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.

Apr 192017

1998 GMC 1500How do you change the intake manifold gaskets? I can smell antifreeze but I can’t see anything leaking on the ground. My brother in-law has the same problem, and he had his checked out and they said the vortex motors were coming for that. I’d like to know how to do this on my own if I can thank you!

Intake Manifold Gasket Replacement


Backfire through TBI 1992 GMC 3500 454 engine

 Auto Repair Questions, GMC  Comments Off on Backfire through TBI 1992 GMC 3500 454 engine
Apr 112017

backfire through TBI 1992 GMC 3500My 1992 GMC 3500 started developing a skip under load. So I did the normal spark plugs, wires, complete distributor, coil, test drove and still skips under load. Truck has 100,000 miles on it, so I replaced the fuel filter then fuel pump. Test drove and still skips at times under load. Now starting to pop up through TBI unit. Any ideas? Have made sure timing is set correctly. Did compression test and all cylinders are at between 145-150.


Backfire through TBI

A pop through the TBI is equivalent to a backfire through TBI.

Anytime an engine backfires through the intake it is from spark occurring while an intake valve is still open. Now this can be from pour timing or an open valve. Since you have done a compression test and received excellent results it makes you want to focus on the ignition timing. I would double check to make sure you have set your timing correctly. Doesn’t hurt to check it more than once. Therefore I have added the timing adjustment procedure below for convenience.

Sticking Intake Valve

I have had sticking intake valves do this from carbon build up on the valve from an RV that had set for a while. It would intermittently cause a valve to stick open. I too would get good compression readings but after testing and retesting found that intermittently one of the engine cylinders compression was ZERO and then would go back up(engine has 14,000 miles). I tried SEAFOAM with no luck. Tried SEAFOAM 4 more times each time driving 200 miles and then changing the oil. Eventually it cleared up on its own after 4,000 miles of driving.

Setting and Adjusting Engine Ignition Timing

  1. Start the engine and allow it to reach operating temperature. Stop the engine and connect the timing light to the No. 1 (left front) spark plug wire, at the plug or at the distributor cap. You can also use the No. 6 wire, if it is more convenient. Numbering is illustrated in this section.
    NOTE: Do not pierce the plug wire insulation with HEI; it will cause a miss. The best method is an inductive pickup timing light.
  2. Clean off the timing marks and mark the pulley or damper notch and timing scale with white chalk.
  3. Disconnect and plug the vacuum line at the distributor on models with a carburetor. This is done to prevent any distributor vacuum advance. On fuel injected models, disengage the timing connector which comes out of the harness conduit next to the distributor, this will put the system in the bypass mode. Check the underhood emission sticker for any other hoses or wires which may need to be disconnected.
  4. Start the engine and adjust the idle speed to that specified on the Underhood Emissions label. With automatic transmission, set the specified idle speed in Park. It will be too high, since it is normally (in most cases) adjusted in Drive. You can disconnect the idle solenoid, if any, to get the speed down. Otherwise, adjust the idle speed screw.The tachometer connects to the TACH terminal on the distributor and to a ground on models with a carburetor. On models with fuel injection, the tachometer connects to the TACH terminal on the ignition coil. Some tachometers must connect to the TACH terminal and to the positive battery terminal. Some tachometers won’t work with HEI.

    WARNING Never ground the HEI TACH terminal; serious system damage will result.

  5. Aim the timing light at the pointer marks. Be careful not to touch the fan, because it may appear to be standing still. If the pulley or damper notch isn’t aligned with the proper timing mark (see the Underhood Emissions label), the timing will have to be adjusted.
    NOTE: TDC or Top Dead Center corresponds to 0�B, or BTDC, or Before Top Dead Center may be shown as BEFORE. A, or ATDC, or After Top Dead Center may be shown as AFTER.
  6. Loosen the distributor base clamp locknut. You can buy trick wrenches which make this task a lot easier.
  7. Turn the distributor slowly to adjust the timing, holding it by the body and not the cap. Turn the distributor in the direction of rotor rotation to retard, and against the direction of rotation to advance.
  8. Tighten the locknut. Check the timing again, in case the distributor moved slightly as you tightened it.
  9. Reinstall the distributor vacuum line or the timing connector. Correct the idle speed.
  10. Stop the engine and disconnect the timing light.
Feb 222017

1999 GMC JimmyThe parking brake will not release, pedal stuck. It worked for hours, but could not get it. I did not want to do too much without guide. any tips ? thanks


The Parking brake system consists of the a brake lever, two cables, adjuster and brake shoes. In order to determine if the lever itself is at fault disconnect the cable and test.

Park Brake Lubrication

Clean and lubricate the park brake lever assembly using Lubriplate GM P/N 1050109 or the equivalent.

Plastic coated parking brake cables do not need periodic lubrication. However, before performing service that involves the adjuster, perform the following steps:
•Clean the exposed threads on each side of the nut.

Important Lubricate the threads of the adjusting rod before turning the nut
•Lubricate the threads of the adjusting rod using Lubriplate GM P/N 1050109 or the equivalent.

Parking Brake Lever Replacement


For complete step by step instructions get an auto repair manual today.

Parking Brake Shoe Adjustment

The park brake must be adjusted any time the park brake cables have been replaced or disconnected, if the park brake shoes have been replaced or if under heavy foot pressure the pedal travel is less than half the pedal total travel. Before adjusting the park brake, check the condition of the service brakes.

  • Raise the vehicle and support the vehicle with safety stands.
  • Remove the wheel and tire assembly.
  • Take off the caliper and the rotor.
  • Remove the park brake cable from the park brake lever.
  • Adjust the shoe diameter using the adjuster nut.
  • Turn the adjuster nut clockwise to increase the diameter until the rear wheel will not rotate without excessive force in a forward direction.
  • Connect the park brake cable to the park brake lever.
  • Install the caliper and the rotor.
  • Install the wheel and the tire.
  • Adjust the rear park brake cables.
  • Install the wheel and tire assembly.
  • Remove the safety stands and lower the vehicle.