Need help : 4WD issue .

[Serge PETIT]

My Blazer is equiped with the 4 buttons 4WD system with RPO NP8 meaning : 2Hi , 4WD auto (Autotrac) , 4Hi , 4Lo .
When in 2Hi, lamp of the button is solid
When pushing 4WD auto button, lamp is flashing continuesly . No small noise of the shaft engagement .
When pushing 4Hi button same thing than for The 4WD auto.
When pushing 4Lo button lamp is flashing and stay flashing even if there is the loud " klang of the 4Lo engaging but I don't have the feeling that I am in 4lo if I take off the car . When pushing back the 2WD button I hear the "Klang of the disengagement of the transfert case.
Concerning the others positions , when I push back the 2Hi button , the 2 Hi lamp flash 1 or 2 times before becoming solid (as usual).

I have checked the vacum lines in the engine compartment and no pipe seems to be leaking .

Before changing any part (transfert case switch, buttons , actuator, vacum electrovalve, transfert case motor, 4wd electronic module ....) I would appreciate your help in the diagnostic and in a check-list indicating me the good schredule for points to investigate .

Thanks in advance .
Serge.

[HenryJ]

Easy fix. Common failure point.
Cut to the chase and replace the transfercase motor (encoder). Expensive, yes.

No loss of fluid is involved. You may need to remove the driveshaft for clearance to remove the bolts. You may need to align the shift shaft to properly clock it for the new installation.

It can lead to a condition where the transfercase will not stay engaged at highway speeds. Might want it fixed rather than walk or be towed.

[Serge PETIT]

Thanks Brule for your fast answer .
What is your recommandation for the part ? Dorman or AC Delco ? Around 150$ for Dorman 270$ for AC Delco both reman ................

[HenryJ]

I do not have a preference in this case. I do lean toward AC Delco most times, but if they are both reman, then they may actually be coming from the same source?

I keep trying to think of something else you can try before spending $$$ ... Maybe reset the module? Pull the battery cables for three minutes. I still think the end of the trail will lead to the transfercase motor.

[Serge PETIT]

Concerning the disconnexion of the battery , I have done it after readind some posts of the forum . Without success .
I am too generaly inclined for AC Delco but what I don't understand is that the rockauto french version of catalogue offer only Dorman and the US version both AC Delco and Dormann .
As you say if both are reman .................and if I consider that I am using rarely the 4wd I think that to choose Dorman is secure and will save more than 100 £ .
Thanks for your help .

[Serge PETIT]

Problem has been solved last week . As suggested by Henry J, we have at first changed the transfertcase motor. It has fixed the trouble but one day after, testing the 4x4 system again .................lights only flashing on the 4x4 positions.........
We have determine that it was the solenoïd valve for vacum on the firewall !!!! I have taken the opportunity of that order to Rockauto to import all the parts concerning the vacum system (same shipping cost) !!!!!!
Now I am ready for immediate fix if actuator or switch falled !!!!

Once more thanks to the forum and mainly to Henry J for the support to a "poor lonesome Frenchy and his Blazer"

[HenryJ]

Totally forgot about the relay on the firewall that the auto-trac vehicles have. Crew cabs don't have that. I think the change to the electric shift front axle eliminated some of the more problematic vacuum system components.
Glad YOU figured out your problem

[Serge PETIT]

Even if you have been thinking as a "Crewcab man" you have indicated the right way !
I think that the failure of the valve hapened after the change of the transfert case motor because just after the change , the lights were solid again after 2 or 3 flashs and the surprise of the new failure has been the day after ..............