Automotive Electrical System Testing
This tutorial is going to be on basic automotive electrical system AVR tests. AVR stands for Amps, Voltage and Resistance. When performing an electrical system diagnosis it will be broken down into three main systems. Your starting system, charging system and capacacity “battery”.
So, were going to start out with the most basic, the battery. The most important and common way of testing a battery is called the battery load test. There are two different types of battery load testers. The older style tester is called a carbon pile load tester and the second type of tester is an impedance style battery tester. Only test lead cell batteries with a carbon pile load tester, there are some newer glass mat cell batteries that can fail if using an carbon pile load on the battery. For a proper load test you need to locate the CCA (cold cranking amps) rating on the side or top labels of the battery. Load test the battery at 1/2 of the CCA rating for 15 seconds while monitoring the battery voltage. If the voltage drops below 9.6 volt the battery failed the load test. Always make sure your working with a fully charged battery. The second type of testing, the impedance tester I’m not so fond of, call me old school I don’t care I just have a hard time trusting these testers but from research they are actually very accurate. This tester is hand held, the concept is the same. The difference is the tester does the load test at a calculated resistance load not actually exercising the battery. The impedance tester will check glass mat cell battery also unlike the carbon pile load. As a technician I prefer to see the actual load test live. If the battery needs recharged pay close attention to the amperage intake during charging. With experience you can hook up a battery charger and know soon if the battery will need replaced or not by just monitoring the amperage intake.
The starting system is usually pretty basic. We’re only going over testing the starter as there are way too many variable when testing a starter control circuit “refer to the factory service manual for the starting system wiring diagram”. It will be up to you to verify if the problem exists in the actual starter itself or the actual starter control circuit. When testing the starter you want to test the starter current draw. This test alone will verify starter failure or not, therefore I will not go into any other irrelevant starter testing. You need to hook up an amp probe to the main power supply and also monitor battery voltage while cranking. Below is a basic starter amperage draw standard to go by.
- 4 cylinder engines 100-120 amps
- 6 cylinder engines 150-175 amps
- 8 cylinder engines up to 250 amps ( keep in mind if working on a diesel engine the current draw may even be higher.
The most common problem with starters is in the solenoid or the contact points(gear reduction starter only). In this case the starter will draw low to no current, make a click noise and do nothing, sound familiar? Verify you have a good battery and cable connection and your diagnosis is complete. Battery cable and continuity testing will be covered under the volt drop test. High current and low voltage? Re-test and/or recharge your battery.
The charging system is you vehicle’s main power supply while running. The alternator is to supply amperage to the electrical system and also keep the battery charged. When testing the charging system there are three main checks that you will need to verify. The first check will be you system voltage. Start vehicle and check voltage, should be anywhere from 13.5 volts to 14.7 volts. If you have a low charging voltage raise the engine RPM’s and retest. Next you will need to check alternator output. Connect amp probe to alternator output wire and voltmeter to the battery. Check and record your voltage and amperage at idle and then at 2,500 RPM’s (NO LOAD). Now you need to check alternator output under load. Turn on head lights and blower motor on high, if you want you can turn on other components or better yet use the carbon pile load tester at the battery (this way you can full field the battery). Full field means checking the peak amperage and voltage output, some ford alternators have a small pin in the back that you can ground and it will full field the alternator. However you do your tests you need to check the alternator output for idle no load and a 2,500 RPM load test. Typical amperage output at idle/no load is around 20 amps. For the full field test you need to check the alternator output rating. There usually anywhere from 80 to 120 amp alternator’s. The full field amperage should be close to the alternator output rating but does not need to be exact. If your alternator has no output that does not mean that the alternator has failed. The alternator needs a system voltage reference, this way the voltage regulator knows the amperage rate required by the alternator. If you have no voltage to this wire the alternator will not work. I have even seen low voltage caused by a week battery cause the alternator to not work so make sure you have system voltage at or above 12 volts. There are typically two or more wires at this connector and it plugs directly into the alternator. Every alternator is different so you will need to refer to the charging system wire diagram for that vehicle. After you become comfortable with your testing you can simply check charging voltage and back probe the alternator reference voltage wire. These two basic tests will take less than 5 minutes and give you an accurate conclusion. All of the other tests you need to know when checking a charging system performance problem.
