Getting Started

By Glen Beanard technical contributor

How It All Started-Trivia

It s no secret that the first automobiles were started by hand cranking. It s also pretty obvious that, somewhere along the way, someone replaced the hand crank for an electric motor. But who? And why? In the days of hand cranking engines, broken arms, broken ribs, and other injuries were a very common result of starting an engine. In 1911 a friend of Henry Leland, founder of Cadillac (and Lincoln, later in 1920), died from injuries sustained while restarting a car for a lady. The need for a starting method that did not risk personal injury became obvious to Mr. Leland. He commissioned Charles Kettering head of Dayton Electrical Company (Delco today) to come up with a better method. By 1912, electric starters were standard on all Cadillacs.

The Bendix on the starter has a bit of it s own history as well. It was named after Vincent Bendix who designed and patented it in 1910. The first car to ever use a starter that contained the Bendix drive was the Chevrolet Baby Grand in 1914.

Between Kettering s electric starter in 1912 and Vincent Bendix s starter drive, we now have the standard starter used in automobiles around the world today. Over the years, size, shape, voltage ratings, and control methods have changed. Overall, the starter operation and design has remained the same for nearly a century.

Electrical Recap

In some previous articles, we discussed how the flow of electricity is much like the flow of traffic on the streets. The number of cars at once represented amperage, the speed of the cars as voltage, the potholes in the road s surface as resistance, one way traffic signs as diodes, and the number of lanes in the roadway represented the gauge of the wire. Finally, AC current flow was illustrated as a car that was stuck in the mud and being rocked out of it.

Keeping all of those illustrations in mind, we are now going to add one more for this article. The newest term to learn is Counter Electromotive Force (CEMF). CEMF is an electrical force that is generated by electric motors. That electrical force is induced into the armature of the electric motor and opposes the voltage from the source that is powering the motor. This may be most symbolized by a wind resistance applied against a moving vehicle.

Understanding CEMF

The above illustration is actually about what happens inside the stator of an alternator. However, this is also what happens inside of a starter (or any electric motor) as well, only the names of the parts really change. Rather than a rotor and stator in an alternator, the electric motor contains an armature and field magnets . The same magnetic fields that change mechanical energy into electrical energy inside of an alternator or generator, is the same magnetic fields that convert electrical energy to mechanical energy in an electric motor. Therefore, as the starter (electric motor) rotates, the magnetic fields collide with each other and produce an electrical current of it s own that it applies back against it s source. That electrical current that opposes it s source is CEMF.

Ever notice that electric motors tend to free spin at a given top speed without going faster? One electric motor, like a starter for example, may spin relatively slow with a tremendous amount of torque when 12 volts are supplied. While in contrast, another electric motor such as a cooling fan motor, may spin relatively fast with less torque on the same 12volt power supply. With no external throttle control , both motors limit themselves to a given RPM range. How do they do that? The top speed and torque characteristics of each electric motor is built into it, not only by the number and type of windings in the armature, but also the amount of CEMF that the designing engineer calculates will be present at different given RPMs.

I have provided an illustration with the mathematical formulas to show the relationships between RPMs, CEMF, and the their effects on the supply voltage and amperage. Are you going to use this formula? No, not really. That s what engineers are for, let them figure out stuff like that right? As technicians, we probably won t find ourselves designing electric motors, but we do need to be aware of the basic principles involved for diagnostic reasons.

Simply put, as the motor speeds up, the amount of CEMF increases. As the CEMF increases, the amperage from the source decreases. That is because the CEMF is acting against the original source voltage (the battery in the this case). Once the RPMs get high enough, the CEMF becomes equal to the source voltage and the motor actually turns off . The motor coasts down to where the CEMF is less than the source and the motor turns on again, and so on.

The opposite is also true. When the motor is stopped, it is not generating any CEMF. Therefore, the battery is free to push amperage amounts that are limited to only by the resistance in the armature and battery cables. That is why start-up amperage of any electric motor is higher than the running amperage. That is also why electric motors draw more amperage when the RPMs are dragged down.

