VW Passat & Audi A4 1990-2000

General Information


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Fig. Exploded view of a typical Bosch® starter

The starting system includes the battery, starter motor, solenoid, ignition switch, circuit protection and wiring connecting the components. On automatic transaxle-equipped vehicles, an inhibitor switch located in the gear selector mechanism is included in the starting system to prevent the vehicle from being started with the vehicle in gear. On later vehicles with manual transaxle, an inhibitor switch is located on the clutch pedal. The clutch pedal must be fully depressed to start the vehicle.

The battery and starter motor are linked by very heavy gauge electrical cables designed to minimize resistance to the flow of current. Generally, the major power supply cable that leaves the battery goes directly to the starter, while a smaller gauge cable supplies other electrical system needs. During starter operation, the positive ( + ) battery power flows from the battery to the starter solenoid, and the starter is grounded through the engine, which is grounded by the battery's negative ground strap.

The starter is a specially designed, direct current electric motor capable of producing a great amount of power for its size. What allows the motor to produce a great deal of power is its tremendous rotating speed. It drives the engine through a tiny pinion gear (attached to the starter's armature), and that, in turn, drives the very large flywheel ring gear at a greatly reduced speed. Another factor allowing it to produce so much power is that only intermittent operation is required of it. Thus, little allowance for air circulation is necessary, and the windings can be built into a very small space.

The starter solenoid is an electromagnetic device that is triggered by the small current supplied by the start circuit of the ignition switch. This electromagnetic action moves a plunger that mechanically engages the starter gear and closes the high amperage switch connecting the starter to the battery. The starting switch circuit is part of the ignition switch. The starting circuit wiring includes a safety circuit, such as a neutral safety switch, or clutch pedal switch, that prevents the engine from being started with the transmission engaged. Also included in the starting circuit is the wiring necessary to connect these in series with the starter solenoid.

The pinion, a small gear, is mounted to a one-way drive clutch. This clutch is splined to the starter armature shaft. When the ignition switch is moved to the START position, the solenoid plunger slides the pinion toward the flywheel ring gear via a collar and spring. If the teeth on the pinion and flywheel match properly, the pinion will engage the flywheel immediately. If the gear teeth butt one another, the spring will be compressed and will force the gears to mesh as soon as the starter turns far enough to allow them to do so. As the solenoid plunger reaches the end of its travel, it closes the contacts that connect the battery to the starter, and then the engine is cranked.

As soon as the engine starts, the flywheel ring gear begins turning fast enough to drive the pinion at an extremely high rate of speed. At this point, the one-way clutch begins allowing the pinion to spin faster than the starter shaft so that the starter will not operate at excessive speed. When the ignition switch is released from the start position, the solenoid is de-energized, and a spring pulls the gear out of mesh, interrupting the current flow to the starter.

To prevent damage caused by excessive starter armature rotation when the engine starts, the starter incorporates an over-running clutch in the pinion gear.

Some starters employ a separate relay, mounted away from the starter, to switch the motor and solenoid current on and off. The relay replaces the solenoid electrical switch, but does not eliminate the need for a solenoid mounted on the starter used to mechanically engage the starter drive gears. The relay is used to reduce the amount of current the start position of the ignition switch must carry.

Never compromise personal safety when working with a vehicle's electrical system.

The starter motor assembly consists of two basic components, the starter motor and the starter relay. Some manufacturers prefer to separate the relay from the starter motor, others prefer to combine the 2 into 1 unit. The Bosch® starter motor found on the A4 and Passat models is a combined unit. The relay used on these starter motor assemblies actually serves 2 purposes. The relay is used to:

Act as a switch to complete a high amperage electrical circuit between the battery and the starter motor.
Engage the starter motor pinion gear with the flywheel ring gear.

The 2 most common symptoms of a combination starter motor failure are:

Nothing happens when the ignition key is turned to START .
A clicking noise is heard when the ignition key is turned to START .

  1. Assuming the battery is fully charged and all of the battery cables are properly attached and their connections clean, if nothing happens when the key is turned to START most likely the starter motor solenoid has failed. In an emergency situation a remote starter switch could override the relay and allow the starter to function temporarily.
  3. If a clicking noise is heard, that's an indication that the solenoid is engaging, but the electric motor portion of the starter motor has failed. In an emergency situation, sometimes the starter motor can be rapped sharply with a screwdriver or hammer handle, which may jolt the starter motor enough to function temporarily.

If the starter motor operation is intermittent, check the operation of the ignition switch. If your starter motor is acting up, use the following steps to troubleshoot the cause and replace the starter motor as necessary. A starter motor replacement is much easier to do in an evening after work or on a day off. It's not something you'll want to be doing on the side of the road.