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  #1  
Old 10-26-2009, 11:49 AM
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Default Service AWD light intermittently on

Hi
I have a 1998 Oldsmobile Bravada Smarttrack

I have scanned through these forums and (finally joined )have done some recommended fixes - but one problem is still lingering.

Here is what happened:

I took the Bravada off roading - and when i got out of the mud track - The suv would not shift past 2nd gear. It would die on me sporadically. Sometimes it went all the way to OD, but then would not switch back to 1, 2 or 3 - and sometimes it would only switch from 1 to 2. I checked the fuse box in the engine bay and found that the 20AMP ATC fuse was blown off. I replaced it - and then drove home limping - with several lights on - ABS, Check Engine, Brake, Service AWD. The speedometer would fluctuate - down all teh way to 0 and then back up to 60 etc...

When I got home I looked at the forums here - and saw that people thought it could be the actuator under the battery - I took the battery out but could not find an actuator. Does my model year not have one?

I then scanned the computer for codes - and found one that said that the vehicle speed sensor A was erratic, also that the EBCM is faulty. Went to the junk yard and pulled an EBCM from another 98 bravada, popped it in - and that took care of the EBCM problem. I then also replaced the vehicle speed sensor in the tail end of the transfer case.

Took it for a 5 min drive - and saw that the service AWD light came on intermittently. No codes were found with my OBD2 and ABS scanner.

So - now what do I do? I do not know what the problem could be. Should I just replace the TCCM in the passenger side kick panel - or replace the transfer case motor?

Does anyone know how to get that skid plate that is under the transfer case off? Please help - I need this truck working. Wife is mad, because this is her truck. I am using our other Buick to get to work...
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  #2  
Old 10-26-2009, 11:59 AM
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The typical code readers cannot access the TCCM used in your truck.

I would look for damage to the wiring harness to the transmission and transfer case.

Did replacing the vehicle speed sensor (VSS) fix the jumpy speedo?
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  #3  
Old 10-26-2009, 12:04 PM
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yeah, the jumpy speedometer issue got fixed with the new VSS

I was under the truck this weekend and could not find the wiring going into the transmission... maybe hidden?

Do you know where the vacuum actuator is located on the 98 Bravada? I cannot find anything circular under/around the battery tray besides the horn...

I can however see some wiring going into the transfer case - visual check this wekeend showed that the wires were not broken or anything
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  #4  
Old 10-26-2009, 12:38 PM
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You'll look for a very long time and never find the actuator on your truck. AWD means that the front axle is always engaged therefore there is no need for an engagement actuator.
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  #5  
Old 10-26-2009, 01:33 PM
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I see, thanks for the info
I will go pull a TCCM from the junk yard and see if that helps
if not I guess the transfer case motor might need replacing?

I do not know if this helps or not - but when I turn the ignition on - sometimes I hear the TCCM making 4-5 clicks in rapid progression...
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  #6  
Old 10-27-2009, 11:14 AM
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ok
So I pulled the TCCM out of the truck and I see two ports on it. A 4 pin Black one and a 32 pin Orange one.
Does anyone have access to the circuit diagram for this module? Or know what each of these ports communicate with?
I am trying to see if I can apply the correct voltage to the wires - will my transfer case motor engage or not, or possibly even rigging this module so I have manual control over 4low, 4hi or 2hi.

Any help is much appreciated. The junk yard replacement module costs $75. But before I spend that money I want to see if this module is toast or not. If I can apply the right voltage to the wires, I can at least rule out the transfer case motor.

I have a Haynes manual, but alas, there is no circuit diagram in that book for this module

HELP!

I will document the help - with photos on how to remove everything for the AWD problem once I solve this. So the moderator can make it a sticky - and other people will have a visual on how to fix this problem.
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  #7  
Old 10-27-2009, 11:49 AM
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It is not that simple with your transfer case. The signal from the TCCM to the encoder motor is a PWM (pulse width modulate) signal which drives the encoder motor to the intended position.

