2000 Stuck in 4wd

I have a 2000 Blazer LS 4x4. It has the push button 4wd with Auto 4wd, 2wd Hi, 4wd Hi, and 4wd Lo buttons. Yesterday the Service 4wd light came on. At this point nothing seemed out of the ordinary with the lights by the 4wd selector buttons. I was curious what was wrong so I pressed the 4wd button. It went into 4wd and the 4wd light came on. I could tell the 4wd was engaged by the way the vehicle operated. I pressed the 2wd button to back to 2wd but it did not go into 2wd happened. The 2wd light flashed, I heard the clicking from the right side of the dash and a motor sound under the Blazer. After a few seconds the the 4wd light came on and it 4wd was still engaguged. I can press the Auto 4wd button and the light will come on but it is still in 4wd. The whole time the Service 4wd light is still on.

I read in this forum to first check the Vacuum Switch on the firewall. What is the procedure to test this? I pulled the supply line and I hear a hiss. 4wd is engauged so I guess this means I have vacuum to the Transfer Case Actuator under the battery. Could the Transfer Case Actuator be frozen up and not releasing the 4wd to go to 2wd? What is the procedure for testing the Transfer Case Actuator. The Transfer Case Actuator looks a little wore out. I had a leaking battery and some corosion a while back and it looks like it might have gotten down there. How do I test the operation of the Transfer Case Actuator? Is there anything else that could be causing this problem? I read about the Transfer Case Control Module, Encodor Motor, and a Shift Motor but I don't know how to test if it is one of these things either.

Is it ok to drive while until the problem is fixed. I have been scared to. I don't want to tear up the 4wd system by driving it around town. However, I am got to have to take it out tomorrow. If I have to take it out should I keep it under a certian speed.

Please bare with me here as I have had a some experience and success working on my cars but I have never fooled around with 4wd problems so I am not familiar with a lot of this. In addition, I have searched this forum but could not find the answers to my problem or anything similar. I'm sorry about the length of this email but I wanted to make sure I had all the details out ther so I could get some help. Thanks.

UPDATE: Took the Transfer Case Actuator and I was able to press it in an out. When I did this I heard a hissing at the Vacuum Switch. I took the hose from the Actuator to the Switch off at the Switch. I press the Actuator in and capped the hose off with my finger and the Actuator stayed in. Does this mean my Vacuum Switch is bad? Is the Transfer Case Actuator going in an out what actually engauges and dis-engauges the 4wd?

UPDATE: I removed the vacuum line from the switch to the actuator. I pressed the actuator all the way in and plugged the vacuum line. I placed an extra section of vacuum line into the switch that went to the actuator and plugged it. This whole time I had the battery unplugged, approx 10 mins. I also removed the ATC fuse and the CTSY LP fuse for 60 secs put them back in and cycled the ignition. I started the vehicle and the 4wd light was now blinking instead of being solid. I pressed the 2wd button and it blinked for a while and then it went back to a blinking 4wd light. This disengaged the front end and I was driving in 2wd but I gues the transmission was still sending power to the front end. Will is cause damage to the front end to drive it like this?

I keep reading on this fourm that it is either the vacuum switch, the actuator, the encoder motor, or the transfer case control module. But I really have not read anywhere that tells you how to isoloate which one it is. I have read about several people having this problem on this forum but has any one had any luck fixing this?

This is the way the system is supposed to operate:

You press your selector button for requested range (if anything slows down the process, the button flashes). This tells the TCCM (transfer case control module) what you want. The TCCM instructs the encoder motor on the side of the transfer case to shift to desired range and if needed, controls the vacuum switch on the firewall to change the front axle engagement. This is accomplished by the vacuum switch supplying or releasing vacuum to the servo under the battery which pulls a cable to engage the right front axle to the differential. There is a switch in the axle which tells the TCCM when the axle is engaged. The encoder motor, at the same time, tells the TCCM what range the transfer case is in. When the front axle in the the correct position for selected range, and the transfer case is in the correct range, the button stops flashing.

It sounds like the TCCM was locked up and you cleared it by letting it power down and clearing the code by cycling the key. Have you tried it since? There are 17 diagnostic codes for the TCCM when the Service 4WD light comes on, but you need a scanner to see them. Without the code, it's a lot of work with a digital volt/ohm meter and hope you find a faulty componant.

