How JustAnswer Works:
  • Ask an Expert
    Experts are full of valuable knowledge and are ready to help with any question. Credentials confirmed by a Fortune 500 verification firm.
  • Get a Professional Answer
    Via email, text message, or notification as you wait on our site.
    Ask follow up questions if you need to.
  • 100% Satisfaction Guarantee
    Rate the answer you receive.
Ask sprinkles08 Your Own Question
sprinkles08, Chrysler/Dodge/Jeep master tech
Category: Dodge
Satisfied Customers: 21439
Experience:  ASE Master and Advanced Certified, Chrysler/Dodge/Jeep Master Certified, Trans and Hybrid Specialist
Type Your Dodge Question Here...
sprinkles08 is online now
A new question is answered every 9 seconds

Dodge RAM 1500 4x4 my 4wd wont stay engaged, it feels like

This answer was rated:

my 4wd won't stay engaged, it feels like the collar on the axle won't stay in place. I've replaed the motor on the axle , the collar and the fork the switch on the transfer case and eliminated the rubber bushing , plugging the vacum lines into the switch directly. Any ideas/

Hello and welcome to JustAnswer!


Have you checked for vacuum at the transfer case with the engine running?


With it in four wheel drive have you checked for vacuum at the vacuum disconnect with the engine running?



Customer: replied 5 years ago.
yes i have checked it at the transfer and never went up to 15 so I am going to my mechanics to check it at the brake booster and straight off the engine
Customer: replied 5 years ago.
I have the vacum tester in my truck, but I have to get to work. so if I don't have 15 psi at the transfer cuold there be problem at the brake booster itself?

You should see closer to 20" of vacuum both at the intake and at the transfer case. Let me know what you find for each and we'll go from there.


Customer: replied 5 years ago.
I will start my truck put it in neutral in 4wd and I should have 15-20" of vacum at the axle motor correct

Leave the transmission in park for safety, instead of neutral. Start the engine and put it in four wheel drive, us a vacuum gauge right at the disconnect actuator and check the vacuum. You should have close to 20".


If it's lower than that, remove a small diameter vacuum line from the intake manifold and check the vacuum there and let me know what you see for both readings.



Customer: replied 5 years ago.

thelines i see on the brake booster are hard lines is there a vacum line on the intake that is rubber?

The hard lines you see are the brake lines coming from the master cylinder. The booster is the large round unit behind the master cylinder and has a large vacuum line going to it. You could either remove that large line and check the vacuum there, or you could disconnect one of the smaller lines at the intake, which would be more convenient.


Customer: replied 5 years ago.
Thanks for your patience I am working outside in a blizzard, it is only 15 degrees out. The actuator is on the front axle correct, I think I called it a motor.

Yes, the disconnect actuator is the vacuum operated unit on the right axle tube that slides the collar to lock the shafts together.



Customer: replied 5 years ago.

I never got a reading above 12 " at idle, increased the rpm no change,applied the brake and reading went up almost to 15,take pressure off brake and reading falls

Where did you see 12" at, the intake or the actuator?



Customer: replied 5 years ago.

The next step was to check the vacuum right at the intake manifold. Have you done this?



Customer: replied 5 years ago.
I't will be done




Customer: replied 5 years ago.
I am not sure where on the intake is the right vacum,I need a schmatic or some utube video to watch if there is such a thing?
Customer: replied 5 years ago.

I figured it out, there is 161/2" at the manifold

Customer: replied 5 years ago.
how do you increase vacum at the manifold?

Remove the breather line that goes from the left valve cover to the air filter box. Remove the PCV valve from the right valve cover and plug the hole. Start the engine and see if you feel vacuum on the breather hose from the left valve cover.



Customer: replied 5 years ago.
I removed the pvc valve and plugged the hole with a rag, removed the only hose off the left valve cover, which went to the intake manifold. ther was no vacum at the valve cover,just on the intake again only 16". Ihope we can figure this one out

The PCV valve is on the right valve cover and it's hose goes to the intake. Remove the PCV valve and plug it with something that will seal - a rag won't seal to do this test. Once that is plugged go to the left valve cover. Near the brake booster you will find the vent hose that goes from there to the airbox. Remove this hose from the airbox end. Start the engine and put your finger over the hose for a couple minutes and see if you feel any vacuum. You could also adapt your vacuum gauge to this large hose.



