4LO does not switch
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New Member
Joined: Jul 2015
Posts: 50
4LO does not switch
2000 Blazer 4x4
NP8 transfer case
tl;dr I've been chasing after 4wd problems eventually replacing the TCCM. Every range works except 4lo won't work unless I disconnect the vacuum hose to the front differential first, then reconnect it.
I am following the parameters when switching to 4wd and out of 4wd listed in the owners manual. The following is a long read, but maybe it may help someone else or give enough information to properly diagnose my problem. Maybe not.
I've been trying to get my 4wd to work correctly for months. First it started to slowly die. It would slowly switch into 4wd then eventually it wouldn't switch at all and just give a S4WD light. Eventually that light would just stay on all the time, but I could still use the 4wd switch, it just wouldn't shift. I ended up replacing the transfer case motor which fixed one problem with the system: it would actually shift into 4wd.
Here is where I found something else: I used to have really bad wheel hubs. They would howl so loud I could hardly hear the radio. I replaced those not too long after replacing the encoder motor and I noticed some other sound, like a light whining and howling. I narrowed it down to the front differential which seemed to be engaged, so I had to disconnect the vacuum hose at the solenoid (if that's the correct word) on the firewall so it wasn't always engaged. Eventually it seemed to be disconnecting the front diff properly in 2wd so I would keep the hose connected. Later on the front diff would always be engaged, but it only seemed to be partially since I can feel the air pulling in through the hole on top of the solenoid, but yet my diff is still engaged. Back to disconnecting the vacuum line! If I left it on I would hear what sounds like grinding or something which gets worse as I speed up. My thought is the firewall switch is bad. Oh well, I can at least switch into 4wd and connect the line! That stopped also.
Another symptom is that it would freeze. What I mean is that if I switch into 4wd I can't get out without restarting my blazer. Problem solved. Can't switch into 4wd? Restart blazer, press button, boom. One day I went through some big puddles as a last hoorah before I went on vacation in California (flying) which I know is a bad idea, but that's what I'm here for. When I go to switch back into 2wd my truck begins to die. The engine would rumble and everything would dim! This went on for about 5 min on the highway to where I was scared ****less that I would be stuck on the side of the road. After 5 minutes though, everything seemed fine. I stopped at Walgreens then started my blazer just fine, no problems at all. I get back from vacation and it seemed fine to connect my vacuum line. I go to my spot on the beach just to find out I can't switch at all! It wouldn't shift into auto or 4hi. When I tried 4lo it would just make a loud metallic BANG and go back into 2wd and show me either a S4WD or freeze after going back to 2wd. I'm stumped now. I admitted defeat for the time being as I pondered the thought that my transfer case may be damaged, the encoder motor dead, or my TCCM is fried.
I decide to take a look at the TCCM, which these Blazers have it located on the pillar right below the glove box behind a trim panel. This is when I realize that the carpet is a little damp there and in the corner there is rust. In short, I knocked out the rust then filled it with bondo, but I still have a big hole in my rocker panel by the area. I noticed that the plug and connector has some corrosion on i so I try to clean the connections and hook it up with no change at all. I put it off till later, mostly focusing on the loud bang. I've never heard that before. Last weekend my friend invited me to go to the junkyard so I get the idea when I'm there to get a TCCM. $46 later, including core charge, I got a new TCCM and most of my 4wd works; 4lo doesn't work though, it's still making the loud bang and switching to 2wd. At this point I'm connecting my vacuum hose before I switch to 4wd which leads to a problem foreign to me.
So, one day with my new TCCM I'm attempting to switch into 4lo when I get this idea that maybe I should disconnect the vacuum line first. 4Lo and behold! It shifts into 4lo! Odd, sometimes I'd switch into 4lo when my front diff was engaged, but now it can't switch unless it's disconnected. I know I probably have to replace the vacuum switch on the firewall, but I was thinking maybe someone else has had similar problems and has more information. Does the front diff have to be disengaged before switching to 4lo? Will replacing the switch work?
NP8 transfer case
tl;dr I've been chasing after 4wd problems eventually replacing the TCCM. Every range works except 4lo won't work unless I disconnect the vacuum hose to the front differential first, then reconnect it.
I am following the parameters when switching to 4wd and out of 4wd listed in the owners manual. The following is a long read, but maybe it may help someone else or give enough information to properly diagnose my problem. Maybe not.
