First post, been searching and reading about a number of issues I'm trying to solve. Hats off to the experts here, great site guys!


1998 Blazer, 3-button 4x4 selector (NV233). Shifting back to 2HI from 4HI happened slower and slower and after a fairly long delay, would usually release with a heavy "clunk" from the front axle. It got to the point where it wouldn't release the front axle for days at a time. Eventually the selector wouldn't even try to put it into 4HI.

Read several of the Tech articles here and started playing with it last night. Could put it in Neutral and get it to go into 4LO and back to 2HI but not 4HI. After putting it in 2HI from 4LO, the 2HI indicator light would go on, it would shift it into hi range, but the front axle was still engaged (tires would scrub on corners). It would take a long time to get it to disengage, sometimes with a heavy clunk, sometimes not. The last time I tried it, I couldn't get the front axle to disengage.

Next morning, front axle still engaged, jumped pin 13 on the ALDL port to ground, got error code 2, "Encoder Fault". Unbolted the plug attached to the Encoder Motor housing at the transfer case to inspect for corrosion. Looked like brand new inside. Plugged and bolted it back in.

Pulled the ATC fuse. Pulled the battery to inspect the vacuum actuator under the battery tray. Touched the positive lead to the negative lead.

Pulled the tray and unbolted the actuator. Pulled the vacuum hose off and the actuator released the cable. So with the transfer case indicator light showing 2HI, and the t-case definitely in high range (don't know if the front drive shaft was disengaged at the t-case or not), the vacuum line was still pulling vacuum on the actuator.

Tried to find the vent line for the vacuum switch to check if it was clogged. Not sure if I found it or not. There was one zip-tied to the Trans dip stick. Had a piece of foam and a plastic breather cap on the end. Pulled the breather cap and could blow through it. I suppose I should have tried to suck, but it seemed to be working fine. Found another over by the air filter, same drill, same result. I didn't try tracing either one back to the t-case, so the vent line is still a question mark.

Set the tray and battery back in, hooked up the battery and started it up. Front axle had released. Test drove it, would go into 4HI, 4LO, and 2HI almost immediately, although it would take the front axle a fair amount of time to disengage. Seemed like popping it in reverse for about 15 feet helped it let go.

Pulled back into the driveway, 2HI, front axle disengaged, pulled the battery and tray, pulled the vacuum line from actuator and the the actuator had a fair amount of vacuum on it and the cable released at least 1/4 of an inch, maybe more.

So it seems like the vacuum switch is not completely relieving the vacuum from the actuator.

My question is this: Would this cause a DTC 2, "Encoder Fault" all on it's own and throw me into "2HI/4LO only" mode?

Do I have a vacuum switch problem only, or an encoder problem, or both?

Where should I go from here? Check the vent line for sure? Other tests? New vacuum switch? New encoder?

Any help much appreciated.

 

Do a posi lock like I did and eliminate the vacuum all together. I have more info and see pics of it in the first couple pages of my build thread. Others will add insight soon as well I'm sure. Thanks for taking the time to so the reading before posting up a bunch of questions we really appreciate that. I will add more tomorrow when I'm on a computer.

 

After seeing what was under the battery tray, the thought of running a cable to it had occured to me. Came back in and saw the thread in the Tech article section and sure enough, it's already been done.

It's an option I may resort to if I have to, but I would prefer to make it run the way it was intended if possible.

 

Good work on your research...glad you were able to make good use of all we have available on this site. I suspect you have problems with both the vacuum switch and the encoder motor. Sounds like the vacuum switch is sticking open and letting vacuum to the 4WD actuator at all times. This is fairly common, and the switch is not too expensive or hard to replace.

The DTC 2 should be a separate issue. As far as I can tell, the only overlap between the vacuum system and the electrical system (i.e. TCCM, encoder motor and dash switch) is indirect, via the sensor that detects if the hubs are locked. I suspect that if the vacuum switch is stuck open and the hubs are always locked, the TCCM will be ok with that when you press one of the 4WD buttons. So the DTC 2 is telling you something else about the 4WD system - it's detecting faults with the operation of the encoder motor. That too is pretty common (but probably more expensive to fix, unfortunately, if you're sure the electrical connections are ok).

