The military employs some very talented pilots. The best of these pilots are sent to special schools where they learn the techniques of air-to-air combat. It is the variety of techniques they learn that separates them from the rest. Honing your diagnostic skills is similar, in that the more testing techniques you know, the more ammunition you have to take down any electrical problem that comes your way.
Those Pesky Parasites
You know the ones I mean — those little sources of drain on the battery while the car is sitting idle. Finding the sources of parasitic draw used to be easy. Connect an ammeter in line with the battery and start pulling fuses until the drain went away. But modern electronics is making the hunt for the causes of battery drain a little more complicated. What if the source of the draw is a control module that won't time out, or as we say in the car repair biz, won't "go to sleep?"
If we disconnect the battery cable to install our ammeter, the very act of disconnecting/connecting the cable might cause the problem to temporarily correct itself. We could use an amp probe instead so we wouldn't have to open the circuit. But amp probes aren't all that accurate when we're looking at maximum draw specifications of 30 to 50 milliamps or so (that's 0.050 amp), and might provide us with misinformation. What to do?
One alternative is to use a separate power supply to keep the circuits alive while you plug in your multimeter. I first read of this technique while looking over some Honda service bulletins on keeping the various memory presets alive while replacing a battery. It uses a dedicated jump box that is equipped with a diagnostic link connector (DLC) instead of the normal jumper cable ends. Simply connect the box to the car's DLC, look for the green indicator light that shows you are connected properly and then hit the toggle switch. It is now safe to disconnect the battery negative cable and place your digital multimeter (DMM) in series with the cable and the battery post (the car's modules are being powered by the jump box). Once you have the meter in place, toggle off the jump box and perform your tests.
Forget about plugging into the cigarette lighter or power outlet as we did years ago. Too many cars today do not share that circuit with their control modules, and you'll just be wasting your time. And be sure to check the service information for the car you're working on for the time it will normally take for all modules to go offline.
Another Drain Detection Method
The whole idea in troubleshooting parasitic drain is to find the circuit(s) that still are working when they aren't supposed to be, right? So it would be reasonable to say that we can find it by looking for current flow, right? And that's the usual approach, using an ammeter or low amp probe.
There is another alternative: voltage drop.
Voltage drop occurs across any resistance in a circuit when current is flowing. It's a dynamic test, meaning things have to be working for it to happen. So I should be able to measure voltage drop of some kind in any circuit when it's working, right? And the procedure for performing a voltage drop test requires the circuit to remain intact — it's a non-intrusive test. If it is a module that has insomnia, I won't do anything to interrupt it or reset it.
I wish I knew who originally came up with this test, because it's a great addition to my own diagnostic arsenal. The procedure is simple enough. Check the power distribution diagram for the vehicle you're working on, and isolate all the fuses that are first in line from the battery to the rest of the car. Think of them as the first "toll booths" on the "power highways" leading to the rest of the "roads" current will travel if something is on.
Now with the key off and the car idle, start measuring voltage drop across those fuses, one at a time. Fuses have resistance, and that resistance varies with the rating of the fuse. Therefore, there will be a voltage drop across them if current is flowing. Use your millivolts scale because it won't be much. Place one meter lead on the one side of the fuse, accessing the little exposed elements on the top of the fuse itself, and the other lead on the other side of the fuse. If no current is flowing, the meter should read zero. But that's in a perfect world. More likely, you will see anything from zero to a reading in the single digits.
If there is current flowing, you'll see it in a higher reading. If you know the resistance of the fuse, you can actually calculate the current flowing through it. Personally, I don't really care. I just want to know what circuit is on. With the circuit that's flowing identified, the next step is to isolate which leg in that fused circuit is the culprit. If there are fuses downstream from the first one, repeat your tests there to further pinpoint the cause.
Two Birds, One Stone
Doesn't troubleshooting EVAP purge and vent solenoid problems drive you crazy? Electrical failures in most solenoids occur either all out, or only after they've gotten hot. And with pulse width controls on many solenoids, they are constantly being turned on and off. How can I simulate that condition and tell if it's failing?
Current Is the Key
Current is a convenient measurement to take. It is the same everywhere in the circuit, so I can measure it at the most convenient spot, usually the fuse. It also is a window into the health of the circuit I'm troubleshooting. If current is too high, I know there is less than normal resistance and if it's too low, there must be higher than normal resistance.
Measuring current is one quick way to gain direction when diagnosing a circuit controlled by a relay. These circuits are really two in one, with one half covering the operation of the relay, usually by a control module, and the other half covering the operation of the actual circuit component, like an A/C compressor clutch or even the horn.
