Jun 252019

pulled the codes po23f fuel relay
cant find the relay

P023F – Fuel Pump Secondary Circuit Open

Code P023F possible causes:
  • Bad fuel pump relay
  • Short to voltage on FP voltage supply circuit or feedback circuit
  • Short to ground on PCM
  • FP relay control circuit PCM
  • FP relay control driver shorted to ground internally

There are several fuel pump fuses and relays located in the underhood fuse block.


  • 18 – Fuel Pump Relay (20A) – Fuse(2 pin micro fuse)
  • 41 – Fuel Pump Relay – Secondary (20A) – Fuse(2 pin micro fuse)
  • 59 – Fuel Pump Relay – Primary(Micro Relay)
  • 66 – Fuel Pump Relay – Secondary(Micro Relay)
Dec 092018

2006 GMC Envoy

Did the 2006 GMC Envoy XL-SLT ever come with Tow Hooks either front or rear? I cant get a positive answer from the local GMC Dealer. My almost former Chevy Trailblazer LT model had them. The Envoy has the Towing pkg but no towing hooks of which I’m surprised>

Envoy did not come with factory tow hooks….

Envoy is the “Luxury* edition

Trailblazer is the more “sport” edition

That’s the reason behind Envoy’s not getting factory tow hooks, even the Denali did not have them.

I know, as I asked every place I went when shopping around for my Envoy, they never had that option available. I was told that if I wanted tow hooks, I’d have to buy the Trailblazer.

Source: Voymom@gmtnation.com

Oct 252018

2005 GMC Yukon

Instrument Cluster, Radio, Door Locks, and Windows quit working on my 2003 GMC Yukon XL SLT 4×4. Believe it to be in the BCM. Need schematics, Connector Charts, and Diagnostic connector Diagram to Help in Diagnosing. Thanks, Patches

Here is the information that I have including wiring diagrams. A blown fuse that is used by all systems described would be the first thing I would look for. And it would be the most common cause if they all went out at the same time. Since the vehicle is more than 10 years old, rusted ground connections may also be a factor.

Body Control System Description and Operation

BCM Functions

The Body Control Module (BCM) performs multiple body control functions. The BCM can control devices directly connected to its outputs based on input information. The BCM evaluates this information and controls certain body control systems by commanding outputs on or off. The BCM control inputs can be:

  • Sensors and switches that are directly connected to the BCM
  • Class 2 serial data received from other control modules connected to the class 2 serial data link.

The BCM is also capable of controlling other vehicle systems that are not directly wired to the BCM. The BCM does this by sending specific messages on the class 2 serial data link. The control module capable of performing the required function will respond to the BCM message.

The BCM controls these functions:

  • Audible warnings
  • Automatic door locks
  • Automatic headlamp control
  • Interior lighting
  • Keyless entry (AUO option)
  • Passlock theft deterrent
  • Retained Accessory Power (RAP)

Serial Data Power Mode

On vehicles that have several control modules connected by serial data circuits, one module is the power mode master (PMM). On this vehicle the PMM is the Body Control Module (BCM). The BCM uses 3 signals from the ignition switch. These are the Ignition 0, Ignition 1, and Accessory.

To determine the correct power mode the BCM uses:

  • The state of these signals
  • Next , the sequence of switch closures received by the BCM
  • Finally, the status of the engine run flag

The chart indicates the power modes detected and transmitted by the BCM:

Correct Ignition Switch Inputs

Ignition Key Position

Power Mode Parameter

Engine Running

Sampled Ignition Signal State


Ignition 1

Ignition 0

OFF Off-Awake or RAP No Inactive Off Off
UNLOCK Unlock or RAP Unlock No Inactive Off On
ACC Accessory No Active Off On
ON Run No Active On On
START Crank No Inactive On On
ON Run Yes Active On On

Fail-safe Operation

Since the operation of the vehicle systems depends on the power mode, there is a fail-safe plan in place should the BCM fail to send a power mode message. The fail-safe plan covers modules with discrete ignition signal inputs as well as those modules using exclusively serial data control of power mode.

Serial Data MessagesThe modules that depend exclusively on serial data messages for power modes stay in the state dictated by the last valid BCM message until they receive the engine status from the PCM. If the BCM fails, the modules monitor the serial data circuit for the engine run flag serial data. If the engine run flag serial data is true, indicating that the engine is running, the modules fail-safe to “Run”. In this state the modules and their subsystems can support all operator requirements. If the engine run flag serial data is false, indicating that the engine is not running, the modules fail-safe to “OFF-AWAKE”. In this state the modules are constantly checking for a change status message on the serial data circuits and can respond to both local inputs and serial data inputs from other modules on the vehicle.

