Applying principle to reality

Feb. 3, 2016
While principles of electricity work well in the classroom, the application to reality can sometimes be hard to see. Basic principles of electricity are used in a multitude of diagnostic applications whether you realize it or not.

Experts in every field have something in common: They possess the ability to perform at a high level repeatedly. In our field, diagnostic technicians like John Thornton, Scott Manna and Bernie Thompson come to mind as top performers. While automotive is their craft, they share traits with the greats in other areas such as sports and music. The secret to their success started early in each one of their careers with mastering the fundamentals. If you master electrical fundamentals, you will fix cars right the first time on a consistent basis. Here we will take a different look at some fundamental concepts that apply to your daily diagnostic routine, whether you are the newbie in the shop or the seasoned foreman. While principles of electricity work well in the classroom, the application to reality can sometimes be hard to see. Basic principles of electricity are used in a multitude of diagnostic applications whether you realize it or not. Let’s look at some examples.

Voltage essentials

Whether you are fixing a simple circuit problem or an advanced computer input or output fault, knowledge of voltage and voltage testing is essential. Voltage is a unit of electrical pressure that is necessary for current to flow. Without voltage there is no flow, therefore testing available voltage is typically a good starting point in any electrical diagnostic routine. When using voltage testing, keep in mind that voltage is only the command as the work in the circuit is done by electron flow, or current/amperage. Voltage can be measured digitally with your DVOM or graphically with a graphing meter or lab scope. Knowledge of your equipment is key.

When diagnosing ECU inputs and outputs, always remember that ECUs communicate only in voltage. Sensor inputs can be both analog and digital, but an ECU can only process a signal that is digital or on/off. For that reason, analog to digital (AD) converters are used. A common scenario when diagnosing input faults is that data will not always make sense. For example, if a two-wire analog wheel speed sensor reading jumps between speed values yet the scope waveform at the ECM looks normal, you may have a bad A to D converter.

Knowing how and when to use your electrical diagnostic tools, such as a digital multimeter, is a necessity for getting to the correct diagnosis quickly.
Using voltage drop to test for parasitic draw is built upon foundational knowledge of Kirchhoff's law.

Scoping inputs such as crank, cam, wheel speed, etc. is a great way of diagnosing input problems. While these sensors are outputting a voltage of some sort, it is important to remember that a repeatable pattern’s frequency and signature is often used by the ECU to determine input speed and position. Also keep in mind that triggering and slope come in to play when an ECU is interpreting a signal. I remember a case study a few years back of a wiring repair on a two-wire crank sensor. The tech mixed up the polarity of the wires and wound up with an inverted slope, which led to all sorts of problems for the poor truck owner.

Dealing with voltage outputs on ECUs can also be thought about fundamentally. Output signals can really only be on or off voltage signals. It is a fairly simple concept. Where it gets tricky for most techs is understanding that pulse width modulation (on time), amplitude (height of the Voltage pattern), frequency (speed at which the cycle is repeating) and duty cycle (the percentage of on-time of a cycle) are all control mechanisms that utilize voltage for a specific outcome. One example is a fuel injector. We turn it on for a specified duration (PWM) to match fuel to incoming air. In another example, duty cycling controls your instrument cluster illumination dimming. A low percentage duty cycle results in dim lights while a high percentage duty cycle will yield brighter illumination. You may ask: how does a signal that is duty cycled not blink on and off if it is on for only a portion of the cycle? The answer is simple: They are duty cycled at a frequency that is too quick for your eye to really see.

When we think of voltage testing, we all too often think about basic circuitry. Challenge your fundamental voltage testing skills by using a scope to view inputs and outputs to ECUs. If you have questions, ask them. Learn to take scope captures and screenshots and share them on forums and with other seasoned techs.

Kirchhoff’s Law of Voltage Drop

Kirchhoff’s Law states that voltage drops across a load. A load is basically anything that has resistance and does work in a circuit and is typically a device such as a bulb, inductor, motor, etc. Resistance is also found in connections and other places that we may often overlook. The beauty of understanding voltage drop testing is that we can easily pinpoint the location of unwanted resistance in a circuit or connection. While this concept isn’t new to most, applying it in other areas of diagnosis can provide some new insight on the subject. Let’s look at a few.

An ECU uses voltage drop by reading a voltage after a fixed resistor to determine the resistance and temperature at the Coolant Temperature sensor.

MAF rationality testing
If you have ever tested an MAF sensor for rationality, you may have used a plug-the-numbers type formula or other methods to determine if the MAF sensor is performing properly. Scoping an MAF snap-throttle event is a method that can come in handy to identify MAF problems. Using this type of testing will require knowledge of known good waveforms; however, I like to use this method if I suspect a problem with resistance in an MAF connector or harness. Using this technique, you will scope the signal wire of the MAF circuit both at the sensor as well as at the ECM. For example, most Toyota MAF sensors will have a peak voltage of around 4 Volts on a WOT snap. Anything significantly less can be a great indicator of MAF problems. If the Voltage on the WOT snap at the sensor is different from the Voltage measured at the ECM the problem is typically resistance somewhere in the harness or connections. This test is more or less a graphed Voltage drop test and provides a visible confirmation of voltage drop problems.

