4WD light off, under full throttle

Gentlemen,

Last Sunday, I took my old 1984 XJ out for some wheeling and I noticed that the 4WD light would turn off, when accelerating full-throttle.

Let me add a couple of details. When I got the truck, I removed all the emission control devices, except the catalytic converter. I also changed the carb with a Weber one. I therefore removed several pieces of tubing, but I made sure that the vacuum tube which goes to the transfer case remained connected to its original position on the intake manifold. Original, in this case, should be read "where I found it connected", not where it should be connected.

Now, I should assume that the 4WD goes off, because under full throttle, there is no vacuum? Maybe the vacuum pipe should be connected to the vacuum canisters (there are two of them in the truck, on the passenger fender)? I do not care that much about the light itself, obviously, what I care about is the truck shifting out of 4WD when I need it in 4WD.

Any advise would be greatly appreciated.

Rgds

Now, I should assume that the 4WD goes off, because under full throttle, there is no vacuum?

Click to expand...

Yup. Not enough vacuum to keep the front axle engaged. Check for leaks.

My understanding is that a lack of vacuum will turn the light off but does not shift either the t-case or the front axle disconnect. In order to shift the front axle disconnect you must apply vacuum to the other side of the shift motor diaphragm. Proof of this is the fact that after parking your jeep in 4wd and shgutting it off the shift motor doesn't shift back in to 2wd mode, if it did it might cause problems when you came back to go again.

http://www.drivetrain.com/../4x4PosiLock.html

actually the vacume and electric controlled disco on front axle will diss engage on you when you shut it down. The electric one, key off no power can and might let go. The same with the vacume disco, no vac, no lock.

If the vacume hose on the intake is for the t-case, you should not loose vac at full throtle. Light throtle=light vacum WOT=gobs of vacum. Put you hand on intake at idle and then rev it up. The only way to loose vac at WOT is with a turbo, which the block is not SUCKING air, but it is being pushed in, and that would give an oppisite air pressure in vac line.

I am sorry to refute you M.lake, but the rush of air you feel on WOT does not equate to vacuum. The opposite is true. vacuum drops in proportion to the amount of throttle opening. In fact, the is the resrtriction of the air supply by the throttle plate that CAUSES the vacuum. on WOT there would probably not be enough vacuum to light the switch that controls the dash light. tI have the same effect on my XJ. On the other hand, while the collar inside the axle might not be held in the locked position very tightly, it should not disengage because to unlock it requires vacuum on the other side of the diaphragm, and this is not the case. I have tested this in my jeep. On WOT in gravel, even though the light ges off, I am still spitting gravel with front and rear tires.

mountaindrew said:

The opposite is true. vacuum drops in proportion to the amount of throttle opening. In fact, the is the resrtriction of the air supply by the throttle plate that CAUSES the vacuum.

On the other hand, while the collar inside the axle might not be held in the locked position very tightly, it should not disengage because to unlock it requires vacuum on the other side of the diaphragm, and this is not the case.

Click to expand...

First, I didn't think of that way with vac :anon:

Second, I read an artical about the, engine off, no lock in front thingy, it is not by experience, but by write up. Either you can't always believe what you read, or not every one has read it

The transfer case is mechanically shifted, a disco axle is shifted by vacuum. Also, the switches for the indicator lights are also vacuum-operated, so if you lose vacuum (like at WOT) you will lose the light and the shift actuator in the axle (if you have it.)

Check the vacuum reservior ("nothing holder") in the front bumper. It is supposed to hold "nothing" for use under WOT for the heater plenum controls and vacuum shift, and if it's leaking - a distinct possibility... - it will allow control loss under WOT.

5-90

The axle disconnect motor only moves under vacuum...either way. It will not return to disconnect if vacuum is lost, it needs vacuum to connect or disconnect the axle. The light is a different story and, as stated above, will go out if no vacuum is present at its sensor.

Rev

Well, I guess mine is an odd one... I have a '90 XJ W/ NP231, NHO motor, and Selec-Trac, and it has been doing virtually the exact OPPOSITE... Funny thing, I just figured I have a vacuum leak somewhere too...

My system says "Part time" (meaning 4WD engaged) all the time, even of course when I am in 2WD, EXCEPT under WOT. I would expect from the comments above that when I was in 4WD under WOT the light would go OFF, but mine stays ON...

Could this be from the same cause? My 4WD works fine, but the stupid light is a PITA blaring all the time at me...

My VAC canister is perfect BTW, I replaced it last year when my other one got crushed...

Help, Tom

hockeyfan_019 said:

Well, I guess mine is an odd one... I have a '90 XJ W/ NP231, NHO motor, and Selec-Trac, and it has been doing virtually the exact OPPOSITE... Funny thing, I just figured I have a vacuum leak somewhere too...

My system says "Part time" (meaning 4WD engaged) all the time, even of course when I am in 2WD, EXCEPT under WOT. I would expect from the comments above that when I was in 4WD under WOT the light would go OFF, but mine stays ON...

Could this be from the same cause? My 4WD works fine, but the stupid light is a PITA blaring all the time at me...

My VAC canister is perfect BTW, I replaced it last year when my other one got crushed...

