Update: 99 Yukon NP8 codes 1-2 shift scan tool?

[Smooth_Rider]

Hoping someone can send me in the right direction. My 99 4wd Yukon had two previous owners. Original owner was OCD on maintenance, put 130k on it, second owner was good on maintenance, but harder on truck, put 70k. I bought at 200k, and been "fixing" since. Now at 202k. I'm an old gear head and do most work myself, but not a modern/computer car savy guy. I did a trans service on her about 1k ago and was pleased with the lack of gunk in the pan and a relatively clean magnet. Prior service 30k back. At this point the biggest issue was the dash is gone/rotten and shakes to hell over potholes, and the front end is shot- everything was worn out. Including axle boots, CA bushings, tie rods, just all of it.

So before I get to installing this pile of new front end parts, the truck starts shifting strange. Mostly just when its cold, and mostly just 1-2 shift. Like its short shifting into second...lugging and almost stalling out. I can manually shift it and it's ok. After it warms up, it shift better, but still not perfectly correct. So I'm thinking wtf, did I mess something up with the service I did? At some point the CEL came on and off, the "service 4wd" light came on and off a few times then stayed on.

I took the truck to a local/reputable trans shop and they came back with "its not your transmission, it has to do with the Tcase" they said they had "multiple codes" but that's where the $125/hr diagnostic charge starts.

One thing I should add here is that after the shifting issue started, but before any dash lights (and before I took it to the transmission shop) the trucks alarm system, that was dealer installed when the truck was sold new, went haywire, going off in my driveway with no way to shut if off than to disconnect the battery. Leaving the batt disconnected for a few days didn't help/reset anything. I couldn't start the yukon or turn the alarm off. So I cut the siren, and "uninstalled" the guts of the whole thing. Ever since then, seemingly randomly, I can hear a relay clicking under the dash, which I correlate to the alarm removal. I don't have any idea what caused the alarm system to freak out initially, or if it has had any other impact on anything.

In my semi ignorant run up to addressing this, and because I have another identical truck, and because they were relatively cheap, I acquired the following parts with this problem in mind: TPS, VSS, 4 button 4wd selector, Sonnex 1-2 pinless accumulator kit.
My gut says this is a sensor/computer issue.

So, before I blindly throw money I dont have at the transmission shop, here are my thoughts and questions- since the shop wont volunteer the codes, would a second opinion be in order? Would ANY shop volunteer the codes? Can anyone recommend an affordable/easy to use scan tool capable of reading transfer case codes? I also assume from my rudimentary understanding of this tcase that it can't be bypassed or "unmarried" from the trans as older units can. Any other solutions, thoughts, ideas?

 

[89Suburban]

@NickTransmissions

 

[NickTransmissions]

Hoping someone can send me in the right direction. My 99 4wd Yukon had two previous owners. Original owner was OCD on maintenance, put 130k on it, second owner was good on maintenance, but harder on truck, put 70k. I bought at 200k, and been "fixing" since. Now at 202k. I'm an old gear head and do most work myself, but not a modern/computer car savy guy. I did a trans service on her about 1k ago and was pleased with the lack of gunk in the pan and a relatively clean magnet. Prior service 30k back. At this point the biggest issue was the dash is gone/rotten and shakes to hell over potholes, and the front end is shot- everything was worn out. Including axle boots, CA bushings, tie rods, just all of it.

So before I get to installing this pile of new front end parts, the truck starts shifting strange. Mostly just when its cold, and mostly just 1-2 shift. Like its short shifting into second...lugging and almost stalling out. I can manually shift it and it's ok. After it warms up, it shift better, but still not perfectly correct. So I'm thinking wtf, did I mess something up with the service I did? At some point the CEL came on and off, the "service 4wd" light came on and off a few times then stayed on.

I took the truck to a local/reputable trans shop and they came back with "its not your transmission, it has to do with the Tcase" they said they had "multiple codes" but that's where the $125/hr diagnostic charge starts.