A variable resistor and fixed resistor are used by the ECM to determine voltage input.
ECU voltage ouputs can only be on or off, and duty cycling is one way of accomplishing variable speed or position of an ECU output.

Voltage drop as a representative of parasitic draw

Parasitic draw testing on late-model vehicles can be a time-consuming task. Waiting for modules to “go to sleep” and connecting and disconnecting circuits by removing fuses is a recipe for a time-suck of a job. My favorite technique for finding current flow in a circuit is using voltage drop across the top of a fuse. The reason that this concept works is simple as long as you remember one of the essentials of voltage drop. There will only be voltage drop in a circuit when the circuit is closed and flowing current. For that very reason, we can eliminate circuits with no voltage drop and narrow our search for the drain down to the fuse that is showing the highest amount of drop, typically only millivolts of drop.

ECU voltage detection

 Another important concept that utilizes voltage drop is the concept of ECU voltage detection. On circuits that utilize an analog voltage signal such as a coolant temperature or throttle position sensor circuit, the computer utilizes voltage drop to determine the temperature or position.

If you notice in the circuit pictured, there is a fixed value resistor inside of the ECU. The voltage supplied in the circuit is 5 Volts; however, the important voltage reading is the one between the fixed resistor and the ECM. In this type of Voltage detection circuit the ECM is reading the voltage drop after the fixed resistor. As the resistance value of the temperature sensor, or thermistor (temperature sensitive resistor) changes, so does the voltage value read by the ECU. The ECU then compares the voltage value to a programmed lookup table and determines a temperature to be output to a data PID or sent as a signal to the Instrument Cluster for the temperature gauge.

The next time you have a free moment, unplug a coolant or air temperature sensor. When looking at the data PID it should read -40 degrees F. The reason for this? The voltage detected after the fixed resistor is 5 Volts because of the principle of voltage that tells us that there will be available voltage up to the point of an open in a circuit.

Once you have the sensor unplugged, install a jumper wire across the harness. The Temperature PID should now read 284 degrees F. This is because the entirety of the five volts will be used up or “dropped” across the fixed resistor.

The ECM/PCM uses these numbers as set points for diagnostic trouble codes. It is really as simple as that. Remember computers are not smart. Someone programmed them and told them how to respond. The bottom line here is they are simply voltage in and Voltage out devices. Wrapping your mind around this principle will make fixing even the most complex of input or output faults a lot easier.

Scoping voltage and amperage provides a view of the command and the result of the comand. Notice the opening of the injector pintle in the amperage waveform.

Amperage

Amperage or current is the flow of electrons through a conductor. Amperage provides a better picture of the work being performed in a circuit and should be looked at when performing electrical diagnosis on electric motors, inductors or solenoids. The best tool for the job here is an inductive current probe. In the pictured scope capture of a fuel injector, the blue trace is voltage, or command, and the red trace is amperage. When looking at different outputs it is often a good idea to think command and response. As you can see in the image the voltage pattern looks textbook. The amperage pattern does as well. Pay particular attention to the incline or ramp as the coil of the solenoid builds towards saturation. You will notice the buildup is even and happens slowly. You will also notice the classic “seagull” effect in the middle of the ramp. This is the point in which the pintle is lifted off the seat.

Experimenting with current ramping can yield great results. For more information on the subject, be sure to search the archives of Motor Age and have a look at resources out on the web that specialize in scope testing, such as Autonerdz.

Advice for beginners

So you are the new tech in the shop and are wondering where to begin. First off, get your hands on as much info as you can whether it is printed materials such as textbooks or articles such as this one. You can often pick up some older edition textbooks rather inexpensively online. Popular authors for Automotive electrical and electronics include Barry Hollembeak, James Halderman and Jack Erjavec for starters. To get an idea of all the topics that you will need to be fundamentally sound in start by having a look have at NATEF.org. The National Automotive Technicians Education Foundation (NATEF) is the accrediting body that governs secondary (high school) and post-secondary (college) automotive programs. NATEF specifies task lists for each service area and the lists are available on their website. A good honest look through the electrical task list can give you a good idea of where your strengths and weaknesses lie. If you want to get good at something, practice the concept that you are the worst at and go from there. While the NATEF tasks are incomplete, they provide a good starting point. Another suggestion is to write down concepts, ideas and terms that you don’t really understand. Then do some homework to find the answers you are looking for. I always tell my students that if they rely solely on someone to teach them to gain knowledge that they will never get to the level where they want to be. Don’t rely solely on someone else to teach you. Dig deep and do your homework.

If you are just beginning in electrical diagnosis, it's time to do your homework.
A MAF voltage signal throttle snap test is another great way of using voltage to create a picture on your scope.

Training Events

Fantastic training opportunities are happening all over the country. Live training events will provide you with the opportunity to learn from some of the best in the business. If you didn’t have a chance to make it to Automechanika Chicago, you best put it on your calendar for July of 2017. Last year’s event included some fantastic classes on electrical testing by some of the best trainers in the business. Make it a point to get to these events and most importantly, apply what you learned when you get back to the shop.

I now leave you with a simple challenge. Become an expert in electrical principles and apply them to even your most complex of diagnostic scenarios. In the end you will be glad you did.

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