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Jerry
Jerry, Master Mechanic
Category: GM
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Experience:  ASE Master tech, 30 years experience GM certified
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1997 GMC Yukon SLT: with 4WD...it flashed but did not engage

Customer Question

1997 GMC Yukon SLT with 4WD. Went hunting this weekend and got into some farm land where I needed 4 HI. I hit the 4 Hi button and it flashed but did not engage. After fooling around with it and romoving and reinserting the #2 fuse on the side it finally engaged and I was good until later that evening when I got back to black top. I was able to get it back to 2WD, but there is defintely someting wrong. I tried putting it back in 4Hi or Lo and won't go unless I keep hitting the button and moving the transmision in and out of gear. Help!
Submitted: 4 years ago.
Category: GM
Expert:  Jerry replied 4 years ago.

Hello and welcome to Just answer.com, Super mechanic here. About your vehicle.

 

It sounds like it may be the shift motor, (encoder motor) in the transfer case, but you will want to see what trouble code(s) may be stored in it.

 

 

 

graphic

 

Reading Diagnostic Trouble Codes - NV 243 (Electric Shift)
Notes

The diagnostic trouble codes are displayed on the three transfer case shift select buttons. The Shift select buttons are located on the instrument panel. When connector pin 13 on the data link cable is grounded, and the ignition switch has been off for at least five seconds prior to positioning the ignition switch to run, the shift select buttons will blink various times together to indicate diagnostic trouble codes from 1 to 4.

  1. Position the ignition switch to "OFF".
    • Ensure the ignition switch is positioned to "OFF" for at least 6 seconds.
  1. Connect pin 13 on the data link cable to a vehicle ground source.
    • The data link connector is located in the cab under the instrument panel on the drivers side.
    • Look for an unplugged cable lead with a connector that is housed by the bottom of the instrument panel trim.
  1. Position the ignition switch to "RUN".
  2. Note the shift select buttons for blinking codes. Refer to DIAGNOSTIC TROUBLE CODES. If the shift select buttons all blink one time and stop, and do not continue blinking, no fault codes are stored in the TCCM. The transfer case shift select buttons will blink to identify any stored DTC. If only one code is stored in the TCCM memory, that code will blind repeatedly with a three second delay between blinking sequences. If more than one code is stored, the first code will blink once, then after a three second delay, the next code will blink. This sequence will continue until pin 13 is no longer grounded. When reading the diagnostic trouble codes, the number of shift select buttons blinking will indicate the code number.

 

 

 

Notes

DIAGNOSTIC TROUBLE CODE (DTC) 1
When the ignition switch is positioned to "RUN", the TCCM test to determine if RAM Standby Power (maintained battery power) to the TCCM was lost since the ignition was last turned off. When power is interrupted on pin C6 of the TCCM, the TCCM stores a "loss of RAM Standby Power" DTC 1.

DIAGNOSTIC TROUBLE CODE (DTC) 2
During electronic shifting, the TCCM checks the motor/encoder for normal operation. If the motor/encoder does not function correctly enough times, the TCCM stores a "motor/encoder failure" DTC 2.

DIAGNOSTIC TROUBLE CODE (DTC) 3
The TCCM performs a test each time the electric-shift motor is turned on or off. If the motor is not functioning properly the TCCM stores a "motor circuit failure" DTC 3, and the shift select buttons blink to inform the driver of a problem.

DIAGNOSTIC TROUBLE CODE (DTC) 4
Each time the ignition is turned on, the TCCM tests it's memory, program, and internal system to ensure that it is operating properly. If the TCCM detects a fault within itself, the TCCM stores a "RAM/ROM memory failure" (DTC) 4.

  • When the TCCM is running a diagnostic, and a code 4 is displayed by the shift select buttons, replace the TCCM.

