Ch. 2
After testing the relay for power, we found that there wasn't any loss of power and the relay is operating, allowing the power to pass to the grid. What should you do next?
Check that the circuit has power from output of the relay to the rear
Why would you test circuits by performing a voltage drop?
Dome lights that don't light up or door locks that aren't operating. doing a volt drop test can help you identify where the problem lies on the circuit
What is the last step in measuring resistance?
Hold the test leads in parallel across the component or circuit where you're trying to test the circuit.. The resistance measurement will appear on the screen. if its an open circuit will appear OL.
What is step six in measuring amperage? placing the wires
Open the circuit at a point between the power supply, which is the battery, and the load in the circuit. remember to always break the circuit when measuring amps with a digital multimeter.
When diagnosing a vehicle with slow crank condition, you first verify the concern, and then you verify that the battery is fully charged and the cables are tightly connected. What is the next step?
Perform an amp draw test of the battery cables.
What is step four and five in checking resistance?
Set your black lead into the com of the DMM and then set your red lead into the volts Ohms jack of the DMM (far right)
what are the parallel circuit laws? like head lamps or tail lights
There are multiple paths to flow. all loads consume source voltage. in order to get total resistance you have to follow the one over one over formula. total resistance will be smaller than the smallest load. current from each leg will add up to the total current value. And when a leg is added in a parallel circuit amperage will go up and resistance will go down
What does a high current flow or a higher amperage reading indicate?
This indicates that the circuit has lower than normal resistance
When measuring amperage, you should do which of the following?
Use the manufacturer's service information to determine the amount of current that can be expected in the circuit.
What happens when you have high resistance?
Your voltage goes down, and something is sucking the power away from what it's supposed to flow to
What kind of path does a series circuit have?
a single path
Poor circuit performance like a working but dim headlight would be checked by performing what kind of check?
a volt drop check
When diagnosing a bulb that does not illuminate, in addition to isolating the 12 volt positive switch, you should also test the bulb with visual inspection or perform a
continuity check
resistance goes up what goes down?
current
if you have a 12 V power source on a series circuit and you have two different devices that produce a curtain amount of resistance. How would you find out how much the total voltage will between both devices?
Both devices voltage will be added up to source voltage, which in this case is 12 v
In a parallel circuit when performing a voltage drop, what is the voltage going to be between the two devices being used in this example? The source voltage is 12V
Both devices would read source voltage
A customer reports that the engine does not crank and the vehicle will not start. After verifying the concern by attempting to start the car by turning the key, what is the next step?
Check available technical service bulletins or recalls.
The technician connects one test lead to the chassis ground and the other to the door chime negative connection. Then, the technician performs a voltage drop test on the ground side circuit to the door chime module and finds that the ground side circuit voltage has a 3V drop reading. What is the next step?
Inspect the ground wire from the door chime module connector to the chassis ground.
There is no related service information or TSBs open for this symptom. The concern is still present. What is the next step?
Isolate what fuse and circuit powers the rear defrost using an electrical schematic
IF you measure current flow in a series circuit what will the reading be?
It will be the same at any part of the circuit
source voltage is 12 v How do you find voltage across two resistors in a series circuit one that has a resistance reading of 10 Ohms and one that has 50 Ohms according to Ohms law? You have a current .2 amps
Multiply resistance by your amperage. You multiply 10 x .2 and get 2 and you multiply 50 x .2 and get 10. 10 + 2 is 12 v
What is step one in performing a volt drop? (on a DC circuit)
use manufacturers service info to find maximum allowable volt drop for circuit under test
In a parallel circuit how do you find out how much amperage is flowing through each leg in the parallel circuit?
You first find your amperage by dividing voltage by resistance and then you divide your total amperage by how many legs you have in the circuit
You may have a circuit wire that is so chafed that only one copper strand remains intact. If you measured the resistance with a DMM set to ohms, this wire would pass the resistance test. The ohm readings of the chafed wire could be the same for a similar wire that wasn't chafed. However the chafed wire could not support the current flow for an electrical device such as a light bulb or motor. So in this case what kind of test would you have to do?
a voltage drop
Remember ABC when checking amperage. what is ABC?
always break the circuit
If something just wont work or turn on what is probably the problem?
an open circuit
What is step 9 the final step on measuring amperage?
get the amperage measurement
What are examples of parts of the car where people experience poor circuit performance. You would perform a volt drop to find the problem on the circuits
headlights, starter motor, charging system
If something doesn't work very well but still kind of works what kind of problem would that be most likely ?
high resistance
What are the three types of failure modes?
high resistance, shorts and open circuits
In an amperage test after you've test the battery with a load test what should you do?
perform an amp draw test
What is step five in measuring amperage?
prepare a fused jumper wire. the fuse installed in the jumper wire should have a rating that is less than the maximum allowable current indicated on the ammeter. This additional step is a safety measure to prevent damage to your multimeter when testing the circuit.
