Michael Thomas, automotive and diesel technician, is the author of the McGraw-Hill textbook "Truck and Trailer Systems."
AC Relay and Switch Circuit
Switches on our dash used to control electrical circuits directly. When you turned on a switch, it was responsible for sending power or grounding a circuit. Most dash switches we operate today are an input into a computer, and the computer will turn on or off the component. Most of the late model vehicles use a body control computer, along with the engine, dashboard, transmission and ABS computers. The body computer will monitor and control the power or ground to the; lighting, AC and heater, trailer lighting and a variety of other functions.
Example: When the “Snow Flake Button” is pushed on the dash, the AC should turn on. What actually happened is the computer received a request for AC from the switch. It does a system check to make sure it can operate the AC. If everything is ok, it will turn on the AC. If it senses something is wrong, it will not turn on the AC.
For Basic Electrical see my previous article DIY Auto Service: Basic Electrical and Electronics Testing
Single Pole Single Throw (SPST)
This switch is a single position single circuit that is on (closed) or off (open) When used in a basic electrical circuit it would turn one circuit on or off. In a computer circuit, it would either complete a power circuit or ground to send a high voltage or low voltage signal to the computer. The computer would then use this signal as a request for that circuit to be turned on or off.
- Single Pole Single Throw (SPST) Testing: To test an SPST Switch, an Ohmmeter or Voltmeter can be used. With two terminals, they will have contact in one position and no contact in the other.
Single Pole Double Throw (SPDT)
This is a three position switch. It could be off in the middle and turn on 2 separate circuits, or it could be used in 3 positions. A typical application would be a 3 position Engine Compression Brake or Jake Brake” This would be used with the SPST as an on/off switch and the SPDT as the level selector. In the engine computer, the switch positions would tell the computer what level of Compression Brake was selected and then to activate that level. Thru a Scan Tool or PC Software, the technician can move the switches and watch the information change in the display. This information should match the level selected.
- Single Pole Double Throw (SPDT) Testing: To test an SPDT Switch, an Ohmmeter or Voltmeter can be used. With three terminals, they will have continuity in one position and no contact in the other. The switch may also have a no-contact middle connection. The contact typically moves opposite of the terminals. With the switch up the lower contact is connected. With the switch down, the upper contact is connected.
SPST Hi/Low Switch
- Ganged Switches: Ganged switches allow one movement to move multiple contacts at the same time. A headlight switch turns on parking lights and head lights even though these two circuits are separated and do not come in contact. The movement is usually indicated by a dotted line connecting the contacts of the switch.
- Momentary Switches: Momentary switches are spring loaded in one direction. Usually, they are spring loaded off but could be spring loaded in the on position. A cruise control Resume/Accel button is spring loaded in the middle. If you select Accel and let off, it returns to the middle. The ignition switch has a spring loaded start position. The ignition switch is a ganged switch with a momentary position. Momentary switches are used as; horn, clutch, brake, throttle, cruise control, and trailer light interrupt indicator switches.
- Switch Packs: Some of the truck manufacturers, like International, sell additional add-on switches in Switch Packs. These are groups of 4 to 6 switches in a panel for optional exterior lighting and other electrical controls. Tow truck light bars would be an example of when extra switches would be needed. These switches are then programmed into the body computer, so it knows what is meant when the switch is turned on. The computer then turns on or off that circuit as requested.
Relay Numbering System
A relay is a special solenoid that instead of moving a valve, it moves an electrical contact or switch, sometimes referred to as a magnetic switch. A relay has at least 2 circuits. One of the circuits is for the relay coil and at least one more controlling the electrical device. The relay is rated in the amount of voltage and amperage it is to operate in maximum. The relay is used in an electrical circuit where the control device of the circuit cannot handle the high amperage that operates the load, like an AC clutch coil. The AC clutch coil (magnet) uses around 10 to 15 amps of current flow.
- The control circuit for the AC Clutch is a computer controlled circuit. For the example, the computer circuit can only handle 2 Amps.
- If the relay coil is controlled by the computer and closes the switch to connect the 15Amps needed by the clutch coil, the resistance in the relay coil will reduce the amperage in the control circuit.
- If the relay coil had 80Ω of resistance, according to Ohm’s law a 12V circuit with 80Ω of resistance would flow about .15 Amps (12V ÷ 80Ω = .15A) This is well within the computers control capability. This would also affect the size of the wire needed to operate the control circuit. Lower amperage means the wiring to the control circuit could be much smaller.
- A relay’s job is to control a large amperage draw circuit with a low amperage draw control circuit.
Relays are used so a low amperage control circuit (computer) can control a higher amperage device. The only part of the relay circuit that the computer sees is the relay coil.
Relays can also fail two ways.
- The relay coil could have high resistance or a broken wire.
- The moving contact could have a mechanical problem, or the contacts could have become burnt or corroded.
Relay Testing De-energized
Relay Coil testing is done across terminals #85 and #86 or terminals #1 and #2 depending on the relay numbering.
- Remove the relay and with an Ohmmeter test across these terminals and you should see 70 to 100Ω. If not replace the relay.
- While the relay is not powered or De-energized, test with the ohmmeter between pins #30 and #87A or #3 and #4. There should be very low resistance usually .1Ω.
- While the relay is not powered or De-energized, test with the ohmmeter between pins #30 and #87 or #3 and #5. There should be not contact or Infinity.
- Using jumper wires or an old relay harness, power the relay coil. You should hear the relay “Click.”
- While the relay is powered or energized, test with the ohmmeter between pins #30 and #87 or #3 and #5. There should be very low resistance usually .1Ω.
Service Tip: If the relay is hot while in operation, the component it controls should be checked for excessive amperage draw.
Relay Testing Energized
Clamping Diode or Resistor
Whenever a coil of wire is powered up, a magnetic field is produced around the coil as described earlier. When the coil is de-energized, the “Magnetic Lines of Force” collapse. When they collapse across the coil of wire, an AC voltage is “Self Induced” into the coil. The polarity is also reversed, meaning the positive is the negative. This voltage spike, depending on the coil, could be 300V AC. This high voltage could damage the electrical circuit. To deplete the “Induced” voltage, a resistor or diode is placed across the two terminals leading to the coil.
- Clamping Diode: Since a Diode is a One-way electrical check valve it will only allow electricity to flow thru the Diode when a Positive Voltage is on the Anode, and a Ground or Negative is on the Cathode. When the Anode (+) has a ground and the Cathode (-) has a Positive Voltage, there will be no flow.
- The Clamping Diode is wired into the circuit with the Cathode (-) attached to the Positive Lead to the Coil, and the Anode (+) is attached to the negative lead. In this configuration, there will be no flow thru the Diode. When the coil is de-energized, the Induced Voltage is produced and has the opposite polarity. This means that the Diode Anode (+) and Cathode (-) are now connected properly, so there will be flow thru the Diode. This allows the Voltage Spike to flow back thru the resistance of the coil and be depleted without trying to flow back into the circuit.
- Clamping Resistor: The Clamping Resistor works the same way except the resistor is not directional. The resistor has a very small current flowing thru it when the coil is energized and then depletes the voltage spike when the coil is de-energized.
Service Tip: Example: If the AC Clutch Coil Clamping Diode or Resistor is bad, there may be a popping noise in the radio from the voltage spike.
This article is accurate and true to the best of the author’s knowledge. Content is for informational or entertainment purposes only and does not substitute for personal counsel or professional advice in business, financial, legal, or technical matters.
© 2014 Mike Thomas