 

 

 

 

Jerry, Master Mechanic
Category: GM
Satisfied Customers: 13011
Experience: ASE Master tech, 30 years experience GM certified
Jerry and 8 other GM Specialists are ready to help you
Customer: replied 4 years ago.
I have a couple OBD II diag code readers, is that what you are suggesting I check? How are these different than the codes generated by the ECM after two drive cycles?
Expert:  Jerry replied 4 years ago.
They are trouble codes for the transfer case control module, normally a code reader won't pick them up. You would need a tech2 or similar scan tool, or use the method that I posted to get the codes
Customer: replied 4 years ago.
ok, sounds good. Assuming the motor/encoder is bad, is that an expensive part and and or big job?
Customer: replied 4 years ago.

When performing the DTC code test above, I get no lights on the switches when pin 13 is grounded and when the switch is turned to run. What does that mean?

Expert:  Jerry replied 4 years ago.

You would then want to start with the functional check. (may want to check the fuses first)

 

Symptom Diagnostic Tables - NV 243 (Electric Shift)

 

 

 

graphic

 

 

Table 2: Functional Test Failed

 

 

graphic

 

Customer: replied 4 years ago.
Ok, I started following the procedures and it refers me tot table 3 which you did not include. Can I get these tables?
Expert:  Jerry replied 4 years ago.

Sure can...

 

Table 3:

 

graphic

 

Table 4:

 

 

 

graphic

 

 

 

Let me know if you need more

Customer: replied 4 years ago.

Well I replaced both fuses and got a system ok or all three lights blinked once and the a pause of three seconds and then again. Following the tables, it points me to table 8, which I don't have? Any thoughts?

 

 

Expert:  Jerry replied 4 years ago.

Let me give you the next few tables. It sounds like your making some progress with it. Then you can try the diagnostic again to see what codes may be there.

 

 

Table 8:

 

graphic

 

 

Table 9:

 

 

 

graphic

 

 

 

Four Wheel Drive Indicator Schematic

 

 

graphic

 

 

graphic

 

 

graphic

Customer: replied 4 years ago.
Ok, I was able to get the unit to intermitently shift in and out of 4 hi and 4 low gear and then it stopped working at the switch and I got the following code pattern test -- all three blinked once, 3 sec then all three blinked twice and then repeat.
Expert:  Jerry replied 4 years ago.

The code #1 may have been set by taking out the fuse. It looks like a code #3, which is the encoder motor, (most likely) let me see if there are any tests that can be done on it.

 

 

 

DIAGNOSTIC TROUBLE CODE (DTC) 1
When the ignition switch is positioned to "RUN", the TCCM test to determine if RAM Standby Power (maintained battery power) to the TCCM was lost since the ignition was last turned off. When power is interrupted on pin C6 of the TCCM, the TCCM stores a "loss of RAM Standby Power" DTC 1.

 

 

DIAGNOSTIC TROUBLE CODE (DTC) 3
The TCCM performs a test each time the electric-shift motor is turned on or off. If the motor is not functioning properly the TCCM stores a "motor circuit failure" DTC 3, and the shift select buttons blink to inform the driver of a problem.

 

 

 

 

Whenever the electric-shift motor is turned on or off, the motor and its electrical circuits are tested both in the de-energized and energized condition. If one or both voltage relays fail to detect the proper voltage after energizing or de-energizing, the shift is aborted by the Transfer Case Control Module (TCCM) and a DTC of 3 is stored to indicate a faulty TCCM motor circuit.

If one or both relays fail to detect the proper voltage after de-energizing, both relays are turned on by the TCCM (even with the ignition switch positioned off) to prevent the motor from running. The TCCM then stores a DTC of 3 to indicate a faulty TCCM motor circuit. The all the status lamps blink three times, stop, and repeat the blinking continuously to alert the driver that the electronic shift system requires immediate repair. The battery will drain when this condition occurs, regardless of the ignition switch position.

 

Customer: replied 4 years ago.