Thanks for the reply. I don't think I actually cleared anything. The Blazer is still stuck in 4wd. I just disengaged the front end by manually backing out the actuator/servo. This gave me 2wd but I guess power was still going to the front from the transfer case. The ABS acted strange too. Basically the front end was disengaged but the vehicle still thought it was in 4wd.

I have a digital volt/ohm meter but I don't know how to test each part. Nor could I find on the forum how to test each part. I don't want to just start replacing parts one at a time hoping to luck into a solution. I am familiar with placing a jumper wire on the diagnostic port on my other car to get the check engine light to flash the service code. However, I could not find how to do that with the Blazer.

I need to get the Blazer up and running so I took it to the shop for a diagnosis. Depending on what they tell me the problem is I might try to fix it myself. Judging by the Haynes manual replacing the vacuum switch, actuator, encoder motor, or TCCM seem to be fairly straight forward. Has anyone out there replaced any of these parts and found otherwise? Is there anything else it could be?

One concern I have is that all the parts except the TCCM are mechanical and you expect a mechnical part to break. However, what makes a TCCM go bad, a short in the electrical system? How do I know if I put another one in there in isn't going to suffer the same fate right off the bat?

The only way to get codes out of the Auto 4WD system is with the scanner. If you get the code, I'll send you any diagnostics you want.
If the transfer case is staying engaged in 4WD, then either the dash switch is giving the wrong signal to the TCCM, the TCCM is bad or the encoder motor is bad. I've done mostly TCCM's and encoder motors. TCCM replacement's easy, encoder's not too bad, but they're both expensive.

Ready for this one. I took my Blazer to the local Chevy dealer to get them to hook it up to the scanner and diagnose the problem. The service tech said he got in, pressed the 2wd button, and it went into 2wd. He pressed 4wd and it went into 4wd. Then it went back to 4wd. Everything operated normally. The whole time the Service 4wd light was off. He hooked it up to the scanner and there were no codes in the computer.

In checking everything out myself before I took the Blazer to the shop I did the reset procedure detailed on this forum (pull the ATC and CTSY LP fuses and cycle the ignition from start to run five times) and this didn't fix the problem. I also disconnected the battery for approx 30 minutes and this didn't fix the problem.

The whole time I drove to the shop the Service 4wd light was on and the Blazer was stuck in 4wd. While driving it to the shop I tried a couple times to shift to 2wd and it wouldn't shift. The service tech gets in it today and there was no problem. When I was driving home I went from 4wd to 2wd and into Auto several times and everything seemed to function properly. Anybody ever heard of this happening before?

What is going on? I took the Blazer out this morning. I got 2 miles down the road and the Service 4wd light came on. I didn't even think about pressing the 4wd button. So I don't know if it will be stuck in 4wd if I press the button. It is in 2wd now so I can at least drive it.

As I stated earlier, when I picked it up from the shop yesterday the Service 4wd light was off and the 4wd operation seemed normal. It would go from 2wd to 4wd and back as normal. Now the Service 4wd light is back on.

What you have is know in the business as an intermittent concern. It's the worst kind of problem to have because as you know, sometimes it works normal. It could be caused by a loose connection somewhere in the system or a component beginning to fail. If the Service 4WD message is being displayed, it should leave a history code to give some kind of clue. Unless the TCCM is bad and erasing the codes by itself

I took the Blazer back to the Chevy dealer to get them to diagnose the problem. The Service 4wd light stayed on but the mechanic said he could not get any codes out of the computer. He said he could not get the scanner to communicate with the Blazer. The mechanic said he wanted to replace the Transfer Case Control Module (TCCM) to the tune of $595 (US). He said he did not know if this would fix the problem but it was necessary for the Blazer to communicate with the scanner. Would a bad TCCM keep the Blazer from communicating with the scanner? Wouldn't the scanner get some type of code if the TCCM was bad? The mechanic said he thought I might need a whole new transfer case. I have not read anywhere on this fourm about anyone having to go to that extreme to fix a 4wd issue. My Blazer is a 2000 and it only has 60K miles. I am thinking about replacing the TCCM myself and if that does not make the light go off I would try the Encoder Motor on the Tansfer Case. Does this make since. What causes the TCCM to go bad? Should a TCCM go bad at only 60K miles? How do I know whatever made this one bad isn't going to make a new one go bad?