One thing I should add here is that after the shifting issue started, but before any dash lights (and before I took it to the transmission shop) the trucks alarm system, that was dealer installed when the truck was sold new, went haywire, going off in my driveway with no way to shut if off than to disconnect the battery. Leaving the batt disconnected for a few days didn't help/reset anything. I couldn't start the yukon or turn the alarm off. So I cut the siren, and "uninstalled" the guts of the whole thing. Ever since then, seemingly randomly, I can hear a relay clicking under the dash, which I correlate to the alarm removal. I don't have any idea what caused the alarm system to freak out initially, or if it has had any other impact on anything.

In my semi ignorant run up to addressing this, and because I have another identical truck, and because they were relatively cheap, I acquired the following parts with this problem in mind: TPS, VSS, 4 button 4wd selector, Sonnex 1-2 pinless accumulator kit.
My gut says this is a sensor/computer issue.

So, before I blindly throw money I dont have at the transmission shop, here are my thoughts and questions- since the shop wont volunteer the codes, would a second opinion be in order? Would ANY shop volunteer the codes? Can anyone recommend an affordable/easy to use scan tool capable of reading transfer case codes? I also assume from my rudimentary understanding of this tcase that it can't be bypassed or "unmarried" from the trans as older units can. Any other solutions, thoughts, ideas?

I'm fixing to start a thread on the issue of folks not being able to diagnose and troubleshoot their own vehicle problems due to lack of a suitable scan tool(s). If folks had what they needed in that dept, they would be able to read codes, run guided component tests on suspect parts/systems, perform "adapt relearns" and thus reducing or even eliminating any dependencies folks currently have on other people (shops, technicians or friends/associates/family with the tooling, knowledge etc).

For now I will recommend that you and those in your position thing seriously about getting a good, well-rounded bi-directional scan tool. For example, the shop is asking $125/hr to diagnose the problem...figure that you will likely need such diagnosis services at least 6-10 times more while you own the vehicle unless you plan on immediately dumping it. Assuming you keep it, invest 600-1000 in a two-way scanner, that thing will have paid for itself within three-five instances where it helped you diagnose your own issue vs $125/hr that you'll never see again. Search 'Autel' scanners on Amazon or used Snap On Solus, Modis, Verus as well as Autel on ebay for deals on scanners.

Onto your specific problem: The transfer case isn't going to cause your transmission to suddenly mis-behave as you're describing. If the tcase has an issue, it will usually show up as a snapped chain (total loss of movement in all range settings including reverse, forward, manual 2 or low, etc), failed bearing or blocking ring preventing proper shifting in and out of the various transfer case ranges (2H, 4H, 4L ,etc) or failed transfer case encoder motor sensor or assembly (this is what typically causes a chassis 'C' code to set and 'Service 4WD' message to appear in the driver info center. These chassis codes cannot be read by basic/budget scan tools.

The TPS can impact the transmission's behavior but it's usually a pattern of very inconsistent early, late or harsh shifting in all points of the drive cycle (i.e. all gears). Stack shifting from first to second gear (i.e. short shifting) indicates a cross leak of some sort in the intermediate feed circuit that feeds the 2-4 servo and enables it to apply the band. It can potentially also indicate a faulty shift solenoid but not likely (seeing shift solenoid diagnostic trouble codes P0753 and/or P0756 would confirm or deny for sure). Other things that can adversely affect the 1-2 shift include valve body (TCC regulator valve bore wear and P1870 sets and stores), 1-2 accumulator (broken springs - usually shows as a late, harsh 1-2 shift with no DTC, 2-4 servo assembly (worn sealing rings/seals), 2-4 band (worn band), pump (excessively worn rear stator bushing).

If you're interested in learning how to test/diag those that I mentioned, reply back (QUOTE THIS POST) and i'll do my best to lay it out for you (ill also link you to specific videos on my and/or others' channels that show the procedures).

 

[Smooth_Rider]

I'm fixing to start a thread on the issue of folks not being able to diagnose and troubleshoot their own vehicle problems due to lack of a suitable scan tool(s). If folks had what they needed in that dept, they would be able to read codes, run guided component tests on suspect parts/systems, perform "adapt relearns" and thus reducing or even eliminating any dependencies folks currently have on other people (shops, technicians or friends/associates/family with the tooling, knowledge etc).

