Electrical Diagnosis and Testing
#1
01-04-2009, 06:38 PM
Electrical Diagnosis and Testing
Electrical Diagnosis and Testing
I just wanted to share some information regarding electrical diagnosis and testing procedures in order to help my fellow Dodge owners in diagnosing their own electrical issues, so here goes nothing!
General Information
In order to test electrical circuits you have to understand the principles of operation in order to be able to see what can’t be seen. Electricity is a lot like water with voltage being electrical pressure, ohms being the amount of resistance to flow, and amperage being the amount of current flowing through a circuit. Also like water electricity always takes the path of least resistance.
To see what can’t be seen you need to get a digital multimeter. These can be had for as little as $20 at your local auto parts store. They will not be nearly as accurate as a $300-400 job but for simple diagnostics they will perform the task at hand. Do not rely on a cheap meter for testing to make sure certain electrical components (such as injectors) are within tolerance as they have too high of an error rate at +/- 2% if I remember correctly. The cheap meters also should never be used on the PCM. They do not have high impedance like the more expensive brands (10,000+ ohms) and can fry the PCM. With that being said there is still a ton of use for a cheap meter so don’t let that scare you out of doing your own work and saving quite a bit of money in the process.
Series and Parallel
To test voltage and ohms you must go in parallel to the circuit.
(+)-------Meter----------(-)
V....................................V
----------Circuit--------------
(ignore the periods I could not place my arrows in the right place without them)
To test amps it must be done in series to the circuit.
----Circuit-----< (+) Meter (-) >------Circuit-----
Ohms
When testing electrical circuits for continuity you can use the ohms setting (omega symbol). You must have all power shut off to the circuit you are testing in order to do an ohm test and the test must be done in parallel. Ohms testing is not a reliable means of determining how much resistance there is in a circuit. As wire diameter increases this is even more true. This is due to the meter only sending out a small signal and measuring voltage drop. With this small signal going through a larger diameter wire the electrons are able to find ways around resistance thus preventing the meter from getting an accurate representation of the resistance in the circuit. When testing for continuity it is farily normal to see a few ohms of resistance and even more when the signal travels through the circuit’s load (ie motor, light, etc). If the meter reads OL (over limit) this is an indication that the circuit is open (light switch turned off).
Voltage
Voltage testing is a great way to find problems in a circuit. This is done with power going to the device/circuit being tested. As stated before voltage is a measure of electrical pressure. This pressure can be measured with the multimeter set to DC V. The first thing to remember is that all voltage testing does is tests the difference in voltage between the 2 points being tested not the overall voltage. In order to find the voltage at any given point in a circuit put the positive lead on the point you are wanting to test and the negative lead to ground. This testing is to be done in parallel to the circuit. As electrons flow through resistance whether it is a motor or a light voltage drops. On the ground side of a load there should little (.1) to no voltage left over. If there is excessive voltage found on the ground side of a circuit you should start looking for a poor ground. If the electrical device is not operating testing then for voltage drops allows you to find where the circuit is open. By placing one lead at the beginning of a wire/switch/etc. and placing the other lead at the other end of the wire/switch/etc. When testing wires, splices, fuses, and switches there should be minimal to no voltage drop on the area of the circuit being tested. If it is the same reading as your source (battery) voltage then you have found your opening in the circuit. If excessive voltage drop is found on a switch, relay, or wire that part of the circuit should be repaired or replaced. If you are testing a series circuit (more than one load in the circuit) you should find voltage left after all loads in the circuit aside from the last load which will drop the remaining voltage to 0.
(+)------2 ohm load-------2 ohm load-----(-)
.......^.........................^................ ..........^
.....12 V....................6 V......................0 V
In this example the two loads are the same which means that the voltage drop would be equal across each load.
(+)------2 ohm load-------1 ohm load------(-)
........^.........................^............... ...........^
......12 V...................4 V.......................0 V
In this example the loads are unequal. The larger resistance load drops more voltage than the lower resistance load. Any way that this circuit is arranged will have the same result.
Amperage
To test amperage the meter needs to be in series with the circuit and turned to amps (A) or milliamps (mA). The positive lead must also be plugged into the amp port. This is the only time that the positive lead should be in the amp position. On the A scale most meters are good to test up to 10 amps before blowing a fuse. On the mA scale this is only up to 1 amp. Always set the meter to A before mA if the amperage is unknown. One nice thing about testing amperage is that it does not need to be tested in multiple locations just one as amperage is constant throughout the entire circuit. It is important that you do not leave the meter on A after you are done testing amperage and moving on to voltage. If you use a meter in parallel on accident you will blow the fuse and may very well damage the meter itself as there is no resistance in the meter for the amperage testing position.
