1999 GMC Suburban Encoder Motor Sensor

MADMOOSE

MEMBER
1999 GMC SUBURBAN
V8
4WD
AUTOMATIC
155,000 MILES

My transfer case suddenly will not go into neutral, 4 hi or 4 lo. When the button on the dash is pressed, the front axle actuator motor hums and engages, but the actuator motors runs and appears to "hunt" for a few seconds, then stops, leaving the transfer case in 2 wheel drive. From all the research I have done, this sounds like it is probably the encoder motor sensor, and not the motor itself. (I don't have any way to check for the "CO327 code to confirm this theory.)

Is there anything else you would recommend I check before ordering and replacing the sensor?

Thanks!

7 Replies

FACTORYJACK

MECHANIC
4,159 POSTS

From what I see, your 99 Suburban is possible to be equipped with two options, a selectable NVG-243 case, or an automatic NVG-246 case. Neither of which have a separate encoder sensor, it is integral to the motor. Does the switch LED's indicate a change in mode or range? It would be necessary to replace the complete unit, there is no option for serviceability. It is also possible that something has failed internally, and the motor is driven, and hall sensors are changing state indicating the motor has moved, but mechanical failure is preventing actual shift.

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Wednesday, September 29th, 2010 AT 3:19 PM

MADMOOSE

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Thank you so much for your reply!

This model has the 4 push buttons to the right of the steering wheel on the dash; 2 hi, 4 hi, Auto 4WD & 4 Lo.

When I push the "4 hi", the led on the switch flashes for a few seconds while the transfer case motor "hunts". (The front differential actuator motor runs for a second and engages during this time.) Then, the LED on the "4 Hi" button stays on, even though it is not engaged at the transfer case. (If I attempt to go to "4 Lo" at this time, the "4 Lo" LED flashes for about 6 seconds and a relay clicks under the dash, then the light goes out on "4 Lo" and comes back on at the "4 Hi" button.)

While the "4 Hi" LED is on, If I push the "2 Hi" button, the front differential actuator motor runs and disengages and the LED on the "2 Hi' button comes on.

It has also been suggested to me that the connectors on the TCCM may be corroded or loose, or the TCCM itself may be bad. Do either of these items sound like a possibility on this model? If so, where is the location of the TCCM? (On older models it was behind the passenger kick panel, but it is not there on this model.)

If the symptoms mostly point to a new motor, I'll just go that route. (I just hate blowing money on the part to find a bad connection later.)

Thanks again for all your help!

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Wednesday, September 29th, 2010 AT 3:49 PM

FACTORYJACK

MECHANIC
4,159 POSTS

If it is showing completion of some shifts, and the front axle is engaging, it sounds like the module is operating. It is located under the left side of dash, above the junction block(convenience center). Was the 4 lo shift attempted in neutral or park, with 0 vehicle speed? Is the service 4wd/awd light coming on? I would suspect an encoder, but like I say, there is a possibility that something internal might be at fault. I believe if you remove the encoder, the shift shaft has flat sides. You can try tu turn it back and forth, it should have some resistance, and maybe even detent in a shifted position. That could indicate there is integrity in the transfer case. If I remember correctly, you do not want to try commanding shifts through the vehicle, with the encoder removed from the transfer case, I think that destroys it. If there is a warning light, one might at least seek a diagnosis to determine what the trouble code may reveal.

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Wednesday, September 29th, 2010 AT 10:41 PM

MADMOOSE

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Thanks again for your reply.

The car was in neutral when the 4 lo was attempted.

The "Service 4WD" light has not come on during any of this.

Does that tell you anything different, or is it still most likely the motor?

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Thursday, September 30th, 2010 AT 12:57 PM

FACTORYJACK

MECHANIC
4,159 POSTS

I am going to suspect a motor, because they are occasional failures. The fact that you have some motor operations and the front differential locks, indicates that there is some operation attempting to occur. Like I say, when(if) you have the motor off, manually move the shift shaft with an adjustable wrench. See if you can feel it actually doing something.