Construction and Operation

A lot has to happen before the start signal from the ignition switch can reach the starter assembly. The functional diagrams here show some of the most typical forms of starter control. The diagram to the left is typical to older vehicles. The ignition switch simply sent a voltage signal through the safety switch to the starter solenoid. Some of these systems used a starter relay, and some did not. The system to the right represents most modern starting systems. In the modern starting systems, the PCM is now key to the operation. The starter relay will not be energized without a signal from the PCM.

Once the start signal reaches the starter assembly, more must happen before engine begins to crank over. The start signal voltage enters the smaller diameter lug on the starter solenoid. The starter solenoid windings are energized and pulls the plunger inside the solenoid rearward. The front of the plunger is connected to a fork that pivots and engages the starter drive pinion into the engine s flywheel. The teeth of the pinion gear are tapered in order to ease engagement into the flywheel. At the farthest length of the plunger s travel, the rear of the plunger pushes a contact plate against the 2 heavy gauge lugs in the rear of the solenoid. That then completes an electrical path between the heavy gauge battery cable and down into the starter motor brushes. The brushes then transfer the electricity into the contacts on the armature, called the commutator . The armature is then magnetically charged. The magnetic fields in the armature push against the magnetic fields of the permanent magnets. After all of that, the starter begins to rotate, and in turn, rotates the engine. The starter drive (or Bendix) contains an overrunning clutch (one-way clutch) that protects the starter when the engine fires up.

As small of a package as what a starter is, it is a little power house. When measuring an electric motor, horsepower is rarely used. Usually, they are rated in watts . The typical starter for a gasoline 4 cylinder engine has a wattage rating between 1,000 watts to 1,200 watts. How much power is that exactly? One horsepower is equal to 745.699 watts. That means that a 1,200 watt 4 cylinder starter is approx 1.6hp. Not impressed yet? Ok, think about this. One horse power is the ability to lift 33,000 pounds of weight one foot in the air in one minute. That means that a 4cylinder starter motor, around 1.6hp, has enough force to lift about 52,800lbs(or 26.4 tons) of weight one foot within one minute. That is not even taking gear reductions into account. That s a bit more impressive when looked at the way; Isn t it? When it comes to the larger starters for larger diesel engines, horsepower readings can reach 13hp! When bench testing a starter, care must be taken not to let anything get caught in the Bendix area. Damage to the starter and/or personal injury could result. If you are unfortunate enough to have a finger caught in the Bendix, the starter probably won t even slow down.

The sectional diagram of starter supplied here is not intended to replicate any particular brand of starter for any particular make of vehicle. It is simply a generic example that I drew for this article. Actual starter operation and design may vary slightly from this picture for various makes.

Diagnostics Tips

As with anything else, you first need to know about the system you are working on. Is it like the older system to the left in the function diagram? Or is it like the more modern system where the PCM can lock out the starter? Just because it has a factory security system, doesn t mean the system has the ability to deny the starter. Be sure to familiarize yourself with the make and model by reading the description and operation listed in your information system. If you are good with reading wiring diagrams, a glace over the diagram can reveal it s secrets of operation.

For this discussion, relays and solenoids will be defined as such.

1. Relay: A device that uses an internal electromagnet to cause an internal switch to overcome spring tension and either complete or break an electrical circuit. A relay is only capable of transferring electrical energy. An example of a relay would be a cooling fan relay.

2. Solenoid: A device that uses an internal electromagnet to cause an internal plunger devise to overcome return spring tension in order to move and transfer mechanical energy. A solenoid may also be used to complete an electrical path in conjunction with a mechanical output, such as in the case of a starter solenoid on top of a starter. The high amperage fender mount relay used by Ford motor company is NOT a solenoid, although it is often mistakenly referred to as such, even by manufacturers. It only transfers electrical energy and constitutes as a relay.