I've been trying to get my 4wd to work correctly for months. First it started to slowly die. It would slowly switch into 4wd then eventually it wouldn't switch at all and just give a S4WD light. Eventually that light would just stay on all the time, but I could still use the 4wd switch, it just wouldn't shift. I ended up replacing the transfer case motor which fixed one problem with the system: it would actually shift into 4wd.
Here is where I found something else: I used to have really bad wheel hubs. They would howl so loud I could hardly hear the radio. I replaced those not too long after replacing the encoder motor and I noticed some other sound, like a light whining and howling. I narrowed it down to the front differential which seemed to be engaged, so I had to disconnect the vacuum hose at the solenoid (if that's the correct word) on the firewall so it wasn't always engaged. Eventually it seemed to be disconnecting the front diff properly in 2wd so I would keep the hose connected. Later on the front diff would always be engaged, but it only seemed to be partially since I can feel the air pulling in through the hole on top of the solenoid, but yet my diff is still engaged. Back to disconnecting the vacuum line! If I left it on I would hear what sounds like grinding or something which gets worse as I speed up. My thought is the firewall switch is bad. Oh well, I can at least switch into 4wd and connect the line! That stopped also.
Another symptom is that it would freeze. What I mean is that if I switch into 4wd I can't get out without restarting my blazer. Problem solved. Can't switch into 4wd? Restart blazer, press button, boom. One day I went through some big puddles as a last hoorah before I went on vacation in California (flying) which I know is a bad idea, but that's what I'm here for. When I go to switch back into 2wd my truck begins to die. The engine would rumble and everything would dim! This went on for about 5 min on the highway to where I was scared ****less that I would be stuck on the side of the road. After 5 minutes though, everything seemed fine. I stopped at Walgreens then started my blazer just fine, no problems at all. I get back from vacation and it seemed fine to connect my vacuum line. I go to my spot on the beach just to find out I can't switch at all! It wouldn't shift into auto or 4hi. When I tried 4lo it would just make a loud metallic BANG and go back into 2wd and show me either a S4WD or freeze after going back to 2wd. I'm stumped now. I admitted defeat for the time being as I pondered the thought that my transfer case may be damaged, the encoder motor dead, or my TCCM is fried.
I decide to take a look at the TCCM, which these Blazers have it located on the pillar right below the glove box behind a trim panel. This is when I realize that the carpet is a little damp there and in the corner there is rust. In short, I knocked out the rust then filled it with bondo, but I still have a big hole in my rocker panel by the area. I noticed that the plug and connector has some corrosion on i so I try to clean the connections and hook it up with no change at all. I put it off till later, mostly focusing on the loud bang. I've never heard that before. Last weekend my friend invited me to go to the junkyard so I get the idea when I'm there to get a TCCM. $46 later, including core charge, I got a new TCCM and most of my 4wd works; 4lo doesn't work though, it's still making the loud bang and switching to 2wd. At this point I'm connecting my vacuum hose before I switch to 4wd which leads to a problem foreign to me.
So, one day with my new TCCM I'm attempting to switch into 4lo when I get this idea that maybe I should disconnect the vacuum line first. 4Lo and behold! It shifts into 4lo! Odd, sometimes I'd switch into 4lo when my front diff was engaged, but now it can't switch unless it's disconnected. I know I probably have to replace the vacuum switch on the firewall, but I was thinking maybe someone else has had similar problems and has more information. Does the front diff have to be disengaged before switching to 4lo? Will replacing the switch work?
Moderator
Joined: Oct 2014
Posts: 4,336
From: North Central Indiana
2000 Blazer 4x4
NP8 transfer case
tl;dr I've been chasing after 4wd problems eventually replacing the TCCM. Every range works except 4lo won't work unless I disconnect the vacuum hose to the front differential first, then reconnect it.
I am following the parameters when switching to 4wd and out of 4wd listed in the owners manual. The following is a long read, but maybe it may help someone else or give enough information to properly diagnose my problem. Maybe not.
I've been trying to get my 4wd to work correctly for months. First it started to slowly die. It would slowly switch into 4wd then eventually it wouldn't switch at all and just give a S4WD light. Eventually that light would just stay on all the time, but I could still use the 4wd switch, it just wouldn't shift. I ended up replacing the transfer case motor which fixed one problem with the system: it would actually shift into 4wd.