For now I will recommend that you and those in your position thing seriously about getting a good, well-rounded bi-directional scan tool. For example, the shop is asking $125/hr to diagnose the problem...figure that you will likely need such diagnosis services at least 6-10 times more while you own the vehicle unless you plan on immediately dumping it. Assuming you keep it, invest 600-1000 in a two-way scanner, that thing will have paid for itself within three-five instances where it helped you diagnose your own issue vs $125/hr that you'll never see again. Search 'Autel' scanners on Amazon or used Snap On Solus, Modis, Verus as well as Autel on ebay for deals on scanners.

Onto your specific problem: The transfer case isn't going to cause your transmission to suddenly mis-behave as you're describing. If the tcase has an issue, it will usually show up as a snapped chain (total loss of movement in all range settings including reverse, forward, manual 2 or low, etc), failed bearing or blocking ring preventing proper shifting in and out of the various transfer case ranges (2H, 4H, 4L ,etc) or failed transfer case encoder motor sensor or assembly (this is what typically causes a chassis 'C' code to set and 'Service 4WD' message to appear in the driver info center. These chassis codes cannot be read by basic/budget scan tools.

The TPS can impact the transmission's behavior but it's usually a pattern of very inconsistent early, late or harsh shifting in all points of the drive cycle (i.e. all gears). Stack shifting from first to second gear (i.e. short shifting) indicates a cross leak of some sort in the intermediate feed circuit that feeds the 2-4 servo and enables it to apply the band. It can potentially also indicate a faulty shift solenoid but not likely (seeing shift solenoid diagnostic trouble codes P0753 and/or P0756 would confirm or deny for sure). Other things that can adversely affect the 1-2 shift include valve body (TCC regulator valve bore wear and P1870 sets and stores), 1-2 accumulator (broken springs - usually shows as a late, harsh 1-2 shift with no DTC, 2-4 servo assembly (worn sealing rings/seals), 2-4 band (worn band), pump (excessively worn rear stator bushing).

If you're interested in learning how to test/diag those that I mentioned, reply back (QUOTE THIS POST) and i'll do my best to lay it out for you (ill also link you to specific videos on my and/or others' channels that show the procedures).

Thank you for your detailed reply! I totally agree with you- it IS frustrating for folks like me, who are not professional mechanics but have respectable "hobbyist" skills and enjoy working on their own vehicles, to be held hostage for lack of a fancy scan tool. If it were up to me, everything I own would be 1973 or older and get about 6mpg...but I digress. As a largely self taught gearhead, I have aquired my tools over decades, buying a specialty tool here and there as the need arose. I might finally be at that crossroad with a scanner, if, like you said, the ends justify the means. I will look into the scanners you mentioned.

I do realize there's a point where things fall outside my wheelhouse, and I'm ok with that. However, I own my house and I own my vehicles and I don't owe payments to anyone partly because I'm driven to do things myself (self sufficient or cheap, depending on how you want to look at it) and I'm not afraid to learn new things or fail trying.

What irked me about the transmission shop is I couldn't speak to a tech, so everything was filtered through a salesperson, who doesn't really know about the business they're in, and that $125/hr diagnostic time is nebulous. Could be one hour, could be ten, who knows.

Three years ago I had never owned a GM product, now I'm kicking myself I didn't get into them sooner. I love the gmt400 platform, the parts interchangeably/availability/affordability and how easy they are to wrench on compared to ford and dodge (I own them all) So yes, I would definitely watch your videos and follow any diagnostics. In the mean time I will be on the hunt for a scan tool.

 

[NickTransmissions]

Thank you for your detailed reply! I totally agree with you- it IS frustrating for folks like me, who are not professional mechanics but have respectable "hobbyist" skills and enjoy working on their own vehicles, to be held hostage for lack of a fancy scan tool. If it were up to me, everything I own would be 1973 or older and get about 6mpg...but I digress. As a largely self taught gearhead, I have aquired my tools over decades, buying a specialty tool here and there as the need arose. I might finally be at that crossroad with a scanner, if, like you said, the ends justify the means. I will look into the scanners you mentioned.

I do realize there's a point where things fall outside my wheelhouse, and I'm ok with that. However, I own my house and I own my vehicles and I don't owe payments to anyone partly because I'm driven to do things myself (self sufficient or cheap, depending on how you want to look at it) and I'm not afraid to learn new things or fail trying.

