Electronics use devices to manipulate the flow of electricity. Most of the actions performed by today's vehicles are commanded by an electronic control module (ECM) using inputs from numerous sensors and switches. An ECM is only as dependable as the information it receives. When an ECM makes a bad call, it's generally because the module received wrong information from an input or attempted to command a faulty output. In some cases, an ECM will recognize an erroneous input and report it to the user in the form of a malfunction indicator lamp and a diagnostic trouble code (DTC). Other times it will believe the inaccurate input and continue to output invalid commands.
This article will offer tips and pointers that will be helpful when diagnosing an ECM-controlled system. This isn't about the inner workings of transistors, diodes, and thermistors. There will be no in-depth discussion about the movement of electrons between atoms, nor will we dive into electrical theory formulas. The purpose of this column is to help you diagnose the vehicle that's sitting in your shop. It's always crucial that theory, description, and operation are studied and understood for the system you're servicing before the onset of diagnosis.
ECMs are not smart
Every decision an ECM makes is based on its programming; it will never think out of the box. Regardless of how many times you try to type the word "train" on a keyboard with an inoperative "T," it will type "rain." The computer doesn't know what your intentions were and it doesn't care. It will do what it's programmed to do. So, the man at the station will board the "rain" every time. Computers really aren't smart. They are obedient, reactive, and quick to process, but they're not smart.
Look before you leap
Before proceeding with diagnosis, perform a thorough visual inspection. The visual inspection process is the most crucial step of all automotive diagnoses. Eliminating the obvious should be the first step when diagnosing an electrical concern.
Look under the hood, under the vehicle, and the dash for poorly installed aftermarket devices. Inspect harnesses for wire rub-through or critter chew-through. Check the area under the battery for a harness that might be corroded due to past battery leakage. Was the vehicle sunk in a puddle, causing water intrusion in connectors, or was it in a crash pinching a harness between bent metal? Spare yourself the frustration of locating the issue following a prolonged diagnosis when you could have found it at the beginning during a competent visual inspection.
Hands-on testing
Electronic diagnosis starts with pulling DTCs. Even if you know which module the concern originates from, it's best to start by pulling DTCs from all modules. The operation of a module can be affected by false information transmitted over the network by another module. Pulling all DTCs allows you to determine if there's a network issue and if other modules are reporting a concern related to the issue you're diagnosing.
Pinpoint tests (PPT), also called diagnostic flow charts, take you step-by-step through a well-thought-out series of tests that should eventually lead to a diagnosis. Unfortunately, the final result of a PPT isn't always the correct diagnosis. The problem with the PPT is that the author of the test isn't in the service bay diagnosing that particular vehicle. This means that you are the eyes, ears, and hands of the test.
Don’t get caught in the trap
Pinpoint tests procedures make many assumptions when they calculate results. It's up to the technician to recognize the discrepancies between what the PPT says and what he sees. It's not unusual for a PPT to display a wrong wire color or show a connector cavity to be empty when it's occupied by wire. Don't perform a PPT like you're putting together a piece of furniture from Ikea. Tab "A" doesn't always fit into slot "B", and the result is never predetermined. When directed by a PPT to check a circuit, it's essential to understand why you're testing it, what it does, and how to interpret the results.
For instance, when pursuing an Ambient Air Temperature Sensor (AAT) issue: Pinpoint test step A5 may direct you to check for voltage at pin 1 of connector C714 at the AAT. It may then it ask, "Is any voltage present?" The result of this step determines whether you're repairing a circuit or moving to the next step of the PPT.
If you're simply following directions, since no voltage measured at pin 1, you answer "no" and move on to step A6. The problem with this robotic approach to testing is that the test doesn't always tell you why you performed this step, and what you eliminated as a possible cause of the concern.
