DIAGNOSTIC EQUIPMENT FOR DRIVEABILITY DIAGNOSIS

MOOELS: 1985-1987 TRUCKS EQUIPPED WITH THROTTLE BODY INJECTION (TBI)

With the introduction of the 2.5L Throttle Body Injection (TBI) in the S/T model trucks in 1985, the diagnosis of some driveability conditions must be performed by the method of using diagnostic charts and service tools such as a tachometer, digital voltmeter, test light, etc. While the tests performed with these various tools are accurate, there are some disadvantages to testing the TBI system by this method. They are difficult to use, for example, in the diagnosis of intermittent problems.

Several tool companies have released special siters that can be attached to the ALCL (Assembly Line Communications Link) connector located under the dash of the vehicle. These special ALCL "Scan" tools offer several advantages in the diagnosis of driveability complaints. They are designed to read a variety of information that is supplied by the ECM to terminal "E" in the ALCL connector.

When a diagnostic chart calls for a sensor reading, the "Scan" tool can be used to read the following directly:

Park/Neutral Switch in volts

Throttle Position Sensor in volts

Manifold Absolute Pressure Sensor in volts

Coolant Temperature Sensor in degrees Centigrade

Vehicle Speed Sensor in MPH

Oxygen Sensor Voltage

The "Scan" tool is also helpful in cases of intermittent system operation. The tool can be plugged in and observed while driving the vehicle under the condition where the "Check Engine Light" comes "ON" momentarily, or the engine driveability is poor momentarily. If the problem seems to be related to certain areas that can be checked on the "Scan" tool, then those are the positions that should be checked while driving the vehicle. If there does not seem to be any correlation between the problem and any specific circuit, the "Scan" tool can be checked on each position, watching for a period of time to see if there is any change in the reading that would indicate intermittent operation.

"Scan" tools do not make the use of diagnostic charts unnecessary. They do not tell exactly where a problem is in a given circuit. However, with an understanding of what each position on the equipment measures, and knowledge of the circuit involved, the tools can be very useful in getting information which would otherwise be more time consuming to obtain.

NOTE: When the tool is plugged in, it takes out the timer that keeps the system in "Open Loop" (cold engine) for a certain period of time. Therefore, it will go "Closed Loop" (warm engine) as soon as the vehicle is started, if all other "Closed Loop" conditions are met. Most "Scan" tools set the engine at 1,000 rpm and the timing to its base setting while performing the test.

Starting with the 1986 Light Duty Service Manuals, the driveability diagnostic charts will have procedures for both the multiple tool test and for the "Scan" tool test. Starting in 1987, the Light Duty Service Manuals for all models will only contain "Scan" tool diagnosis procedures. With the introduction of the 1987 Light Duty trucks, all gasoline powered models 1500-3500 will be changed from carbureted to Throttle Body Fuel Injection (TBI). The 1987 S/T/M vehicles equipped with the 2.5L TBI engines will also include the new GMP4 ECM which has a "Baud Rate" (speed at which the ECM operates) of 8192 versus the current ECM which has a "Baud Rate" of 160. Dealers who already have "Scan" tools should check with their manufacturers to make sure their equipment can be updated to the new higher speed ECM's, and if they cannot be updated, dealers should consider purchasing a new "Scan" tool that can read the new high speed ECM.

Complete information on the "Scan" tools can be provided by their individual manufacturers. The following list contains known sources for "Scan tools.

GM Dealer Equipment Renaissance Center Tower 400, Suite 1500 Detroit, MI 48243 Telephone (313) 567-5780

Kent-Moore Tool Group 29784 Little Mack Roseville, MI 48066 Telephone (313) 774-9500

Microtron Product, Inc. 2355 Avon Industrial Drive Aubum Heights, MI 48057 Telephone (313) 852-4042

Owatonna Tool Company Major Motors Division 2013 Fourth Street N.W. Owatonna, MN 55060 Telephone (507) 451-5310

Micro Processor Systems, Inec. 7641 Nineteen Mile Road Sterling Heights, MI 48078 Telephone (313) 731-6410

General Motors bulletins are intended for use by professional technicians, not a "do-it-yourselfer". They are written to inform those technicians of conditions that may occur on some vehicles, or to provide information that could assist in the proper service of a vehicle. Properly trained technicians have the equipment, tools, safety instructions and know-how to do a job properly and safely. If a condition is described, do not assume that the bulletin applies to your vehicle, or that your vehicle will have that condition. See a General Motors dealer servicing your brand of General Motors vehicle for information on whether your vehicle may benefit from the information.