My 2000 LT 4.3 recently started displaying the "Service 4WD" lite but only after driving for a few miles -- then if I stop and turn off the engine and take off again it will be off - then come on again after a few miles. I thought maybe the fluid was a bit low because it has a slight leak from the transfer case so I added about 1/4 quart of fluid but it seems to have made no difference. Should I try adding more fluid? I'm not sure if I got it all the way full because I was in a bit of a hurry and already had a mess of fluid on my hands and couldn't really tell if I was feeling the fluid level correctly through the "level check" hole. I was thinking light may be due to low fluid level -- that once the fluid got distributed through the transfer case after a few miles that the level by a sensor maybe dropped low enough to lite the service 4wd light. Could it be due to a different cause? Any thoughts guys?
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Does the 4x4 switch work?I once hada4x4 switch go.A GM tech buddy borrowedthe dealersTech2 and the 4 button switch was bad. It was $42.00 or so. I haven't noticed a low tranny fl lamp or sensor in any of my Blazers.

 

I copied this from the owners' manual.
The SERVICE 4WD light comes on to indicate that there may be a problem with the four-wheel drive system and service is required. Malfunctions can be indicated by the system before any problem is apparent, which may prevent serious damage to the vehicle. This system is also designed to assist your service technician in correctly diagnosing a malfunction.
It might be a good idea to have the transfer case codes read. If there are no codes present, there may be a history code.