Discrete Ignition SignalsThose modules that have discrete ignition signal inputs also remain in the state dictated by the last valid BCM message received on the serial data circuits. They then check the state of their discrete ignition input to determine the current valid state. If the discrete ignition input is active, B+, the modules will fail-safe to the “RUN” power mode. If the discrete ignition input is not active, open or 0 voltage, the modules will fail-safe to “OFF-AWAKE”. In this state the modules are constantly checking for a change status message on the serial data circuits and can respond to both local inputs and serial data inputs from other modules on the vehicle.

Body Control Module Programming


The procedures below are designed to set-up the body control module (BCM) correctly during BCM related service. Before you start, read these procedures carefully and completely.


The following procedures must be followed:

  1. BCM Programming
  2. Programming Theft Deterrent System Components, if equipped with Passlock System.

Programming the BCM


After the procedure is completed, the personalization settings of the BCM are set to a default setting. Inform the customer that the personalization must be set again.

Programming Theft Deterrent System Components


This procedure has to be performed only for vehicles equipped with Passlock System.

Perform the Programming Theft Deterrent System Components in Theft Deterrent after successfully finishing the Setup New BCM procedure. If the Programming Theft Deterrent System Components in the Theft Deterrent procedure is not performed after a BCM replacement, one of the following conditions will occur:

  • The vehicle will not be protected against theft by the PASSLOCK system.
  • The engine will not crank nor start.


After programming, perform the following to avoid future misdiagnosis:

  1. Turn the ignition OFF for 10 seconds.
  2. Connect the scan tool to the data link connector.
  3. Turn the ignition ON with the engine OFF.
  4. Use the scan tool in order to retrieve history DTCs from all modules.
  5. Clear all history DTCs.

> > Need more, access a 2003 GMC auto repair manual now!

Jul 092018

GMC Envoy

Passlock Key No Start

I recently bought this 2002 GMC Envoy. It was sitting for around 6 months after the previous owner had a problem with it and couldn’t afford to fix it. My Envoy did not come with any sort of chipped key and the key I have will work in the ignition but not to unlock the doors. It cranks but won’t start. And the shifter locks and wont go into any gear. Is it the passkey security issue with it? I’ve check fuel pump and it’s working but no fuel at the fuel rail. So I’m figuring it’s got to be something to do with the security system. Is there a way to bypass that for now so I can at least get it started and check for it to run before spending all the money on a new passkey? Any hints or tips would sure help. Thank you very much.

Vehicle Theft Deterrent (VTD) Description and Operation

Passlock™ System

The Passlock™ System is provided in order to prevent vehicle theft if the ignition lock cylinder is forced to rotate or the ignition switch is operated while separated from the ignition lock cylinder case. If starting is attempted without authorization from the Passlock™ System, the powertrain control module (PCM) will disable engine starting. Start disable may be in the form of fuel disable or starter disable, depending on engine application.

The components of the Passlock™ System are as follows:

  • The ignition lock cylinder and key
  • Ignition lock cylinder case, including the Passlock™ sensor
  • The ignition switch
  • The body control module (BCM)
  • The security indicator on the instrument cluster
  • The powertrain control module (PCM)Ignition Lock Cylinder Case, Including the Passlock™ Sensor

The ignition lock cylinder fits inside the ignition lock cylinder case and operates the ignition switch when turned by a key with the proper mechanical cut. When the ignition key is used to turn the ignition lock cylinder to crank, start, a magnet on the lock cylinder passes close to the Passlock™ sensor within the ignition lock cylinder case. The magnet activates the security hall effect sensor in the Passlock™ sensor which completes a circuit from the security sensor signal circuit through a resistor to the security sensor low reference circuit. The resistance value will vary from vehicle to vehicle.

If a magnet from outside of the ignition lock cylinder case is used to attempt to steal the vehicle, the tamper hall effect sensor will be activated. This completes a circuit from the security sensor signal circuit through a tamper resistor to the security sensor low reference circuit bypassing the security resistor. If the ignition switch is forced to rotate without the correct key, or if the ignition lock cylinder is removed by force, the Passlock™ sensor will be damaged and will not operate.

Ignition Switch

The ignition switch contains the wiring and electrical switching portion of the column mounted ignition assembly. The switch includes wiring pigtails which connect it to the base of column connector, the Passlock™ sensor on the ignition lock cylinder case, and other components. The wiring for the Passlock™ sensor is unaffected by ignition switch position. The electrical switch portion is operated by the key and lock cylinder when they are rotated within the ignition lock cylinder case. The ignition switch operates the crank relay regardless of the status of the Passlock™ System.

