Electrical 2 - Starter Motor & Solenoids

    Starter Motor & Bendix Gear .......Starter Motor & Solenoid ............Starter Motor & Solenoid 

The starter motor on the far left is common for lawn mowers, it is simply a dc electric motor and on the left end in the picture is a gold looking gear. This gear slides into the matching flywheel gear teeth on the engine crankshaft when the starter motor rotates. This starter motor is made to rotate by supplying dc voltage to the lug terminal shown on the far right of that starter motor. A lug is usually a stud with threads to screw on a nut and this lug is in the brush housing on this starter motor. On a DC electric motor it does not matter which way you hook up the positive and negative from the battery to make it rotate. Clamp one cable to the mounting bracket and one to the threaded lug and it will rotate one way or the other. Reverse the battery cable leads and the direction of the shaft rotating will also change direction. This is the first starter we will use in the following tests providing a load to our battery.


The other two starters are with a solenoid attached and we are covering them as well. These two are also able to rotate clockwise or counterclockwise and depending on the battery cables and what they are attached to.


All starters draw current that is directly proportional to the resistance applied to the Bendix gear that restricts the rotation of the starter motor. This is called a load on the electrical system when the starter motor is drawing current from the battery. The load from the starter motors above follows the path directly back to one of the large lug terminals on the starter solenoid and back to the battery post. The case housing on the starter motor goes back to ground through the engine block metal and into the negative cable attached to the battery. With this type of load test you will be able to find a bad cable, a broken starter solenoid or, a burnt out starter motor.

The picture above on the far left is a picture of our battery, the large red cable coming from the positive (+) post of the battery to one large terminal lug on the Starter Solenoid, another large red cable attached to the other large terminal lug on the starter solenoid and continuing to the large terminal lug on the bottom and going into the brush holder housing on the dc electric starter motor.


On starter solenoids there are normally two large terminal lugs, these are all attached to one main battery cable, do not have a fuse or breaker with the other terminal lug and cable attached to the main terminal lug on a starter motor. The cable is larger than any other cable due to the current (volume) the starter motor could use.

NOTE: All starter motor's have a rated maximum power they can develop and normally indicating watts capability. A one kilo watt starter has the capacity of pulling 1000 watts @ 12 Vdc. It requires 746 watts approximately to produce one horsepower so it's an easy equation to calculate out when we do our resistance tests with no applied source voltage.


In the center picture shows the DVOM wire probes pressed onto the large terminal lug of the starter solenoid where the cable is connected to the starter motor input lug. This will be the + or red probe on the meter, and the other meter lead (black) - is pressed on the starter case itself to provide a ground circuit. The meter will read 0 Vdc, if you attach the positive probe to the other starter solenoid lug you will read voltage from the battery so the one you need to be on at this time is the one with 0 voltage and going to the starter. Far right picture indicated a voltage drop from the original voltage measured at the battery and shown in the first set of pictures. This drop is measured and is really a good meter reading. Most battery systems that are bad will show a very low voltage and are shown below in the picture examples. If your system source voltage is 12.75 Vdc and you turn the ignition key to start you could possibly read as low as 8 Vdc on the voltage drop test and this indicates a battery load capacity as well as testing for a no start situation.

The two pictures above show a voltage drop test and what this test is supposed to do is identify the capacity capabilities of the battery as well as to indicate a condition of the cables. If your battery voltage was 12.5 Vdc and above and your voltage drop test shows as these two readings you can pretty sure tell that the battery is bad or the ground cable from the metal frame to the battery post negative is loose or corroded beyond repair, or a cable is rotted in the inside and that cable would feel very stiff to bend in comparison to a good new cable the exact same diameter and made of the same metal inside.


If you want to eliminate the battery as the problem then do the voltage drop test while placing the meter leads directly on the battery posts, not the terminal ends attached to the cables on the battery posts, directly on the top lead posts of the battery. If your battery voltage drops to a low level as seen you can be sure your battery is bad. That indicates the capacity not the pressure of a battery, they are directly related to one another in a load test.


If your battery Voltage doesn't drop you have a problem, either the starter solenoid is not getting it's proper voltage from the starter switch at the key, or your cable ends and cables going to and from the battery need a good cleaning. In fact most all electrical problems begin as lose or corroded terminals attached to one another and end up in a burnt situation. Customer modified wiring and terminal ends are the first places to look for an electrical problem. The next is connection points like terminals or fuse ends which become corroded here due to the metal they are made of. Clean them and inspect the connections first after these primary tests with the DVOM.