# '02 2500 Service 4WD light on



## Motorman 007 (Jan 11, 2005)

Good Morning Guys,

One of the fleet here at work is an '02 2500 non HD 6.0 gas ext cab. First snow of the season here in Mass. Truck will not go into 4WD and the service 4WD light is on in the dash. When I press the 4Hi button, it flashes a few times and then goes back to 2Hi. Same with 4Lo. Any advice?

Thanks in advance.


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## B&B (Nov 4, 2006)

There's only three components that ever cause an issue on the push button 4X4 systems, the first and most common on the 99-02 trucks is the switch assembly itself. Think I've replaced about 100 of them now. Second in line is an internally corrode shift encoder motor in the T-case. This is the actuator that actually shifts the T-case when you send the command down to it after a button is pressed. And lastly is the front axle actuator. Those used to die quite often on the old body style trucks but the newer ones are much more reliable and seldom have a problem.

It's not easy to diagnose which components has failed without a good scanner to view if each component is performing the command asked of but with your ears you can narrow it down to two of them.

1) Place the key in the run position without starting the engine.
2) Press the 4HI button while a helper pushes the truck a few feet in either direction.

As you press the button listen for a good solid "cluck" from under the truck. This would be the encoder motor on the T-case shifting the T-case into 4HI (as you commanded). If it clunks then everything is working correctly EXCEPT the front axle actuator. But if you hear no good solid "clunk" then the issue is either the switch panel or the encoder motor. But there's no backyard test to differentiate those two to further narrow it down but _usually_ its a bad switch panel. Although as those trucks age corrosion will get the best of the encoder motor also.

A good scanner would tell you what is and isn't functioning in about 2 minutes.


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## Motorman 007 (Jan 11, 2005)

*Now that's service....*



B&B;890928 said:


> There's only three components that ever cause an issue on the push button 4X4 systems, the first and most common on the 99-02 trucks is the switch assembly itself. Think I've replaced about 100 of them now. Second in line is an internally corrode shift encoder motor in the T-case. This is the actuator that actually shifts the T-case when you send the command down to it after a button is pressed. And lastly is the front axle actuator. Those used to die quite often on the old body style trucks but the newer ones are much more reliable and seldom have a problem.
> 
> It's not easy to diagnose which components has failed without a good scanner to view if each component is performing the command asked of but with your ears you can narrow it down to two of them.
> 
> ...


Who's better than you, Mike. No one in my opinion. I'll get on it right away.
As an aside, my mechanic here has been bugging me to get him a good scan tool. Any recommendations?

Dave


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## BigLou80 (Feb 25, 2008)

I had my encoder motor go but it was still making noise. I think the computer was not getting a signal that said "im in 4wd" so it would try to go in to 4wd and then go back. It sounded like a pressed the 4hi button and then immediatly pressed the 2hi button 

I never could have diagnosed it my self. It was well worth every penny I gave to the mechanic
I don't know if my old (new to me) snap on scan tool would have helped. But parts changing gets expensive fast


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## RichG53 (Sep 16, 2008)

I too would like to know about Scanners.... Which are good and how much $$$ they cost ??
Do all of them do the same quality job as far as diagnosing.????


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## articstorm (Dec 15, 2005)

I'm sure there are much cheaper scanners on the market, but my mechanic bought one and was showing it off in my truck last spring. This was after he changed the Crank Position Sensor and was teaching the new sensor to the computer with the new scanner. It had so much information I was amazed. It told readings of each sensor in the truck and what they were doing. Not like the old ones that just gave you a simple code and let you figure out why it threw that code. This scanner was a multi-vehicle one from Snap-on and it cost him around $2500, but he said they normally go closer to $4000. I have worked on many vehicles for many years and I will continue to do the service and simple maintenance, but when warning lights come on, it's going in now.


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## B&B (Nov 4, 2006)

Which scanner to buy depends entirely on what you're want to spend and how much info you feel you need to know now and in the future. You can buy some pretty decent scanners now that have good capability for far more than simply pulling a generic code (like a $40 code reader for example). There's some out there now geared more toward the hard core DIY crowd that are very powerful for under $700 that have many of the pro features that only a $5000 OTC, Snap-on or Tech2 scanner once had. Full code retrieval including B, U, and C series codes (main 4X4 codes are in the B and C categories) in addition to the usual P series codes that any generic reader can retrieve, along with some excellent freeze frame data reading and recording . I hear Auto X-ray recently released a new version of their scanner that even has some bi-directional control capabilities which can be very very handy. Believe it's their new 7000 model if I recall correctly. And although I haven't had my hands on that specific model I did demo a few of their scanners at a show not long ago and was very impressed with their capabilities and features for the price range they're in. If I was looking for a low cost well stocked scanner for occasional or DIY use I'd definitely take a serious look at what they have.

Now if you want one that will tell you everything except the color of the truck then you'll need to spend considerably more.


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## Motorman 007 (Jan 11, 2005)

I haven't started looking at it yet but I can tell you that the dash switch panel makes the same clicking and relay noises that it normally does when I press the buttons. Does that mean anything?


