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post #1 of (permalink) Old 10-03-2000, 10:34 PM
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my 4 wheel drive won't engage the light won't come on when i even put it in high or low, do u think i might just need oil,or what could be the problem????

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post #2 of (permalink) Old 10-04-2000, 05:00 AM
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Re: help

Sounds like you may need a vacuum switch, which is the most common item to go bad in the 4x4 system. Just to check, jack up one front tire, block the rear wheels, put the t-case in 4hi, and try to turn the front driveshaft by hand, if it turns you have a t-case problem, if it doesn't, you have a vacuum problem or a front axle problem. If the driveshaft doesn't turn start with replacing the vacuum switch on the drivers side of the t-case just above the front driveshaft, it had three hoses plugged onto it. If you need help with further diagnosis, let me know. Hope this is helpful. Talk to you later.[img]/wwwthreads_images/icons/wink.gif[/img]
David G.

'91 S15 Jimmy 5.7L 4X4
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post #3 of (permalink) Old 10-04-2000, 02:41 PM
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Re: help

If the front diff doesn't engage a cracked vacuum line is the most likely culprit. Disconnect the vacuum line on the bottom of the vacuum actuator, it is located on the passenger side inner fender. Engage 4wd (with the engine running). You should feel a strong vacuum in the line, I never put a gage to it, but I would guess 20 lbs. If you don't feel suction, or a weak one, replace the line. If that doesn't help, change the vacuum switch mentioned above.
If you feel a strong suction, the diaphragm in the vacuum actuator maybe cracked or something further down the line.
Let us know what you find.

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post #4 of (permalink) Old 10-04-2000, 02:52 PM
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Re: help

It doesn't need to be a strong vacuum, in fact my truck's vacuum actuator engages with only a slight vacuum... Barely enough to feel it...

What I'd do is find your vacuum actuator, start the engine and observe the actuator... If it doesn't engage, then it's time for more checking, with the engine still running, pull the vacuum line off of the actuator, and put your finger across the vacuum line to feel for vacuum there. If there is vacuum there, then try moving the cable that's attatched to the diaphragm by hand. If you cannot move that cable in the direction it's supposed to go (the cable pulls towards the vacuum diaphragm), then it's time for some more checking with the front diff. You'll have to pull where the cable goes into the front diff and make sure it can move freely, greasing if necessary. If there was no vacuum to the actuator and/or the cable pulls freely, then the problem is either in the actuator or in the vacuum switch on the t-case or vacuum lines... To test the actuator, put a vacuum pump on the actuator and pump it up to see if it pulls in. You can substitute a hose of the appropriate size and suck on it to get the vacuum to pull it also. The vacuum pump method works better since you can pump it up to whatever vacuum you need, and then walk away from it for a little while. When you come back the guage on the vacuum pump should be exactly the same or nearly the same as when you started. If not then your diaphragm likely has a tear in it... If the actuator checks out to be ok, then the problem is likely caused by the vacuum line or switches... Anywho that's a slightly out of order version of the diagnosis of the vacuum engage system for these trucks that I've been working on for the Chevy page... Not quite ready to submit it yet though...

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post #5 of (permalink) Old 10-04-2000, 04:40 PM
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Re: help

Yours works with a slight vacuum? Mine requires a fairly stong vacuum to engage 4wd, I can definitally feel it. I had the hardest time figuring why my 4wd would ingage 'cause the vacuum line had a very small crack and still had good suction. I even tried sucking the vacuum actuator in with good old lung power, but that wasn't strong enough.

Hmmm, maybe I have a small crack in my diaphram... I'll have to look into that.

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