4 whl drive - '97 Blazer

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  #1  
Old 02-16-04, 06:55 AM
dmontague
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Question 4 whl drive - '97 Blazer

My "97 Blazer will not release from whl drive. Control button on dash properly activates 4 whl but will not disengage. If I back up about 2 ft., it drops out. Local shop has been all over the mechanism - computer control, shifter fork & spring in differential, encoder motor, actuator, vacuum, etc. Unit works properly - in and out - while on lift, and with wheels on jacks. However, once back on the ground, doesn't disengage. Any thoughts would be appreciated.
 
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Old 02-16-04, 08:21 AM
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are you saying...

....that after you select 4WD and try to go back to 2WD that you have to back up a little bit to get it to release? If so, you don't have a problem...this is pretty normal...with a load againt the shifting mechanism, there isn't enought force generated by the diaphragm to overcome the load of the drivetrain...reversing momentarily releases load and allows the shifting mechanism to slide back to the disengaged position. the reason it works fine on the lift is because there is no load on the drivetrain. i've looked in the service literature and can't find where is tells you that...but I know I've seen it somewhere
 
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Old 02-16-04, 08:57 AM
dmontague
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carguyinva:
Thanks for taking the time to reply. My only prob. is that, according to my manual, this activation cycle should be able to take place "on the fly". I know when automatics first came in that backing up was the way to disengage, but my understanding is that this '97 souldn't require that.
 
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Old 02-16-04, 11:08 AM
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i believe on the fly will disengage the drive line to the front, but your hubs remain locked until you back up. its a centrifical force thing.
 
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Old 02-16-04, 02:58 PM
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Do a rolling circumferance check on a open lot area.Park with the wheels straight ahead,mark all 4 tires at the 6 oclock position,gab a helper and drive straight ahead and have the helper count when 1 tire has turned 10 times and stop that tire at the 6 oclock position.If any tire is not within 1 inch or less or the 6 oclok position you may have found the problem.Tires that are differant in size can cause the front axle actuator to not disengage properly.Sometimes it will disengage rather abruptly sounding like something is broken.If it fails the check deal with it as is or buy 4 tires.
 
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Old 02-17-04, 04:17 AM
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correction...

i believe on the fly will disengage the drive line to the front, but your hubs remain locked until you back up. its a centrifical force thing.

it doesn't have locking hubs
 
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Old 02-17-04, 04:19 AM
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Davo's...

...hit it right on...after talking with a colleague, it all came back. check rooling circumference
 
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Old 02-17-04, 06:45 AM
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duh!

rooling = rolling
 
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Old 02-17-04, 06:53 AM
dmontague
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You're probably right

Thanks guys,

When I bought the truck used about 3 yrs ago, the dealer had put on two new Michelin tires (same model as the other two) and I thought he had put on 4 new tires - they looked so much the same. Anyway, I will try the ten revolutions test and see what happens. Question: if there is a significant difference between the new and older tires, but the two new and two old are the same, if I swap front to rear on just one side for the test, should the differential make up the difference and the 4 whl drive disengage? I know it isn't a good idea to run like this, but just to do the test - what do you think?
 
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Old 02-17-04, 08:28 AM
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just do the test...

as davo specified...

If any tire is not within 1 inch or less or the 6 oclok position you may have found the problem.Tires that are differant in size can cause the front axle actuator to not disengage properly.Sometimes it will disengage rather abruptly sounding like something is broken.If it fails the check deal with it as is or buy 4 tires.

it's not going to make any difference whether you switch them or not...if they are different circumferencially AND nothing else is worn in the engagement system, you've probably found the culprit
 
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