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'91 YJ . Move the transfer case lever to 4h, the front axle engages and the indicator light comes on. All works fine at slow speeds, but as speed increases (toward 40 mph) the indicator light will flick off and then back on accompanied by "clunk" noise from the front and slight jerk felt in the chassis. Since the shift motor only has 2 vacuum lines, I assume that one applies vacuum for engagement and the other vacuum for disengagement. Both lines lead to the control switch on the transfer case. I suspect the control switch is failing. Any thoughts.

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Jeep North...Sounds like a vacuum supply issue.  This could be either a leak, bleed off of vacuum or a low vacuum supply as engine load increases.  Vehicle speed might be coincidental with the drop in available vacuum.

I had an issue with my '99 XJ's cruise control kicking off and A/C malfunctioning while driving up a grade.  The problem was a vacuum leak at the supply side, actually a damaged plastic pipe between the vacuum reservoir (bulb) and its manifold vacuum source.  (Corrosive battery case debris had eroded the plastic pipe beneath the battery.)  A new section of vacuum hose ended the problem.  

Check the vacuum supply and reserve tank.  Also, you can check the switch and motor hoses for a leak with the use of a simple vacuum gauge/pump.  Use logic to isolate the flow of vacuum on the circuit, pump up the vacuum, and trace for vacuum losses or leaks.  Of course, the transfer case switch could be leaking and therefore defective, but I would check for a simpler connection and tubing issue.  The plastic is brittle and vulnerable over time, rubber end connections get worn, loose and stretched or overheated.  Start with a vacuum bleed-off test before spending more on hard parts.

Also check the actual engine manifold vacuum and for leaks at the manifold vacuum fittings and hoses.  Let us know what turns up!

Moses

 

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  • 4 weeks later...

Moses

Had some nice weather so did some vacuum testing using my vacuum brake bleeder. Applied 15in vacuum to the engage side of the vacuum motor and rocked the jeep a few inches, the front drive shaft would move. Applied same 15in  to the release side of the v motor, rocked the jeep and the front shaft didn't move. Also checked for bleed down on both sides but they held steady > so the motor doesn't have an internal leak and it drives the engaging fork.

Next checked the v motor connector fittings. With the engine running, a steady vacuum  (no needle flutter) reading of 17in. HG on both the engage and disengage sides was correctly indicated when either 4h or 2h was selected on the transfer case. I guess I was surprised to see a steady vacuum reading on the disengage side so obviously the v motor is not spring loaded to the off position.

Next applied 17 in of vacuum to the ports on the v motor connector. Both held fairly well with just a very slow bleed down which I assume was coming in through the vacuum check valve in the supply line between gauge and the manifold (engine off).

So much for static tests. Next I'm going to T a vacuum gauge into the v motor engage line and take her out on the road to see if the vacuum on the engage side is really dropping out and then suddenly re-engaging. If so, i'll assume the switch is either bad OR perhaps the transfer case linkage is enough out of adjustment that 4h will engage in the case but the v switch button is not being fully depressed. Not sure what the button rides on in the case so just guessing.

Any thoughts.

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Jeep North...Great troubleshooting...Logical and systematic, you're on it!

Narrowed down, I expect you'll have a solution or answer shortly, your last tests were revealing, there's an end in sight.  

Take a look at the right side axle shaft parts in a schematic drawing.  There is the possibility of parts damage or fork/clutch hub wear that could create a disengage situation while under load.  Try your simpler possibilities first, they don't involve removing the axle shaft...I can provide diagrams if needed. 

Moses

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  • 1 month later...

Moses

Teed a vacuum gage into the "engage" side of the front axle vacuum motor. Select 4H while parked and the gage indicated 16in. Hg. so I took it on the road. Good vacuum readings pulling through 1st and 2nd gear. Good reading when going into 3rd and putting engine under load and that's when the clunk and jerking began. When pulling in 3rd the gage needle was steady and when the clunking occurred the needle only quivered, +/- 2 to 3 in. Hg from it's steady reading. I'm fairly sure not it's not the vacuum switch on the transfer case.

I'm thinking you called it your last message regarding damage or wear in the fork/clutch area, although I haven't investigated it yet. I'm hoping it's only the vacuum motor not actuating far enough to allow the fork to completely engage the axle halves.

Stay tuned.

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Jeep North...I like your diagnostics approach and thoroughness!  It's easy to remove the vacuum motor shift mechanism from the front axle to inspect the shift fork.  Inspect the two-piece axle shaft cogs to make sure the shaft and clutch hub are intact as well.

A one-piece axle shaft (right side) would be one way to cure the problem if you find too much shaft damage.  If you do the one-piece axle shaft, be sure that there's is a polished surface on the shaft for the outboard (beyond the vacuum motor) axle shaft seal.  On your axle, unlike the Dana 30 axles without a vacuum disconnect, the seal at the right side of the differential carrier is not present.

Of course, it would be easier to restore your shaft disconnect if the damage is only at the vacuum motor or its actuator...You're on the right diagnostics track!

Moses

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