I recently purchases a 99 Tahoe and have already replaced the fuel pump and relay.

Every so often the "Service 4WD" light comes on...The lady I bought it from said it did that as long as she could remember and that she never had any problems with using 4WD.

When that light isn't on I'll sometimes notice that the 2WD HI light on the panel isn't illuminated...if I give the panel a little tap it usually comes on...but if I tap the panel again it'll go off. Either way, the 4WD appears to be working fine.

When the "Service 4WD" light first came on I brought it to the dealership but could no duplicate the problem...the tech ran a diagnostic and said that there might be something wrong with the control unit (ECM?) but not to worry about it if everything seems to be working...he even test drove it with no problems.

So, any ideas? Do you think it is the ECM...a very expensive fix...or could it just be a bad panel in the cabin? Where do I get a replacement panel and how do I put it in?

Any help is greatly appreciated!

I had what sounds to be the exact same problem. Try putting the vehicle in 4 wheel high WHEN THE SERVICE 4x4 LIGHT COMES ON. If it won't go into 4 wheel drive, you obviously have a problem. My dealer put my 2000 Tahoe on the computer and found some sort of valve problem (sorry, can't find my copy of the work order. It may be the same valve you mentioned). They replaced it and it works fine. It cost approximately $80.00 to repair (plus labor). Mine, fortunately, was under warranty at the time. Hope this helps. I'll see if I can locate the work order or contact my dealer for you.

Hope this helps.....Best of luck.

I recently purchases a 99 Tahoe and have already replaced the fuel pump and relay.

Every so often the "Service 4WD" light comes on...The lady I bought it from said it did that as long as she could remember and that she never had any problems with using 4WD.

When that light isn't on I'll sometimes notice that the 2WD HI light on the panel isn't illuminated...if I give the panel a little tap it usually comes on...but if I tap the panel again it'll go off. Either way, the 4WD appears to be working fine.

When the "Service 4WD" light first came on I brought it to the dealership but could no duplicate the problem...the tech ran a diagnostic and said that there might be something wrong with the control unit (ECM?) but not to worry about it if everything seems to be working...he even test drove it with no problems.

So, any ideas? Do you think it is the ECM...a very expensive fix...or could it just be a bad panel in the cabin? Where do I get a replacement panel and how do I put it in?

Any help is greatly appreciated!replaced switches in my 03 fixed the problem

I had the exact problem, so have a lot of people. You just need the 4x4 switch changed out...that's it...there is nothing wrong with your 4x4.

Actually Mulaka is right on the money...I had the same problem with my Silverado...you can change out the switch or you can remove it from the dash and resolder the main pin connections inside. It was easy, the hard part was getting the back off. To replace it you'll need to remove it anyway so give it a try. The vibrations of the truck crack the solder around the pins.
Sonny