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## jrjr2u (Oct 5, 2009)

I was battling a check engine light on a car a while ago. It kept throwing the O2 sensor code and I would replace it, swap it with a known good one, etc. I bought the Auto XRay 6000 so I could read the codes and watch the live data on the sensor that was giving trouble. Always the same one. Watching the voltage suddenly go out of spec for no reason made me decide it was something in the harness. New wires were run from computer to the sensor and all has been good for a couple years now. 

I would recommend the auto xray line for the money, mine has treated me very well.


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## B&B (Nov 4, 2006)

Motorman 007;892761 said:


> I haven't started looking at it yet but I can tell you that the dash switch panel makes the same clicking and relay noises that it normally does when I press the buttons. Does that mean anything?


Well it means that the buttons are at least sending some info to the T-case control module as that's what you hear clicking, it's mounted in the dash right behind the switch panel.

Don't hear a good solid clunk from below the truck?


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## Motorman 007 (Jan 11, 2005)

*Update*

So we diagnosed as best we could without the scan tool. Although not totally convinced at first we went with the encoder motor in the T-Case. Completely corroded as described by Mike that it would be. VERY expensive dealer part at $535.00. 2Hi, 4HI, and 4LO all work now with the desinctive clunk. The only question now is why does the 4HI and 4LO light continue to flash on the switch panel after the 4wd is engaged. Is there a code that has to be cleared or do I still have a hidden issue?


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## B&B (Nov 4, 2006)

Motorman 007;898787 said:


> So we diagnosed as best we could without the scan tool. Although not totally convinced at first we went with the encoder motor in the T-Case. Completely corroded as described by Mike that it would be. VERY expensive dealer part at $535.00. 2Hi, 4HI, and 4LO all work now with the desinctive clunk. The only question now is why does the 4HI and 4LO light continue to flash on the switch panel after the 4wd is engaged. Is there a code that has to be cleared or do I still have a hidden issue?


Yes the encoders are expensive which is why I recommend being sure that it's the actual problem before simply replacing it...shot gunning parts gets expensive fast. And that's where having a scanner will pay for itself in short order.

As for the indicator light blinking...

Have you actually verified that it's in 4WD? I know you can hear the T-case engage now but have you actually taken the truck out to see if the front tires are pulling? Usually when the indicators blink continuously it's due to the T-case control module not receiving confirmation back from the front axle actuator that it has engaged. And if it hasn't you'll have no power to the front wheels whether the T-case itself is now functioning or not. So you may have more than one issue.


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## Motorman 007 (Jan 11, 2005)

B&B;899292 said:


> Yes the encoders are expensive which is why I recommend being sure that it's the actual problem before simply replacing it...shot gunning parts gets expensive fast. And that's where having a scanner will pay for itself in short order.
> 
> As for the indicator light blinking...
> 
> Have you actually verified that it's in 4WD? I know you can hear the T-case engage now but have you actually taken the truck out to see if the front tires are pulling? Usually when the indicators blink continuously it's due to the T-case control module not receiving confirmation back from the front axle actuator that it has engaged. And if it hasn't you'll have no power to the front wheels whether the T-case itself is now functioning or not. So you may have more than one issue.


Yes the 4 wheel drive now works in hi and low. The clunk you told me to listen for was absent prior to encoder replacement and now is absolutely definitively there. Hope this helps. We pulled the encoder and found it to be corroded to the nines. With the encoder disconnected from the switch, the lights appear to funtion the same as a properly operating system in one of our other similar trucks.


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## B&B (Nov 4, 2006)

Sounds like a weak connection in the encoder plug, thus the feedback signal isn't being received by the T-case module. 

Did you checked it very carefully for any corrosion in the plug itself? Often times when the encoder is found to be very corroded internally the connector side of the pins are also. COuld have a bent pin as well.

If you shift the T-case into neutral does the neutral light glow steady, or also blink as the 4HI and Lo lights do?


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## Motorman 007 (Jan 11, 2005)

B&B;899653 said:


> Sounds like a weak connection in the encoder plug, thus the feedback signal isn't being received by the T-case module.
> 
> Did you checked it very carefully for any corrosion in the plug itself? Often times when the encoder is found to be very corroded internally the connector side of the pins are also. COuld have a bent pin as well.
> 
> If you shift the T-case into neutral does the neutral light glow steady, or also blink as the 4HI and Lo lights do?


OK I'll bite the hook. How do you put it in Neutral with the push button system? Can't seem to make that happen and for that matter, have never done it.


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## Motorman 007 (Jan 11, 2005)

*Call off the wolves!*

Sorry Mike,

When I put it back up in the air on the lift I found that my mechanic:realmad::realmad: left the actuator unplugged. All is well now. I still would like to know how to put it in neutral though


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## B&B (Nov 4, 2006)

Motorman 007;900550 said:


> Sorry Mike,
> 
> When I put it back up in the air on the lift I found that my mechanic:realmad::realmad: left the actuator unplugged. All is well now. I still would like to know how to put it in neutral though


My next question would have been if you checked the actuator connector. Sounds like you did. 

