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Electrical Primer - Last updated: December 17, 2004 ©By: John T. Blair (WA4OHZ)  1133 Chatmoss Dr., Va. Beach, Va. 23464;
(757) 495-8229
It's very frustrating when your car won't start. But if it the engine won't turn over the problem is usually simple. Luckily the starting system isn't very complicated! The problem could be any of the following: 1. A low charged or dead battery, We'd like to be able to figure out where the problem is, without having to replace every part in the system. To trouble shoot this, you will need either a multimeter (either digital or analog) or a mechanics test light (which you can make your self for about 50 cents). I prefer the volt meter as you get a more accurate reading than the brightness of the light bulb.
The Basics When you turn on the ignition switch, current flows from the battery through the ignition switch and to the solenoid's coil. Very simply put, the coil makes a magnetic field which closes a mechanical switch inside the solenoid case. There are two basic types of starter solenoids: 1. A remotely mounted solenoid. These solenoids are not mounted on the starters. These solenoids are a small box with 2 very large studs and usually 2 small studs coming out and were usually mounted on the fender well. They can be found on most of the cars from the 1970s and before. 2. The "integrated or attached" solenoid. My way of describing solenoids that are attached or bolted to the actual starter motor. These can be found on pre-engagement starter or on most of the newer front wheel drive cars. Simple isn't it? But I did leave out a couple of items. Cars equipped with an automatic transmission have a "Park/Neutral" switch that is in series with the ignition switch. If the car is not in Park or Neutral the switch is open and no current will flow to the starter solenoid. The car will not start, nor will you hear the "click" from the starter solenoid. On 1974 model year cars, U.S. Federal Law required that the seatbelts be connected before the car was allowed to start. So there is effectively a 2nd switch in series with the ignition switch. If this one isn't closed, the car will not start. Finally, on later cars with manual transmissions (circa 1985) there was a neutral safety switch put on the clutch pedal. If the clutch pedal was not depressed, the car would not start. Now to the trouble shooting. Assuming that this is not your problem, you will need to have a helper sit in the car to turn on the ignition switch to "start" when you tell him to, as you will be under the hood with the volt meter. Make sure the meter is set to read at least 12V DC. (Many meters don't have a 12V scale. Therefore use the 1st setting above 12V. When trying to perform many of the tests below, you will want to connect the negative (black) lead from the meter to any metal part of the engine. This is called the ground. It might be difficult to hold this negative lead to a ground while you are trying to position the positive lead to take a reading. To make things easy, I use an alligator clip attached to the probe end of negative meter lead, and clip the free end to the ground. (Note: The alligator clips are readily available at your local Radio Shack store.) Testing the Battery Try using a set of jumper cables and attaching to another car's battery. If it starts, then your battery needs recharging or replacing. Did you leave the parking lights, headlights, or an accessory light like the trunk, hood or interior lights on. Or you could have a problem with your charging system and it is not charging the battery. In both of these cases, you only have to recharge the battery. However, if your battery is several years old, it could be dying and need replacing. For help here, try jump-starting the car and drive to the closest auto parts store. Most of them can test your battery and charging system and will sell you the necessary parts. Testing the Ignition switch If you didn't hear the click, we need to find out why. Start by testing the ignition switch. You will have to find the starter solenoid. The easiest way to do this is to find the starter and follow the large black wire back until it connects to something. This should be the solenoid. The starter solenoid will have several wires attached to it, 2 very large diameter wires, and 1 or 2 small wires. (Note: The larger the wire, the more current it can handle. Consequently, there will be two large wires - usually black - going to the solenoid. These are the high current wires that carry the current to the starter. The small wire connecting to the solenoid is the low current wire and is the control signal for the solenoid.) Take the positive lead and attach it to one of the 2 "SMALL" terminals on the starter solenoid. Have your helper try to start the engine. Did the volt meter go to 12V? If yes, then that's a good thing. Your ignition switch is working. If no, then try the other small terminal. (Depending on the type of solenoid you have there may be only 1 terminal with a small diameter wire going to it.) Did you now get 12V? If not, and you are working on a '74, ensure that the driver seat belt is connected and test again. Or if you are dealing with a later model car with a manual transmission, is the clutch depressed? You can check the clutch switch with an ohm. This switch is usually located on the clutch pedal. Disconnect the wires from the back of the switch and connect the probes from the ohm meter to each of the terminals on the switch. There should be infinite resistance. Now move the switch to the other position (some switches are located so that when