DVOM Not-So Basics

Digital High Impedance VOM

Meter Selection Features V Voltage DC DC is Direct Current Reads the circuit pressure Best meter for troubleshooting Voltage AC AC is Alternating Current Use for Magnetic Pickup Outputs V ~

Meter Selection Features Diode Check Applies voltage to turn on Diode in Forward Bias .3 to .5v in one direction and OL in the other means the diode is OK Resistance / Ohms Ω Last meter to use OL is open or over the range of the meter 0.1 – 0.3Ω circuit is shorted or completed

Meter Selection Features Frequency Hz Measures the Hertz or pulses, cycles per sec. Percent Duty Cycle % Measures the percent “on” time for Pulse Width Modulated signals Continuity Buzzer “Beep” to indicate continuity (maybe) Be careful – some meters will sound at 100Ω

Meter Selection Features Amperes (Amps) A Measures the current through the circuit CAUTION - The probes complete the circuit MilliAmperes mA Same as Amps only lesser current Auto Range Auto is easier – Manual is harder

Digital High Impedance VOM

Probe Sockets COM Black cord goes here VΩ Red cord here for Voltage and Resistance A Red cord here for Amps – 10A - 20A Fuse Protected mA Red cord here for Lower Amps Protected with a smaller fuse

Probe Safety

Since there is no current flow, there is no voltage drop Voltage Check 1 12.5v M Open V - + Since there is no current flow, there is no voltage drop 12.5

Electricity Compared to Hydraulics 12.5 V - + Voltage = Pressure Current = Flow (gpm) Resistance = Restriction .15 A - + 240 Ω - + * Electricity behaves very much like fluid *

Electricity Compared to Hydraulics Battery = Reservoir + Pump Voltmeter = Pressure Gauge Ammeter = Flow meter Load/Resistance = Load/Restriction * Electricity behaves very much like fluid *

Since there is no flow, there is no pressure drop Pressure Check 1 1000psi 1000psi Pump w/1000psi internal relief valve Control valve Motor P M blocked 1000psi Reservoir Since there is no flow, there is no pressure drop

Voltage Check 2 Completed Circuit 12.5v Voltage Drop is normal on Non-Load components as long as the maximum drop is ≤ 0.5v on 12v systems M 12.45 12.5 12.47 1200 RPM 0.00 V - +

Minor pressure drops across Non-Load components are normal Pressure Check 2 1000psi 998psi Pump w/1000psi internal relief valve Control valve Motor P M 1200 RPM 0psi Reservoir Minor pressure drops across Non-Load components are normal

Voltage Check 3 Green Connection 12.5v M 12.47 9.45 12.5 300 RPM V - + Voltage Drop that is Significantly less than full system voltage indicates High Resistance

Major pressure drops across Non-Load components indicate restriction Pressure Check 3 1000psi 275psi Pump w/1000psi internal relief valve Control valve Motor P M 300 RPM Reservoir Major pressure drops across Non-Load components indicate restriction

Pressure Check - Example 20psi P 18psi 0psi Pressure drop across filter indicates the level of Restriction Tank

Other Concepts of Operation Ghost voltage & zero voltage indicate opens and shorts and can be used in place of an ohmmeter A digital VOM will indicate full system voltage, even through high resistance Continuity buzzer will buzz or beep even if there is up to 100Ω in circuit – must check ohms to verify Circuits must be isolated to check ohms

Your body even generates an electrical field!! Ghost Voltage The meter is so sensitive that it is picking up voltage from the atmosphere Your body even generates an electrical field!! Ghost V - +

Ghost Voltage This is eliminated by touching the probes together giving you Zero volts. 0.00 V - +

If the meter reads “True Zero” you have Continuity Voltage Check 4 12.5v + - V - + V - + 0.00 0.00 If the meter reads “True Zero” you have Continuity

Ghost Voltage indicates that you have an Open Circuit Voltage Check 5 Open Connector 12.5v + - Blown Fuse V - + V - + Ghost Ghost Ghost Voltage indicates that you have an Open Circuit

Could you make these checks Voltage Checks Review Could you make these checks with an Ohmmeter?? V - + 12.55 12.55 V V - + Ghost V - + 0.00

Voltmeters 12.5v V - + Voltmeters are the safest of all meters. They don’t complete the circuit. The meter has an extremely high internal resistance or impedance and therefore draws extremely low amperage. Low Amps – Low Danger.

