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Discussion Starter · #1 ·
Been having problems with my 4wd. For a couple years now, when going up a hill, sometimes there will be a truck shaking thunk. When this happened, the 4wd light stayed on.

Shift motor is 2 years old. Vacuum line pressure is normal. Not a slipping TC chain.

Now when that thunk happens, the 4wd light goes out for less than a second, then pops back on.

Any ideas, guys?

Thanks
 

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"Shift motor" I do not think you have an electronic shifter so no motor. You must mean the 4x4 vacuum actuator? If it is only two years old check all the line connections for good seal. Soulds a lot like a vacuum leak in the actuator diaphragm, lines and connections or switch. If you have a carburetor vacuum tester you can test each line for leaks. Make sure fork and collar are properly installed.
 

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Discussion Starter · #3 ·
Thanks, Big. Correct, it is the vacuum actuator. I tested the lines with a vac tester-- they seem fine.

Just funny that it does it episodically and only when going up a bumpy hill.

Tempted to just replace the actuator-- have had a spate of newish Duralast parts go bad on me.
 

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You can test it both ways by sucking it either way on the tubes and see if it leaks down. Have you had the transfer case apart recently before issue? Was the right axle out at all? It is also possible you/whoever put the fork on backwards during last actuator replacement.
 

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Discussion Starter · #5 ·
Thanks, Big.

Transfer case was taken apart after issue-- thought it might have been stretched chain. Nope. Right axle-- no. I put the for on... I assume I did thefork the right way, but I think it's time to start over.

Best,
 

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Last time I did mine I put the fork in backwards and it acted something like you are having.
 
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Discussion Starter · #7 ·
Big, do your remember to which side the fork goes? I remember it has a bend/curve-- does that go towards the drivers or passenger side? Having trouble finding decent info on that. thx.
 

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Looks like long end away for vacuum pod. Make sure vacuum lines have not been switched on it or on the crossmember.


Motor vehicle Font Line Parallel Engineering
 
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Bigfish,

Out of curiosity, why did you ask if the right axle was taken out?
 

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Discussion Starter · #10 ·
Took the actuator off. Fork is in correct orientation. I guess next step is to re-check vacuum and if that is fine, rebuilt the actuator. Vac lines are correctly oriented.

Axle has never been off.

Thanks,
 

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Because if the axle had been out the color could have been put in backwards. Try switching the vacuum line on the crossmember. If that makes it work backwards it is not the issue. put them back. Actuator could have a leak or maybe elswhere. If you have a harness leak I bout a new one from Dealer for 45 dollars recently. Make sure a tit on the switch on top of the TC. is not broke.
 
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Discussion Starter · #12 ·
To finish this tale, I replaced the vacuum line assembly from the TC up to under the hood. I blew air from the hose at the firewall that goes to the TC, and I could hear air.

Problem fixed. Thanks for all the help!
 

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Most likey a leak somewhere, maybe the big joint above the TC.
 

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Discussion Starter · #14 ·
Crap... thought I fixed it, then it did it again. So what next-- replace that transfer case switch-- the one with the four prongs that the vac assembly harness plugs into?

This problem is so damned frustrating: replaced vac shift motor (correctly), replaced vac line forward of transfer case... cannot find why it will pop out and in 4wd (that cycle occurs in less than a second.

TIA
 

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You replaced the vacuum actuator on the CAD on the axle? It is not a motor it is a diaphragm. Unless broke the switch is not a common problem. Is the light coming on and off when it goes in and out? Have you done a vacuum leak test on all the lines? Does heater vent/defrost and cruise work well?
Does shift lever remain in proper position for both low and high 4x4?
 

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Discussion Starter · #16 ·
Yes, I replaced the actuator. Switch isn't broken. Light goes off and on. Tested all lines, and they test fine-- it only does it when I'm driving, say every 1/2 mile it will do it. Sitting in the driveway, it won't. Heater vent/def and cruise all work fine. Shift lever is in proper position. The fork is in its proper position, with new pads.

I tell ya Big, this is driving me insane.

And I can tell it does go out of 4wd when the light goes off.

I guess I need to start over and test everything again. Maybe blow the metal lines out.

Anyway, thanks!
 

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That almost certainly has to be a vacuum issue. I acts like my last bad actuator. Do you have an old style fuel pump/vacuum tester? You put on the new harness that goes from the main line on the firewall to the TC and back to the actuator, several hoses?
Your location?
Have you had the TC apart?
Did you remove the collar and/or axle when working on it?
 

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PS could one oftehsnap rings have come off the actuator shaft during or after you assembled it?
 

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Discussion Starter · #19 ·
Thanks Big. Have a vac tester.

Yes, new octopus-like harness, from TC to firewall, to the first set of steel vacuum tubes.

In Moscow, ID

Have had the TC apart-- initially I thought I had a slipping chain. It would slip out of gear, but the 4wd light stayed on. The TC was fine, new chain, etc didn't solve the issue. Months later when it slipped out of gear, the 4wd light would go out too, and it was more frequent.

I've had a snap ring come off before-- I opened the CAD up and all was well. Both actuators test fine-- the one I took off the truck and the new one, holding a steady 20psi.

Collar/axel has never been removed.

Thanks!
 

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You and I have not had a long back and forth with the TC last month did we?
How much vacuum is the vacuum pump putting out to the main line at idle?
Have you hooked/pumped it up to BOTH sides of the acutator and pumped it up and left it to see if it holds or leaks off, both old and new? It should not bleed off at all if connection good.
 
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