Body Control Module (BCM)

The body control module (BCM) contains the logic of the Theft Deterrent System. The BCM provides the battery positive voltage to operate the Passlock™ sensor. The BCM also measures the voltage of the security sensor signal circuit. The voltage measured will indicate whether the Passlock™ sensor has been activated and whether the resistance value from the sensor is a valid value or the tamper value. If voltage measured is in the valid range, the BCM compares this voltage, voltage code, to a previously learned voltage code. If the voltage codes match, the BCM sends a class 2 message containing a password to the powertrain control module (PCM). When the voltage codes do not match, or the voltage is in the tamper range, or there is a circuit fault, the BCM will not send the correct password to the PCM, and the vehicle will not start.

Powertrain Control Module

The powertrain control module (PCM) contains the remainder of the logic of the Theft Deterrent System. If a class 2 message containing a valid password is received from the body control module (BCM), the PCM will continue to allow the fuel injectors to operate. The PCM will allow the fuel injectors to operate until it decides there is no valid password coming from the BCM. If the PCM does not receive a class 2 message, or receives a class 2 message with an incorrect password, the engine will crank and will not run or will start and stall immediately.

Theft System Indicator

The IPC illuminates the theft deterrent indicator as determined by the theft deterrent system. The IPC receives a class 2 message from the BCM requesting illumination.

  • The vehicle theft deterrent (VTD) system requests the IPC to illuminate the indicator only when the ignition switch is ON.
  • The content theft deterrent (CTD) system requests the IPC to illuminate the indicator only when the ignition switch is in the OFF or ACC positions or during RAP.
  • The body control module performs the displays test at the start of each ignition cycle. The indicator illuminates for approximately 3 seconds.


Fuel Lockout Cycle

When it receives a password which is incorrect or a password which indicates tamper and the powertrain control module (PCM) disables the fuel injectors, the fuel injectors remain disabled for 10 minutes even if the ignition switch is turned from the RUN position to the OFF position.

Changing the Passlock™ Components

The following components contain codes or passwords, or must learn codes or passwords for the Passlock™ System to allow the vehicle to start:

  • The ignition lock cylinder case
  • The body control module (BCM)
  • The powertrain control module (PCM)

If any of these parts are replaced, a learn procedure must be performed. Refer to Programming Theft Deterrent System Components . If parts are replaced and a learn procedure is not performed, the engine will crank and will not run or will start and stall immediately.

Jun 012018

GMC Sierra

Heater and Air Conditioning blower doesn’t work.
Causes, and location of blower motor?

Manual Heating and Air Conditioning

If the blower motor only comes on in High speed, replace the blower motor resistor. If the blower motor doesn’t work at all, you will need to replace the blower motor and possibly the resistor also.

Automatic Climate Control

If the blower motor only comes on in High speed, replace the blower motor controller.

Blower motor location

Under the passenger side dash. Access from inside the cab.

Blower motor location diagram

Blower Motor Replacement

  • If equipped, remove the sound insulator panel.
  • Remove the blower motor insulating cover screws.
  • Disconnect the electrical connector from the blower motor.
  • Remove the blower motor insulating cover.

Blower Motor

  • Pull the retaining tab down while turning the blower motor counterclockwise in order to disengage the blower motor from the heater/ventilation module.
Feb 262018

1995 GMC Sierra

Battery cables keep coming loose

Positive battery cables keep coming loose. I have a new dual post battery I would like to replace side post, can I separate the two positive cables, one one side post, one on top post, or do they have to remain in contact?

You may separate them. As long as they are both connected to the positive terminals there essentially still connected.

Feb 022018

Chevy Suburban

My Suburban wasn’t starting so I changed the Distributor and Rotor.  It started but it runs at 4,000 RPM’s. I put a code scanner on it and got a P0102 code so I used Mass Air Flow Cleaner on it and tried again. Same thing, 4,000 RPM’s but no codes this time. Can you give me an answer because I’m stumped! I looked at the throttle spring and it seems to snap back OK, please help. It is a 1996 GMC Suburban 4×4 4 door Automatic with a 5.7 liter engine.

The engine running with a high idle is a classic description of a vacuum leak.

Chevy Code P0102

Mass Or Volume Air Flow A Circuit Low Input

Probable causes

  1. Sensor or solenoid faulty
  2. Connector terminal contact is damaged or corroded
  3. Wire harness – Check harness for correct voltage, open, short to ground or short to voltage
  4. Update control unit software – Check for the latest control unit update
  5. Powertrain or Engine Control Module faulty
  6. Battery voltage low

Door Lock Problem 1996 GMC Suburban

 Auto Repair Questions, Chevy, GMC  Comments Off on Door Lock Problem 1996 GMC Suburban
Dec 082017

Chevy Suburban

Door Lock Problem

I have a new to me Suburban, it is all power. This vehicle has a Smart Start Security System and the key fob will not lock the door. The right rear door is missing the up/down button to manually lock from inside. The right rear door will not lock using to front power lock button.