When the tech is saying that the scanner will not communicate, he is saying that the TCCM will not communicate with the scanner.
If the TCCM is dead, there should be code U1026 in some other computer such as the PCM. This code says that the PCM can't communicate with the TCCM. Did you try removing the ATC fuse for a minute to see if it resets itself? Now if you do remove the fuse, it will set the U1026 since the TCCM will have lost power.
Now assuming that the code is set and there is no communication, you would check at the TCCM for power (battery orange wire, connector C2, terminal D and ignition brown wire, connector C1, terminal E10), and grounds (black wire, connector C2, terminal C, and black/white wire, connector C1, terminal E16). If that's good, check the communication line (dark green/white wire,connector C1, terminal E9 to the DLC purple wire, terminal 2). I would try that first. If all else fails, have them replace the TCCM but only if that fixes the communication problem. That way if it doesn't fix that part, at least you won't be out that much more.

I took the Blazer back to the Chevy dealer to get them to diagnose the problem. The Service 4wd light stayed on but the mechanic said he could not get any codes out of the computer. He said he could not get the scanner to communicate with the Blazer. The mechanic said he wanted to replace the Transfer Case Control Module (TCCM) to the tune of $595 (US). He said he did not know if this would fix the problem but it was necessary for the Blazer to communicate with the scanner. Would a bad TCCM keep the Blazer from communicating with the scanner? Wouldn't the scanner get some type of code if the TCCM was bad? The mechanic said he thought I might need a whole new transfer case. I have not read anywhere on this fourm about anyone having to go to that extreme to fix a 4wd issue. My Blazer is a 2000 and it only has 60K miles. I am thinking about replacing the TCCM myself and if that does not make the light go off I would try the Encoder Motor on the Tansfer Case. Does this make since. What causes the TCCM to go bad? Should a TCCM go bad at only 60K miles? How do I know whatever made this one bad isn't going to make a new one go bad?
Take it to a Reputable transmission shop for computer diagnosis, their equipment is specific for trans/4wd problems. Local dealership may not be able to find trans codes. Throwing parts at it is never the answer, and yes if you add new parts that don't fix original problem, you're left where you started.

ricksza, thanks for your replies and your help. I'm afraid you lost me when you were talking about how to test the TCCM. In your post you asked about pulling fuses. Before I took the Blazer to the dealer the first time I tried pulling the ATC and CTSY LP fuses for 60 seconds and then cycling the ignition 5 times but that did not work. I also pulled the battery for 30 minutes and that did not fix the problem. When I took it for a diagnosis the first time is when the problem fixed itself. The Service 4wd light went off and the 4wd system operated normally. I did not pull any fuses between the first and second trip to the dealer for a diagnosis. The second time is when they said the scanner would not communicate. I won't bore you with the whole history. I have pretty much documented the whole experience in my earlier posts.

You seemed to indicated in your last post that pulling the fuses would have made it look like the TCCM was not communicating because removing the fuse would set the U1026 code. The mechanic did not indicate a U1026 code. He just said that the scanner would not communicate with the TCCM. If I pulled the fuses and the mechanic is now saying the scanner cannot communicate with the TCCM could this mean the TCCM is possibly ok and my problem is the Electronic Shift Motor (aka Encoder Motor I guess?).

If you remember after I picked up the Blazer the second time the Service 4wd light was on but it was at least in 2wd. Tonight, I tried pulling the fuses again and I cycled the engine 5 times. I also pulled the battery for 30 minutes. After I did all this the Service 4wd light was still on but now the Blazer is stuck in 4wd again. It did this when I started it up without me pressing the 4wd button. When I pressed the 2wd button I hear the clicking of the TCCM and the Electronic Shift Motor (aka Encoder Motor) makes its noises. It almost sounds like the whole system tries twice to engage.

Here is another question, when the Blazers 2wd/4wd system is operating corectly and the Blazer is in 2wd does the drive shaft from the Transfer Case to the front end turn. I was under the impression it does not. However, after the second time I picked up the Blazer with the Service 4wd light on and the Blazer in 2wd the shaft was spinning. I indexed the shaft and I saw it had moved. In addition, while this was driving it my MPG was down about 1.5 to 2.0 MPG. I know the front axel was not engaged but it seems the drive shaft was turning.

I am leaning toward my problem being the Electronic Shift Motor (aka Encoder Motor). If pulling the fuses will make it look like the TCCM can't communicate with a scanner it will make me feel a lot better about this problem being the Electronic Shift Motor. I don't know if this will help but the tag on the Transfer Case in my Blazer is 236.