Short circuits
Ok now to the types of short circuits. There is short to ground, this is also referred to as a short to steel. This is a short circuit that bypasses a load completely and is usually a break in the wiring. This usually occurs in places that wiring moves around and can rub insulation off or near sharp edges. This will always blow a fuse if the short occurs after the fuse block which is always (99.9% of the time) the case. If for some strange reason the short occurs before the fuse the wiring would burn up possibly catching the vehicle on fire. This is why it is imperative that when a fuse blows you do not put a larger rated fuse in when you blow smaller fuses. If the fuse does not blow something else will. A short to ground can be located by fashioning a simple yet effective test light with an old sealed beam headlight and some leads from the positive and negative terminals. A sealed beam is the best because it is sturdy and won’t get as hot as fast as an open bulb will. If the circuit powers low current draw device an actual test light can be used as well. By placing one lead in each slot for the fuse it puts a load on the circuit preventing it from burning up any wiring. The circuit must be energized which may require turning the ignition on or turning the component on. Now with the light in place you move along the harness and wiggle the wiring until the light goes out or dims. When this occurs you have found the general area of the short. Open the harness and begin looking for the short and repair it.
The other type of short is a short to voltage. This is also called a short to copper. This problem arises when two (or god forbid more) wires insulation rubs off and the wires come into contact with each other. This causes more than one circuit to energize when activating one circuit. An example of this is when you hit the brakes it activates the horn. This will NOT always blow a fuse unless the current draw exceeds the rating of the fuse. This can be a very dangerous short if one wire is significantly larger than the other wire. When the circuit with the larger wire is activated the circuit with the smaller wire cannot handle all of the current and can overheat and burn. In order to locate this type of short a tone generator can be used but this is an expensive option. Your only other option is to look at a wiring diagram and see where the wires to the individual circuits intersect each other and look for shorts in the areas indicated. You can also try wiggling the harness and hope that when you are near the short it breaks contact.
If you have any questions please feel free to ask. I will be adding to this post as I feel up to writing more.
Hydra or Ken, can this get a sticky we’ve been getting a lot of questions regarding electrical issues lately and this will help those who have questions at least understand the basics of electrical testing when we tell them what they can do.
I just wanted to share some information regarding electrical diagnosis and testing procedures in order to help my fellow Dodge owners in diagnosing their own electrical issues, so here goes nothing!
General Information
In order to test electrical circuits you have to understand the principles of operation in order to be able to see what can’t be seen. Electricity is a lot like water with voltage being electrical pressure, ohms being the amount of resistance to flow, and amperage being the amount of current flowing through a circuit. Also like water electricity always takes the path of least resistance.
To see what can’t be seen you need to get a digital multimeter. These can be had for as little as $20 at your local auto parts store. They will not be nearly as accurate as a $300-400 job but for simple diagnostics they will perform the task at hand. Do not rely on a cheap meter for testing to make sure certain electrical components (such as injectors) are within tolerance as they have too high of an error rate at +/- 2% if I remember correctly. The cheap meters also should never be used on the PCM. They do not have high impedance like the more expensive brands (10,000+ ohms) and can fry the PCM. With that being said there is still a ton of use for a cheap meter so don’t let that scare you out of doing your own work and saving quite a bit of money in the process.
Series and Parallel
To test voltage and ohms you must go in parallel to the circuit.
(+)-------Meter----------(-)
V....................................V
----------Circuit--------------
(ignore the periods I could not place my arrows in the right place without them)
To test amps it must be done in series to the circuit.
----Circuit-----< (+) Meter (-) >------Circuit-----
Ohms
When testing electrical circuits for continuity you can use the ohms setting (omega symbol). You must have all power shut off to the circuit you are testing in order to do an ohm test and the test must be done in parallel. Ohms testing is not a reliable means of determining how much resistance there is in a circuit. As wire diameter increases this is even more true. This is due to the meter only sending out a small signal and measuring voltage drop. With this small signal going through a larger diameter wire the electrons are able to find ways around resistance thus preventing the meter from getting an accurate representation of the resistance in the circuit. When testing for continuity it is farily normal to see a few ohms of resistance and even more when the signal travels through the circuit’s load (ie motor, light, etc). If the meter reads OL (over limit) this is an indication that the circuit is open (light switch turned off).
Voltage
Voltage testing is a great way to find problems in a circuit. This is done with power going to the device/circuit being tested. As stated before voltage is a measure of electrical pressure. This pressure can be measured with the multimeter set to DC V. The first thing to remember is that all voltage testing does is tests the difference in voltage between the 2 points being tested not the overall voltage. In order to find the voltage at any given point in a circuit put the positive lead on the point you are wanting to test and the negative lead to ground. This testing is to be done in parallel to the circuit. As electrons flow through resistance whether it is a motor or a light voltage drops. On the ground side of a load there should little (.1) to no voltage left over. If there is excessive voltage found on the ground side of a circuit you should start looking for a poor ground. If the electrical device is not operating testing then for voltage drops allows you to find where the circuit is open. By placing one lead at the beginning of a wire/switch/etc. and placing the other lead at the other end of the wire/switch/etc. When testing wires, splices, fuses, and switches there should be minimal to no voltage drop on the area of the circuit being tested. If it is the same reading as your source (battery) voltage then you have found your opening in the circuit. If excessive voltage drop is found on a switch, relay, or wire that part of the circuit should be repaired or replaced. If you are testing a series circuit (more than one load in the circuit) you should find voltage left after all loads in the circuit aside from the last load which will drop the remaining voltage to 0.