This is why you should be following every PPT step with an open wiring diagram. This allows you to see what is being tested and why. In the scenario above, pin 1 is the ATT signal wire to the Powertrain Control Module (PCM), and it should not be powered up. The wiring diagram shows that only the AAT would be affected if there's a short to power on this circuit. It also reveals an inline connector between the PCM and the AAT, which is a possible source of an intermittent issue. By viewing the schematic, you understand the test you performed and the system you're diagnosing.
Be careful not to change the state of anything between PPT steps unless the test directs it. The PPT step above could have tripped you up if you didn't carefully read the steps leading up to it. Step A2 requires disconnecting the connector at the PCM and does not reconnect it. The test asks if "any" voltage is present. There could be less than a volt creeping down that circuit with the PCM connected. Reconnecting the PCM after performing step A2 could cause a false result, leading you down entirely the wrong path.
The outlined procedure isn’t necessarily the most efficient
The main rule of performing a PPT is to carry out the steps without skipping an operation. The problem is that these tests usually make minimal effort to streamline the diagnosis. It's not uncommon for a test to require access to a module to check for power, just to find out that you have a blown fuse in the next step. Sometimes ripping through interior trim panels to access a module takes more time than the actual testing. Unfortunately, the test doesn't mention the fuse until you've removed the center console and discovered that there's no power to the module connector. This is why you need to be proactive.
When a PPT requires checking power feed at a hard-to-access module, reference the wiring diagram and back your way into the module connector. If the fuse has power and isn't open, check for power at an easy-to-access inline connector between the fuse and the module. This way, you're eliminating easy to access components and connectors. If all tests well, you'll have to gain access to and check power at the module.
If the module has no power, you've already eliminated the circuit section between the fuse and the inline connector as being the problem. If the module is powered up, you are right where you need to be for the next step of the pinpoint test.
The same goes for grounds. Reference the schematic and put your hands on all easily accessible grounds that pertain to the module you're pursuing. Finding a loose or corroded ground during a visual inspection is a "touchdown".
A logical approach will be a time-saver
You can also save time by performing numerous tests at once. Pinpoint tests will have you check the circuits in a connector for an open, then go back and check those circuits for a short to ground, then the test will send you back to the same connector pins to check for a short to power and a short to each other. For the sake of streamlined diagnosis, let's bang out all these tests at once.
After using an ohmmeter to verify that the circuit is closed, move the probe to the other wires in the connector, checking for continuity between circuits. Then move one probe and check for continuity to ground. With the probe still connected to ground, turn on the ignition, switch the meter to DCV, and check for a short to power. You've completed four tests on the single circuit with virtually one hookup.
In an ECM-controlled system, the amount of load that a wire carry can vary significantly between circuits. The wire that commands the HVAC module to move the temperature blend door carries only a fraction of the wire's current that moves the blend door actuator, yet the PPT has you check both circuits with an ohmmeter. Ohmmeters send minimal current through a wire. Circuits that fail under a heavy load during operation can easily pass with flying colors when tested with an ohmmeter. Load testing a circuit uses the tested wire to operate a load, forcing it to carry a current similar to what it endures during regular operation. We like to use one or two brake lamp bulbs for the load when testing a suspect wire. Again, it’s recommended to use a load similar to the intended load.
A suggested way to load test a wire is to attach one end of the suspect wire to the positive battery terminal and the other side to the bulbs using jumper leads. Connecting the alternate side of the bulbs to the negative battery terminal should illuminate the bulbs. The suspect wire is now supplying current for the bulbs; therefore, it's assuming the load. Compare voltage at the bulbs with battery source voltage. Due to resistance in the jumper leads and the suspect wire, you could see about a 1V difference or voltage-drop. Much more than that means that there's a problem. If the voltage drop is over a volt, substitute a known good jumper lead for the suspect wire. If the drop decreases, the suspect wire isn't handling the load. Test the ground side the same way by using the suspect ground circuit to power up the bulbs.
Real life application
We were diagnosing inoperative heated seats on a newer model Ford Taurus. The PPT directed us to check power at the Seat Control Module (SCM), which is integral to the Driver’s Seat Module (DSM).