What irked me about the transmission shop is I couldn't speak to a tech, so everything was filtered through a salesperson, who doesn't really know about the business they're in, and that $125/hr diagnostic time is nebulous. Could be one hour, could be ten, who knows.

Three years ago I had never owned a GM product, now I'm kicking myself I didn't get into them sooner. I love the gmt400 platform, the parts interchangeably/availability/affordability and how easy they are to wrench on compared to ford and dodge (I own them all) So yes, I would definitely watch your videos and follow any diagnostics. In the mean time I will be on the hunt for a scan tool.

You're welcome, man.

Yep, the scanner helps you make the unknown known. You may find out that a given problem is beyond the scope of what you're willing or able to take on but at least you know what the problem is vs being at the mercy of someone else to tell you (someone who may or may not have your interests 100% front and center). So by doing your own diag work, you come into the conversation with any shop or dealership much more informed and empowered vs being in the dark. So the likelihood of getting taken for a ride drops bigly!

That by itself makes the bi-directional scanner worth it, at least for me (I have accumulated a small collection of them, lol).

 

[89Suburban]

You're welcome, man.

Yep, the scanner helps you make the unknown known. You may find out that a given problem is beyond the scope of what you're willing or able to take on but at least you know what the problem is vs being at the mercy of someone else to tell you (someone who may or may not have your interests 100% front and center). So by doing your own diag work, you come into the conversation with any shop or dealership much more informed and empowered vs being in the dark. So the likelihood of getting taken for a ride drops bigly!

That by itself makes the bi-directional scanner worth it, at least for me (I have accumulated a small collection of them, lol).

Thanks Nick.

 

[Smooth_Rider]

You're welcome, man.

Yep, the scanner helps you make the unknown known. You may find out that a given problem is beyond the scope of what you're willing or able to take on but at least you know what the problem is vs being at the mercy of someone else to tell you (someone who may or may not have your interests 100% front and center). So by doing your own diag work, you come into the conversation with any shop or dealership much more informed and empowered vs being in the dark. So the likelihood of getting taken for a ride drops bigly!

That by itself makes the bi-directional scanner worth it, at least for me (I have accumulated a small collection of them, lol).

That's exactly right. I am totally willing to pay a fair price for fair work when need be. We all have to eat. And I don't intend on tearing into the guts of an 4L60E anytime in the immediate future, but ancillary parts under the pan or other sensors are very doablefor me with those diagnostic skills.

 

[Smooth_Rider]

One detail I realized I left out, is that the transfer case is now, in fact, non responsive. The dash buttons do nothing, and no engagement can be heard. It worked as expected the last time 4wd was needed, which was before the shifting issue began.

 

[east302]

If a scanner isn’t in the immediate future, I’d try dropping by a tire shop or similar chain-store type place and ask the guy if they’d pull the codes for you real quick.

You might also consider getting a copy of the GM service manual. It has the diagnostic procedure tables associated with each code (once you get them) and are pretty straightforward. It’s largely written around the Tech2 scanner and can include references to special ($) Kent Moore tools, but it’s certainly worth looking into as you can usually workaround those things.

I don’t think that they have them for 1999, but the site below does have free PDF copies for 1998. While that should suffice for the majority of things on the truck, I couldn’t speak to any differences in the 4l60e for those two years. Best look for a 99 manual if you’re getting into the weeds on a transmission.

88-98 Service Manuals

Over the years I have collected a bunch of GM service literature. Links to PDF Scans of service and STG manuals and a links to the Parts manuals. You'll need the FREE Adobe Acrobat reader to open these files. Get it directly from Adobe here {http://get.adobe.com/reader/} If you're not...

www.gmt400.com

Alldatadiy.com is a subscription service which has nearly verbatim copies of the GM manual and is pretty easy to use. Used paper manuals (eBay) are another option.

Good luck!

Edit: Here are some sample pages from the 98 manual…

 

[Eman85]

If you are going to any more than put gas in and check the oil you should own a scan tool and a digital multi meter just for starters. The Autel is a good one for basics and very reasonably priced.
If I was a guessing man the first guess for the T case would be the switch on the dash, they are a common failure.
I always like the references to a not a modern/computer car deal. Kind of limits you to over 40yo vehicles.