1999 Jimmy 4WD drive issues - Please help

[jr_greatwhite]

Hi I hope someone can help me here. I have a 99 Jimmy with the Auto 4 WD Drive system. It has the NP8 transfer case. When I am in Auto 4 WD drive it works When I engage just 4 Hi or 4 Lo I have no traction or very little. I have now installed a new Transfer case but still have the same issue. I had GM check the TTCM, I also checked the Vacume and the actuator. I am at a lost here. I also noticed that when it is in 4 Hi or Lo all 4 wheels are now turning, why is this. The wheels will turn some what but when I apply a force they slip. Does any one have any suggestions. I am now running out of money?

[MT-2500]

Welcome to AF.
That sounds about normal to me.
4W drive does not say you wheels will not spin.
4W auto mode applies traction to rear and front as needed.
4W low or high locks front and rear drive shafts together.
Or applies power to front and rear the same.
If you wheels or on ice or slick payment they will slip.

How are you testing it?
In 4W low or high is a rear and front wheel spinning at the same time?
Do you have positive traction rear or front or both.
Positive traction should spin both wheels on the rear end assembly even if it is working right.
Post back with more info on how you are testing and what wheels are spinning.
MT

[jr_greatwhite]

Originally Posted by MT-2500

Welcome to AF.
That sounds about normal to me.
4W drive does not say you wheels will not spin.
4W auto mode applies traction to rear and front as needed.
4W low or high locks front and rear drive shafts together.
Or applies power to front and rear the same.
If you wheels or on ice or slick payment they will slip.

How are you testing it?
In 4W low or high is a rear and front wheel spinning at the same time?
Do you have positive traction rear or front or both.
Positive traction should spin both wheels on the rear end assembly even if it is working right.
Post back with more info on how you are testing and what wheels are spinning.
MT

Well I had it tested at GM they said it was slipping under load. Not exactly sure how they tested it. I tested by jacking it up on 4 Stands putting it in 4HI and if I grabed the wheel in the front I could stop it by applying actually grabbing it. Under 4 LO as well. IThe first time this spring out 4X4 got stuck in a small amount of mud I got out while my buddy tried it in 4LO and I noticed only the rear turning, we then tried 4 Hi the same when put in Auto 4 I was able to get out. I understand in 4 Auto your axel in engaged put power is only sent when the system noticed slipping. Not sure how my machanic tested it. I almost think it's the TTCM even thought I cant read any codes and GM didn't get any. But if 4 Auto works, it would seem to me there is no signal being sent to the encorded. I know that your tires will sometimes slipp in some bad mud areas, but this really shouldnt be munch of a challeng for 4Hi let alow 4 Low. I've been through somwe pretty bad stuff. Also I notice. You can hear when the transfer case ingages, it's like the Power is not be sent to the wheels.

GM also said the saw noting wrong with the encorded. Their diagnosis was the clutchs were probably burnt.

So I got a new one and still the same problem.

Something else I heard was that this actuator is different then the one that dont have Auto 4.

I only get positive traction on the back in 4Hi 4 Lo, but in 4Auto mode I do get traction in the front

[MT-2500]

Was it a GM transfer case that you put in it.
If it has problems a good transfer case should show the proble or set a code.
Has it got any codes?
If so post back code no.
To test for 4W low or high jack all 4 wheels up and put it in 4W and see if front and rear drive shafts are locking in.
Also check front axel for locking in.
If front axel is not locking in you will not have 4W drive.
Post back results of test.
MT

[MT-2500]

Also a quick test for 4W low or high is to make a sharp turn on dry payment.
If you have a crow hop it is working.
MT

[jr_greatwhite]

Originally Posted by MT-2500

Was it a GM transfer case that you put in it.
If it has problems a good transfer case should show the proble or set a code.
Has it got any codes?
If so post back code no.
To test for 4W low or high jack all 4 wheels up and put it in 4W and see if front and rear drive shafts are locking in.
Also check front axel for locking in.
If front axel is not locking in you will not have 4W drive.
Post back results of test.
MT

.

Its a GM Transfer case, but there are no codes. The axel lock when in 4Auto and I think it is for Hi and Low. I will have to double check. Its as if there is now power to the wheel unless in 4Auto

[MT-2500]

Originally Posted by jr_greatwhite

.

Its a GM Transfer case, but there are no codes. The axel lock when in 4Auto and I think it is for Hi and Low. I will have to double check. Its as if there is now power to the wheel unless in 4Auto

Front Axel and front drive shaft both have to lock in in 4W.

[jr_greatwhite]

This is the issue I am having My axel is engaging in 4Auto, 4Hi and 4Lo. 4Auto appears to work, when I apply resistance (Load) to the front the 4 Auto works like it should. However when I apply a Load to 4Hi or 4Lo they slipp. To me this appeasr that no power or very little is being sent to the front. What controls sending power to the wheels?? Is it the TTCM?? What does the encoder do??

[MT-2500]

The TTCM should lock the cluch solid in 4WL or 4WH.
If it was a GM transfer case ask them to check it out.
A transfer case scanner should pick up on the problem.
The transfer case computer or wiring or clutch in transfer case is no working right.
Did you use the GM auto trac blue fluid in transfer case?

[MT-2500]

Symtoms says it is pointing to a tarnsfer case control module.
But proper testing .
Get it on a transfer case capable scanner and get a factory repair manual or a All Data online sub and go threw the repair flow charts.
http://www.alldata.com/products/diy/index.html
It should be setting a code.
get it rechecked for codes.
Good Luck and let us know how it goes.
MT