98 Sonoma 4x4 that acts like a 4x2 now

98 Sonoma with the whole 4.3 Vin W towing package, 3.73 gears, posi.
within the last month I went from having all 4WD choices, to having only 2HI and 4LO, and now, all I have is 2HI. anyone know what that might be? PCM?
Vacuum hoses? TC problems?
Jason
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Electric or manual 4WD switch/shifter?
Doc

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Sorry, Doc....electric
push button 4WD
Jason
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There are folks in here more knowledgeable than I in this matter, but my guess would be the switch itself or the actuator on the TC itself.
Have it scanned and see if any codes pop up.
Doc

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Jason Sobol wrote:

What happens when you actually try to shift to a different range? Does the light (range that you are shifting into) blink for a while and then revert back to 2hi? Or does it stay blinking indefinitely until you shift it back to 2hi?
Ian
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no, the shift light switches to 4WD, but the TC doesnt seem to engage......
J
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Jason Sobol wrote:

If that's the case, you may have some sort of mechanical problem. The light won't go solid unless the encoder motor has moved to the correct position, and the front diff has achieved it's lockup.
Ian
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but what could it possibly be?
once I have an idea what to look for, *I*, since I am a mechanic, and went to school for it...will for the first time in ages, do work on this POS...which waited till it's almost paid off b4 goin to hell on me (next month my Sonoma will FINALLY be fully paid off :) )
165,300 miles, had it for 4 1/2 years, bought it in aug of 00 with just under 25K on it, so you all see where the mileage has come from....
4.3 Vin W, towing package, very strong motor....only ever needed new oil cooler lines and WP, everything else has been in AT, steering/brakes or body issues......
J
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Jason Sobol wrote:

Well, Jason, what do "you" think it could possibly be? If you want to work on the vehicle, you are going to have to do some of the leg work. You say it "appears" to shift into 4hi, so the next logical step would be to put it up on jack stands or a hoist (get the wheels off the ground) and see what is actually happening mechanically when it's in 4hi. If you are a mechanic, you should be able to get a better idea of what's going on. Remember, just because these vehicles have encoder motors and modules doesn't mean that you don't have to some basic mechanical style diagnosis once in a while.
Ian
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it doesnt shift into 4WD anymore....the light goes solid when it shifts, but I dont hear the TC engaging, and right now, all I can surmise is that A) there are vacuum lines loose/broken, b) something has gone wrong in the TC or C) maybe the PCM has gone bad
the weather here is so bad right now I can not get under it and look, but I will think of some way to get a look
Shinden, I appreciate your help and comments. I have recently had someone, who thinks that they know everything about automobiles go around flaming me in another NG, which has stressed me out (I was supposed to work as a tech for this guy....long story...) so its a tad bit hard on me mentally right now... I do know that if I can get a code reader that says I have a code 4 transmission problem, then that is my PCM needing replaced. What should I do if that is the case?
J
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Like, put 'err in the mud and see if the front wheels spin? More fun and IMO safer than the jack stand approach.... hehehe
GMC Gremlin

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wrote:

that's much much less fun than the jackstands if the fronts *don't* spin.
-Bret

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Done it...fronts do not spin
J
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Jason Sobol wrote:

Ok, does the front driveshaft spin? This will tell you if you have t/case problems or front diff actuation problems. I can't remember off hand for this year, but I think the vacuum control for the front axle is right above the distributor on the firewall. You would see a small solenoid sitting there with vacuum hoses going to it. They often fail. If it is there, remove the two hoses, stick them together temporarily and see if you get 4x4 action. This is assuming that the front drive shaft is turning when you put it in 4hi.
Ian
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