Solenoid's & Coil's - Testing

The picture on the far left is a relay, we are using this relay to connect the starter to the battery when we put 12Vdc to the coil spade terminals. This is then called a starter relay, this is a common and generic relay. The two large terminal lugs protruding from the top side of this starter relay connect the battery cable not grounded (input) from the battery to the input terminal lug on the starter motor housing. This is just exactly like a large switch connecting two points together, the difference is that this relay has a coil of wire attached at each end to two flat spade terminals on the lower end of the relay. This coil of wire (one piece) has two ends, each end is connected to the 12 Vdc from the ignition switch. It takes the positive connected to the negative to complete a circuit and using a one wire coil with two ends each connected to one post of the battery. It normally does not matter which end you connect to the coil terminals, they will still work as they get dc (direct current). For this next picture we are using the wires on the equipment we are testing (John Deere D-110). As seen we left the starter relay attached and unplugged the female spade terminal ends from the male spade terminals on the lower part of the starter relay. Purple wire is + in this application and the black is -, in this test the meter probes are stuck into the female spade terminals. It doesn't matter on a voltage test which probes are stuck into either the - or + wires, if you have it backwards the meter just shows a - sign in front of the readings. This is due to the fact dc (direct current) has a specific direction it flows into.


As seen the meter reading in this test is 12.17 Vdc which is plenty of voltage to make the relay coil produce a magnetic field strong enough to pull the switch "on" (closed) to connect the large terminal lugs to the starter motor from the battery +. What this test needs is someone setting on the machine or the emergency brake on and turning the starter switch to start. With the meter plugged in to the terminal ends one person can turn the key and read the meter. The test shows that the ignition switch and wiring are good and getting supply voltage from the battery. If no voltage is present or it is low (10 Vdc) or lower the coil will not be able to develop sufficient magnetic gauss (pressure) to pull the switch against the battery terminal lugs that close the connection between the battery and the starter motor. This test turned out good in this picture.

Above are examples of a coil of wire, they are called coils. When electrical current flows through the wires it creates a magnetic field with gauss (pressure) and two poles in magnetic strength (gauss). The coil of copper wire on the far left would be a really strong magnet when voltage and current are supplied. You will see that a coil is one wire wound around and around what is called a core (hollow area in center). This wire appears copper but there is a thin coating of clear paint (insulation) just like any wire, it's coated with an insulator keeping it from shorting out when against another part of the wire. This is very thin and melts away very easily, once a coil has been overheated it smells real bad, like burning paint. So you can see that coils come in all shapes and sizes. These are called induction coils and induce a magnetic strength when energized (dc current flowing).


Here we can test our coil in the starter solenoid without electrical energy, we simply use the OHM scale and those are considered resistance to the flow of electrical current. Any coil can be tested with an OHM meter so above we see where to set our meter when no voltage is connected to coils and circuits. The far right picture above is the resistance or OHM scale, the meter reads 4.5 Ohm's. This will be detailed in the electronics portion of the electrical repair. As you can see the dial on the meter has been rotated counter clockwise an additional few clicks and positioned pointing directly down to the Ohm scale on this meter, 200 Ohm scale setting.

The picture on the far left shows the dial rotated 10 clicks counter clockwise from the OFF aligned to the arrow on the meter dial. The symbol for Ohms (Ω) is a measurement for electrical resistance and is represented by R in electrical formula. Resistance in the electrical circuit will be checked in the 200 to 2000 ranges. For regular 12 volt electrical charging and starting systems the 200 range is recommended. 


This DVOM we are using displays 1      .  when it is on and in the ohm setting, some meters will display other things like OL for open lead. An open lead is a wire that is not connected to the other lead going to something intended to operate from dc electricity . One is considered a supply and the other is called the load. Using the Ohm's setting you can test all kinds of sensors, sending unit's like fuel gauge, and the list is to long to keep listing them. It is the most useful setting to determine If, when, and why not anything electrical is working and you do the tests with no power on so it seems "safer". Safer for you and safer as to not burn out or damage other electrical components. The Ohm's setting is a must to further progress into all electrical appliances.