Customer: replied 5 years ago.
so I did it backwards,sorry about that my bad
Customer: replied 5 years ago.
so I did it backwards,sorry about that my bad There is about 1psi of vacum coming from the valve cover

Is it 1 PSI, or is there vacuum?



Customer: replied 5 years ago.
there is vacum
Customer: replied 5 years ago.
Its starting to get dark,my blue sky shop is closing any ideas

I have an hour drive home from work, which is why I couldn't respond to you sooner.


Your vacuum at the intake is a little low, and 12" is pretty low at the disconnect actuator, but 12" could be enough to get the actuator to move.


Did you replace the actuator with a new one or used one?


Did you inspect the splines on both shafts and the collar when it was taken apart?


Are you positive that the lines are on the vacuum switch at the transfer case correctly? If two of the lines got swapped around then you would be sending vacuum to the wrong side of the actuator.



Customer: replied 5 years ago.

I was very careful when I did that. Nothing changed when I switched out that rubber gromet to vacum lines

Did you replace the actuator with a new one or used one?


Did you inspect the splines on both shafts and the collar when it was taken apart?


Customer: replied 5 years ago.
It was a new actuator. they were worn but I also have replaced that half shaft that goes from the pumpkin to the collar
How were the splines on the outer axle shaft, in the area that the collar engages?

Are you using just a vacuum gauge, or is it a pump/gauge with a handle on it to be able to draw vacuum? If you're using a pump then I'd like to see if you can run the disconnect actuator with it. With it all together, remove the apply side vacuum line from the actuator and pull about 20" of vacuum, spin the right tire and see if the axles lock together.
Customer: replied 5 years ago.
I am using the Mac tools vacum/fuel pump pressure kit gauge. But tomorrow might be able to get the pump style .

If you could get a pump on it then we would know for sure if we had an actuator/shaft/collar problem, or if we are fighting a vacuum supply problem. Your vacuum is low, but I'm torn as to whether or not 12" is enough to run the actuator. If we find that it will engage ok with the pump then we're going to draw vacuum on the rest of the vacuum supply to see if we have a pinhole in a line between the actuator, transfer case, and to the engine.



Customer: replied 5 years ago.
I'm leaning towards a vacum leak since I have replaced everything on the actuator side ie;collar, fork, half shaft the clips on the fork with no change.I,snt 16 a little low on the motor side.,

I'm assuming that we have a vacuum problem too if the actuator, fork, intermediate shaft and the vacuum switch have been replaced. Just to verify we're not fighting bad parts or an outer shaft spline problem I'd like to run it with a vacuum pump to make sure it will engage.


Yes, 16" from the engine is a bit low.



Customer: replied 5 years ago.
Just a thought cuold I run a vacum line straight off my intaketo the acuator?, because we know we have 16" off the intake, and that should engage the collar

Yes, you could definitely try that if you had enough vacuum hose available.



Customer: replied 5 years ago.
I have to get some vacum line since this may be a temporary fix until I find the leak. it's dark now and the oregon duck game is on, must go hope you are online tomorrow thanks for your time

I'll be at work in the morning, and then I'll be online for the rest of the weekend. If you reply while I'm offline I'll get back to you as quick as I can!



Customer: replied 5 years ago.
I'll gather some stuff plug some things in check to see if my 4wd works with the direct line and get back to you in the afternoon with my results, it needs to warm up to dbl digits to start in the morn. Chat with you tomorrow, thanks

Sounds great. Talk to you tomorrow!



Customer: replied 5 years ago.
I have more input, the 16" did not make a difference at the actuator.I opened up the actuator to find the clips broken off the fork so I replaced them tested the 13" vacum line and moved the fork just fine. I also removed the 4wd switch thinking it my prevent the fork from moving freely all the way out, not an issue. My thoughts are going toward the front diff. could I have worn out my side gears something with my limited slip. There were fine metal shavings in the gear oil but not big chuncks of metal. Let me know what you think, would like to keep working with on this to the finish.If you have the patience. I believe we can woop this one.