The first step in this technique is to identify which fuse powers the relay, and which powers the component the relay controls. Often it’s the same fuse. By measuring the current at this fuse, you can quickly tell which half of the circuit has the problem. Replace the fuse with a fused jumper wire, or "fuse buddy" that will allow you to place your amp probe into the circuit. You can also use your DMM with proper adaptors if you don't have a low amp probe.
Next, operate the circuit and read the current flow. If there is no current flowing at all, then the problem is probably in the relay side of the circuit. The relay isn't being energized. If it were, you would see somewhere around 0.50 amps of current flow as current passed through the relay windings. If that's all you see, then focus your attention on the device side of the circuit. That circuit is open, and it might be as simple as a relay contact that isn't closing. Last, if you see current in the single digits (3 amps or more), then look at the circuit device for a mechanical fault. This much current flow tells you that, electrically, everything appears to be working as it should.
Should there be two separate fuses, test the relay fuse first then move on to the fuse powering the component the relay controls.
PAGE 4Did Someone Say ‘Voltage Drop?’
I would not be meeting my responsibility to you, our faithful readers, if I didn't mention voltage drop testing. This is likely the best single technique you can master for finding circuit faults. What is voltage drop? Voltage is the force that overcomes the resistance of an electrical component so current can flow through it. Once that resistance is overcome, the force is no longer needed. Try this experiment to see for yourself. Grab your DMM and take it over to the car in your bay. Turn the headlights on. Now use your DMM to measure the voltage going into the bulb, as close to the bulb as you can, and with your negative meter lead firmly attached to the battery's negative post. With the car off, it should read about the same as the battery voltage.
Next, measure for voltage on the ground side of the bulb while leaving the negative meter lead right where it is. This reading should be pretty low, somewhere between 0.50v and 0.0v. See? The voltage dropped after crossing the bulb. That's the short version.
Voltage is needed to overcome resistance. Any resistance. If there is a source of unwanted resistance — like corroded wiring, or a loose connector — there will be a voltage drop across it as well. And it will show up in one of the two readings you just performed.
Learning how to perform voltage drop tests isn't the hurdle, its understanding what the meter readings are trying to tell you. That's why we're hosting a live, interactive webcast with G. Jerry Truglia and TST later this month, and we invite you to attend. Hey, it's free! For registration information, go to our community website at http://workshop.search-autoparts.com. You will also find lots of video and written information on learning this and the other techniques we've discussed today. See you there!
The military employs some very talented pilots. The best of these pilots are sent to special schools where they learn the techniques of air-to-air combat. It is the variety of techniques they learn that separates them from the rest. Honing your diagnostic skills is similar, in that the more testing techniques you know, the more ammunition you have to take down any electrical problem that comes your way.
Those Pesky Parasites
You know the ones I mean — those little sources of drain on the battery while the car is sitting idle. Finding the sources of parasitic draw used to be easy. Connect an ammeter in line with the battery and start pulling fuses until the drain went away. But modern electronics is making the hunt for the causes of battery drain a little more complicated. What if the source of the draw is a control module that won't time out, or as we say in the car repair biz, won't "go to sleep?"
If we disconnect the battery cable to install our ammeter, the very act of disconnecting/connecting the cable might cause the problem to temporarily correct itself. We could use an amp probe instead so we wouldn't have to open the circuit. But amp probes aren't all that accurate when we're looking at maximum draw specifications of 30 to 50 milliamps or so (that's 0.050 amp), and might provide us with misinformation. What to do?
One alternative is to use a separate power supply to keep the circuits alive while you plug in your multimeter. I first read of this technique while looking over some Honda service bulletins on keeping the various memory presets alive while replacing a battery. It uses a dedicated jump box that is equipped with a diagnostic link connector (DLC) instead of the normal jumper cable ends. Simply connect the box to the car's DLC, look for the green indicator light that shows you are connected properly and then hit the toggle switch. It is now safe to disconnect the battery negative cable and place your digital multimeter (DMM) in series with the cable and the battery post (the car's modules are being powered by the jump box). Once you have the meter in place, toggle off the jump box and perform your tests.
Forget about plugging into the cigarette lighter or power outlet as we did years ago. Too many cars today do not share that circuit with their control modules, and you'll just be wasting your time. And be sure to check the service information for the car you're working on for the time it will normally take for all modules to go offline.
Another Drain Detection Method
The whole idea in troubleshooting parasitic drain is to find the circuit(s) that still are working when they aren't supposed to be, right? So it would be reasonable to say that we can find it by looking for current flow, right? And that's the usual approach, using an ammeter or low amp probe.
There is another alternative: voltage drop.
