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Discussion Starter · #1 ·
I have a 1990 S-10 with 4WD and need to know the right process to engage the 4WD. It is an automatic with the 4.3 V-6. When I have tried to engage 4WD, I have stopped it, put it in neutral, and tried to pull the shifter into 4WD, but it clashed the gears. I have tried to put it in 4WD in Park, with the motor off, restarted it, put it in gear in 4WD (with it on jack stands) and only the rear wheels turn. I had to rebuild the rear u-joints and while the driveline was out, I tried putting it in 4WD and the speedometer indicated it was moving, but the front wheels did not turn at all.

I have never had a 4WD before and want to learn how to use it correctly. I don't know if there is a problem with the system, or if I am doing something wrong in the way I have tried to engage it. I don't have an owners manual to read/learn how to do it. Thanks for any help. Dan
 

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Is the front driveshaft turning?

If so, suspect the actuator on the front differential, actually that may be a vacume servo under the hood on that one IIRC.....

If the front shaft isnt turning suspect maybe the transfer case is trash.
 

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Discussion Starter · #3 ·
I had it up on jacks and yes, the front driveshaft is turning, but wheels still don't turn. I will have to check on the vacuum relay or sensor next week. I had it 4WD and tried to put it in Park, but it clashed gears real bad, so I turned it off to put it in Park and 2WD. At least the rear axle drives to it can be my backup vehicle. Any other ideas? Thanks, Dan
 

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I would suspect the front diff actuator is bad (not uncommon). I dont recall if it is electric or vacume tho, it is one way on fullsize and other on s series IIRC.....
 

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I would suspect the front diff actuator is bad (not uncommon). I dont recall if it is electric or vacume tho, it is one way on fullsize and other on s series IIRC.....
its vacuum

fire up the truck put it in 4wd but in park or neutral and watch it there will be a diaphram that moves when you unplug the hose and and again when you plug the vacuum line back in or have someone shift it in and out of 4wd it takes a few seconds to move if it doesn't move unplug the line there and put your finder over the end it should be a light suction on it if there isn't there's a broken vacuum line between the t case and the actuator or between the tcase and the intake manifold. or the peice where the lines go into that tcase is screwed up
 
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