Vacuum transfer case willNot stay engaged (2 Viewers)

[toddconley]

My vacuum transfer will engage for a few seconds then comes back out. The rubber lines that connect the metal are both good. I clamped both ends securely to make sure there were no leaks. I have tons of vacuum coming out of the valve cover. The light comes on and stays on the whole time the knob is pulled.

How does the system work? Does the vacuum pull the system in gear then the switch is reversed and the vacuum and pulls it back out? Or is the system spring loaded and requires vacuum to to stay in then in the absence of vacuum it springs out?

 

[middlecalf]

Vacuum moves the diaphragm, so it's either engaged (and light is on - it's tripped by a depression in the shaft that the diaphragm moves, not the pull switch itself) and not. Vacuum keeps the diaphragm in one of the positions, there is no spring return. The diaphragm-operated shaft moves a fork that slides a splined connector on or off the TC output shaft and the FD shaft.

Is the loss of engagement very brief, like popping in and out? If so, then likely an issue with the fork - worn surfaces - that moves the splined connector. If continuous, and the light stays on (the shaft isn't moving to disengaged position) then in addition to the fork maybe the splined connector and/or the TC shaft, FD shaft. Mark @65swb45 is an expert with these, I've only rebuilt a couple of them. Hopefully he'll help address this.

 

[Living in the Past]

The vacuum should be off the intake manifold not the valve cover. I would use a vacuum gauge and checked the vacuum in each line going to the diaphragm. Then cap both lines and check each one with the switch in both positions. The reading should be the same on each line. I wouldn't rule out a crack in the diaphragm that opens as it moves. I like the vacuum 4WD on the 40 series. I just wish my FJ62 was as reliable. When I take it off road I usually play with the switch a few times until it switches regularly. That I know when I need it not going play with it to engage.

 

[Solace in Solitude]

I have tons of vacuum coming out of the valve cover.

I don’t know a thing about the vacuum transfer system.

But, your statement, above, seems to conflict with the word “vacuum”.

Are you really pulling vacuum from the valve cover? Or, did you misspeak?

The only two sources of vacuum, of which I’m aware, are manifold (pulled from the intake manifold) and ported (vacuum that is low at idle and increases as you give it gas).

Examples:

Manifold vacuum is constant and is used by the brake booster.​

​

Ported vacuum is vacuum that is low at idle and increases as you give it gas.​

Use manifold vacuum for your vacuum transfer.

 

[Living in the Past]

Valve cover vacuum would be very unreliable. The hole attached to the valve cover is pulling filtered from between the air filter and carburator. This is to make up for the air being pulled out of the crankcase by a hose going to the base of the carburator which has good vacuum. Both of which are part of the PCV system. If your vacuum is truly coming off the valve cover that is the first thing to need to fix.

 

[middlecalf]

Agree with all on the source of the vacuum, hopefully the OP misspoke. But the problem as stated is disengagement, and the FD light stays on. That means the diaphragm shaft moved, that’s what turns the light on. So whatever vacuum the OP is getting is enough to move the diaphragm shaft to turn the light on. By stating “the light comes on..” I’m assuming it goes off when the dashboard switch is moved to off position, which means the diaphragm shaft moves in both directions.

OP, are you checking this functionality with the rig on stands, or how do you know the FD is disengaging?

 

[toddconley]

Yes you are all correct with the fact it comes off the intake. Sorry for the confusion. Yes light stays on when knob is pulled and off when it is pushed. So if I am hearing you all correctly the gizmo must be actuating if the light is on. Is this correct?

Sometimes I go straight to thinking things are the most complicated of problems but I have not considered the hubs could be malfunctioning. I’ve done a ton of reading. There are a couple of things I need to check. You guys stand by for a day. Ill check back. Thank you all so much for your replies.

 

[toddconley]

Vacuum moves the diaphragm, so it's either engaged (and light is on - it's tripped by a depression in the shaft that the diaphragm moves, not the pull switch itself) and not. Vacuum keeps the diaphragm in one of the positions, there is no spring return. The diaphragm-operated shaft moves a fork that slides a splined connector on or off the TC output shaft and the FD shaft.

Is the loss of engagement very brief, like popping in and out? If so, then likely an issue with the fork - worn surfaces - that moves the splined connector. If continuous, and the light stays on (the shaft isn't moving to disengaged position) then in addition to the fork maybe the splined connector and/or the TC shaft, FD shaft. Mark @65swb45 is an expert with these, I've only rebuilt a couple of them. Hopefully he'll help address this.

It engages when you first pull the knob. Then comes out within a few seconds. The reason I know this is just through use. It almost stuck me the other day coming out of the River.

 

[thebigredrocker]

You put the transmission in neutral before pulling the vac shifter correct? I think it needs to be. @Living in the Past could confirm that.

 

[toddconley]

You put the transmission in neutral before pulling the vac shifter correct? I think it needs to be. @Living in the Past could confirm that.

No I have not been. You think that may be the reason it isn’t fully engaging?

 

[tornadoalleycruiser]

They are simple devices, this will sound weird but test it manually, hose off the device on the firewall. Suction on one side, spin front shaft by hand till engaged. See if it works. Then apply suction to other side see if disengages.

 

[middlecalf]

It engages when you first pull the knob. Then comes out within a few seconds.

Does the light go off and does it stay disengaged? Or does it “pop” back in and then out again in a few seconds, and repeats this while under load? If the later that’s a good indication the shift fork is worn excessively.

 

[middlecalf]

A couple of quotes from Obi Wan

Intermittent popping under load is a fair indicator of a worn shift fork.

A collapsed needle bearing on the output shaft could also be causing the front drive to disengage.

On edit - just read you need to check hubs as well and to stand by. Standing by!

 

[toddconley]

First of all thanks for all your help. T case is fixed. Had to end up pulling the actuating mechanism off the side of the case and tearing it all apart. Took quite a bit of detective work but I finally found it. For some reason 74 had come all the way loose from the inside of 72. This in turn would not force the collar over the front drive shaft. No idea how or why.

 

[middlecalf]

Awesome! Now you’re an expert on FD units!

Cotter pin missing maybe?