You replaced the intermediate shaft because of spline problems, what about the outer axle shaft? I've seen the ends of the splines wear there and the collar has nothing to grab when it tries to engage.



Customer: replied 5 years ago.
I have replaced it as well, when I checked the collar it seemed to be engaged firmly on both splines

And you were able to hold the disconnect actuator in your hand, apply vacuum to it and watched it move full travel? I've seen some that would move but they wouldn't fully move to lock the shafts together.



Customer: replied 5 years ago.
Yes I held it in my hand and applied the vacum line with 13"[stock] and applied the 16" off the engine and got the same results, thats when I removed the engagement switch to see if that was holding it up,it was not the case.

I never asked you to elaborate on why you thought the shafts weren't locking together. Obviously you have a four wheel drive problem of some sort. Did you get all four wheels of the ground and put it in four wheel drive to verify that the axles aren't locking together?



Customer: replied 5 years ago.
Yeah I had it on lift and all the wheels spin, could the limited slip be giving me problems?

All four wheels do spin in four wheel drive? If the wheels will all spin in four wheel drive then the axles are locking together. If they weren't locking together and you had it in four wheel drive, in gear, all wheels off the ground, only the rear wheels would be spinning.


Customer: replied 5 years ago.
But if the collar does not engage the limited slip just spins I thought

If the collar didn't engage when you were in four wheel drive the only thing that will spin is the front driveshaft. The front driveshaft will drive the front differential carrier, which will then spin the component with the least resistance - the intermediate shaft that you can't see.


There isn't any limited slip in the front differential. Limited slip was optional in the rear, but all front differentials are open type.





Customer: replied 5 years ago.
So if the front drive shaft is spinning and the collar is locked both front wheels should spin in the same direction

Both front wheels aren't going to be locked together and driving at the same speed because it's an open differential and not limited slip. The only time they would spin at exactly the same speed is if it was limited slip.


If you had all four wheels off the ground, in gear and in four wheel drive, the front and rear driveshafts should be driving. If the collar is locked in then you will have at least one front wheel driving, the other front wheel may be spinning a little or stopped. If you have either front wheel driving when it's in four wheel drive, then the truck is operating normally.


Customer: replied 5 years ago.
We at times have no front wheels spinning with the front drive shaft spinning

Then at those times the axle shafts aren't locked together. At those times is when testing would need to be done. Only when they aren't locking together would be checking for vacuum, etc, be valid.


But you applied 16" of vacuum right to the actuator, drove the truck with all wheels off the ground and nothing in the front would drive but the driveshaft? If that's the case then the problem has to be the actuator, shaft splines, or internal in the differential carrier. Everything but the differential internals you say are new, but if you had damaged gears in the carrier the problem shouldn't be intermittent, and if it was then you would have some pretty extreme noise up there.



Customer: replied 5 years ago.
I am just as baffled, I actually drove the truck with the 16" on the actuator and the problem was still present. I agree about the front diff making noise if that was the problem.
What do you feel when you drive it that makes you think something is wrong? What was the original symptom that you felt and were trying to repair?
Customer: replied 5 years ago.
The front four wheel drive would disengage, at the same time you could hear gears grab then slip then grab and stay engaged for a short time then it would happen again,or it would stay engaged for hours at a time
You didn't mention that there was any noise. Your description of "you could heard gears grab then slip then grab" do go along with a possible front differential problem. If the front driveshaft is driving, the intermediate and right axle shaft are new, the disconnect actuator does lock in for sure, yet the front wheels don't drive, then you either have a problem with the splines between the left axle shaft and left side gear in the differential, or you have side gear or spider gear failure. The next step is going to be to inspect the spider and side gears in the front differential. Based on all the information you've given the problem has to be in the differential.
sprinkles08 and 4 other Dodge Specialists are ready to help you

Related Dodge Questions