4wd not engaging

[Cwarrenm]

Hey guys, I'm reviving this old thread because I have yet to find an answer after searching/reading through several threads on the topic. My ADD isn't engaging correctly and thanks to all the information i've gleaned here I was able to identify that the issue is Vacuum.

I've tested the VSV's, the actuator, the check valve, and T-case. All are working normally, but the system isn't getting any Vacuum from it's source. I traced the lines to a round thingy on the back side of the intake near the firewall and put a vacuum gauge on it with the engine running. There's no vacuum coming from it. What is it? I can't find it in any diagram of the engine's Vacuum, or 4wd components. It looks like there's another hose that comes out of the bottom and maybe runs to the gas tank but it's really hard to see back there.

I plugged the vacuum line with the check valve in it into another port on the intake that did have a measurable vacuum on it and tested the 4x4 system. Everything worked, however this is not permanent solution. Does anyone know what the round thing is on the back of the intake that seemingly has no vacuum? I'm not pulling any codes and the engine is running normally.

I would include a picture of the part in question but for some reason I can't include an attachment. Please help! Thanks!

 

[denlion81]

I've tested the VSV's, the actuator, the check valve, and T-case. All are working normally, but the system isn't getting any Vacuum from it's source.

Hey, I am glad you have found your issue and it is quite easy (and cheap I believe too) to fix. I have recently was involved in troubleshooting of issue with my 4WD not engaging and that was quite a task I would say!

So, hopefully nobody objects if I post here all steps I have taken to do that on my 1996 T4R LTD. Hopefully it will help to someone to do the same.

So, here is the full list of what I have done to diagnose my issue:

Started with car not running, 4WD button is switched off.

1. Get down under the car and try to move short (front) driveshaft. It moved free - that is how it should be in 2wd.

2. Take out rubber hoses from both VSV and started the engine - vacuum appeared in one of those hose.

3. Pushed 4WD button - vacuum still on the same hose, so our 4WD CU doesn't give a command to switch it. Noted.

4. Cross-switched the vacuum hoses, ADD is engaged, light is on. So, that concludes that ADD is working. Your driveshaft should NOT be moving free as well. It means that passenger side of the axle is connected now.

5. Returned all back, went to read more on different forums.

6. Found wiring and place where to find 4WD CU on some forum, printed out the wiring diagram.

7. Opened the drives side panel and found that 4WD CU, took out the plug from it.

8. Ignition is ON, plug is out from 4WD CU, started "check-ups" on wiring/pins : check that there is positive 12v on #16 and negative on #8.

9. Check that there is negative 12v on #15 when 4WD button is pushed and nothing otherwise.

10. Ignition is ON, plug is IN 4WD CU, but it is out of the box, so I can "check" how it works (really careful, don't create short circuit on a plate).

11. Check that after 3WD is pushed on there is a 12v between #2 and #3 for a some time (command to actuator).

12. Ignition is OFF, plug is out of 4WD CU and just check circuit between #2 and #3 on the plug site. It is there.

13. Stop right here since there is a signal, there is circuit but actuator isn't moving (you can hear when it works), which means actuator is fried (

For you, I guess it would be obvious on the Step #3 that your ADD isn't working properly.

Hope that helps to someone...