What is step one in measuring resistance ?
remove power from the circuit; disconnect the negative battery terminal to begin measuring for resistance.
What is step one in measuring amperage?
using manufacturers service info determine the amount of current that can be expected in the circuit. Expected current is identified as "I" and can also be calculated using Ohms law. Measure the source voltage identified as "E" and total circuit resistance, identified as "R" and then calculate the current using the formula I=E divided by R.
A customer brings a vehicle in and complains that the rear window defroster is not defrosting properly. What is the first step in diagnosing this customer concern?
verify the concern
A parallel circuit has multiple branches that allow current to flow. Will the DMM show a different reading on a branch that has different resistance from the other branches due to having a light bulb consuming wattage?
yes
Will additional load devices affect the current flow throughout a series circuit?
yezzir
What happens in a short to power?
you lose control of the circuit. You can't turn something off, or you turn something on and something else turns on.
What should you never do with your leads when checking amperage?
you should never place them in parallel across a load
You have 12 volts and two resisters in a series circuit. one resistor is 10 Ohm and the other is using 50 Ohms what is the amperage?
12v divided by 60 Ohms giving us 0.2 A
Circuits in vehicles typically have voltages that span across a large range, usually between
50 mV and 15 V
What are the reasons you would want to troubleshoot the circuit and test the voltage?
A head light or other light that is dimmer than usual or a power motor may not be functioning well. You also might need to check sensors like the one for the fuel tank. Other reasons include interior light problems, dashboard problems. pretty much problems with electrical equipment in the car
Now that we have verified that the glass will not clear the frost away by taking the vehicle for a test drive, what is the next step in this diagnostic process?
Check for any related service information or TSBs
Now that we have verified the concern, what would be the next best step?
Check for any related service information or TSBs.
After isolating the 20-amp fuse that powers the rear grid, you checked voltage to the fuse, verified that the fuse is not open, and inspected the fuse terminals. What is the next step to isolate the concern?
Check the control relay found on the electrical schematic for power.
You have checked the 12-volt power terminal attached to the grid and you haven't lost voltage to the fixed terminal on the rear window. The ground to the grid is complete. Now that you've verified power from the switch through the relay and to the grid, what is the next step?
Check the grid for any opens in the element.
What is step 8 9 and 10 in performing a volt drop? (on a DC circuit)
Connect the black lead to the end of circuit being tested and place meter leads across component being tested. then close the switch and you should have power flowing through, and your load should be consuming power properly. You will have a voltage reading on your meter. It will be reading how much of the source voltage is used across the wire or component being tested.
What is step 8 in measuring amperage?
If applicable turn on the DMM using the on and off button
You performed a volt drop test on the circuit from battery positive to power terminal in the door chime module. It's a 12 volt circuit from the battery and when testing the voltage drops 3 volts on the positive side circuit, what does this mean?
It means the door chime module power supply wire has excessive resistance and current passing through the wire produces significant voltage drop. therefore there is not enough voltage left to adequately power the door chime module.
What are the laws of series circuit?
One path to flow. resistance adds up so you add all the resistance reading of the loads. voltage is shared between all the loads. The speed or the current is the same throughout the whole circuit. The load with the most resistance will consume the most voltage. and all voltage must be consumed by the time it reaches ground
The technician discovers a corroded ground circuit wire between the door chime module connector and the chassis ground. As a result, the technician replaces the wire. What is the next step
Reassemble the vehicle and verify the repair by inserting the key into the ignition cylinder and opening the door.
A customer reports that the door chime does not sound when the door is opened. After verifying the concern and checking available technical service bulletins or recalls, the technician locates the electrical diagrams. What is the next step?
Refer to the service manual for proper testing procedures and isolate the power and ground circuit wires at the door chime module.
Next, you check the defrost grid ground circuit for an open or high resistance. Using the DMM, you measure resistance between the ground terminal of the defrost grid and chassis ground. The DMM display reads 'OL'. What is the next step?
Repair the open in the ground circuit.
How do you find total resistance in a parallel circuit? Say you have two loads that both read a resistance of 10 Ohms
The formula for this is 1 over 1 divided by each load. So you divide 1 by 10 for the first load and then 1 by 10 again for the the other load. you get .1 for both so you get .2. Then finally you divide the one over top of everything by the .2 to get 5 Ohms.
What are some parts you might need to check or test the circuit for?
The headlamp bulb, a blower motor, a thermistor when testing the resistance in diagnosing a sensor for concerns with temperature. you might also need to check for resistance on a fuel injector after a vehicle misfires or sets a code that is linked to improper fuel injector operation.
Why would you test the leads of the DMM before completing a resistance test?
To verify the resistance of the DMM leads
A customer brings a vehicle in and complains that the ambient temperature display reads 101°F on a 59°F fall day. What is the first step in diagnosing this customer concern?