(+)------2 ohm load-------2 ohm load-----(-)
.......^.........................^................ ..........^
.....12 V....................6 V......................0 V
In this example the two loads are the same which means that the voltage drop would be equal across each load.
(+)------2 ohm load-------1 ohm load------(-)
........^.........................^............... ...........^
......12 V...................4 V.......................0 V
In this example the loads are unequal. The larger resistance load drops more voltage than the lower resistance load. Any way that this circuit is arranged will have the same result.
Amperage
To test amperage the meter needs to be in series with the circuit and turned to amps (A) or milliamps (mA). The positive lead must also be plugged into the amp port. This is the only time that the positive lead should be in the amp position. On the A scale most meters are good to test up to 10 amps before blowing a fuse. On the mA scale this is only up to 1 amp. Always set the meter to A before mA if the amperage is unknown. One nice thing about testing amperage is that it does not need to be tested in multiple locations just one as amperage is constant throughout the entire circuit. It is important that you do not leave the meter on A after you are done testing amperage and moving on to voltage. If you use a meter in parallel on accident you will blow the fuse and may very well damage the meter itself as there is no resistance in the meter for the amperage testing position.
Short circuits
Ok now to the types of short circuits. There is short to ground, this is also referred to as a short to steel. This is a short circuit that bypasses a load completely and is usually a break in the wiring. This usually occurs in places that wiring moves around and can rub insulation off or near sharp edges. This will always blow a fuse if the short occurs after the fuse block which is always (99.9% of the time) the case. If for some strange reason the short occurs before the fuse the wiring would burn up possibly catching the vehicle on fire. This is why it is imperative that when a fuse blows you do not put a larger rated fuse in when you blow smaller fuses. If the fuse does not blow something else will. A short to ground can be located by fashioning a simple yet effective test light with an old sealed beam headlight and some leads from the positive and negative terminals. A sealed beam is the best because it is sturdy and won’t get as hot as fast as an open bulb will. If the circuit powers low current draw device an actual test light can be used as well. By placing one lead in each slot for the fuse it puts a load on the circuit preventing it from burning up any wiring. The circuit must be energized which may require turning the ignition on or turning the component on. Now with the light in place you move along the harness and wiggle the wiring until the light goes out or dims. When this occurs you have found the general area of the short. Open the harness and begin looking for the short and repair it.
The other type of short is a short to voltage. This is also called a short to copper. This problem arises when two (or god forbid more) wires insulation rubs off and the wires come into contact with each other. This causes more than one circuit to energize when activating one circuit. An example of this is when you hit the brakes it activates the horn. This will NOT always blow a fuse unless the current draw exceeds the rating of the fuse. This can be a very dangerous short if one wire is significantly larger than the other wire. When the circuit with the larger wire is activated the circuit with the smaller wire cannot handle all of the current and can overheat and burn. In order to locate this type of short a tone generator can be used but this is an expensive option. Your only other option is to look at a wiring diagram and see where the wires to the individual circuits intersect each other and look for shorts in the areas indicated. You can also try wiggling the harness and hope that when you are near the short it breaks contact.
If you have any questions please feel free to ask. I will be adding to this post as I feel up to writing more.
Hydra or Ken, can this get a sticky we’ve been getting a lot of questions regarding electrical issues lately and this will help those who have questions at least understand the basics of electrical testing when we tell them what they can do.
#2
01-07-2009, 06:00 PM
Oh man, I foresee future posts about someone frying something because they didn't have the meter set right, or didn't have the power on/off as needed. LOL!!!!
I understood everything and it is a great right up, and I do mean great, but if any of the comments above even confuse you the slightest, please get some help with your problem.
I can't stop laughing. I keep recalling myself trying to talk a novice person through the simplest tests and hearing not so kind words later in to the talk... LOL!!!!
I understood everything and it is a great right up, and I do mean great, but if any of the comments above even confuse you the slightest, please get some help with your problem.
I can't stop laughing. I keep recalling myself trying to talk a novice person through the simplest tests and hearing not so kind words later in to the talk... LOL!!!!
#3
01-08-2009, 12:27 PM
L&R you are absolutely right and I am one of the lucky ones who just "got it". Definitely ask questions if you are not 100% sure on something and it will save you a headache later.
