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I'm banging my head against the wall and hoping someone can help. I purchased a 96 4.3l 4wd pickup in decent condition, but the 4wd isn't working. I'll list the things I've tried as I've narrowed it down significantly.
  • The transfer case shifts in and out fine. When engaged, the front drive shaft spins as expected.
  • I replaced the vacuum switch on the transfer case, which seemed to be stuck.
  • Replaced a corroded vacuum line from the reservoir to the vacuum switch.
  • The actuator under the battery actuates as expected when I connect a vacuum pump to the hose running from the vacuum switch.
  • When using vehicle vacuum, the actuator moves about 1/4", not enough to engage.
  • When I place the vacuum pump on the hose running to the vacuum switch, the actuator moves the same 1/4".
  • When actuated, the hub locks and engages with the front drive shaft.
The vacuum switch appeared to be working when tested before installing. What else could be interfering with passing the vacuum to the actuator? I'm at a loss and not sure what else to check. Thanks for any suggestions.
 

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98 2.2 Extended cab, 2001 V6 Extended cab Sport Suspension, 2001 V6 Extended Cab 4x4
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I'm banging my head against the wall and hoping someone can help. I purchased a 96 4.3l 4wd pickup in decent condition, but the 4wd isn't working. I'll list the things I've tried as I've narrowed it down significantly.
  • The transfer case shifts in and out fine. When engaged, the front drive shaft spins as expected.
  • I replaced the vacuum switch on the transfer case, which seemed to be stuck.
  • Replaced a corroded vacuum line from the reservoir to the vacuum switch.
  • The actuator under the battery actuates as expected when I connect a vacuum pump to the hose running from the vacuum switch.
  • When using vehicle vacuum, the actuator moves about 1/4", not enough to engage.
  • When I place the vacuum pump on the hose running to the vacuum switch, the actuator moves the same 1/4".
  • When actuated, the hub locks and engages with the front drive shaft.
The vacuum switch appeared to be working when tested before installing. What else could be interfering with passing the vacuum to the actuator? I'm at a loss and not sure what else to check. Thanks for any suggestions.
Sounds like the vacuum switch is not being opened up fully from the transfer case. If your old switch was stuck it could have damaged the arm that depresses it when the T-case is activated. Try removing the switch and actuating it my hand and see if the differential actuator moves completely.
 

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If you used a Dorman vacuum switch the connector doesn't fit tight and leaks loosing vacuum. Very common error. They've been bad for 10 years and never corrected them.
Get the good GM one. Part is 89059420 and has a blue stripe on it.
 
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