Another problem the alternator can have is excessive AC voltage output. The alternator generate an alternating current (AC) then converts it to an direct current (DC). The alternator diodes are what causes this AC to DC electrical conversion. When the diodes start to fail the alternator will start bleed AC voltage into the electrical system. The big problem with this is that a lot of your vehicles electrical sensors generate a low signal AC voltage. The low signal is AKA a sine wave. The sine wave is an electrical signal you see on a DSO ( digital storage oscilloscope). Bottom line, it causes problem and you need to know how to check for it. If you have a charging system analyzer it usually has this test built into it. What you’re looking for is excessive ripple in the DC voltage output. If the DC voltage is creating even a weak sine wave fluctuation then your diodes are starting to fail. Don’t worry it’s pretty uncommon to have diode allure problems. the alternator’s usually fail before they have diode failure problems.
TIP: if working on an older style charging system make sure that you do not have a external voltage regulator, if so some of the tests need to be performed at the voltage regulator. The only model I can recall that has an external voltage regulator is FORD. Imagine that! Most everything built today have internal voltage regulators.
A parasitic draw is a load on the electrical system while all electrical components are off. All vehicle have a draw you just need to know if it’s within an acceptable range or not. The acceptable range will depend on the type of vehicle your working on. The normal draw is caused from the control module and alarms. So, just know that the more electrical accessories the vehicle has the higher the acceptable draw will be. I would have to say that 20 to 40 milliamps is ok. Until your very comfortable with testing and diagnosing a parasitic draw refer to the factory service manual for an acceptable draw rate. When testing a draw there are two accurate ways to test for it. This one will be using an ammeter in line with the positive or negative main circuits. The other test requires a low amp probe to be connected to positive or negative battery cables. Keep in mind your low amp probe will not fit all battery cables. When testing in-line you need to make sure you don’t blow the ammeter fuse and end up with a false reading. To prevent this you hook up a jumper wire then connect the ammeter and disconnect the jumper wire. If you suspect a high draw start with the 20 amp scale and if you want a more accurate low draw reading reconnect jumper wire and change the scale to milliamps, remove jumper wire. The low amp probe is pretty self explanatory and I will not go into specifics as it is beyond the scope of this article. If a draw is present locate the main power distribution and start eliminating fuses one at a time while monitoring the meter. Now you have located the circuit that has the draw but you still need to eliminate the draw. Locate that fuse on a power distribution wire diagram and eliminate all components one at a time while monitoring the meter until you find the source of the draw or circuit that has a short. Keep in mind if there is no specific fuse that’s causing the draw make sure your not losing it through the alternator. Sometime the alternator diodes will fail causing a parasitic draw. Clean the battery surface to prevent surface discharge and your done.
The volt drop test is going to make sure you have a good connection between your main battery cables and the ground/components they feed. This test can be performed on smaller circuits also. Use this test anywhere you believe to have a connection problem (high resistance). This test is very simple and straight forward. Whatever circuit your testing hook voltmeter up around that circuit and test while under a load. For example, you want to test the ground cable. Hook up you positive meter lead to negative battery post and your negative to a good engine ground. Crank the engine and measure the voltage. The industry standard is 0.3 volts. The concept is simple. Electricity follows the least resistance path to ground. If you have a bad battery terminal connection or high resistance in the battery cable the voltage will attempt to go around through your meter instead of the cable. Typically if you have a resistance problem with the circuit that your testing the voltmeter will read way above 0.3. volts. Sometime even at or around battery voltage so don’t trick yourself on this simple test. If you completely understand the whole concept of the test it can become very useful in many ways.