you push the clutch pedal you press the switch closed. Others can be positioned so that when the clutch pedal is pressed the switch is allowed to move out and close.) The meter should go towards 0 resistance. If the switch did not change resistance, the it is probably bad. You can also try to jumper across the 2 wires on the connector that attached to the switch. I've used a paper clip to jumper these before. Try to start the engine again. If not, I'd look at replacing the ignition switch. Since this is a generic article, I should mention that there are a lot of different ignition switches. In the older cars (pre 1970), they were on the dash and usually a round switch. On most newer cars (post 1970) where you put the ignition key is not the ignition switch. They are usually mounted on the steering column down near the floor. When you turn the ignition switch there is a rod that goes down to the switch and actually controls the position of the ignition switch. Sometimes the ignition switch get loose and is allowed to move slightly. Thus when you try to start the car, it won’t. Try moving the ignition switch either up or down the steering column slightly, and re tightening the hold down bolts. Also worth mentioning at this point is that there is a difference in the ignition switches for steering columns that have tilt or non tilt steering wheels. Testing the Solenoid If yes, then the coil in the relay is OK. IF no, and you have checked: then chances are that the coil in the solenoid is burned out or you have a bad ground connection. You can test the solenoid’s coil. Ensure the ignition is off. Test the solenoid's input wire from the ignition switch. If it reads 0V you can proceed with the next test. Set the multimeter to the Ohms position. Connect the black lead to a "ground" - almost any metal part on the car. Touch the red lead to the connector that has the wire from the ignition switch. If it reads 0 ohms, then the coil is probably OK. If not, try moving the black lead, from the "ground", to the metal mounting tab on the solenoid. If the meter reads infinite resistance, then the coil is open, burned out. Replace the solenoid. If the meter reads 0 ohms then you have a bad ground or the coil is shorted. Clean the area around the holes on the mounting tab and the metal that the solenoid sit on with a piece of sandpaper until the metal it is shinny and clean. Try starting the engine again. If you don't here a click then the wire in the solenoid has shorted out and you will have to replace the solenoid. Testing the Solenoid
If no, you either have a weak battery or a bad connection on one of the battery cables. Pull both the positive and negative battery terminals, and clean both the posts on the battery and the connectors on the cable. (Note: you can purchase a nice little wire brush for cleaning the upright battery posts at most auto parts store for under $5. Also, is there some white powder all over the battery posts? Of so, mix some baking soda and water and pour the mixture over the battery posts. This will neutralize the powdered acid on the posts.) Repeat the test. If the voltage drops you probably have a weak battery. - Try jumping the car from another car. If the voltage is above about 9.5 V, then the battery is OK and so are the connections at the battery are fine. Move the positive meter lead to the other large nut on the solenoid and have your helper try to start the car. Do you get a above 9.5V? If yes, the contacts in the solenoid are fine. If no, did the meter show any voltage? If yes, there is a bad connection at one of the solenoid contacts. (Disconnect the negative lead of the battery before working on the solenoid.) Remove both of the large wires from the solenoid. Clean the area around the holes in the cable with a piece of sandpaper until it is shinny and clean. Remove the low current wires from the solenoid and remove the solenoid from the car. Clean the flat metal area at the base of the large studs, the washers, that go on either side of the cable, and the end of the battery cable with sandpaper until they are nice and shinny. Reinstall the solenoid and the wiring. If no voltage was observed, then the contacts of the solenoid
are probably burned and will have to be replaced. On some
solenoids, you can get replacement contacts, some solenoids you
simply have to replace the solenoid. For an explanation of the burned contacts look at the diagram. Notice how the input contact has burned away. When the plunger pulls down, the right side hits the full height output contact and stops moving downward. However, it can't contact the input contact because it has burned away. Testing the Starter Motor Move the positive lead to the large nut on the starter. This is probably easiest accessed from under the car. (Remember to use jack stands.) When your helper tries to start the car, do you get a strong voltage reading (i.e.. greater than 9.5V). If yes then the output of the solenoid is OK, you have a bad starter. Most likely it needs new brushes. If no, then you either have a bad connection at the output of the solenoid, or you have a bad cable from the solenoid to the starter. End of testing If you attempt to work on the solenoid or starter, DISCONNECT the NEGATIVE SIDE OF THE BATTERY BEFORE YOU DO ANYTHING!!
John
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Connect the black (negative) meter lead to a ground
(any metal part of the chassis). Connect the red (positive) lead
to the large nut on the back (pointing to the rear of the car).
There should be 12V there all the time. Have you helper try to
start the engine. Did the voltage stay above 9.5 volts? 