Other Concepts of Operation Ghost voltage & zero voltage indicate opens and shorts and can be used in place of an ohmmeter A digital VOM will indicate full system voltage, even through high resistance Continuity buzzer will buzz or beep even if there is up to 100Ω in circuit – must check ohms to verify Circuits must be isolated to check ohms

There is No Voltage Drop High Resistance Without Current Flow There is No Voltage Drop V - + 12.55 12.55 V V - + 12.55 Corroded Terminal

Voltage Check 3 (Review) Green Connection 12.5v M High Resistance causes a Significant Voltage Drop but only if there is Current Flow Problem: How do I insert the meter’s Probe into the circuit? V - + 9.45

Why NOT to Puncture the Wire Puncture the Insulation Remove the Probe, a Hole is Left As the Wire Heats, the Hole allows the Insulation to Expand As the Wire & Insulation Cools and Heats, it Draws in Moisture thought the Opening Corrosion Develops Moisture

High Resistance Checking Tool TESlite Voltmeter Leads TESlite ® Enhanced Voltmeter Leads - www.teslite.com

High Resistance Checking Tool Without Current Flow there is No Voltage Drop V - + 12.55 3.75 PUSH the Button 12.55 Corroded Terminal TESlite ® Leads 12.55 V Now a Circuit is Completed, there is Load, Current flows and Voltage Drop can be read

First Corrosion Check Corrosion 12.55v Remove load Test across Load Connection Low Voltage – Bad Circuit M M 4.75v 4.75v V - + Retest w/ Good Ground Low Voltage – Power Circuit is Bad If Good Voltage – Ground is Bad

Power Side Corrosion Check 12.55v V - + M 12.52 12.53v 12.55v 3.78v TESlite ® Leads

Ground Side Corrosion Check 12.55v V - + 12.53v M 4.75v Corrosion

Other Concepts of Operation Ghost voltage & zero voltage indicate opens and shorts and can be used in place of an ohmmeter A digital VOM will indicate full system voltage, even through high resistance Continuity buzzer will buzz or beep even if there is up to 100Ω in circuit – must check ohms to verify Circuits must be isolated to check ohms

Continuity “Beeper” To safely check for Continuity, use the Ohmmeter function or Voltmeter, don’t trust the Beeper 100Ω BEEP Ω - + 100.0Ω

Other Concepts of Operation Ghost voltage & zero voltage indicate opens and shorts and can be used in place of an ohmmeter A digital VOM will indicate full system voltage, even through high resistance Continuity buzzer will buzz or beep even if there is up to 100Ω in circuit – must check ohms to verify Circuits must be isolated to check ohms

Ohmmeter Limitations HOW?? For an Ohmmeter to work accurately you MUST isolate the circuit – EVERYWHERE “OL” or ∞ indicates that the meter is not able to read the resistance in the circuit. Go to a higher scale. Ohmmeters use about 3 volts to operate – over 50 feet of wire this is not much to work with Minimum Resistance is usually about 0.3 – 0.5 Ω which can approach actual values on some components HOW??

However, it is not the best troubleshooting tool for circuits. Ohmmeters Ω - + Ohmmeters are used for checking separate components for shorts, opens or proper values. However, it is not the best troubleshooting tool for circuits. 87.0Ω 2.03Ω 254.0Ω OL Sensor M Motor Relay

Troubleshooting Relays 12V 75 – 110 Ω Coil Resistance (12V Estimates) ACME 87 30 86 85 87A 20 – 40 Ω ~ 130 Ω

ACME 30A 12V 12 V+ Why use a Relay?? If I Short 87 to 30 What Would Happen? 87 30 86 85 87A ACME 12 V+ Horn (1Ω) (Socket) (80Ω) 85 30 87 87A 86 30A 12V 85 86 30 87 87A A Volt Meter across Socket positions 85 & 86 should show what when the switch is closed ? A=V/Ω Ohm’s Law 85 & 86 = Coil 30 = Common 87 = N.O. 87A = N.C. Switch A=12/80 = .15A Horn A=12/1 = 12A

Troubleshooting Relays With the Battery Across 85 & 86 30 to 87 Should Read 0Ω 30 to 87A Should Read OL With the Battery Removed 30 to 87 Should Read OL 30 to 87A Should Read 0Ω (Base of Relay) 86 85 30 87 87A 9 Volt 87 30 86 85 87A 87 30 86 85 87A

Troubleshooting Micro Relays With the Battery on Brass Pins Copper Pins Should Read 0Ω With the Battery Removed Copper Pins Should Read OL (Base of Relay) 9 Volt

Ammeters 9.0a A - + Ammeters complete a circuit. It is a jumper. Whatever is connected will become active. It is therefore the most dangerous meter. The meter is protected with a fuse but you aren’t. Amps Kill. M

Relay Amperage Ratings 12V 12V 30A/40A Coil Voltage - 12 Volts 30A for N.C. Contacts 40A for N.O. Contacts

Checking Amp Flow with a Compass Align the needle with the cable to start Current Flow Current Flow Magnetic North Crank Run Negative We just proved that the alternator is charging Power Off Power On Solenoid Electro-Magnetic Actuator

Arc Suppression in Relays 12V - + The Diode Points to the Positive Wire + - ( 700Ω ) No Polarity