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Author Topic: Buick 3.8 Transaxle shift help!!!  (Read 889 times)
SBJeepn
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« on: March 07, 2019, 12:22:51 PM »

I built a trans axle buggy with my kids with a gm 3.8 out of a 2001 buick lesabre.  I have not driven it much except on and off a trailer,  but I am using it for snow removal this winter with a utv plow and noticed it shifts really smooth into second but then doesnt shift up to third or fourth.  In thinking back i remember the kids struggling to keep up with me in moab and think this might be why.  Is there anything from a wiring standpoint I should look for that might affect this?   Maybe a sensor, I missed, or that may have been removed that would prevent it from shifting into a higher gear?

If you have any input that would be great. 

Thanks in advance.   
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SBJeepn
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« Reply #1 on: March 07, 2019, 06:46:22 PM »

I was thinking it might be because the trans thinks it's going slow due to the speed sensor location and the addition of larger tires and axle gear ratio but then I realized I had it backwards it thinks it's going super fast.
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simpletoy
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« Reply #2 on: March 07, 2019, 07:33:56 PM »

I’m not sure the trans but I had that issue with my aw4. Ended up my plug to the wiring solenoids wasn’t plugged in all the way and the trans would shift from 1st-2nd without an issue as default but wouldn’t shift any further


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TBItoy
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« Reply #3 on: March 07, 2019, 07:53:26 PM »

I’m pretty sure it has to have a good (reasonable speed range) vss signal to shift above 2nd.

Did you move the vss location from stock?


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Nick Cooper - Dodson Branch, TN

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SBJeepn
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« Reply #4 on: March 08, 2019, 04:39:34 AM »

The vss is in its stock location.  I learned my lesson with that on my other buggy.  I was told to put it on the output shaft of the tcase. So when I put it in high it shifted perfect. But when I put it in low range it wouldn't shift because it thought it was going too slow so it wouldn't shift. 
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SBJeepn
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« Reply #5 on: March 08, 2019, 04:40:42 AM »

I'll check the trans plug and make sure it's in tight.  Maybe disconnect it and reconnect it to make sure it didn't somehow get some debris
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LightBnDr
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« Reply #6 on: March 08, 2019, 06:39:00 AM »

I used to work on them all the time. It should be a 4t65. Though they were somewhat my bread and butter, they really didn't have major issues.

I put pressure control solenoids in them all the time for shuddering on accel from a stop.

As well as stripped 4th gear shaft splines. Other than those they would have TCC codes and I was putting valve bodies in them.

Almost guarantee you don't have a 4t60 in it but if for some reason you have a unicorn, it has a vacuum modulator that controls line pressure. Those would corrode and get sticky, causing the same shudders and shift issues.

Basically that being said, if you have a 1-2 shift but nothing else, for sure stick to the basics. 2-3 shift solenoids would crap sometimes but it had to have a million miles on it.


You have a DLC connector to pull codes ? You should.  I reprogrammed these PCMs all the time.   

Plus you can watch data display and it will show you output speed RPM so you can tell right away whether it's an input issue or not.
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SBJeepn
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« Reply #7 on: March 08, 2019, 07:28:55 AM »

I have an obd reader. I wonder if the torque app can get that data?!
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SBJeepn
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« Reply #8 on: March 13, 2019, 04:10:39 AM »

Ok so I finally got some time to mess with this today for a few minutes.  My big concern was maybe after the engine rebuild that a sensor wasn't plugged in or working.  So I got my torque app hooked up and drove it around.  It seems to consistently read speed so that would tell me that the speed sensor is at least getting to the ECU.  I checked fault cosea and didn't have any however I can't tell if the trans fauly codes would show on that or if I need a different style obd?   So here's a couple thoughts first what would be the easiest and cheapest starting point to get this narrowed down.  Is there a separate computer for the trans internal maybe or is it run by the main ECU? 
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LightBnDr
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« Reply #9 on: March 13, 2019, 06:37:09 AM »

The PCM controls both engine and trans.

I'd put it on jack stands and manually command gear changes with your scanner. I'm not familiar with the one you have but you'll need one

You'll need to be able to first watch whether the PCM commands 1-2, 2-3, 3-4 shifts. Then be able to manually command them yours self. Mainly so you can listen to the side cover with a stethoscope to hear the solenoids click then be able to probe the harness to be sure the signal is making it to the solenoid

Otherwise this is gonna be a tough prob to diagnose.
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SBJeepn
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« Reply #10 on: March 13, 2019, 08:05:38 AM »

Ok when I get my jack stands back this weekend I'll get it jacked up so I can see what it's doing and get one of the kids to run the gas/brake. 

What sends the command to shift is it based on rpm or load or speed or some magical combo of them. 

Here's another question. The original computer failed.  Jim from Jim's performance replaced it. Would it be possible for the computer to be not compatible with the trans or not be programmed to run the trans after the flashing of it?. It so how could I tell. 
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SBJeepn
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« Reply #11 on: March 13, 2019, 10:36:34 AM »

The PCM controls both engine and trans.

I'd put it on jack stands and manually command gear changes with your scanner. I'm not familiar with the one you have but you'll need one

You'll need to be able to first watch whether the PCM commands 1-2, 2-3, 3-4 shifts. Then be able to manually command them yours self. Mainly so you can listen to the side cover with a stethoscope to hear the solenoids click then be able to probe the harness to be sure the signal is making it to the solenoid

Otherwise this is gonna be a tough prob to diagnose.