Here is where I found something else: I used to have really bad wheel hubs. They would howl so loud I could hardly hear the radio. I replaced those not too long after replacing the encoder motor and I noticed some other sound, like a light whining and howling. I narrowed it down to the front differential which seemed to be engaged, so I had to disconnect the vacuum hose at the solenoid (if that's the correct word) on the firewall so it wasn't always engaged. Eventually it seemed to be disconnecting the front diff properly in 2wd so I would keep the hose connected. Later on the front diff would always be engaged, but it only seemed to be partially since I can feel the air pulling in through the hole on top of the solenoid, but yet my diff is still engaged. Back to disconnecting the vacuum line! If I left it on I would hear what sounds like grinding or something which gets worse as I speed up. My thought is the firewall switch is bad. Oh well, I can at least switch into 4wd and connect the line! That stopped also.
Another symptom is that it would freeze. What I mean is that if I switch into 4wd I can't get out without restarting my blazer. Problem solved. Can't switch into 4wd? Restart blazer, press button, boom. One day I went through some big puddles as a last hoorah before I went on vacation in California (flying) which I know is a bad idea, but that's what I'm here for. When I go to switch back into 2wd my truck begins to die. The engine would rumble and everything would dim! This went on for about 5 min on the highway to where I was scared ****less that I would be stuck on the side of the road. After 5 minutes though, everything seemed fine. I stopped at Walgreens then started my blazer just fine, no problems at all. I get back from vacation and it seemed fine to connect my vacuum line. I go to my spot on the beach just to find out I can't switch at all! It wouldn't shift into auto or 4hi. When I tried 4lo it would just make a loud metallic BANG and go back into 2wd and show me either a S4WD or freeze after going back to 2wd. I'm stumped now. I admitted defeat for the time being as I pondered the thought that my transfer case may be damaged, the encoder motor dead, or my TCCM is fried.
I decide to take a look at the TCCM, which these Blazers have it located on the pillar right below the glove box behind a trim panel. This is when I realize that the carpet is a little damp there and in the corner there is rust. In short, I knocked out the rust then filled it with bondo, but I still have a big hole in my rocker panel by the area. I noticed that the plug and connector has some corrosion on i so I try to clean the connections and hook it up with no change at all. I put it off till later, mostly focusing on the loud bang. I've never heard that before. Last weekend my friend invited me to go to the junkyard so I get the idea when I'm there to get a TCCM. $46 later, including core charge, I got a new TCCM and most of my 4wd works; 4lo doesn't work though, it's still making the loud bang and switching to 2wd. At this point I'm connecting my vacuum hose before I switch to 4wd which leads to a problem foreign to me.
So, one day with my new TCCM I'm attempting to switch into 4lo when I get this idea that maybe I should disconnect the vacuum line first. 4Lo and behold! It shifts into 4lo! Odd, sometimes I'd switch into 4lo when my front diff was engaged, but now it can't switch unless it's disconnected. I know I probably have to replace the vacuum switch on the firewall, but I was thinking maybe someone else has had similar problems and has more information. Does the front diff have to be disengaged before switching to 4lo? Will replacing the switch work?
NP8 transfer case
tl;dr I've been chasing after 4wd problems eventually replacing the TCCM. Every range works except 4lo won't work unless I disconnect the vacuum hose to the front differential first, then reconnect it.
I am following the parameters when switching to 4wd and out of 4wd listed in the owners manual. The following is a long read, but maybe it may help someone else or give enough information to properly diagnose my problem. Maybe not.
I've been trying to get my 4wd to work correctly for months. First it started to slowly die. It would slowly switch into 4wd then eventually it wouldn't switch at all and just give a S4WD light. Eventually that light would just stay on all the time, but I could still use the 4wd switch, it just wouldn't shift. I ended up replacing the transfer case motor which fixed one problem with the system: it would actually shift into 4wd.
Here is where I found something else: I used to have really bad wheel hubs. They would howl so loud I could hardly hear the radio. I replaced those not too long after replacing the encoder motor and I noticed some other sound, like a light whining and howling. I narrowed it down to the front differential which seemed to be engaged, so I had to disconnect the vacuum hose at the solenoid (if that's the correct word) on the firewall so it wasn't always engaged. Eventually it seemed to be disconnecting the front diff properly in 2wd so I would keep the hose connected. Later on the front diff would always be engaged, but it only seemed to be partially since I can feel the air pulling in through the hole on top of the solenoid, but yet my diff is still engaged. Back to disconnecting the vacuum line! If I left it on I would hear what sounds like grinding or something which gets worse as I speed up. My thought is the firewall switch is bad. Oh well, I can at least switch into 4wd and connect the line! That stopped also.