If I replace the handle/lock assembly on inside will I be able to lock the door from the inside if the actuator is bad?

Yes, if you replace all the missing “manual” components in the door. You will be able to manually lock the door. Even if the electrical actuator portion does not function.

Oct 282017

 GMC Sierra

Idles a little rough and when pressing the accelerator doesn’t have power and stalls sometimes.

Doesn’t have power

The first thing to do would be to have any check engine light codes scanned. Post them below in the comments. If there are no check engine light codes then the most common cause for this on your vehicle would be a lack of fuel pressure. This can be from a faulty fuel pump or a restriction in the fuel line. The restriction could be a clogged fuel filter or a kink in the line. Could also be from a faulty sensor signalling the PCM to use less fuel such as a faulty MAF sensor.

Quick test

A quick test would be to have someone press the accelerator and when it bogs, spray some starting fluid in the air filter. If the engine revs up then you know it is a lack of fuel that is causing the issue.

Jul 302017

When the truck sets for an extended period of 24 hours or longer, it takes many engine crank overs before the fuel is pumped back up to the carburetor for the engine to start. Thus far have changed out the sending unit in the gas tank, the fuel pump, and sent the Q-Jet carb off for rebuild (twice) to fix the problem but no luck so far problem still exists. Any ideas? Thanks.


The carburetor has a float bowl and will have fuel in it at all times. So thinking the carburetor is out of gas would be incorrect. It would be easy to test by simply working the throttle on the side of the carburetor and visually seeing fuel squirt.If the float bowl is empty, then you have a leak. Either an external leak or an internal fuel pump leak. An external leak should be easy enough to find by putting a piece of cardboard under it and leave it set after you park it for the night. An internal fuel pump leak you will not be able to see but the fuel pressure will bleed off from an internal leak.

I am thinking more along the lines that the choke is not functional or is not being set properly before initial start up.

The proper way to start a non-fuel injection engine would be to press the accelerator pedal to the floor and then let off completely. With your foot still off the pedal turn the ignition key to the “START” position and hold for no longer than 10 seconds.  By pressing the pedal to the floor before cranking it sets the choke and squirts a bit of gas into the intake manifold.

Carburetor Adjustments

8-350, 8-400, and 8-454 Engines


1.All adjustments should be made with the engine at normal operating temperature, air cleaner on, choke open, and air conditioning off, unless otherwise noted.
2.Set the parking brake and block the rear wheels.
3.Automatics should be place in Drive, manuals in Neutral, except as noted in the Tune-Up Specifications chart.
4.Check the underhood emissions sticker to determine which hoses, if any, must be disconnected.
5.For two barrel carburetors, make sure that the idle speed screw is on the low( L) step of the fast idle cam.
6.For vehicles without air conditioning, do the following: A.Turn the idle speed screw to adjust idle speed to the specification found in the Tune-Up Specifications chart or on the underhood emissions sticker.
B.Open the throttle slightly to allow the solenoid plunger to extend.
C.Turn the solenoid screw to adjust the curb idle to specification, as given in the chart or on the emission control sticker in the engine compartment.
D.Then disconnect the electrical connector from the solenoid. The idle speed will drop.
E.Turn the idle speed screw to set the slow engine idle to the figure given on the emission control sticker.
F.Reconnect the solenoid and shut off the engine.

7.For vehicles equipped with air conditioning, do the following: A.Turn the idle speed screw to set the idle to specifications.
B.Disconnect the air conditioner compressor electrical lead at the compressor, and turn the air conditioner on.
C.Open the throttle slightly to allow the solenoid plunger to fully extend.
D.Turn the solenoid screw and adjust to the specification on the emission control label and the Tune-Up Specifications chart.
E.Reconnect the air conditioner compressor lead.

8.Reconnect any hoses that were disconnected.


Mixture is adjustable on heavy duty emissions V8s with the four barrel M4MC carburetor. This procedure will not work on light duty emissions trucks.
1.The engine must be at normal operating temperature, choke open, parking brake applied, and the transmission in Park or Neutral. Block the rear wheels and do not stand in front of the truck when making adjustments.
2.Remove the air cleaner. Connect a tachometer and a vacuum gauge to the engine.
3.Turn the idle mixture screws in lightly until they seat, then back them out two turns. Be careful not to tighten the mixture screw against its seat, or damage may result.
4.Adjust the idle speed screw to obtain the engine rpm figure specified on the emission control label.
5.Adjust the idle mixture screws equally to obtain the highest engine speed.
6.Repeat Steps 4 and 5 until the best idle is obtained.
7.Shut off the engine, remove the tachometer and vacuum gauge, and install the air cleaner.