I am trying to example a connection in a circuit with the middle picture and the one on the far right. If a wire or connection in a circuit is a solid one there is no more resistance other than what is in the wire itself and that's a starting point. A solid copper wire 6" long will have very little resistance in Ohms for example .02 meter reading and when you cut the wire in half and hold it apart the meter will show infinite resistance and on this meter a 1 on the display in the far left picture. When you lightly touch the cut wires together the resistance then reads like the middle picture where the probes are simply laying together and blowing on them may cause them to fall apart. In the far right picture the probes are firmly held together but have a coating with a resistance value of .8 Ohm. The better the connection the lower the resistance, the failed connection started out as a loose one and as the resistance got higher it restricted more current and caused a voltage drop in the circuit.

All of the relays pictured above have a small coil of wire that produces a magnetic field which pulls together a flat plate and two terminal lug posts. The relay on the far right is found on most automotive and equipment starter motors. It is called a solenoid because it does one more different thing than the others. It mechanically has an attached metal piston that when exposed to the magnetic field of the coil inside when energized pulls the Bendix gear into the car flywheel starter ring.


So these relays simply connect the input wires of a large current draw to the load while using very small amounts of current to hold them together. This is done with the coil of wire attached to the two smaller terminals on a relay.


The far left picture has small threaded lugs instead of spades but works exactly the same way as the spade relay picture on the top of this page. Relay in position 2 from the left has one small threaded lug for one end of the coil wire and the mounting case has the other end attached. This is called an internal ground so you only need one wire to make it operate and mounted to a good ground on the vehicle engine.


The two wire relay has an external ground allowing you to mount it to a plastic piece like fender or something not grounded. Relay 3 & 4 are also used for starter motors and essentially can be substituted for the other relays except for the last one on the far right.


Since we are working with the simple dc starter motor with drive gear right now a cool thing that a dc motor does is generate dc electricity  as the shaft is rotated, and it also will change direction of rotation when the + & - from the battery are switched. So you can use a dc motor to start an engine and when the engine starts and rotates the motor shaft dc electricity will be a result. It is an easy way to make an electric start for a regular engine because you can use a belt to drive it which is very quiet compared to a gear or chain.

In the above picture it shows the two spade terminal ends on the relay we are testing and is the same equipment that has been pictured in the previous tests. We tested the wires to the solenoid in the voltage drop test at 12.17 Vdc so that section was fine and able to energize the coil windings inside the starter relay. Now our Ohm test will show if the coil inside the relay will work after the wires are plugged back into the spade terminals of the relay.


Even though all coils are different the important part to know here is the math behind electrical current. This meter is reading 4.5 Ohms resistance and the voltage is 12.75 Vdc. When you take your volts or E and divide the Ohms R into them you get I or amperage (current) flow potential. So in our scenario here we take 12.75/4.5=2.83 amps. That's about the current flow of a 1157 tail light bulb with both elements burning. Rule of thumb states that is a good solenoid coil. Now if the test was 1 ohm that would be 12 amps and you know for sure the coil is shorted out inside. You could look it up in a manual but if your wire attached to the coil is tiny then 3 amps is the limit before the wire melts down, the larger ones like 20 to 18 gauge will have a limit slightly larger. A coil drawing 3 amps @ 12.75 volts will consume 38.25 watts, I x V = Watts and the point being you can calculate the heat to anticipate by knowing the watts consumed. We will get into that later on an additional page.

The thing this test does is verifies that the system should work, this system is electromechanical meaning there are mechanical things that work or caused motion by the addition of dc electrical energy. When I have supply voltage from the ignition switch in the start position at the relay coil terminals and my ohm test shows good for the pull in coil then I know it should provide source voltage to the starter input terminal lug. If it doesn't I hold the key switch to the start position and tap on the relay. If everything is good electrically then the problem is the mechanical lever inside is stuck. 

In the above picture shows the two large battery cables, one comes from the positive (+) on the battery post and the other goes directly to the starter motor. The important thing to notice in this picture is the white looking FUZZ around the nuts and threaded terminal lugs that the two large red cables are attached to. This is a sure sign that the terminal nuts have been run loose. This creates additional resistance between the points the cables are attached to and the by product is heat. Heat causes a metal to corrode due to ionization, and as electrical current flows through a highly resistive environment it crates this white looking FUZZ, and is actually oxidation to the metals.


When looking over an electrical problem always look at all the connections first to see if the white oxidation is present. If it is first clean and re connect the terminals prior to attempting an electrical diagnosis.