Many of my mobile diagnostic calls begin with, “I replaced all of these parts and it still does the same thing.” After spending time and money, the tech’s diagnostic road has now come to a dead end. He began by blindly following two roads, one leading to pattern failures and the other to symptom diagnostics. With no end in sight, he began “shot-gunning” the problem with a multitude of related parts. His final destination will be to find refuge with a technical hotline, mobile technician or dealership. A dreaded scenario for any technician, his road back home will be long, painful and expensive.
But there’s another diagnostic road less traveled. When individual parts are placed in the context of a complete operating system, the concept of what I call “systems logic” begins to appear. Instead of dealing with individual parts, we now look at how individual parts relate to each other as an operating system. That operating system requires a logical sequence of events to occur before a particular electronic or mechanical function happens, which is why we call it “systems logic.”
ENGINE STARTS, STALLS
Good examples of systems logic occur each September when I serve as a technical advisor for a church-sponsored single mom’s car care clinic. We sometimes, for example, accidentally bump hidden cranking “disable” switches installed by suspicious ex-husbands or encounter engines with bad fuel pumps that will start only by spraying a liberal dose of starting fluid into the air intake.
This year, we immediately encountered a vehicle that would “mysteriously” crank and start, but wouldn’t stay running. Using basic systems logic, I noticed that the tachometer would hang at about 1,500 rpm, which indicates that the crankshaft position sensor, ECM, and ignition coils were operating even as the engine stalled. Systems logic further tells us that fuel systems logic is the specific road to follow.
The car was driven to the church parking lot, so the problem occurred after arrival. A quick inspection revealed that the key in the ignition was cut from a hardware store key blank. Using systems logic, I knew that a non-programmable key would mechanically open the door locks, but not be recognized by the vehicle security system. See Photo 1.
In some applications, a key recognition issue will allow the engine to start, but will deactivate the fuel pump or fuel injectors after a few seconds of run time. Having found my diagnostic road map, a quick search of the console revealed a programmed or “chipped” key, which allowed the engine to keep running. Not a complicated problem by any means, but one that illustrates the basic principle of systems logic.
ENGINE CRANKING SYSTEMS
Let’s apply systems logic to a “generic” engine cranking system. To crank the engine, the starter motor needs these functioning parts: 1) a fully charged battery, 2) an ignition switch, 3) a neutral safety switch or clutch safety switch, 4) a starter relay, 5) a solenoid or relay/solenoid assembly to mechanically engage the starter drive, and 6) clean battery connections and wiring harness integrity. With the exception of hybrid or stop-start technology, automotive engines require an input or output from each of the above half-dozen components to crank the engine.
STARTING SYSTEMS LOGIC
Using starting systems logic, it’s relatively easy to identify many failed component parts from the driver’s seat. Working within the confines of a basic cranking system, no starter relay or starter drive engagement noise indicates a power supply problem at the battery, fuse box or ignition switch (see Photo 2). If the starter relay located in the under-hood fuse box clicks when activated by the ignition switch, we know the ignition switch operates. The same applies if the relay/solenoid assembly mounted on the starter clicks.
Again, using starting systems logic, if the starter relay doesn’t activate, we know that current to the ignition switch flows from battery positive. Once current flows through the ignition switch, it’s routed to the starter relay through a transmission neutral or clutch safety switch to the relay. If the engine cranks in neutral but not in park, the neutral safety switch is worn or misadjusted. If the clutch safety switch doesn’t engage, make sure that the clutch pedal safety switch has continuity with the pedal depressed. Using systems logic, we can assume that, in most cases, ignition switch and neutral or clutch safety switch electrical failures first appear as intermittent, rather than hard, failures.
Once again using starting systems logic, a loud click indicates that the starter drive is engaging the flywheel, but the engine isn’t turning over. A draw of between 100 to 200 amperes from the battery should result in the starter cranking an average-sized, 3/.0-liter engine at room temperature (see Photo 3). Using empirical data gathered through an inductive amp probe, a very low amperage might indicate that that the battery is low on charge, the battery terminals are corroded, or the starter solenoid contacts or starter brushes are badly worn. A very high amperage draw might indicate a locked-up engine.
ECM-BASED STARTING FUNDAMENTALS
What do more sophisticated computer-operated cranking systems need to crank the engine? Let’s remember that, while the electrical architecture might be more complex, all of the above cranking functions mentioned above must still take place. To begin, the vehicle security system must recognize a “chipped” ignition key or a programmed key fob. Once the vehicle security system recognizes the key or fob, the engine control module (ECM) usually prepares the engine to crank by activating the fuel pump and priming the fuel injectors.
Next, the driver either turns the ignition switch or presses the start/stop button. Let’s remember that, in computer-operated starting systems, the ignition switch or start/stop button merely commands the ECM to crank the engine through a relay mounted in an under-hood fuse box or power distribution center. The relay activates the relay/solenoid assembly mounted on the starter. Not only does the relay/solenoid assembly engage the starter drive pinion to the flywheel, it connects the battery to the starter motor, which in turn supplies the amperage needed to crank the engine.
As for starting systems logic, the ECM must “see” the park or neutral transmission gear position or the clutch released pedal position, which is relayed to the ECM by the transmission gear range or clutch pedal position sensor (see Photo 4). The ECM might also want to see an applied brake pedal. Depending upon application, the ECM might require various inputs from the vehicle security system before it will allow the engine to crank. To illustrate, a vehicle security system might actually disable the cranking circuit after several attempts to crank the engine with a faulty key.
In the above examples, starting systems logic has provided the road map, now it’s time to connect a scan tool and let a wiring schematic lead us to the ultimate Diagnostic Solution.
Following the Road Less Traveled: Learning How to Think Like a Computer
1.Looking at systems logic on a higher level, a local transmission shop had a Mitsubishi passenger car that wouldn’t shift into reverse unless the electronics connector was disconnected from the transmission. A new engine/transmission control module (ECM/TCM) didn’t fix the problem.
2. That said, I didn’t get the diagnostic job. Perhaps the shop found a TSB addressing the problem or maybe they sent the car to a dealership.
3. But let’s use systems logic to explore how a computer might “think.”
4. A “mysterious” problem? Decidedly, yes. But once we use systems logic to understand how the ECM/TCM thinks, we’re on our way to arriving at the correct Diagnostic Solution.
5. Using systems logic, we know what the ECM/TCM should “see” before it will allow the transmission to be shifted into reverse.
6. Systems logic tells us that reverse gear is normally controlled by a manual shift control valve.
7. Systems logic also tells us that the reverse lock-out function on older mechanical transmissions is controlled by governor oil pressure, which indicates output shaft or vehicle speed.
8. From there, systems logic might indicate that reverse lock-out occurs when the ECM/TCM “sees” a forward vehicle speed through the transmission output shaft speed sensor or through an antilock braking wheel speed sensor.
9. The above diagnostic scenario would explain why disconnecting the transmission electronics allows the transmission to shift into reverse.
10. Using systems logic, I’m thinking that the failure to go into reverse is an electronic, rather than a mechanical, failure.
11. Maybe it’s a faulty input from a wheel speed sensor? Or maybe the ECM/TCM is looking for a signal from the brake switch.
12. At this point, I would use a scan tool to review all the transmission data parameters and verify data inputs to the TCM. We’ll never know until we look…