# Sticky  Troubleshooting common problems



## octaneman

Introduction to Digital Multimeter

This article should be used as a guide to help apprentice mechanics get acquainted with the digital multimeter and its application in automotive troubleshooting. The real learning of the DMM though will be accomplished through hands-on practice and experience. As you become more proficient in using the DMM to troubleshoot, you will learn very quickly how certain electrical problems can relate to many drivability problems. There are many styles and types of meters that are available in the open market, but no matter which one you use, all of them incorporate the same fundamental principal of having a positive and negative probe and being able to record Voltage, Resistance, Frequency, Diode, and Current. Some meters have special features built into them to facilitate a more in depth analysis of circuits, such as Temperature, RPM, Bar graph, Milli Amp/Volt scale and so on. 

Automotive electrical problems can be divided into several categories. Depending on the system causing the fault, the real problem can exist in one system, while the symptoms you are testing appear in another. Symptoms such as current drains, shorts, ground problems are the classic signs that the engine components will exhibit when they begin to fail. Knowing how to test the systems with the DMM will help the mechanic repair faults faster and cheaper rather than just tossing parts at the problem. 

Automotive Electronics

Battery – Batteries are often blamed for a no start condition when in fact the real culprit is the charging system. When there is a charging system problem, the battery will become discharged and will not be able to supply enough current for the starter to start the engine. To start a diagnosis of the battery, its electrolyte has to be tested with a hydrometer and it also has to be fully charged.

Testing - Since all batteries are maintenance free, disconnect the battery and perform the no-load test by measuring the voltages across the battery terminals. Turn on the meter and set the DMM on DC volts, a fully charged battery should have a reading between 12.65 volts or higher. This test only shows the charge state of the battery not its condition. A hydrometer reading on non-maintenance free batteries should show the sulfuric acid density of 1.299 or higher. To test the performance of the battery, a load test must be done to verify its condition. 

Charging system - To diagnose alternators it has to be determined if the alternator has an internal or external regulator. If the alternator has an external regulator the regulator has to be by-passed. By doing this you are full fielding the alternator. Be sure that you know which type of alternator you are testing and that the voltage does not exceed 15 volts or damage to the alternator and regulator will result. 

Testing – To properly test the alternator the battery must be fully charged and it has passed all previous tests. Turn on the meter and set it to DC volts, start the engine and connect the meter to the terminals of the battery with respect to its polarity. With engine running at idle the voltage output from the alternator should be between 14.2 to 14.5 volts.
Testing the alternators current is different from testing its voltage. It will require the DMM to have a minimum 10 amp current jack. Testing for current requires that the meter is set up in series not in parallel. If a current clamp is not available, set your meter to Amps DC scale in the 10amp jack. Connect the meter in series by disconnecting the positive alternator cable from the battery. Connect the positive lead of the meter to the cable end that is disconnected and the negative lead to the battery positive. A typical reading should be between 2 to 7 Amps with engine running.

Note: The vehicles accessories such as lights must be turned on to allow maximum current draw. Follow manufactures recommendation for testing procedures and specifications. 

Warning! ; Most DMM’s do not have the capacity for testing high current. Connecting the meter in parallel while testing in the Amps setting will destroy the meter beyond repair!


Diodes – Diodes are one way valves which means they allow current to flow in one direction only. Its symbol on the meter is an arrow with a line in front. Diodes are used in many different configurations but they are simple devices which can be tested very easily. An alternator for example has a diode pack which converts AC into DC. 

Testing – The best method to perform an alternator diode test is to remove the diode pack from the alternator. Turn the meter on and set it on diode. Touch one side of the test probe to one side of the diode and touch the other probe to the other side. The readings should be 0.8 volts. Once you get a reading reverse your leads the meter should show infinity in the other direction. Only one side of the diode should be showing readings. If readings are found on both sides when the leads are reversed then the diode is shorted. If no readings are found on either side when reversing the leads then the diode is open. 

Starter System – Allot of times starting system problems can be confused with a charging system problem. If the starter turns slowly it means that the starter is drawing more current than it actually needs. When this happens a full inspection of the starter is needed to determine if the starter circuit has high resistance, or that the starter solenoid is cracked or worn out. Because of the starter’s capacity to draw high current, an ammeter is better suited for the task. The DMM is best suited to do a voltage drop test on the starter.

Testing the voltage drop - Disconnect the fuel pump and run the engine until it dies. To test the starter’s voltage drop by using the DMM, set your meter on DC volts, connect the negative lead of the meter to the solenoid terminal on the starter. Connect the positive lead of the meter to the positive side of the battery. Crank the engine and record the readings. 

Ignition systems - There are two types of position sensors, Hall Effect and magnetic. The magnetic type has a permanent magnet with a wire coiled around it with two connections. Hall Effect sensors usually have three connections; power, signal output, and ground. 

