rjblack87

My 4 wheel drive isn't engaging, it will drop into low gear but the front tires won't engage. I noticed there is a 2wd written on my igniter under the hood. Could that be part of the problem.i have a spare and could swap it out. What could be the other issues. It's an a.d.d. 4x4 3.0 v6

RJR

Don't think it's the igniter - that shouldn't have anything to do with it.

Automatic or manual?
Does the green 4WD light come on on the dash (under the tachometer)?
Most likely is that the vacuum or electrical to the ADD is messed up, so the differential never connects to the driver side front CV axle. In that situation the open differential just spins freely and you get no torque to the front wheels.

Couple of things to check:
- Put it in 4wd and have someone (carefully - don't get run over) watch to see if the front drive shaft turns when you slowly move the truck forward. If it does, the problem is almost surely the ADD switching.
- Check the vacuum hoses going to the front differential, and also to the two ADD vacuum solenoids on the right hand fender in the engine compartment. Make sure none of the hoses are dangling with no apparent home.
- With the ignition on but the engine off, listen carefully as you shift from 2wd to 4wd. You should hear the ADD relay click under the dash by the steering column, and if you have the hood raised and your head out the window (or an assistant by the engine compartment), you should hear the vacuum solenoids click.

Run these checks and let us know what you find out.

rjblack87

I'll do those checks but to answer a couple things, it's a Manual transmission. And I can hear something click while I shift between the 4high and 4 low and no the 4wd light does not light up.

RJR

You're probably hearing the ADD relay under the dash. That's good. It means your transfer case is sending the shift signal to the ADD relay. Next step is to check if you hear the vacuum solenoids shifting under the hood on the right fender. If those are switching, it's a matter of tracking down where the vacuum is getting lost.

The ADD mechanism in the differential is very reliable and seldom gives trouble. Usually it's a vacuum or electrical issue.

Basically here's what happens when you shift to 4wd (assuming a manual transmission. Auto has a few more components.)
- You move the xfer case lever to 4wd
- xfer case shifts into 4wd, closing the xfer case shift indicator switch
- xfer case switch applies power to ADD relay
- ADD relay switches power to vacuum solenoids, sending vacuum to ADD actuator in front differential
- Vacuum pulls sliding ADD collar into place, locking driver's side CV shaft to differential stub shaft.
- At the same time, switch closes in ADD actuator, lighting the 4wd light on dash.

Sounds like your system is getting lost somewhere in the last three steps. Check and see if the vacuum is switching at the output of the vacuum solenoids. It's easy to tell just by pulling the hoses and putting your finger over the nipple while someone cycles the xfer case lever. (Engine has to be running).