frankjake

I did a seach on this problem but amost eveyone said when they shift into 4wd but the front does not engage, the 4wd light DOES come on.

I have a 1992 4Runner, 3.0, auto trans, stock auto locking hubs, 4x4 shifter (not a button) right next to the auto trans shifter.

Yesterday I get to the beginning of the trail, go to shift into 4wd (4H) and I hear it "clunk" in but no light and the front wheel do not engage. So I shift it down into 4L an hear it "clunk" into the lower gear but still no light and the front wheel still not engaged.

So I do the obvious and check the front drive shaft and it is turning. When I take it out of 4wd and back into 2H, the front drive shaft stop turning. I try shifting in and out many times with the auto trans shifter in all positions. Can't get front wheels to engage at all.

So I head home about 20 miles away. Get into the driveway and try it again and now everything works. The 4wd light comes on and the front wheels engage.

What happened? How can I prevent it from happening again? If I install manual hubs will it prevent this in the future or does some electrical/vaccum actuator actually engage the 4wd?
Thanks for any advice.
Frankjake@gmail.com

Elton

those auto hubs suck and the add works of a vacum line may want to check that out

Glenn

I don't think that you have auto hubs. It's probably an ADD issue. Automatic Differential Disconnect.

frankjake

Okay guys, sorry for the stupid questions, but what is the difference between the ADD and auto locking hubs and how can I tell what I have?

ChickenLover

With ADD, your cv shafts have a solid/constant connection to your hubs. If your wheel is turning, your cv is turning with it. With auto hubs, you should be able to reach under there and turn your cv shafts while the wheel is still.

regularguy412

You may have a vacuum leak in the ADD system. If you were sitting still with the engine idling, you might not have been drawing enough vacuum to engage the ADD.

You could try engaging the 4WD shifter with the tranny in Park and revving the engine slightly, to see if it engages. You might also try switching the vacuum lines on the ADD actuator ( on the front axel housing near the front diff.). That 'should' force the ADD to engage, regardless of the transfer case shifter position.

Mike in AR

frankjake

Thank you guys for the useful info. I think I understand the difference now. Is the ADD engaged only with vacuum? I notice there is also an electrical connection on the front of the front differential. Does this have anything to do with engaging or is it maybe just the switch for the 4wd dash light?

In some other topics on another forum, someone mentioned the "shift selector sensor" and also said this sensor can fail causing the differential not to engage.

Thanks again

TNRabbit

Yes, the ADD engages with vacuum switch which tells the ADD (electrical connection on the diff you saw) to engage. I bought my 94 with 92k on the odo and the ADD was bad and had to be replaced to the tune of $158. It sounds like you have an intermittent issue with either the electricall connection at the diff or possibly at the shifter. I'd clean everything thoroughly with ELECTRONIC (NOT electrical) cleaner and see if that doesn't clear it up.

PeanutRanger

MY truck wouldn't go into L4 one time, i'm pretty sure it was because of all the mud. Pressured washed it off the bottom of my truck and its never done it again!