Testing Magnetic Pick-Up – Open the distributor cap and disconnect the magnetic pick-up from the ignition module. Turn on the DMM and set it to DC volts. Connect the leads of the DMM across the magnetic pick up. When you crank the engine a good pick-up will produce a pulse that the meter will read. If no pulses are produced then the pick-up is bad. 

Testing Hall Effect Sensors - Hall Effect sensors need to be connected to power in order to check the reference voltage coming from the battery at the connector of the sensor. To test the sensor outside the car, connect power and ground to the proper terminals. Turn on the meter and set it to DC volts, connect the positive lead to signal output and the negative lead to ground. Insert a metal blade between the sensor and magnet. When sliding the metal blade across the sensor the voltage will vary between 12 to 0 volts. This indicates that the sensor is good.

Testing Crank Sensor – Step 1 Disconnect the crank sensor plug and connect the positive and negative leads on the sensor pins. Turn on your meter and set it on ohms scale, the sensor should read between 200 to 500 ohms. Step 2 - Re-bolt the sensor back on and with the leads attached set your meter to AC volts. Crank the engine and the readings should be between 0.2 to 2.0 volts AC volts. If either reading fail then the sensor is bad.

Testing Cam Sensor – Cam sensor has three pins; it has a 12 volt pin, a 5 volt reference to the computer, and ground. Step 1 Turn on the meter and set your DMM to DC volts. Connect your positive lead to battery positive and the negative lead to negative pin on the sensor. The meter should read 12 volts. Step 2 Reverse the leads so that the negative is ground and the positive to 12 volt pin on the sensor. The meter should read 12 volts. Step 3 Connect your positive lead to 5 volt reference pin and negative lead to ground pin. Crank the engine and there should be a change in DC voltage. If there is no voltage change when cranking the sensor is bad.

Ignition Module - There are many types of ignition modules that vary from manufacturer to manufacturer. Whether the ignition module is from domestic or import models, all of them perform a single function and operate in the same way. The ignition module acts as a switch which triggers the ignition coil to fire when cranking the engine. The pickup coil or cam sensor tells the module when to switch. To list them all will be an exercise in futility, for the sake of practicality the test will be on a typical GM module with a DIS system.

Testing Ignition Module (DIS) - Step1 Disconnect the ignition wires from the coils and remove the hold down bolts. Then extract the coils from its position on the module. Turn the meter on and set it to DC volts, place one lead of the meter to one of the spade pins and the other lead to the other pin. With ignition off the meter should read battery voltage. Step2 With leads still connected crank the engine and the voltage should fluctuate, this indicates that the module is switching.

Note: Due to the quick switching action of the module, a test light should be used instead of a DMM to verify operation. The test light ground should be connected to one pin and the probe on the other pin of the module. The test light should be flickering on/off when the engine is cranked. 

Testing Ignition Coil (Conventional) - Disconnect the spark wire and terminal wires from the coil. Step1 To test the primary side turn on the DMM and set to ohms scale, connect the positive lead to the positive terminal and the negative lead to the negative terminal. The readings on the meter should be a few tenths of an ohm. Step2 To test the secondary connect the positive lead to the tower and the negative lead to the negative terminal, a typical reading should be 10K ohms.

Isolating Current Drains, Shorts and Ground Problems.

One of the most frustrating problems mechanics encounter is when a new battery is installed and then the customer complains that it’s dead within a few hours. As the mechanic in charge you already suspect that there is a short circuit somewhere. Fuses check out ok and then you ask yourself the million dollar question; where is it?! How do you find it?! There is a way out of the mess you are in and here is how to find it.

Parasitic Load - All vehicles leak battery power and have a certain amount of parasitic load that is considered normal. Anything that exceeds the recommended limit should be stopped as quickly as possible. The biggest culprits of killing the battery are the alternator and starter and should be treated as hostile when large short circuits are suspect. 

Testing for Parasitic Load - Disconnect the positive of the battery and set your DMM to DC Amps. With ignition off connect the positive lead to the positive battery cable and the negative lead to the positive of the battery. The meter will display the current drain. Remove fuses one at a time to isolate the short circuit. 

Testing Alternator Parasitic Leakage – Disconnect the alternator‘s positive cable. Turn on the DMM and set it to the Milliamp scale, make sure that the positive lead is in the milliamp jack and the negative lead to common on the meter. Connect the negative lead to the positive of the alternator cable and the positive lead to the positive terminal on the alternator. The meter should have a reading of a few milliamps. 


Warning! When measuring current directly with your DMM, in order to avoid blowing the meters fuse, use the 10 amp input and never crank the engine or operate any accessories that draw more than 10 amps permanent damage to the meter will occur. To check the entire system for current drain with the key off, connect the DMM in series with battery and make sure you set up the meter to read 10 amps. This test will show the total current drain in the system when the key is off. This is considered normal parasitic load. 


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This article assumes that the technician has knowledge in the fundamentals of mechanics, electricity, and how they are complimentary to one another, in troubleshooting engine and a host of other electrical problems. The information provided serves only as a stepping stone into diagnosing common drivability problems and it is no substitute for shop manuals.


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