Here are a few descriptions for you. If you really want the wiring diagrams, I can dig those up sometime, but unless you like doing more harm than good, extreme caution should be used when dealing with your NV136 transfer case and its controls. You really have to gain an understanding of how it works prior to fiddling around with its I/O's.
Quote:
Transfer Case Description

The NV136 transfer case is an automatic transfer case with two drive ranges. Shifting from rear wheel drive to four wheel drive is done automatically when the transfer case shift control module receives wheel rotating slip information from the speed sensors. The transfer case shift control module then engages the transfer case motor/encoder to position the transfer case from rear to four wheel drive. When the transfer case shift control module receives information that the wheel rotation is the same on both axles, the transfer case shift control module sends position information to the motor/encoder to put the transfer case back into rear wheel drive.
Quote:
Transfer Case Circuit Description

Transfer Case Shift Control Module
The transfer case shift control module uses the VIN information for calculations that are required for the different calibrations used based on axle ratio, transmission, tire size, and engine. The system does not know which calibration to use without this information.
The direct battery supply line provides the power to the module and motor circuitry.

Transfer Case Encoder Motor
The transfer case encoder motor is a permanent magnet D.C. motor and gear reduction assembly. The encoder motor is controlled with a Pulse Width Modulated (PWM) signal provided by the transfer case shift control module. It is located on the left hand side (drivers side) of the transfer case. When activated it turns the sector shaft of the transfer case to shift the transfer case. This circuit consists of a power supply relay (Motor Feed A circuit) that supplies voltage to the motor. The Motor Control B circuit is a PWM driver that varies the duty cycle to control the amount of current through the motor to ground.

Transfer Case Speed Sensors
There are three speed sensors on the Automatic Transfer Case (ATC) , two on the rear output shaft and one on the front output shaft. Each speed sensor is a Permanent Magnet (PM) generator. The PM generator produces a pulsing AC voltage. The AC voltage level and number of pulses increases as speed Increases.

Vehicle Speed Sensor
One of the two speed sensors on the rear output shaft is the Vehicle Speed Sensor (VSS) . It is an input to the Vehicle Control Module (VCM) . The VCM sends this information to the transfer case shift control module via the Class 2 Serial Data bus.

Rear Propshaft Speed Sensor
The transfer case shift control module converts the pulsating AC voltage from the rear Propshaft speed sensor to a Rear Propshaft Speed in RPMs to be used for calculations. The Rear Propshaft Speed can be displayed with a scan tool.

Front Propshaft Speed Sensor
The transfer case shift control module converts the pulsating AC voltage from the front Propshaft speed sensor to Front Propshaft Speed in RPM to be used for calculations, and to monitor the difference between the front and rear Propshaft sensor speed. It is also used in the AUTO 4WD mode of operation to determine the amount of slip and the percent of torque to apply to the front axle. The Front Propshaft Speed can be displayed with a scan tool.

SERVICE Indicator (AWD/4WD) Lamp
The SERVICE indicator (AWD/4WD) lamp is an integral part of the instrument cluster and cannot be serviced separately. This lamp is used to inform the driver of the vehicle of malfunctions within the Automatic Transfer Case (ATC) system. The SERVICE indicator (AWD/4WD) lamp is controlled by the transfer case shift control module via a Class 2 serial data message.
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  #8  
Old 10-27-2009, 12:00 PM
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Oh no
the TCCM is caliberated to the VIN???
So I cannot use one from the junk yard?
Sucks that the motor is driven by a PWM signal...
Is the caliberation done on just the first 10 letters or so of the VIN, or the entire string? Because of it is just the first few, then I ought to be able to use a junk yard part

Also
do you know the location of the three speed sensors in the ATC? The one I replaced was on the tail end of the TC- going into the rear prop shaft. Maybe I can replace the other two as well?

Last edited by swartlkk; 10-27-2009 at 12:18 PM. Reason: Combining Consecutive Posts - Please use the EDIT feature to add additional information to your posts if another member has yet to reply.
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  #9  
Old 10-27-2009, 12:19 PM
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You are getting WAY ahead of yourself. Your truck is likely telling you exactly what its problem is. Without reading the codes, you are just poking and hoping. You will waste money in the process.
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  #10  
Old 10-28-2009, 10:42 AM
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What sort of a scanner do I need? Cannot fin any codes with an OBD2 and ABS scanner. Where do I get one of these scanners?
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Old 10-28-2009, 10:42 AM
 
 
 
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1998, 1999, awd, bravada, case, center, fix, fuse, light, location, sensor, speed, stays, tccm, transfer, wheel


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