To engage neutral place the transmission in neutral and with the truck running or the key in the run position, press and hold the 2Hi and 4Lo buttons simultaneously for a minumum of 15 seconds. You'll hear it shift into neutral. To shift it from 4Lo simply press any other button.


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## TJS (Oct 22, 2003)

Not to Hi-jack, but B&B my Bro in laws truck is a 2000 2500 GMC. His 4 wheel drive lights on the switch buttons do not light up and neither does the 4x4 system. He replaced the fuse and now the buttons lit up, but after pressing them again it looks as if the fuse blew again. I read and re-read this whole post. Called my parts buddy and he thought is was the common actuator till I gave him the exact specs on the truck. He said he was not sure. Got any ideas. How much are the switches and are the switches one assembly.
Thanks.
T.J.


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## B&B (Nov 4, 2006)

The switch assembly or the encoder are the two that will cause a blown fuse most often when a command is pressed, but the actuator or it's wiring can too. The switches because they had many issues with the early ones, and the encoder due to age/corrosion thus shorting out the system when a command is pressed, and the actuator is the same.

You can test for a bad actuator by simply disconnecting it at it connector plug near the front diff and then press the 4WD button a few times. It will blink the 4WD light continuously with the actuator unplugged but what you're after is if it blows the fuse again. If it doesn't, then the chance is good that the actuator is shorted. But if it still blows then it's narrowed down to the switches or the encoder. You can try the same test with the encoder but it's not always a guaranteed test. 

Normally I'll simply try a spare switch assembly to narrow it down as its a 5 minute easy test to swap the switches (they're all in one housing and take minutes to change). You can do the same if you have a switch assembly to borrow from a like year truck. Makes for a quicker diagnosis.

Also be sure to check the ground connections on the cab mount right under the driver door. One is ground for the 4WD functions and can cause the same scenario. Good idea to remove them and clean the contact points with a wire brush and add some Fluid Film periodically on any truck.


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## TJS (Oct 22, 2003)

B&B;977421 said:


> The switch assembly or the encoder are the two that will cause a blown fuse most often when a command is pressed, but the actuator or it's wiring can too. The switches because they had many issues with the early ones, and the encoder due to age/corrosion thus shorting out the system when a command is pressed, and the actuator is the same.
> 
> You can test for a bad actuator by simply disconnecting it at it connector plug near the front diff and then press the 4WD button a few times. It will blink the 4WD light continuously with the actuator unplugged but what you're after is if it blows the fuse again. If it doesn't, then the chance is good that the actuator is shorted. But if it still blows then it's narrowed down to the switches or the encoder. You can try the same test with the encoder but it's not always a guaranteed test.
> 
> ...


Thanks. The fuse did not blow again. However, this is what we did:
Turned the key on engine off, shifter in "N" and then held both buttons for 15 seconds. Did not really hear anything. Started the truck up and tried 4Hi. No click noise, and no back-lit buttons. Shut truck off again and did key on engine off and I pressed the 4Hi button and I heard the encoder click and the 4Hi button lit up. Hmmm, this was wierd. We started the truck with the 4Hi button still lit up and wanted to really see if it was in 4 wheel drive. So I had him drive forward and turn hard to the left to get that normal 4x4 hop. Yup we had 4 wheel drive. So then we pressed the 2wheel drive button and you could really "feel" the encoder dis-engauge the transfer case. Tried 4Hi again and could not get it to work or lite up. So to me it seems like it is the switch. We will be getting one by this weekend.
Thanks for the help B&B.
T.J.


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## B&B (Nov 4, 2006)

With that extra detailed description I would agree it's the switch assembly itself that's faulty.


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## andcon83 (Dec 10, 2005)

I have a new encoder motor if you need one....


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## rjpjnk (Jan 28, 2010)

*A new issue. Stuck in 4H. Please help*

B&B,

It sounds like you are quite an expert in this area of GM 4WD shift issues. I could really use your help on a similar issue I am having with my 2001 Suburban where I am stuck in 4H. I have the same 4-button system you guys are troubleshooting above. Before getting into the details I would like to ask should I start a new thread or continue in this one?

Thanks for any help!

Rich



B&B;901044 said:


> My next question would have been if you checked the actuator connector. Sounds like you did.
> 
> To engage neutral place the transmission in neutral and with the truck running or the key in the run position, press and hold the 2Hi and 4Lo buttons simultaneously for a minumum of 15 seconds. You'll hear it shift into neutral. To shift it from 4Lo simply press any other button.


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## rjpjnk (Jan 28, 2010)

I decided to start a new thread for my issue.

Please take a look here:

http://www.plowsite.com/showthread.php?p=978449#post978449



rjpjnk;978421 said:


> B&B,
> 
> It sounds like you are quite an expert in this area of GM 4WD shift issues. I could really use your help on a similar issue I am having with my 2001 Suburban where I am stuck in 4H. I have the same 4-button system you guys are troubleshooting above. Before getting into the details I would like to ask should I start a new thread or continue in this one?
> 
> ...


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