Service 4WD light came on last night, causing shifting problems

[Chris_B]

On my 1999 GMC Yukon Denali (5.7L V8 ~213k miles) The Service 4WD light came on last night while I was driving, ever since the light came on when it tries to shift from 1st to 2nd the engine almost dies due to the rpm dropping so low, it has a fair amount of acceleration loss compared to before the light came on, it just happened out of nowhere the Temperature outside last night was about 30 degrees, there were no sounds when the light came on, just light came on and shifting problems began

 

[the 18th letter]

On my 1999 GMC Yukon Denali (5.7L V8 ~213k miles) The Service 4WD light came on last night while I was driving, ever since the light came on when it tries to shift from 1st to 2nd the engine almost dies due to the rpm dropping so low, it has a fair amount of acceleration loss compared to before the light came on, it just happened out of nowhere the Temperature outside last night was about 30 degrees, there were no sounds when the light came on, just light came on and shifting problems began

Were you in 2 or 4 wheel when it happened? Is light on the 2/4wheel drive selector lit and corresponding correctly?

 

[Fless]

Get a capable scanner on it to pull the 4wd code(s). If there's a message, there's a code. Not all low level scanners can show them.

 

[Chris_B]

Were you in 2 or 4 wheel when it happened? Is light on the 2/4wheel drive selector lit and corresponding correctly?

2WD, The light on the selector has the little red light on 2WD High

 

[the 18th letter]

2WD, The light on the selector has the little red light on 2WD High

Was hoping there were no lights so I could suggest you try changing the selector out which solved a similar issue on my 99 Yukon SLE. The red light is concerning unless the selectors are different in the 2 models as it should be yellow. There’s a red light that comes on briefly at start up. Wil it change in and out of 2 wheel drive? Getting the code scanned is the best advice at this point.

 

[Chris_B]

Were you in 2 or 4 wheel when it happened? Is light on the 2/4wheel drive selector lit and corresponding correctly?

So heres an update, I managed to drive it to my local shell gas station and put some shell plus (88) and fired it up, service 4wd light is gone and shifting issues are gone, I don't know how but it's fixed as of right now
Update 2: the light came back on same issues once again after I had to turn it off and restart it

 

[east302]

There should be a code set for it, you’ll need a scanner that can read chassis codes. Any shop with a good ($) scanner could pull them, it’d be worth dropping by one to see if they would do it for you real quick.

GM publishes diagnostic procedures for each code in the shop manual, so that would be the best way to resolve it. It could be something as simple as a damaged wire.

PDFs of these manuals are available for download at the link below. Year 1999 manuals are in post #77:

88-98 Service Manuals

Does anyone have newer official CK Illustrated Parts Catalogs? I have seen online scans of the diagrams but never pdfs of them. I have found the 1988 to 91 CK Illustrated Parts Catalog...

www.gmt400.com

Somewhere, and I cannot find it now, I remember hearing something about transfer case module fault codes throwing the transmission shift points off or into limp mode.

As a shotgun attempt, the transfer case control module might be on the fritz. It’s under the dash:

 

[the 18th letter]

So heres an update, I managed to drive it to my local shell gas station and put some shell plus (88) and fired it up, service 4wd light is gone and shifting issues are gone, I don't know how but it's fixed as of right now
Update 2: the light came back on same issues once again after I had to turn it off and restart it

My issue was similar, the service 4wd light would come on but the lights on the 4wd would go out and restarting it seemed to reset it. Next time the service light goes out try switching drive modes to see if system is responding or not until you can get code scanned. Like others have said a better level scanner is needed as my basic scanner never pulled a code when I had my issue.

 

[waveryd]

Symptoms of a faulty transfer case encoder ring. I would attempt to verify with a good scan tool but it sometimes will not trigger a code unless you catch it during a malfunction.

 

[2591tdj]

There are several threads here about the "Service 4WD" message and solutions. Mine sounded a bit like yours in that it seemed to only appear when it was really wet and/cold outside. It turned out to be a faulty encoder ring (easy repair) that was shorting out when it got wet or damp.