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sprinkles08, Chrysler/Dodge/Jeep master tech
Category: Dodge
Satisfied Customers: 23105
Experience:  ASE Master and Advanced Certified, Chrysler/Dodge/Jeep Master Certified, Trans and Hybrid Specialist
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4wd on 2000 2500 ram not staying engaged. vacuum measures 80

Customer Question

4wd on 2000 2500 ram not staying engaged. vacuum measures 80 inHg on black line in 2wd and 35 inHg on red line when 4wd engaged. Same at transfer case. Vacuum pump output line at 20 inHg. any ideas for me to research in figuring out why 4wd is not staying engaged?
Submitted: 1 year ago.
Category: Dodge
Expert:  sprinkles08 replied 1 year ago.

Hello and welcome to JustAnswer!

I'm not sure exactly what you're seeing at the transfer case and disconnect actuator but it's not possible that it could be 80 and 35 inches with 20 inches at the vacuum pump. Just under 30 inches at sea level is 100% vacuum, I've never seen an automotive gauge that was labeled lower than 30.

Not knowing what you're using I'm wondering if it was a digital gauge it was possibly 8 and 3.5", which wouldn't be enough to correctly operate the disconnect actuator. You may also have accidentally read a metric scale.

Does the four wheel drive light come on?

Does the front driveshaft spin when in four wheel drive?

Is there any abnormal noise in four wheel drive?

Customer: replied 1 year ago.
Had one of my guys at the shop take the measurements. Actually was mmHg not inHg. So the black line is at about 80 mmHg in 2wd and red line is at 35 mmHg when shifted to 4wd. The vacuum pump is about 80 mmHg. So there is quite a difference in vacuum coming out of the transfer case when shifted to 4wd.
The axle shift motor is engaging the 4wd and the indicator light, however while driving the light will start to blink and or go out and when the light goes out the 4wd goes also.
I am not much of a mechanic but seems to me that the low vacuum is keeping the 4wd from staying engaged. I thought that maybe the shift motor was bad but the vacuum difference seems to point to the transfer case switch or whatever mechanism sends the vacuum to the two shift motor lines. (Red and Black) Note: Usually when the indicator light/4wd drive disengages shifting the tranny down or up manually will cause the system to function again. If vacuum is the problem, shifting may be changing the vacuum slightly causing the shift back to 4wd.
I don't know what the vacuum lines should be reading when I test them. I haven't had any luck with finding vacuum specs or vehicle specific diagrams online.
To answer your other question, there are no abnormal noises associated with the problem.
If the problem is in the switching mechanism in the transfer case, how difficult is that to remedy. I haven't had a chance to research that end of it yet.
Expert:  sprinkles08 replied 1 year ago.

The low vacuum is allowing the disconnect actuator to move then. The low vacuum is either the vacuum switch on the transfer case or a leak between the switch and disconnect actuator.

You can visually inspect the vacuum lines, if you don't see anything then if you have a hand vacuum pump, remove the vacuum harness at the switch and pull at least 20" of vacuum on the line and actuator. If it doesn't hold then you have a leak, if it pulls down easily and holds then you have a bad vacuum switch.