Toyota Pick-ups/Land Cruiser/4Runner 1989-1996

General Information


See Figure 1

The battery is the first link in the chain of items which work together to provide power of the automobile engine. In most modern trucks, the battery is a lead/acid electrochemical device consisting of six two-volt subsections connected in series so the unit is capable of producing approximately 12 volts of electrical pressure.

Each subsection, or cell, consists of a series of positive and negative plates held a short distance apart in a solution of sulfuric acid and water. The two types of plates are of dissimilar metals. This causes a chemical reaction and it is this reaction which produces current flow from the battery when its positive and negative terminals are connected to an electrical appliance such as a lamp or a motor. The continued transfer of electrons would eventually convert sulfuric acid in the electrolyte to water, and make the two plates identical in chemical composition. As electrical energy is removed from the battery, its voltage output tends to drop. Thus, measuring battery voltage and battery electrolyte composition are two ways of checking the ability of the unit to supply power. During the starting of the engine, electrical energy is removed from the battery. However, if the charging circuit is in good condition and the operating conditions are normal, the power removed from the battery will be replaced by the alternator which will force electrons back through the battery, reversing the normal flow, and restoring the battery to its original chemical state.

The battery only stores electrical energy, it does not produce it. The battery can only deliver power equal to what is stored within it. Thus, a low battery may not be able to supply enough power to crank the starter but will still operate the lights, horn and other lower amperage circuits. This should eliminate the famous line, "I know it's not the battery because the radio works...''

The battery and starting motor are linked by very heavy 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 other electrical needs are supplied by a smaller cable. During the starter operation, power flows from the battery to the starter and is grounded through the truck's frame and the battery's negative ground strap.

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Fig. Fig. 1: Starting system circuit diagram

The starting motor is a specially designed, direct current electric motor capable of producing a very great amount of power for its size. One thing that 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), which 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 required, and the windings can be built into a very small space.

The starter solenoid is a magnetic device which employs the small current supplied by the ignition switch. The magnetic action moves a plunger which mechanically engages the starter and electrically closes the heavy switch which connects it to the battery. The starting circuit consists of the starting signal controlled by the ignition switch, a transmission neutral safety switch or clutch pedal switch, and the wiring necessary to connect these with the starter solenoid or relay. Since the circuit has multiple switches, all must be in the ON position for the system to operate.

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. (This overrun condition is similar to riding a bicycle downhill; the rear wheel is turning faster than the chain sprocket.) When the ignition switch is released from the starter position, the solenoid is de-energized, and a spring in the solenoid assembly pulls the gear out of mesh and interrupts the current flow to the starter.

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 to mechanically engage the starter drive gears. The relay is used to reduce the amount of current the starting switch must carry.