Help, Tom

Click to expand...

did you swap the 231 in? "Selec-Trac" is the 242 and the 242 does NOT use ANY vacuum for anything. it is 100% mechanical

the switch for the light is in the transfer case, and neither of mine ever worked right, even after rebuilding the case, and cleaning up the switch, it still never worked right and did some goofy stuff every once and a while.

considering that it goes off under load, I'd suggest checking the wiring that comes from it. a wire may be pinched/burned

Whoops, you're right, I meant to say "Command Trac"... I'm not sure why it would go off under load if it was an electrical issue though, except that the voltage is a bit higher under WOT, causing a voltage-sensitive short? Or the case torque is greater under WOT, causing in intermittent ground-fault?

I'm considering just pulling the stupid bulb out of the cluster...

Thanks,
Tom

Folks,

From my understanding of the operation of the disco and the transfer case, indeed vacuum should be applied to one side to shift the disconnector to lock and the other side for unlocking, so I guess that means that I do not risk to loose traction when I need it.

On the other hand, the 4WD light is controlled by a vaccum operated switch, so when there is vacuum the light is on, no vacuum, the light is off.

And I have to agree with Mountaindrew, under WOT there is no vacuum, hense the light goes off.

My real question now is: Where does the vacuum pipe gets to? Does it start from the intake manifold or from the vacuum canister? If it starts from the intake manifold, it's normal (according to the above) to loose the light under WOT), but if it gets the vacuum from the canisters, it wouldn't. But in this last case, would the vacuum be enough to shift the disco? Anyone with a 1984 2.5 lt who can shed a light here?

Many thanks for all your input and rgds

Rev Den said:

The axle disconnect motor only moves under vacuum...either way. It will not return to disconnect if vacuum is lost, it needs vacuum to connect or disconnect the axle. The light is a different story and, as stated above, will go out if no vacuum is present at its sensor.

Rev

Click to expand...

Not true. Both my old 84 XJ and an 89 XJ (that I used to work on) would disconnect once it lost vac. I don't see why, but they did. They should work like the way your talking about. The splines could have worn a bit, maybe.

hockey fan...
check for a short on the wire that runs to the sensor under the pressure bottle...or a bad sensor.

Bottleworks...
The point I was trying to make is that the vacuum motor on the disconnect requires vacuum to move either direction, it has no "spring return" to 2WD. However, without any vacuum present to hold it in the current mode it is possable the drive cog could "walk" out of engagement, considering it only engages like 1/4 inch anyway.


Hope I am clear.

Rev

Rev,

There's supposed to be a sensor associated with the pressure bottle??? I guess It must've been ripped off somewhere along the line or during the collision! I replaced the old "baseball" resevoir a few years back with a "football" style one when I rebuilt the front end, but I just plugged the vac lines in and WHAMO, the vac was restored, and the heater valves all work...

Sheeh, I guess I better look for a sensor, or at least for where a sensor USED to be...

Thanks,
Tom

On my 90 it is a little sensor witha single vacuum line running to it mounted just below the pressure bottle....I seem to remember it being blue.


Rev

Darn, I just checked mine and it goes out under full throttle also. Mine is an 89 with a 231 and AW-4. I nailed it in a sharp turn and it was obvious that both axles were pulling even during the "no light" periodduring WOT. My control harness had softened with oil from my rear main leak so I replaced it about six months ago, but I notice that the heater control valve is a little slow. Also engagement always occurs, but I have to stop and back up sometimes to get the axle to disengage. I'm thinking its all those 15 yr old vac. tubes.

Does anyone have a vac. system they know is in good shape and still have the light go out under WOT? In other words, should I expect the light to stay on under WOT if everything is good? Will a worn 165K engine pull enough vac. to keep it lit?

While the axle shift is done under vacuum, it apparently (by consensus - I've not checked personally) stays put until shifted the other way.

The light, however, must always have vacuum to stay on. At WOT, vacuum is minimal (there is little to no obstruction to the intake air flow,) so there is little manifold vacuum. Highest vacuum occurs at idle, when the throttle valve is nearly closed and the displacement of the engine is unchanged (it is the reduced airflow that causes the low turning speed of the crankshaft - the engine isn't moving as much air as it could.) Therefore, if your vacuum reservior is losing its integrity, you will lose vacuum to the switch if at WOT for more than a few moments - like at cruising speed. Therefore, it is possible that the 4WD indicator will go out - but you will still be in 4WD - remember that...

I've been tossing around ideas for going to a mechanical switch, but just haven't been able to decide what sort of switch to use (probably a roller, tho) and where to mount it (probably right next to the selector lever.) This would get rid of an entire vacuum circuit that is mechanically unnecessary... When I finally get round to doing it, there will be a writeup...

5-90

my light goes off, stays in 4wd...'88 4.0l 5-speed



Q: if this vaccume lock was replaced with a solid mechanical link, could I run it in 2LOW with the front axle disconnected?

Thanks.

jjvande said:

my light goes off, stays in 4wd...'88 4.0l 5-speed



Q: if this vaccume lock was replaced with a solid mechanical link, could I run it in 2LOW with the front axle disconnected?

Thanks.

Click to expand...

Yup.


Rev