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99 GMC Envoy 4WD not working

Had some work done under the dashboard to fix the air conditioning, since then the CD player won't work, two speakers don't work, the sun/moon roof only goes half way back unless pushed and the 4WD works when it wants to. I'm thinking it's an electrical problem but I'm not sure. When I start the vehicle, the 4WD lights flash as they should - sometimes. When they do, I can engage the different gears just fine but after I turn the engine off and start it again, it doesn't work. The mechanic was also asked to unplug the air/tire inflation generator because it was running off and on while driving. It still makes noise - at least I think thats where the noise is coming from (could it be the transfer case having problems?) Any suggestions?

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  • j_j_hoover
    j_j_hoover Dec 06, 2012

    This Envoy was give to us so I'm not worried about having to put money into it - and we're trying to do the repairs ourselves if possible - also I'm new to this whole 'being your own mechanic' thing. What is the first thing I should check - the fuses? The what?

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  • GMC Master
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The way to find the problem is electrical testing for the componets that are not working or not working correctly . If someone had the dash a part I would look for a lose connection somewhere ! Finding a wiring diagram for the componets not working . Look to see if wire's share a connector behind the dash or maybe these componets share a common ground . Do you know what a wiring diagram is ? How to use a wiring diagram ? How to do basic automotive electrical testing ? How to use a DVOM - digital volt ohm meter ?

Basic Electricity for Service Techs Ohm law Current Flow Opens Shorts
How to Use Multimeter to Troubleshoot Common Problems

Automotive Electrical System Basics EricTheCarGuy

Posted on Mar 19, 2017

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Posted on Jan 02, 2017

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autodr
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SOURCE: intermittent 4wd on 98 jimmy

the module is what i would suspect. and i do believe it is in the r kick panel. i've seen the button pack on dash go bad too, but module is best bet.

Posted on Nov 16, 2008

kd6rhd56
  • 400 Answers

SOURCE: S-10 Blazer 4wd problem

www.amazon.com/Chiltons-Repair-Manual.../0801981425


Hope this helps you

Posted on Jul 12, 2009

Molson02536
  • 3854 Answers

SOURCE: 4wd does not work

Electronic Shift On the Fly (ESOF) Operation
The ESOF system is an electronic shift 4x4 system that allows the operator to choose between two different 4x4 modes as well as 2-wheel drive. The operator can switch between 2WD and 4WD HIGH mode at speed. To engage or disengage LOW Range, the vehicle speed must be less than 5 kph, the brake depressed, and the transmission must be in NEUTRAL.

The transfer case is equipped with an electromagnetic clutch which is located inside the case. This clutch is used to spin up the front drive-line when shifting from 2WD to 4x4 High mode at speed. When the control switch on the instrument panel is turned, the generic electronic module (GEM) recognizes that a shift has been requested and activates the electromagnetic clutch and the relays which power the transfer case shift motor. When the shift motor reaches the desired position as determined by the contact plate position inputs to the GEM, power to the shift relays and motors will be removed. When the transfer case front and rear output shafts are synchronized, the spring loaded lockup collar mechanically engages the main-shaft hub to the drive sprocket. Finally, the front axle collar is engaged and the electromagnetic clutch is deactivated.

Shift Between 2WD and 4x4 HIGH:
Shifts between 2WD and 4x4 HIGH can be made at speed. Listed below are the inputs and outputs needed by the GEM to execute a change between any of these modes.

Feature Inputs:
*4WD Mode Switch (Various resistances).
*Contact Plate Position Inputs A, B, C, D (grounded when closed, open circuit when open).

Feature Outputs:
*4x4 Shift Motor Relay Outputs
*Transfer Case Clutch Relay (grounded when relay is on, open circuit when relay is off).
*4x2/4x4 Vacuum Solenoid (grounded when activated, open circuit when deactivated).
*Cluster Indicators (ground when activated, open circuit when deactivated).

Shifting Into/Out of 4WD LOW:
When shifting into or out of 4WD LOW, the GEM requires that the vehicle speed be less than 5 kph, the brake is applied, and the transmission is in NEUTRAL.

Feature Inputs:
*4WD Mode Switch (Various resistances).
*Contact Plate Position Inputs A, B, C, D (grounded when closed, open circuit when open).
*VSS Sensor (Sinusoid Signal: 0.7V-20V, 2.2 Hz/mph).
Brake Input (battery voltage when brake is depress, open circuit when not activated).
*Transmission Range Sensor (grounded when transmission is in NEUTRAL, open circuit otherwise).
8Start/Clutch Depressed Input

Feature Outputs:
*4x4 Shift Motor Relay Outputs
*4x2/4x4 Vacuum Solenoid (grounded when activated, open circuit when deactivated).
*Cluster Indicators (grounded when activated, open circuit when deactivated).

4WD Mode Switch
A rotary switch allows the vehicle operator to choose between 2WD, 4WD HIGH, and 4WD LOW modes of 4x4 operation.

GEM Module
The Generic Electronic Module (GEM) controls the operation of the 4x4 system

Vehicle Speed Sensor
The vehicle speed sensor (VSS), mounted in the rear of the transfer case, informs the GEM of the vehicle speed.

Transfer Case Shift Motor Sense Plates

The shift motor sense plate, an integral part of the electric shift motor, informs the GEM to the positions of the transfer case

Transmission Range Sensor
Located on the left side of the transmission, this sensor informs the GEM when the transmission is in the NEUTRAL position.

Electric Shift Motor
The electric shift motor, mounted externally at the rear of the transfer case, drives a rotary cam which moves the mode fork and range fork within the transfer case to select between the 2WD, 4WD HIGH, and 4WD LOW positions.

4x4/4x2 Vacuum Solenoids
These two solenoids are attached to the upper RH side of the engine compartment dash, and alternately route vacuum to the vacuum motor which engages/disengages the center axle disconnect collar in the front axle assembly.

4x4 Shift Motor Relay
A module containing two relays which under the control of the GEM, shift the transfer case shift motor between the 2WD, 4WD HIGH, and 4WD LOW modes.

Transfer Case Clutch Relay
An electromechanical relay is used to activate the transfer case clutch within the transfer case.

Good luck and hope this helps,but start with the simple and check the fuses and wire harness. Both white/light blue stripe wires should have 12V in run. The GEM will ground the left side (closest to engine) solenoid to disengage the front axle. It will ground the the right side solenoid (closest to fender) to engage the front axle. If the GEM doesn't see Neutral it will not let the truck come out of 4Lo. If the GEM had water leaking on it it may well be the root of the problem. The shift motor only has power when the GEM sends it a signal to move the shift lever. Once the shift is completed the motor shuts off. There are 2 wires that control the shift motor. A yellow and an orange. The GEM alternates between 12v and Ground to these wires to control which way the motor turns. The GEM uses the transfer case shift relay module to control the power to the motor. There are contact plates inside the shift motor assembly that indicate the position that the shift lever is in. This is what the GEM used to know what the T-case is doing. Any one of these components can fail.







Posted on Jan 10, 2010

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