Another symptom is that it would freeze. What I mean is that if I switch into 4wd I can't get out without restarting my blazer. Problem solved. Can't switch into 4wd? Restart blazer, press button, boom. One day I went through some big puddles as a last hoorah before I went on vacation in California (flying) which I know is a bad idea, but that's what I'm here for. When I go to switch back into 2wd my truck begins to die. The engine would rumble and everything would dim! This went on for about 5 min on the highway to where I was scared ****less that I would be stuck on the side of the road. After 5 minutes though, everything seemed fine. I stopped at Walgreens then started my blazer just fine, no problems at all. I get back from vacation and it seemed fine to connect my vacuum line. I go to my spot on the beach just to find out I can't switch at all! It wouldn't shift into auto or 4hi. When I tried 4lo it would just make a loud metallic BANG and go back into 2wd and show me either a S4WD or freeze after going back to 2wd. I'm stumped now. I admitted defeat for the time being as I pondered the thought that my transfer case may be damaged, the encoder motor dead, or my TCCM is fried.
I decide to take a look at the TCCM, which these Blazers have it located on the pillar right below the glove box behind a trim panel. This is when I realize that the carpet is a little damp there and in the corner there is rust. In short, I knocked out the rust then filled it with bondo, but I still have a big hole in my rocker panel by the area. I noticed that the plug and connector has some corrosion on i so I try to clean the connections and hook it up with no change at all. I put it off till later, mostly focusing on the loud bang. I've never heard that before. Last weekend my friend invited me to go to the junkyard so I get the idea when I'm there to get a TCCM. $46 later, including core charge, I got a new TCCM and most of my 4wd works; 4lo doesn't work though, it's still making the loud bang and switching to 2wd. At this point I'm connecting my vacuum hose before I switch to 4wd which leads to a problem foreign to me.
So, one day with my new TCCM I'm attempting to switch into 4lo when I get this idea that maybe I should disconnect the vacuum line first. 4Lo and behold! It shifts into 4lo! Odd, sometimes I'd switch into 4lo when my front diff was engaged, but now it can't switch unless it's disconnected. I know I probably have to replace the vacuum switch on the firewall, but I was thinking maybe someone else has had similar problems and has more information. Does the front diff have to be disengaged before switching to 4lo? Will replacing the switch work?
Last edited by LesMyer; Aug 2, 2017 at 07:41 AM.
Thread Starter
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New Member
Joined: Jul 2015
Posts: 50
Try switching the hoses on the vacuum solenoid. If they are on the wrong way, vacuum won't release properly and front axle will be locked in when it shouldn't. Turns out that only one of the two ports is vented when the solenoid is disengaged. Put the hose to the front axle on the wrong port and the vacuum gets trapped. I think it is likely that front axle being locked in already is preventing engagement into 4LO.
Moderator
Joined: Oct 2014
Posts: 4,336
From: North Central Indiana
Nice catch there... It works great now. For some reason I didn't have these problems after I switched the vacuum lines around to apparently the wrong holes which I did a couple months ago. Then my TCCM started acting up even more and finally wouldn't do anything. Thanks for pointing out something I shouldn't have written off as something not possible! I just had an hour to spare and thought a little detail might help others find problems that I've had.
FYI I had my hoses on the front axle solenoid switched for a while some time back (after replacing my engine) and it gave me fits too. Not with refusing to go into 4-lo but just that I could feel the axle being locked in when turning out of parking spaces in 2WD. Knew it wasn't supposed to be that way.
Last edited by LesMyer; Aug 3, 2017 at 08:00 AM.
Thread Starter
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New Member
Joined: Jul 2015
Posts: 50
You're welcome. All your details was what led me to the conclusion. You described things very well. Good job!
FYI I had my hoses on the front axle solenoid switched for a while some time back (after replacing my engine) and it gave me fits too. Not with refusing to go into 4-lo but just that I could feel the axle being locked in when turning out of parking spaces in 2WD. Knew it wasn't supposed to be that way.
FYI I had my hoses on the front axle solenoid switched for a while some time back (after replacing my engine) and it gave me fits too. Not with refusing to go into 4-lo but just that I could feel the axle being locked in when turning out of parking spaces in 2WD. Knew it wasn't supposed to be that way.
Like I said, 4lo never gave me trouble after I replaced the transfer case motor, only in the last couple of weeks. Although electronic 4WD has a lot of failure point's it does work great to be able to just push a button and go. This is the only 4wd truck I've ever driven, so I can't say anything about the stick.
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