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1999 chevy tahoe: service 4wd light..it in drive the light comes on

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i have a 1999 chevy...
i have a 1999 chevy tahoe and the service 4wd light is coming on if i shut the truck off and start it back up the light is not on if i back up the light does not come on but as soon as i put it in drive the light comes on
Submitted: 6 years ago.Category: Chevy
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Answered in 8 minutes by:
10/12/2011
Chevy Mechanic: Jerry Newton, Chevy Technician replied 6 years ago
Jerry Newton
Jerry Newton, Chevy Technician
Category: Chevy
Satisfied Customers: 4,831
Experience: ASE Master Technician, L1, Master GM Technician. Over 20 years of bumper to bumper GM experience.
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There are trouble codes that go along with this system. Here is the procedure for retrieving the codes:


  1. 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 the 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 in order to indicate a diagnostic trouble code from 1 to 4.
1.1. Position the ignition switch to OFF. Ensure the ignition switch is positioned to OFF for at least 6 seconds.
1.2. 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.
1.3. Position the ignition switch to RUN.
1.4. Note the shift select buttons for blinking codes.
  1. If the shift select buttons all blink one time and stop, and do not continue to blink, no fault codes are stored in the TCCM.
  2. The transfer case shift select buttons will blink in order to identify any stored DTC. If only one code is stored in the TCCM memory, that code will blink 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 seconds delay, the next code will blink. This sequence will continue until pin 13 is no longer grounded.
  3. When reading the diagnostic trouble codes, the number of shift select buttons blinks will indicate the code number.


Get the code, let me know what you have, and we'll get this figured out.
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Customer reply replied 6 years ago
yeah i dont know anything about no pin 13 on the data link cable i looked and there is no numbers that i can find on it i thought maybe someone would know off hand what is making the light come but im not even goin to try and go through all that stuff you are talking about way to much bs for me
Chevy Mechanic: Jerry Newton, Chevy Technician replied 6 years ago
Nobody in my business would attempt to diagnose this without knowing the code.

However, for what it's worth, this is "probably" being caused by the transfer case encoder motor. It's the electric motor on the side of the transfer case that actually shifts the case.

On occasion, this is also caused by a faulty transfer case control module. But the heavy hitter here is encoder motors, if you're wanting to take a shot with the most common failure, that's the way I would go.

-Jerry
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Customer reply replied 6 years ago
everything works on it 2hi 4hi and 4low
Chevy Mechanic: Jerry Newton, Chevy Technician replied 6 years ago
The encoder internal sensor fails in a cumulative fashion, as described below:

While the transfer case shifts, the encoder within the electric-shift motor is monitored for the proper operating sequence. If during a shift, the encoder changes to an 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 TCCM produces a DTC of 2 indicating an Encoder Fault. When this happens, the TCCM outputs a signal in order to default the rail shift pattern in the encoder in order to allow for only shifts into 2HI and 4LO.

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



However, I'm still recommending that you have this diagnosed before repairing anything, as the encoder motor is several hundred dollars. Even if you have to pay a repair shop $75 to figure this out, it's a lot cheaper than being wrong by guessing at the encoder motor.

 

The pin 13 thing is not hard to figure out. If you'd like a hand with that, I'm sure I can help.

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Customer reply replied 6 years ago
if the encoder moter was bad it would not shift from 2hi into 4low and 2hi to 4hi and back to 2hi
Chevy Mechanic: Jerry Newton, Chevy Technician replied 6 years ago
That's what I was explaining above. The encoder motor works, but it fails a percentage of the time, and sets the code.


Again, we don't KNOW that this is the case, because we don't know what the code is. This is purely a guess, but it's a guess that's based upon my experiences with these vehicles. I've replaced encoder motors 30 times more often than I've had to replace all other components in these systems combined. Even at that, I wouldn't diagnose this without knowing the code. But I would know before I retrieved the code that it's *probably* a bad encoder motor.
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Customer reply replied 6 years ago
what does the encoder moter do?
Chevy Mechanic: Jerry Newton, Chevy Technician replied 6 years ago
It shifts the transfer case, and it also watches to see which range the transfer case is in by way of an internal sensor inside the encoder motor. This is where the problem usually lies, with this internal sensor. The TCCM knows it commanded the encoder to move, but the feedback from this sensor confuses the TCCM, it's not sure of the reading, so it sets a code.
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Customer reply replied 6 years ago
okay i just went from 2hi to 4low 15 times and 2hi to 4hi 15 times no problems im goin to take it down the road to a buddy that has a scanner then ill get back to u
Chevy Mechanic: Jerry Newton, Chevy Technician replied 6 years ago
That's what I'm trying to tell you. A scanner won't read these codes on a 99.


The only way to retrieve them is by flashing them out, as described above.
Jerry Newton
Jerry Newton, Chevy Technician
Category: Chevy
Satisfied Customers: 4,831
Experience: ASE Master Technician, L1, Master GM Technician. Over 20 years of bumper to bumper GM experience.
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