front end loud humming.

So i finally got all the electrical stuff figured out on my XJ and decided to go to a small local spot with some deep ruts. everything worked good 4hi and 4lo. this was the first time i have used 4x4 in this jeep.

but now i have a humming sound that i think is coming from the front end. sometimes it gets louder and sometimes it goes away. im not really sure what it is but it was not doing it before i took it out. Just trying to get an idea of what it could be so any ideas would be great.

its a 91 sport with a 4inch lift 33's its a 30 front and 35 rear with an aussie locker in the rear.

NP231 or NP242? If an NP231 you may still have the OEM D30 CAD vacuum disconnect axle. If the vacuum system/motor isn't functioning correctly your front axle may still be engaged, and you are hearing the turning parts for the first time.

Post up info on your transfercase/front axle.

Good luck.

To be honest i dont know the differences between the NP231 and the NP242. i think it has something to do with part time 4x4 or something. I had a 84 4runner before the jeep so i dont know much about them.

but if it helps to figure out which case it is,
my transfer case only has 2H,N,4H, and 4lo

as far as i know it is a High pinion dana 30 stock.

I went in to a parking lot today and put it in 4hi and 4lo then back to 2h and it definitely went back into 2H it didnt have trouble turning on the asphalt like it did in 4x4

Thanks.

Ok, 2wd/4hi/N/4lo is the NP231 Command Trac. So, that means your XJ left the factory with the D30 CAD or D30 vacuum disconnect axle. There is a two piece axle on the passenger side, with a sliding collar that engages/disengages the axle using a vacuum motor that is tied into the vacuum switch on the transfercase.

From the FSM 1990:

Raise and support the vehicle. Disconnect the vacuum harness from the axle shift motor and connect a vacuum pump to the vacuum shift motor front port.

Apply 15 in. Hg of vacuum to the front port and rotate the right front wheel to fully disengage the outer and intermediate axle shafts (i.e., shift to two-wheel drive operation).

The shift motor should maintain the vacuum applied to the front port for a minimum of 30 seconds. If the motor does not maintain the vacuum, replace it. If the motor maintains the vacuum proceed to the next step.

Disconnect the vacuum pump from the vacuum shift motor front port. Connect the vacuum pump to the vacuum shift motor rear port, cap the port for the indicator lamp switch, and apply 15 in. Hg of vacuum to the rear port.

The shift motor should maintain the vacuum applied to the rear port for a minimum of 30 seconds. If the shift motor does not maintain the vacuum, replace it. It the motor does maintain the vacuum proceed to the next step.

Remove the cap from the port for the indicator lamp switch and determine if vacuum was present at this port. If vacuum was present, the shift motor functions normally. If vacuum was not present, proceed to the next step.

Apply 15 in. Hg vacuum to the rear port of the shift motor. Rotate the right front wheel as necessary and ensure that the outer and intermediate axle shafts are completely engaged. The axles must be completely engaged (i.e., shifted to four wheel drive operation) to open the port for the indicator lamp switch.

Determine if vacuum is present at the port for the indicator lamp switch again. If vacuum was present at the port, the shift motor functions normally. If vacuum was not present at the port, replace the shift motor.

Ok, if at this point it still won't shift correctly you may have a worn shift fork. OEM and replacements are cast and will wear. Posi-Lok 4x4 makes a cast iron fork, about $55.

does the noise get any louder or quieter when you turn left or right? if so it could be a bearing going out in the front.

notenoughboost said:

does the noise get any louder or quieter when you turn left or right? if so it could be a bearing going out in the front.

Click to expand...

I do not think it changes much when turning.
its a Chirp chirp chirp chirp chirp kind of sound while the tires are spinning doesnt do it when its stopped and its about the same regardless of turning.


And joe, sorry for a possibly newbie question but when you say apply vacuum do you mean with some kind of vacuum tool? i do not have much experience with axles other than installing gears.

Yeah, vacuum pump: http://www.amazon.com/Actron-CP7830-Hand-Vacuum-Pump/dp/B0009XQUK2

They can be had cheaper than that at Harbor Freight, maybe even at your local parts emporium. Some parts stores rent them, some don't.

PS--moving this thread to OEM Tech.

Here is JP Magazine's article on the D30 CAD and alternatives: http://www.jpmagazine.com/techartic...154_0904_dana_30_vacume_disconnect/index.html

Okay thanks for all the help, I am hoping to get the jeep up on the lift at school monday and i will look at some things and see if we have a vacuum pump in the shop i can use to check the lines and what not.

Again thanks for the help.
Mitch.