Again thanks for your help. I am sorry about the length of all these posts but I want to make sure I give all the information needed and I want to leave detailed posts so people with similar problems can fix their Blazers.

The 236 on the transfer case tells me that it is a Two Speed Automatic Transfer Case.

You are correct about the front driveshaft. It should not be engaged in 2WD, but I wonder if it may be moved slightly by the drag on the left front axle. Usually I check these on a lift with all four wheels off the ground.

Pulling the fuse causes loss of power to the TCCM which the other computers will see that the TCCM is not operating and they will show the U1026 (Lack of Communications with TCCM). Once the fuse is reinstalled, the U1026 would be there only as a history code. So if you can't communicate with the TCCM, the other computers should also tell you that you have a communication problem. If the encoder motor is bad, you should still communicate with the TCCM and see that the encoder motor is not operating properly. The whole key here is to establish communication with the TCCM whether the TCCM itself is bad or a lack of power or ground to the TCCM. It could be something as simple as a bad wire terminal to the TCCM (which is something I have found in the past). Impossible to check without the right equipment.

Again I say that if the tech at the dealer says that replacing will take care of the communication problem I would have them do it rather than yourself and see for yourself if it communicates before and after. Because if there is still no communication after, they should remove the new TCCM and look elsewhere, but if you do it, you will be stuck with it.

I understand a bad connection could cause the communication problem but could a bad TCCM cause the comminication problem? Could a bad TCCM be causing Service 4wd light and 4wd problems? Sorry for all the questions but I hate paying $600 for them to replace the TCCM when I could do it for probably half that. In fact, I could replace the TCCM and the Encoder Motor for that price. However, I could still have the communication problem the that's due to some bad connection. Would the TCCM and scanner communication problem cause the 4wd problem. Again thanks for all your help. I am trying to avoid paying some to do this if at all possible.

The scanner is trying to communicate with the TCCM, which controls all the 4WD functions.

I was wondering if you got this solved because Im having the same problem

The dealer first wanted to replace the transfer case control module becuase the scanner could not communicate with the computer. I replaced the transfer case control module myself and was still having the problem. Based on some of the things I read on this site and others I took a chance and replaced the transfer case shift motor (or encoder motor) and the problem went away for a while then came back. I took the blazer back to the dealer and told them everything I had done. Since I had replaced the transfer case control module I guess they were able to get the codes out. This is what my invoice said,

"60619 Daig Checked Codes Found Code CO323 - Transfer Case Lock Circuit Low. Checked Circuit 1689 Found To Be Open. Checked Wire On Other Side Of Connector 100 Found To Be Good. Called TAC Case No 8120414. Ran New Wire From Motor to Wire 1689. Cleared Codes Checked Operation Problem Fixed."

It has been working fine since. I sure hope this helps.

Thanks, I still have to get into it ,havent had a chance lately. I was going to start with the TCCM. I found one locally for $50

I forgot to add the following: When I quoted the statement from the invoice I forgot to add that the wire it talks about replacing is at the Transfer Case Shift Motor (Encoder Motor). I guess a wire or some connection in the harness went bad at the Transfer Case Shift Motor and caused the circuit to lock into 4WD.

I just came across this forum and I am having a similar problem on a 2002 Chevrolet Silverado 1500 Z71 4X4 5.3L V8

The code I got was C0324 "Transfer Case Lock Circuit High"

I was curious what it cost you to have your fixed.

Thanks,
Mike

It was $270 to get it fixed at a Chevy dealer. I don't know how much of that was a diagnostic charge and how much was for actually fixing the problem. The invoice did not break it out. It's an aweful lot to pay to have someone run a jumper wire around the bad circuit but I would never have been able to find it.

Thanks so much for the quick response.

I was afraid to take it to the dealer, I was expecting it to be maybe around $1000.00 before I learned it was just something as simple as a wire.

Mike

My 01 has done this before and I cleared the locked on 4wd by (99up) disconnecting the vac to the front actuator when the engine was idling - heard the front actuator release. On 99up there is an additional vac switch on the firewall the vac passes thru with an electrical connector I assume feeds the TCCM/computer (located in the cab pass side kick panel). I will be replacing the front diff engagement sensor and this vac switch later and post the results. NOTE: front driveshaft always spins at least on the Autotrac (4 button) systems and "shift on the fly" requires the front system be turning at the same speed then the right side axle is just coupled.

Front drive shaft always spins on my 96. I always thought it was normal to do so. Its just got the 3 button switch on the dash.