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4wd light stays on

Old 11-18-2011, 12:47 PM
mccart is offline
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Location: cedar hill mo
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Default 4wd light stays on

Pretty sure it's a sensor but my 4wd light wont go off when in 2wd. I know its not stuck in 4 cause it'll spin the rear tires. What could it be?
Old 11-18-2011, 02:59 PM
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Location: Iowa
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Down on your shift motor on the axle (on the passanger side) there is a switch/indicator that turns the light on when the forks move when 4wd is ingaged. Most likely if 4wd is not engaged but the light is on that indicator is stuck. Heres a diagram to show where it is located, pretty easy to get to. Cleaning it may help, hard to say.
Old 04-21-2014, 10:03 PM
Brushwolf is offline
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Default Same problem here on 1994 Ram, but switch is ok.

The light has been on for a long time (son had put black tape over it), but as earlier post says, it does not drive the front wheels when transfer case is in 2wd tho light is on all the time the ignition switch is on. Just pulled both front axles and replaced axle u joints that were falling apart.

Jacked up right front wheel only and it turns freely in 2wd; put transfer case in 4wd and the wheel no longer turns. To me this indicates the fork and collar are working properly since with other wheel on ground and transfer case engaged the right front axle will not turn when lever in 4WD, but will turn when transfer case is in 2WD.

So thought switch must be defective, removed it (clean, not sticking), then tested with circuit tester which lights up when spring-loaded probe is pushed in. Light goes off when pressure is removed and probe springs back out. Re-installed it with transfer case in 2WD, turned on key and light was no longer on. Then turned to start engine and within a second or two of starting the light came back on.

I suspect the vacuum created when engine started must have moved something but right axle still turns free. Am I drawing the correct conclusion that since the right axle turns in 2WD but not when transfer case is in 4WD that the collar must be actuating correctly even though the switch apparently works yet still thinks it is engaged? This makes no sense to me, but have not worked on a truck with this type of 4WD before. I need to make a 1200 mile run with the truck and want to be sure the right axle halves are not coupling and stress the new axle u joints or the front differential. Do the small bolts holding the assembly to axle break off frequently if the assembly is removed to inspect it? Do I just have some kind of vacuum issue? But if vacuum problem it should not go in and out of 4WD properly like it appears to, should it? I am stumped at the moment..
Old 05-01-2015, 11:48 AM
Brushwolf is offline
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Default Cause was the vacuum switch on transmission..

The screw-in vacuum switch located on top of transmission was the culprit. It would only pass vacuum through one way so despite transfer case not being in 4wd, the collar would only engage and not release. After replacing the whole mechanism on front axle and taking it apart, didn't really seem like there was anything wrong with it. I took vacuum hoses off the axle and found that only one of them had vacuum with engine running, no matter what position the transfer case shift was placed in. I reversed them on axle in case someone had removed and replaced them incorrectly and found that the light then went out. Which indicated the source of the vacuum control was at fault. So, probably replaced a bunch of parts on axle that did not need replacing, but have peace of mind knowing they are all new. Has worked flawlessly since switch replaced.
Old 05-01-2015, 06:17 PM
Brushwolf is offline
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Default Further explanation of vacuum switch operation

Did you ever figure out why your 4wd light stayed on, I am having the same issue[/QUOTE]

It was vacuum switch on top of transmission. It was plugged internally so it only operated in one direction. Sometimes it is leaky vacuum lines too, but mine were OK. Good luck.[/QUOTE]

How did you figure that out? My lines appear to be intact.

All the vacuum lines do is push/pull the mechanism in axle back and forth to engage/disengage axle coupling inside axle housing.

There should be vacuum in one line to axle when truck is running and transfer case disengaged (2WD position), and axle coupler should also be disengaged (though you can't actually see it). When it is in that position there is a rod inside axle vacuum assembly that moves in unison with axle coupler and breaks circuit which runs the 4wd light, so it goes off (or shoul d go off...).

But, when the transfer case is shifted into 4WD position, the vacuum should then be present in the other vacuum line to axle and should move the vacuum mechanism inside axle housing the other direction, making the cicuit complete again and turning the light on as well as engaging the coupler.

All that is happening at the front axle when transfer case is shifted, is the switch on top of transfer case redirects vacuum from the running engine to one side or the other of that vacuum actuated engagement circuit on axle.

A vacuum line goes from the intake to the transfer case switch, the switch (actuated by transfer case lever movement) directs vacuum to one side or the other of that circuit when the transfer case is shifted in or out of 4wd and causes the axle coupler to engage or disengage, depending on which way the vacuum is directing it to go.

I finally figured out the switch was defective when I intentionally hooked the vacuum lines up to the opposite fittings on the front axle vacuum mechanism - just in case previous owner had put them on wrong, since problem already existed when I got the truck.

When I hooked the axle vacuum lines up backwards with transfer case lever positioned in 4wd, the light went out even though it actually was in 4wd at the transfer case (but axle was not coupled, so not pulling at front wheels...).

Shifted transfer case back to 2WD again, yet 4wd light did not come back on, as it might be expected to do hooked up backwards - if both sides of vacuum circuit worked... Switched hoses back into their correct positions again and then 4WD light came back on again. So, the axle coupler and the light activation mechanism could obviously move both directions when it had vacuum at either one of the connections.

So, the long and short of it is that one of the two vacuum lines to axle should have vacuum when transfer case is in 4wd (which engages front axle coupler also..). The other hose should have vacuum only when the transfer case is in 2wd (which should also disengage fornt axle coupler).

If neither line to axle has vacuum in any transfer case shift position, the vacuum source line from motor to transfer case switch could be the problem.

If vacuum line from motor to transfer case switch is ok (has noticeable vacuum when truck running), but one hose or the other at front axle has no vacuum no matter what position the transfer case is shifted into, then the switch must not be opening one or both sides (lines) to vacuum.

I blew through the old switch after I took it off while moving actuator end through its operating range and found that one side was completely blocked inside switch. I already had new switch on hand (bitch to get out, up out of view, but not expensive...). Took the old one out and put new one in before I even checked to see exactly why it didn't work... Been working fine since then (except computer fried last November just like my Durango computer melted two years ago.. 4wd all good though

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