First, we verified the integrity of the fuse that powers the SCM. Then we checked power at the SCM with a voltmeter. The voltage at the connector was 12.5V, battery voltage was 12.6V, all good. We load-tested the power circuit using one brake lamp bulb. The bulb lit up brightly, and the voltage drop at the bulb was only .6V.
We replaced the DSM as per the PPT. Still no heated seats. We went through the PPT again. This time we load-tested the power circuit using two brake lamp bulbs. The bulbs did not illuminate. When we pulled one bulb from the circuit, the other lit up. When we added the second bulb, they both went out. The power circuit to the SCM was able to handle the load of one bulb but failed under the load of two bulbs and failed under the load of the heated seat elements. The cause was a burnt connector at the Battery Junction Box.
Circuits that burden a heavy load, like heating elements or electric motors, should be tested using two bulbs. A circuit that uses a small gauge wire can be load tested using only one bulb. A brake lamp draws about 2 amps, so be careful; the 4 amps load of two bulbs could fry a low current circuit.
Follow the clues
Heat increases the resistance of a metal conductor. This standard of electro physics can cause chaos when diagnosing a concern in an ECM-controlled system. Resistance is the foundation for most module inputs, so any changes in resistance can cause values to go out of range.
When diagnosing an intermittent concern, especially if it's a circuit issue, it's beneficial to know the ambient temperature and the engine temperature at the time of the malfunction. The high passenger compartment temperatures of a vehicle sitting in the sun and the even higher engine compartment temperatures can impact a circuit's resistance, changing a sensor input to the ECM.
Obtaining this information starts with the customer's description of the concern. A good service writer will ask the customer if the concern occurs during the first start-up or when the engine is warmed up. Is it worse in the afternoon than it is in the morning? These clues can help you determine the best conditions to test the suspect circuits.
Freeze Frame Data accompany most electronic engine control DTCs. Freeze frame data is a snapshot of predetermined PIDs when the DTC is set. Included in this data are engine temperature and ambient temperature. This is excellent information to have.
We diagnosed a Ford F150 that was intermittently turning on the check engine light with a stored P0190 (Fuel Rail Pressure (FRP) Sensor Circuit Malfunction). When the technician performed the PPT with the engine cold, no values were out of range. Freeze frame data showed that the DTC set when Engine Coolant Temperature (ECT) was 211 deg-F. We monitored the ECT PID until the engine reached the target temperature, then rechecked. Circuit 1289 from the FRP to the PCM had 22 ohms resistance when hot. We found that the wire harness broke loose from its retainer and was lying on a heater hose. Repositioning the harness is all it took to fix the concern. You have a good argument if you say that we should have found this during the initial visual inspection.
In-car temperature can play a big role when diagnosing an airbag malfunction indicator lamp concern. It takes very little change in circuit resistance for the Restraints Control Module to turn on the warning indicator. It's not uncommon for an airbag circuit to test out of range only after a long soak in the sun.
Network concerns add a whole new dimension to automotive electronic diagnosis. When a module perceives an issue within the network, it will store a U code, but not all U codes mean an issue with the network. Bad information coming over the network will also set a U code.
Let's say the Anti-lock Brake (ABS) module is sending an incorrect vehicle speed value to the PCM. The PCM might store DTC U0415 - (Invalid Data Received from ABS Control Module). This DTC tells us that communication between the PCM and the ABS module is good, but the PCM doesn't believe what the ABS module is reporting. The most likely cause of this DTC is an invalid input to the ABS module. This is why it's important to pull DTCs from all modules.
It's easy to lose track of the fact that automotive electronic diagnosis is electrical diagnosis — the proper amount of current needs to move along the designated wire at the correct time for everything to work smoothly. Understanding the strategies of the module you are diagnosing is critical when diagnosing an ECM-controlled system. Take the time to read Description and Operation before tackling the pinpoint test. Understand the reason for each step of the PPT and the consequences of the results. Don't be a robot. Modules are relatively stupid, and automotive technicians are brilliant. It's no contest.