Voltage drop occurs across any resistance in a circuit when current is flowing. It's a dynamic test, meaning things have to be working for it to happen. So I should be able to measure voltage drop of some kind in any circuit when it's working, right? And the procedure for performing a voltage drop test requires the circuit to remain intact — it's a non-intrusive test. If it is a module that has insomnia, I won't do anything to interrupt it or reset it.
I wish I knew who originally came up with this test, because it's a great addition to my own diagnostic arsenal. The procedure is simple enough. Check the power distribution diagram for the vehicle you're working on, and isolate all the fuses that are first in line from the battery to the rest of the car. Think of them as the first "toll booths" on the "power highways" leading to the rest of the "roads" current will travel if something is on.
Now with the key off and the car idle, start measuring voltage drop across those fuses, one at a time. Fuses have resistance, and that resistance varies with the rating of the fuse. Therefore, there will be a voltage drop across them if current is flowing. Use your millivolts scale because it won't be much. Place one meter lead on the one side of the fuse, accessing the little exposed elements on the top of the fuse itself, and the other lead on the other side of the fuse. If no current is flowing, the meter should read zero. But that's in a perfect world. More likely, you will see anything from zero to a reading in the single digits.
If there is current flowing, you'll see it in a higher reading. If you know the resistance of the fuse, you can actually calculate the current flowing through it. Personally, I don't really care. I just want to know what circuit is on. With the circuit that's flowing identified, the next step is to isolate which leg in that fused circuit is the culprit. If there are fuses downstream from the first one, repeat your tests there to further pinpoint the cause.
Two Birds, One Stone
Doesn't troubleshooting EVAP purge and vent solenoid problems drive you crazy? Electrical failures in most solenoids occur either all out, or only after they've gotten hot. And with pulse width controls on many solenoids, they are constantly being turned on and off. How can I simulate that condition and tell if it's failing?
Current Is the Key
Current is a convenient measurement to take. It is the same everywhere in the circuit, so I can measure it at the most convenient spot, usually the fuse. It also is a window into the health of the circuit I'm troubleshooting. If current is too high, I know there is less than normal resistance and if it's too low, there must be higher than normal resistance.
Measuring current is one quick way to gain direction when diagnosing a circuit controlled by a relay. These circuits are really two in one, with one half covering the operation of the relay, usually by a control module, and the other half covering the operation of the actual circuit component, like an A/C compressor clutch or even the horn.
The first step in this technique is to identify which fuse powers the relay, and which powers the component the relay controls. Often it’s the same fuse. By measuring the current at this fuse, you can quickly tell which half of the circuit has the problem. Replace the fuse with a fused jumper wire, or "fuse buddy" that will allow you to place your amp probe into the circuit. You can also use your DMM with proper adaptors if you don't have a low amp probe.
Next, operate the circuit and read the current flow. If there is no current flowing at all, then the problem is probably in the relay side of the circuit. The relay isn't being energized. If it were, you would see somewhere around 0.50 amps of current flow as current passed through the relay windings. If that's all you see, then focus your attention on the device side of the circuit. That circuit is open, and it might be as simple as a relay contact that isn't closing. Last, if you see current in the single digits (3 amps or more), then look at the circuit device for a mechanical fault. This much current flow tells you that, electrically, everything appears to be working as it should.
Should there be two separate fuses, test the relay fuse first then move on to the fuse powering the component the relay controls.
{C} PAGE 4Did Someone Say ‘Voltage Drop?’
I would not be meeting my responsibility to you, our faithful readers, if I didn't mention voltage drop testing. This is likely the best single technique you can master for finding circuit faults. What is voltage drop? Voltage is the force that overcomes the resistance of an electrical component so current can flow through it. Once that resistance is overcome, the force is no longer needed. Try this experiment to see for yourself. Grab your DMM and take it over to the car in your bay. Turn the headlights on. Now use your DMM to measure the voltage going into the bulb, as close to the bulb as you can, and with your negative meter lead firmly attached to the battery's negative post. With the car off, it should read about the same as the battery voltage.
Next, measure for voltage on the ground side of the bulb while leaving the negative meter lead right where it is. This reading should be pretty low, somewhere between 0.50v and 0.0v. See? The voltage dropped after crossing the bulb. That's the short version.
Voltage is needed to overcome resistance. Any resistance. If there is a source of unwanted resistance — like corroded wiring, or a loose connector — there will be a voltage drop across it as well. And it will show up in one of the two readings you just performed.
Learning how to perform voltage drop tests isn't the hurdle, its understanding what the meter readings are trying to tell you. That's why we're hosting a live, interactive webcast with G. Jerry Truglia and TST later this month, and we invite you to attend. Hey, it's free! For registration information, go to our community website at http://workshop.search-autoparts.com. You will also find lots of video and written information on learning this and the other techniques we've discussed today. See you there!