 

That's exactly what I'm wondering.

If the switch on the axle doesn't flip because the axle doesn't disengage, because the vacuum switch is faulty, would the system assume the encoder is bad and throw a DTC 2?

-OR-

Is it possible the encoder is not positioning the vacuum switch properly which is causing the problem, resulting in a legitimate DTC 2?

-OR-

They both are bad.

Recommended course of action?

 

The DTC 2 diagnostic chart basically says, first test the circuits. If they are ok, then replace the encoder. If that doesn't work, replace the TCCM.

Never mentions a possible vacuum switch failure.


From: OBD2 NV233 Diagnostics by lastcoupe & swartlkk


DTC 2 - Encoder Fault:

Quote:
DIAGNOSTIC CHART

CIRCUIT DESCRIPTION
The transfer case encoder motor is an assembly which houses four separate channels. These channels are used to indicate to the transfer case shift control module the present gear position of the transfer case or if the transfer case is in transition between gears. The encoder motor assembly consists of an electronically conductive inner ground ring in contact with a three legged wiper arm. Each leg's length is such that it makes contact with the conductive regions of the four channels. When any leg of the wiper arm is making contact with the conductive area of any channel, the encoder provides a path to ground to the inner ground ring.
During electronic shifting, the transfer case shift control module monitors the transfer case encoder circuit for the proper operating sequence. If at any time during a shift, the encoder changes from one position to any position other than the next possible position, an error counter in the transfer case shift control module increments by four. After the counter reaches 32, the transfer case shift control module set a DTC 2, a permanent "encoder fault," and reverts to a 2HI to 4L0 or a 4L0 to 2HI shift pattern only. To guard against a transient, random encoder fault due to vibration, dirt, electrical noise, etc., the fault counter reduces by one each time the transfer case shift control module detects a valid encoder value. The encoder must intermittently fail 25 percent of the time for the transfer case shift control module to consider it faulty and store DTC 2.

CONDITIONS FOR CLEARING THE DTC
The condition for setting the DTC is no longer present and a momentary loss of battery power at the battery positive voltage circuit (CKT 40).

TEST DESCRIPTION
The numbers below refer to the step numbers on the diagnostic table.
2. This step measures the voltage between the transfer case encoder circuit channels P, A, B, and C to ground.
3. This step tests the suspect circuit(s) for an open, short to power, or short to ground.
4. This step tests the encoder signal ground circuit for an open or high resistance.

 

After reading through a bunch of info here, it sounds like the DTC 2 is thrown when the TCCM counts a certain number of mis-steps by the encoder.

There are wipers in the encoder motor that send signals to the TCCM verifying position. If the TCCM doesn't get the right sequential sequence from the encoder it increments a counter. Once the counter reaches a limit, it cuts over to 2HI/4LO only mode. You can reset it and clear the DTC 2 and it will function until it racks up the required number of missteps again.

I read a thread here by a guy from the UK who cut one open, found dried up electrical grease on the wipers, cleaned it with sandpaper, epoxied it shut, and fixed it (no more DTC 2s).

I think you're right, the front axle switch doesn't seem to come into play with a DTC 2. Probably have problems with both the vacuum switch and the encoder.

The o-rings in the vacuum switch are probably shot and vacuum is making it to the acutuator even when it is supposed to in a "no vac" state.

The wipers in the Encoder motor are probably not contacting consistently anymore and the TCCM is noticing.

 

Interesting find on the UK guy cleaning up the wipers...prolly worth a try if it's not to time consuming. Otherwise, I think you've got it pretty well diagnosed. If you do end up taking apart the encoder motor, let us know what you find, and if you're able to make it work again.

 

Here's the link to the thread where the guy (error_401) cut open the encoder throwing DTC 2s and repaired it. He's from Zurich, not UK...my bad.

NV233 transfer case / encoding motor switch

Thanks for the great write up error_401!


And thanks swartlkk for creating the Tech Article Index sticky, a gold mine of great info including error_401's thread:

Tech Article (DIY) Index - Check Here first!