I ran some more tests and have continued to be able to get it in and out of 4hi and 4lo, but by playing around with it. Ultimately, it gets to the point where it will only switch from 2Hi to 4Lo and in drive it won't even activate from 2HI to 4HI. After that state I pull the codes and get all three lights one time + 3 secs + all three lights twice + three secs and repeat I did a clear before each test, 45 seconds with #2 fuse out and then 5 times cycling the ign switch to run. I'm thinking that there is something wrong with the TCCM and also possibly the motor/encoder. I checked all the wires to the motor, looks good.

 

Anyway, your thoughts and I would also like to give you a tip for your help....

(edit Mod)

Dave

 

 



Edited by Al-Mod on 11/24/2009 at 11:25 PM EST
Expert:  Jerry replied 4 years ago.

Thanks but we are not allowed to have any communication outside of the posts.

 

It does sound like the encoder/motor is bad. I would go with that before condenming the TCCM first thing. Here is the only thing that the data base has for the testing of it.

 

 

While the transfer case is shifting, the encoder within the electric-shift motor is monitored for the proper operating sequence. If during a shift, the encoder changes to a incorrect position, an error counter in the TCCM starts to count the number of times the encoder fails. If the encoder fails eight times, the Transfer Case Control Module (TCCM) produces a DTC of 2 indicating a (Encoder Fault). When this happens, the TCCM outputs a signal to default the rail shift pattern in the encoder to allow for only shifts into 2Hl and 4LO.

To protect against transient, random encoder faults caused by vibration, contamination, electrical noise, etc., the error counter reduces its count by one each time a good encoder value is detected. The encoder must fail 25 percent of the time for TCCM to store a DTC of 2 and indicate a damaged encoder.

 

 

Customer: replied 4 years ago.

Well, I just replaced the motor/encoder and still have the problem, except that when I put the car into nuetral, I can get into and out of 4lo to 2hi consistantly. After fooling with it for a while, the TCCM throws a code and then I can't get the dash switch to even activate from 2hi to 4hi in gear. It still will go to 4lo in neutral. Code pattern is all three lights flash twice and then repeat. Think it is the TCCM?

 

Dave

Expert:  Jerry replied 4 years ago.
That is possible, of course, however, there aren't any TCCM tests that you can do on it. It would be sort of a "hit or miss", but it almost sounds like a problem in the transfer case.
Customer: replied 4 years ago.

Well I made progress, at least I can depend on it getting me in and out of 4LO for those times when I need it. 4HI is a luxury for now. Don't think it is the TCCM. I'm still not sure how to read the codes. I get all three lights blinking twice and then repeat. Why does that mean 3? How would 4 be shown?

Expert:  Jerry replied 4 years ago.

That should be code 2..code 4 would blink 4 times. Normally, a code 2 will set ater the TCCM sees that there is a problem with going into and out of 2wd and 4wd modes.

 

You might want to try clearing the code(s) if you haven't already. Then see how it does.

 

 

Notes
  • Perform the following to clear the diagnostic trouble codes (DTC) and stored memory from the transfer case control module (TCCM) .
  1. Turn the ignition switch to the "OFF" position.
  2. Remove the 20-amp T-case fuse (Number 2).
  3. Wait at least 40 seconds then replace the fuse.
  4. Cycle the ignition switch from "RUN" to "OFF" five times to clear diagnostic trouble code 1 from the TCCM.

Important:

  • The codes will not clear if the 20-amp T-case fuse is not removed for at least 30 seconds.
  • Code 1 will continue to occur until the ignition switch is cycled from "RUN" to "OFF" five times.
Customer: replied 4 years ago.

If I clear the codes, it allows me to press the 4HI in gear and flashes and then a clunking sound and then it flips back to 2HI. Sometimes it will work if I am moving, but after a few failed times and once code 2 is set, it stops allowing me to try switching to 4HI in gear. 4LO always works now in neutral from 2HI and back. I guess there may be a mechanical problem in the transfer case.

 

 

Expert:  Jerry replied 4 years ago.
That is what it's starting to sound like, internal in the transfer case. Normally a clunk sound is not good.

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