Auto Repair Help

by Kyle McFadden

Todays vehicles place a huge demand on their electrical systems. It is common for a majority of vehicles to use alternators that produce 80 amps of electricity as standard equipment. This has been brought about by the many electrical accessories that are available on vehicles. Rear window defoggers, high powered audio systems, and comfort features such as power seats or windows, can consume large amounts of power from the electrical system. Because of this, electrical systems have become increasingly complex and elaborate. While they still are 12 volt, direct current design. Computer controlled charging systems, advanced battery design, and complex micro-electronics are increasingly in normal use in today’s vehicles.

The battery stores the necessary electrical energy to start the vehicle and provide the additional electrical power necessary during peak electrical surges such as when the air conditioning or electric window defrosters are turned on. All automotive batteries are 12 volts. They have six cells, each cell genterates 2.1 volts so that the actual battery voltage is 12.6 volts. During starting and heavy electrical loads the battery becomes discharged just as a flashlight battery goes dead after a period of operation. Once the engine is started it is the alternators job to regarge the battery. The Alternator converts the rotating mechanical energy of the engine into electrical energy to recharge the battery.

Batteries are rated in Cold Cranking Amps, commonly refered to as “CCA”. This rating refers to the amount of amperage the battery can deliver and still provide adequate cranking performance to start the vehicle. The vehicle manufacturer determines the necessary CCA rating required for the vehicle. A replacement battery must meed or exceed this rating in order to operate properly and maintain a satisfactory life expectancy. Battery life can be affected by many factors. Batteries that operate in extremely cold wheather conditions will usually not last as long as a battery operating in moderate temperature conditions. During the continual discharge and recharge of the battery the water or “electrolyte” in a battery can evaporate in the form of a gas. This water should be checked and replenished on a regualar basis. Older style batteries had removable cell covers that allowed the refilling of the individual cells. Newer “maintenance free” batteries have non-removeable covers and special vents that do not require regular service or refilling of the cells. A clean battery can go a long way in improving battery life. Dirt and acid that can collect on the top of the battery can slowly discharge the battery. A properly tuned engine that starts quickly without excessive cranking time will also improve battery life.

The starting system consists of a high amperage electric motor, a starter solenoid or starter relay, battery, starter cables, and ignition switch. Engine starting is achieved by use of a high amperage electric motor. When the driver turns the key to the start position, current is supplied to the starter relay or starter solenoid. The relay or starter solenoid closes contacts that complete the circuit from the battery to the starter motor. The high amperage current flows through the starter and causes it to rotate the engine at sufficient speed to allow easy starting. The starter motor rotates the engine by means of a small gear attached to the end of the starter motor. This gear is called the starter drive. When the starter is operated, the drive extends and contacts the teeth of flywheel gear. The gear ratio of the starter drive gear is about 15/1. That means that the starter motor will complete 15 revolutions while the engine completes one revolution. When the engine starts, the drive must be immediately withdrawn from the flywheel to prevent damage to the starter.

A starter consumes more current than any other electrical device used on a vehicle. In order to ensure an adequate supply of current to the starter, large cables are used to connect the starter to its power source. Voltage is supplied to the starter from the battery through either a remote relay or a starter mounted solenoid. The design of the starter power source depends on the type of drive used. Starters with a Bendix type of starter drive, will typically use a remote starter relay to actuate the starter. This is because the starter drive will extend automatically with the rotation of the starter. It uses centrifugal force to extend the drive and the drive is retracted by the spinning motion of the flywheel after the engine starts. Starters that use a mounted or integral solenoid use the solenoid to mechanically extend the starter drive and close the circuit that supplies current to the starter. The starter drive used on this system is an overrunning clutch type drive. This type of drive spins free in one direction and locks in the other. This action allows the starter drive to spin the flywheel, when engaged, and spin free when the engine starts. The starter drive is withdrawn when the ignition key is returned to the run position.

SolenoidThe solenoid that is mounted on the starter is an electro-magnetic device. When current is supplied to the starter solenoid, by the ignition switch, the solenoid core becomes magnetized causing a plunger to move inside the starter. The amperage require to operate the solenoid is more than is required to hold the plunger in. This is called a peak and hold circuit and is typical of most solenoid operation. The plunger extends the starter drive while closing contacts of an internal switch that supplies battery voltage to the starter. The remote mounted starter relay operates in a similar fashion, but its function is to supply battery voltage to the starter only.

Because a starter requires a large supply of current, typically about 150 to 200 amps, an adequate power supply is needed to ensure proper starter operation. Battery voltage should remain at or near 9.5 volts when cranking. The difference between voltage measured at the battery and voltage measured at the starter, when cranking, should not exceed .5 volts. This difference is called voltage drop, and is an indication of starter circuit resistance. High starter circuit resistance and/or low cranking voltage can cause shortened starter life.

(Kyle has an affinity for Pale Ale and tooling on his 1956 Chevrolet Nomad Station Wagon.)

2 responses to “Battery Starter Solenoid”

  1. kevin says:

    got 1997 olds 88 car not getting starter power. has power but wont start by key can jumper it will start wont stay running runs for 5 or 6 min shuts off hear solenoid on passenger side

    • Kassim says:

      Why were mechanical starting by rotating handles at the front cars now discarded. When key starter fails in the bush meaning one has to look for a battery.

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