph would be way to soon for 4th gear, but at 10% throttle I would say it would not.

The one problem I've seen that causes shifts to occur way too soon, and I'm talking 4th gear by 10-15 mph, is a 4LO input to the PCM. If the PCM thinks the transfer case is in 4 LO (and you don't have to have 4 wheel drive to have this problem), it will modify the shift pattern and will be in 4th gear REAL QUICK. This is the way it is supposed to shift when in 4 LO.

So, go back to your buddy with the Genesis and see if you can monitor the 4 LO parameter. If it's always indicating it is in 4 LO, that's your problem. Again, it does not matter whether or not your vehicle has 4 wheel drive. The PCM can still get this input.

mikendeb1202 · November 4, 2005

Let me start with tank you for the info. If I'm not mistaken, you hit the nail on the head. I have talked to dealers, tech lines and posted on a bunch of other forums, but no one could even come up with the answer.

Ok, I hook up the scanner, go to trans data 1 datastream.

Shows:

4wd no

4 low yes

Now the thing is no matter what I unplug, or whatever it will still show "4 low - yes".

I took the switch off the back of the actuator motor and turn the switch and no matter what position it was in the scanner still showed me in 4 low.

What exactly tells the computer what gear the transfer case is in?

Or do you have access to a wiring diagram and a diagnostic flow chart of some sort?

I need to find this problem and fix it. This is our only vehicle and I have tried just about every possible avenue.

Please help.

Thank You

Mike n Deb

mikendeb1202 · November 4, 2005

By the way, where in S. Florida?

I used to live in Cape Coral.

GM-Tech · November 4, 2005

Let me start with tank you for the info. If I'm not mistaken, you hit the nail on the head. I have talked to dealers, tech lines and posted on a bunch of other forums, but no one could even come up with the answer.

Thats why they pay me the big bucks here.

What transfer case do you have? If you give me your full VIN, I can look it up. Once I know for sure, I can look at schematics and see how the 4 lo signal gets to the PCM.

GM-Tech · November 4, 2005

By the way, where in S. Florida?

SW miami-dade area.

mikendeb1202 · November 4, 2005

Great, I was going to get the alldata for $25 per year but I would probably use it just once or twice. Anyways here is the vin#:

1GNDT13W1X2219408

I do know when I was checking around for parts they all asked if it was a 3 or 4 button control for the 4WD. It is a 4 button if that helps.

Thanks Again

Mike n Deb

mikendeb1202 · November 4, 2005

By the way, I even tried another trans case, duh.

But now later last night after I put it in and tried tinkering with the actuator, my wife called me from the store to tell me the MIL light was on. I hooked up the scanner 20 minutes ago and came up with a P1875 4WD low switch circuit.

That was after I hooked everything back up and cleared any codes from messing around, at least 2 to 3 hours later.

Thanks

Mike n Deb

GM-Tech · November 5, 2005

By the way, I even tried another trans case, duh.

trans case? What does that mean? A transfer case? You replaced the whole transfer case? I hope not because that would have nothing to do with this problem.

In any case, do this. Disconnect the TCCM (transfer case control module). If the problem is gone, it's the TCCM. That's behind the right front kick panel.

If disconnecting the TCCM doesn't make the "4 LO - YES" go away, then it's most likely the 4 lo circuit from the TCCM to the VCM grounded somewhere. You can verify that by removing that circuit from the VCM. That would be connector 4 (black connector), pin 23, a gray wire with a black stripe. If you remove that wire and it STILL has the problem, then it's gotta be the VCM.

mikendeb1202 · November 6, 2005

By the way, I even tried another trans case, duh.

trans case? What does that mean? A transfer case? You replaced the whole transfer case? I hope not because that would have nothing to do with this problem.

<{POST_SNAPBACK}>

Call it a moment of temporary stupidity

I will be checking out the TCCM and all today.

If it is TCCM , or VCM, I know VCM may need to be reprogrammed, what about the TCCM?

If it does end up VCM, I was looking on the GM-Parts link on this site and looks like a pretty good price. Does it have to be reprogrammed at the dealer or is it just a re-learn procedure.

Don't mind me, most of the time I find a customers vehicle needs PCM/VCM I just send them to the dealer. Mostly because we are not equiped for this work.

Mike

GM-Tech · November 6, 2005

The TCCM doesn't need programming, the VCM does.

mikendeb1202 · November 6, 2005

Well, I disconnected the TCCM and what do you know, The tranny shifts normal again!!

Now that means it is definately the TCCM?

If so, I called Napa, and they have no listing.

I would imagine it is a GM dealer part only.

Let me just say again Thanks a MILLION!!

You have no idea how much crap I have been through trying to isolate the problem.

Cheers to you

Mike

GM-Tech · November 6, 2005

Now that means it is definately the TCCM?

Nothing is definite, but I don't see that it can be anything else. If the transfer case were really in 4 low, you couldn't miss that.

mikendeb1202 · November 6, 2005

Cool.

Thank you so much for your help.

Mike

johnpimentel72 · February 4, 2008

thanks a lot i found this site. im having the same problem.

here's the story:

bought the truck a month ago its got 33k kms on it. the previous owner didnt used it for 2 years coz he's out of the country. his brother just starts the engine for a couple of minutes then off. when i bought it the service 4x4 lighted up. searched the internet, cleaned the 4x4 switch but later found out that the atc fuse was blown and keeps on blowing. we looked for any short but didnt find any. cleaned all, the grounds and contacts. when the atc fuse was now ok another problem arises. the shifting seems to be akward. hooked it up with tech2 and didnt find any codes (except for history - loss of atc class 2 communication U1026) but discovered something funny. when the shifting seemed to jump, the tech2 indicated shifting from 1st gear jumping to 4th and when it seems there's low power, downshifts doesn't happen. tested it with tech2 if the 4lo might be engaged but it says no. somebody suggested that it might be the solenoids being dirty. we changed the transmission fluid and filter, cleaned the solenoids and valve body and true enough it was dirty. while we were at it, we changed the transfercase fluid, differential fluids engine oil and filter, fuel filter, air cleaner and brake pads we also cleaned the throttle body. but still the same annoying problem. somebody suggested that i try to use manual shifting first and later on see if this will solve the problem.

the next day got P1875 - 4WD Low Switch Circuit Electrical and U1026 - Loss of ATC Class 2 Communication, gm technician had it erased coz it might be of history in nature.

after all these, everything seemed normal for a couple of days. then after one day of rest it went back again. no codes as of this time but i know that soon the check engine will light up again.

hope that you could help. i really like the truck and wouldnt want to part ways with it. its still fresh having only about 20k miles on it. no other problems except for the shifting.