Verify the customer concern ambient temp is 59°F and the display reads 101°F.
according to Ohms law what is always on the numerator unless it's the value trying to be found?
Voltage
What are reasons you would need to check circuits for amperage?
You might need to troubleshoot starter circuits if you encounter a motor cranking over too slowly, You might also need to use your DMM to measure milliamp draw while the vehicle is off to assist in finding parasitic drains to the battery such as glove light that stays on or something.
So you performed a volt drop test on the ground side to try and begin troubleshooting a door chime issue. After you volt dropped the ground side and found no problem, what do you move on to? lets say the battery is sending 12 v
You perform a volt drop test on the 12 volt circuit from battery positive to power terminal in the door chime module.
What is step three in measuring resistance? This is the step you'd take if you were unsure of the resistance you're measuring and has to do with setting the meter up differently
You set your meter to the auto range function or the resistance range that you anticipate will be present in the circuit or component.
why would you check a fuse when troubleshooting the circuit and testing the voltage?
You would check a fuse to see if the fuse or other circuit protection device is still intact and not blown.
what should you use if you're unsure of the amount current flow in a circuit?
an inductive meter
What are some common causes of an open circuit?
an open or blown fuse, corrosion, broken wire.
What is step 5 and 6 in performing a volt drop? (on a DC circuit) lead setup
black in COM red in Volts Ohm/hertz positive input jack
Tech A says that unexpected high or low resistance in a circuit can cause the circuit not to operate properly. Tech B says that a visual inspection of a wire may not show a broken conductor under the insulation that a resistance check would show. Which tech is correct?
both
What is step 6 in measuring resistance? 2nd to last step
calibrate the meter by holding the two test leads together and zeroing out the meter
After verifying the customer concern and you figure out you need to perform a volt drop test. What should you do after that?
check the battery perform a battery load test
What is step 7 in measuring amperage? after you break the circuit
connect the DMM and fused jumper wire in series between the power supply and the load. At this point the DMM is part of the circuit. This action will allow the current to flow through the DMM
What is step three and four in measuring amperage? placing your test leads
connect the black lead to the COM jack and connect the red test lead to either the amps or the milliamps and microamp port.
You're performing a volt drop test. you first connect one test lead to the chassis ground and the other to the door chime negative connection ( door chime is problem) . you perform a volt drop test after turning the ignition but not starting motor. you perform a volt drop test on the ground side circuit to the door chime module. As a result, you find that the ground side circuit passes the volt drop test with a .1V drop reading. What does this mean
it means the ground wire is not using an excessive amount of voltage
Your total resistance in a parallel circuit will always be less than what?
it will be less than the resistance reading on each load. Remember you're going to see source voltage when you read a load in parallel circuit so you have to math out your actual resistance
What will happen in a short to ground?
it will overheat and melt opening the fuse (which is the circuit protection device) this results in an open fuse
After the test drive, the malfunction indicator light or MIL, is illuminated and you retrieved DTC code P0201 Cylinder 1 injector in the diagnostic process?
measure the resistance of the injector
If It's determined that a damaged fuse is present after using a DMM, you should
replace the fuse with a new fuse and continue diagnosis
Service publications for code P0201 lead you to a resistance test of the injector windings. The test fails, showing an open winding in the injector solenoid, showing an infinite ohm reading on the DMM display. What are the final steps in the diagnostic process?
replace the injector, assemble and test drive the vehicle, and document the repair and return the vehicle to the customer.
What is step 2 in measuring amperage?
set the mode selector switch to the correct DC amperage setting either milliamps or amps. It is always safest to start with the highest amperage setting available and scale down if necessary
What is step two in measuring resistance?
setting the dial to Ohms
What are the three types of shorts?
short to ground, this happens when there is no load in the circuit so it flows through intensely and can only really happen on the power side of the circuit. The other type is called short to power. This happens when your load is always on no matter what. This will effect on your power side of your circuit. The other type is called shorted together. An example of this would stepping on the brake and the horn going off at the same time. This happens when closing one switch also activates another switch (together).
What does a low current flow or a low amperage reading indicate?
this indicates that the circuit has higher than normal resistance.
If the terminal fit is good, perform a test for resistance to ground using the DMM. After finding a high resistance to ground on the ground side circuit, what should you do next?
trace the ground wire to the chassis ground connection
True or False. low current flow indicates higher than normal resistance
true
What is step 2 3 and 4 in performing a volt drop? (on a DC circuit) (setting DMM up)
turn it on, set the mode selector to mV or Volts DC. Set the meter range (button) to auto range or select a low voltage reading setting.
What are you going to find when you perform a volt drop?
unwanted or excessive resistance
What does excessive voltage drops indicate when performing a volt drop? And what would you do after finding this?
unwanted resistance in the circuit. and you would test the ground side of the circuit by connecting the test leads across the ground circuit wire.
