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Hi folks, I have a strange problem with my 04 trailbazer, 4.2 I6 4x4. I use to car for long distance highway trips about once a month (600 miles round trip). After about 150 miles, the service 4wd light comes on. Some back story on the car, The 4wd has not worked for several years. I put the car in neutral, select a 4wd position, I hear the transfer case making the usual noises and the selector switch properly stays lit on 4wd lo and hi, but I know that the front wheels aren't getting power. Now back to the light. Like I said, the service 4wd pops on after about 150 miles on the highway and will stay lit until I turn the car off. once I turn it back on, the light is gone and will come back after about another 150 miles. I have the car in 2wd, never in auto 4w. Now, last time I changed the oil I noticed some grease spray around the inner front CV boot on the drivers side. Could this be throwing the service 4wd code and have something to do with my 4wd not working? I'm slightly concerned about the front diff now as well. Could the leak be coming from there? Could long high way trips in 2wd be damaging the front diff? Not sure if the front diff is moving when in 2wd. Any thoughts here appreciated.
 

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2004 gmc envoy_slt
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More than likely one of the front wheel speed sensors is getting weak or rust and corrosion has built up under the mounting plate for the sensor (this will raise the sensor enough to lower the sensor's signal). The leaking CV boot probably has nothing to do with this problem.
 

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When in 2WD the internals of the front differential are free to rotate, nothing is 'locked' except that the front left wheel is of course connected to the side gear on the front differential. Now most likely the path of least resistance would be for that side gear to spin the spider gears which would then spin the opposite side gear and the intermediate shaft that runs through the oil pan. That intermediate shaft goes into the front axle disconnect which in 2WD should be disengaged so the shaft would just spin freely.

It would not surprise me if the carrier and front propshaft also spins a little though.

If the leaking fluid you see is not a lot I would just be sure to check the front differential fluid level often. The biggest damage is usually done when that is not checked and runs dry, then the front differential is severely damaged by running dry.

None of that will turn on the service 4wd though. That I am going to guess is a faulty transfer case range position sensor. This part is inside the transfer case shift motor and it is the one and only part that tells the system what mode the 4wd is in.


Your lack of
4wd could be a failed front axle disconnect or the actuator that drives it. When shift into kr out of 2Hi there should be two different noises from two different locations. You mention hearing the transfer case shift which should be heard from under the center of the vehicle. There should be heard another noise from under the passenger side of the engine. That's the front axle disconnect actuator. If that fails there will be no power getting to the front wheels because that intermediate shaft through the oil pan won't be connected to anything so any power sent forward by the front propshaft just spins the disconnected intermediate shaft.
 

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Does the cruise control work OK? Any issue with a wheel speed sensor will disable the cruise control. And any issue with a wheel speed sensor will not enable the service 4WD system.
 

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Discussion Starter #5
Does the cruise control work OK? Any issue with a wheel speed sensor will disable the cruise control. And any issue with a wheel speed sensor will not enable the service 4WD system.
Cruise control works just fine. Spend most of my time on the highway with it on.
 

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Cruise control works just fine. Spend most of my time on the highway with it on.
Same here. That's why I thought of that. I had a speed sensor wire get damaged by rubbing against the tire and kept tossing off my cruise control until I reset the code on the EBCM. Next big bump and the cruise control would shut off again.
 

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That transfer case range position sensor is active always. Always checking and reporting the position of the 4wd system even when you have made no changes to the switch position. And speaking of the switch position, that is also continously monitored and can also get dirty and cause what you are seeing. It is commonly suggested to rotate that switch back and forth fully with the key off in an attempt to clean up the internal contacts. You might try that.
 

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Hi folks, I have a strange problem with my 04 trailbazer, 4.2 I6 4x4. I use to car for long distance highway trips about once a month (600 miles round trip). After about 150 miles, the service 4wd light comes on. Some back story on the car, The 4wd has not worked for several years. I put the car in neutral, select a 4wd position, I hear the transfer case making the usual noises and the selector switch properly stays lit on 4wd lo and hi, but I know that the front wheels aren't getting power. Now back to the light. Like I said, the service 4wd pops on after about 150 miles on the highway and will stay lit until I turn the car off. once I turn it back on, the light is gone and will come back after about another 150 miles. I have the car in 2wd, never in auto 4w. Now, last time I changed the oil I noticed some grease spray around the inner front CV boot on the drivers side. Could this be throwing the service 4wd code and have something to do with my 4wd not working? I'm slightly concerned about the front diff now as well. Could the leak be coming from there? Could long high way trips in 2wd be damaging the front diff? Not sure if the front diff is moving when in 2wd. Any thoughts here appreciated.
Had a similar problem. The outside bearing in the front axle disconnect housing is broken, i.e. it is elongated causing the front CV to be loose, thereby ripping the rubber covering which in turn caused the black grease leak you are seeing. You need to replace the housing and the front CV axle. If it still doesn't work, the actuator will also need to be replaced. What is happening now is that the fork in the housing cannot engage the axle because it is out of position. After doing the above mine works like new.
 
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