GM Full Size Vans 1987-1997 Repair Guide

Description & Operation


See Figures 1 through 5

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Fig. Fig. 1: High voltage switch - 1995 4.3L (VIN W) Engine

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Fig. Fig. 2: HVS ignition coil - 1995 4.3L (VIN W) Engine

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Fig. Fig. 3: HVS camshaft position sensor location

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Fig. Fig. 4: Crankshaft position sensor

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Fig. Fig. 5: High Voltage Switch (HVS) system operational schematic

The High Voltage Switch (HVS) ignition system is used only on the 1995 4.3L (VIN W) engine. The ignition system is controlled by the Vehicle Control Module (VCM-A). The VCM-A obtains information from various engine sensors, then uses it to compute the desired spark timing, as well as control the dwell and firing of the ignition coil by way of an ignition control line to the coil driver. The High Voltage Switch (HVS) assembly resembles a distributor, containing both a cap and rotor. But, unlike a distributor ignition system, ignition timing is preset and cannot be adjusted because the HVS is mounted in a fixed position (it cannot be rotated). The HVS provides spark at exactly the right time to ignite the air/fuel mixture, producing peak performance and fuel economy. The HVS system is comprised of the following parts:

Vehicle Control Module (VCM-A)
Crankshaft position sensor
Ignition coil driver module
Ignition coil
High voltage switch

The Vehicle Control Module (VCM-A, often referred to in the past as an ECM) is located on the right-hand side fenderwell of the vehicle. It is the control center for fuel emissions, automatic transmission control functions and the anti-lock brake system. The VCM-A constantly monitors information from sensors in the engine and controls the component systems. It is designed to process the various input information, then send the necessary electrical responses. The ignition module that is used on the Distributor Ignition (DI) system is not used. All Ignition Control (IC) and bypass functions are controlled by the VCM-A.

The Crankshaft Position Sensor (CKP) is a digital sensor that provides reference information to the VCM-A for spark and fuel delivery. It is located in the front timing chain cover and is perpendicular to the crankshaft target wheel. There is an air gap between the sensor and the target wheel which is not adjustable. The target wheel has three slots 60° apart and is keyed to the crankshaft. As the target wheel rotates, the slots passing by the sensor create a change in the magnetic field of the sensor which results in an induced voltage pulse. One revolution of the crankshaft results in three pulses (3x signal). From these pulses, the VCM-A is able to determine crankshaft position and engine speed. The VCM-A then activates the fuel injector and provides spark to the High Voltage Switch. There is a very important relationship between the crankshaft position sensor and the target wheel in that the sensor must be perpendicular to the target wheel and have the precise air gap.

The Ignition Coil Driver (ICD) module is mounted on a bracket next to the ignition coil. The ICD controls the communication between the ignition coil and the VCM-A. The VCM-A sends a signal to the ICD commanding it to turn current on and off to the ignition coil at the proper times.

The High Voltage Switch is an assembly that looks similar to a distributor. It contains the Camshaft Position (CMP) sensor, cap, rotor and shaft. The High Voltage Switch shaft is driven by the camshaft and rotates like a distributor, providing spark to the correct cylinder via the cap and rotor.

The Camshaft Position (CMP) sensor is located within the HVS. Its operation is very similar to the Crankshaft Position (CKP) sensor, but it provides one pulse per camshaft revolution (1x signal). The VCM-A uses this signal, along with the crankshaft position, to determine which cylinder(s) are misfiring. It connects to the VCM-A through the primary engine harness and provides cylinder identification. The VCM-A controls the dwell and firing of the ignition coil through an ignition control line to the coil driver.

The Camshaft Position sensor does not have any effect on driveability. Its only purpose is to provide the VCM-A with cylinder identification and other needed information for misfire diagnostic trouble codes.


When making compression checks, disconnect the HVS electrical connector to disable the ignition and fuel injection system. Refer to Engine & Engine Overhaul for compression check procedures.
No periodic lubrication of the high voltage switch is required. Engine oil lubricates the lower bushing and the upper bushing is prelubricated and sealed.
There is no manual dwell adjustment.
The spark plug wires are made of a material which is very pliable and soft. It is very important to route the spark plug wires correctly to prevent chafing or cutting.