I downloaded the harness diagram for the trans.  Is there a good way to see if the computer is sending the signal by splicing into the 2-3 shift wire maybe with a volt meter maybe?
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SBJeepn
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« Reply #12 on: March 13, 2019, 10:56:53 AM »

One last thing Smiley  I realized that my obd was not getting a reading for trans temp.  I dont know if this ecu tracks that but if it did I thought you might know, and maybe it would help indicate where I need to look. Or maybe indicate a dysruption of data from the trans. 
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SBJeepn
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« Reply #13 on: March 13, 2019, 02:55:35 PM »

I borrowed a reader tool from a shop friend I have. It is a Snap-on Apollo D8.   it is supposed to show what gear the trans is in and what gear the computer is calling for.  Hopefully this will help narrow some stuff down.  I was thinking maybe the trans was running in limp mode for whatever reason because I know these are set up by default to shift 1-2 and Reverse regardless of input.  But since the ecu didnt have any error codes  I cant imagine that that is very likely.  

« Last Edit: March 13, 2019, 03:19:39 PM by SBJeepn » Logged
LightBnDr
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« Reply #14 on: March 13, 2019, 05:08:20 PM »

I downloaded the harness diagram for the trans.  Is there a good way to see if the computer is sending the signal by splicing into the 2-3 shift wire maybe with a volt meter maybe?
Absolutely. A 12v test light will tell you. The PCM sends a ground not a power so a self powered test light works best but you can just put it on the pos batt lead of the battery. Of course manuals tell you all the time not to back probe on a PCM ground circuit but I did it all the time.

So isolate those 2 wires and make sure you have ign run on the power lead and that it turns your light on at the ground terminal when the scanner says it's commanding 2-3

But I don't have much help if you have a PCM replaced and flashed somewhere
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LightBnDr
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« Reply #15 on: March 13, 2019, 05:16:43 PM »

But it's really not worth probing if you don't have a way to see on the scanner that the PCM is commanding it.

Your entire prob can be related to no command given your circumstances

That's the first thing I did on no shift concerns. Go drive with my tech 2, watch for the command, command it manually then get 3hrs tear down to pull it apart to test solenoids

9 times outta 10 the PCM is commanding it

I found pinched harnesses and bad solenoids. Usually high mileage hydraulic failures. Would bypass fluid under pressure even when commanded.

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SBJeepn
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« Reply #16 on: March 13, 2019, 06:19:51 PM »

I couldn't tell where it was giving the command.  I can see it shift from first to second but never used a scanner this fancy. Here some pics maybe this will help.

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SBJeepn
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« Reply #17 on: March 13, 2019, 06:20:38 PM »

More

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SBJeepn
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« Reply #18 on: March 13, 2019, 06:22:13 PM »

Last

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SBJeepn
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« Reply #19 on: March 13, 2019, 06:40:44 PM »

But it's really not worth probing if you don't have a way to see on the scanner that the PCM is commanding it.

Your entire prob can be related to no command given your circumstances

That's the first thing I did on no shift concerns. Go drive with my tech 2, watch for the command, command it manually then get 3hrs tear down to pull it apart to test solenoids

9 times outta 10 the PCM is commanding it

I found pinched harnesses and bad solenoids. Usually high mileage hydraulic failures. Would bypass fluid under pressure even when commanded.



 Can you tell me where I would see it register the request to up shift.  I have no error codes in the trans.  Not sure if they would show with what your referencing

Thank you for your help with this. I really am clueless here
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SBJeepn
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« Reply #20 on: March 14, 2019, 10:00:15 AM »

I played with this today and got a little info not sure if it will help.  I see on the reader where the shifter is in fourth.  I can see it go into 3rd second and first when I move the lever. 

I noticed that shift solenoid turns from on to off just prior to shifting into second, so I assume that it gets the call from the computer to shift turns the solenoid off and then it shifts.  Then as I accelerate I dont see the 2-3 solenoid of turn off, and I assume this is why it is not shifting up from there? 

If this is accurate What is my problem?
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LightBnDr
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« Reply #21 on: March 14, 2019, 06:51:18 PM »

The "current gear" display is the PCM commanded gear
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SBJeepn
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« Reply #22 on: March 15, 2019, 04:21:47 AM »

The "current gear" display is the PCM commanded gear

So if that never goes to 3 or 4 does that mean the computer isn't calling for the upshift
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LightBnDr
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« Reply #23 on: March 15, 2019, 06:38:17 AM »

Correct. All you really need is "transmission output controls" so you can manually command a gear change

Then you can probe harness with it running on jack stands to be sure the signal is being sent.

Without that ability, your problem is gonna be hard to pin point. About all you can do is get it on stands and run that sucker almost redline and watch output speed.

If it never reaches the 2-3 3-4 speed parameters then the pcm won't command it

If it does reach it, then you gotta figure out why the pcm is not commanding it.

If all this sounds stupid for a kids a rig, then pull the valve body and make it full manual. Be done
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SBJeepn
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« Reply #24 on: March 15, 2019, 06:58:33 AM »

Nope doesn't sound dumb at all.  With all the gear reduction from the trans axle and the acles coupled with the need to be user friendly for the littles I'm happy to trouble shoot the crap out of it.

Do you happen to know at what speeds the command should come for 3rd and 4th?
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