No doubt, some troubleshooting scenarios become nightmares once you get your sleeves rolled up and tear into things. At times, however, you have to be careful so as not to let the beginning of a nightmare turn into a full-fledged episode of sleepwalking into the diagnostic danger zone.
You know it. It's that deep, dark place where logic no longer applies, and you begin to slide down the slippery slope of guesswork and unbillable hours. It’s a painful place. Here, we take a look at some brief, common scenarios to help clear your thinking in case you would find yourself in such an encounter.
MAF mayhem
If you’re suspicious of a bad mass airflow sensor, but you can't get it to act up in the shop, consider the following tip. With your lab scope or DVOM connected to the MAF sensor's signal wire, start the engine and verify a signal on the test instrument. Run the engine at various speeds, provoking the fault. Next, take a hair dryer and warm the body of the sensor to see if it causes a change. Then, tap the sensor housing to aggravate the problem further. Watch the scope or meter display for hiccups in the signal. If you see any loss of signal whatsoever, replace the sensor.
Alternator aberrations
Did you know that a faulty alternator can cause onboard electronics to go haywire? The fact is, AC leakage from the alternator can confuse electronic controllers, so it's a good idea to check for too much AC when chasing weird, ghost-like problems. To test for this, connect a digital voltmeter to the battery with the engine running. With the meter set to the AC range and on the millivolts scale, you should see no more than 400 millivolts (0.4 volt) AC. Repair or replace the alternator as necessary.
Grounds for transmission problems
A common cause of transmission problems that unrelated to the transmission is bad grounds. During a transmission teardown, the clues are usually heavily worn bushings throughout the transmission. The bushings end up serving as the ground path instead of the ground cable. Although the carmakers have added additional grounds and nylon bushings to combat this problem, it still exists.
You can do your customers a favor checking for bad grounds by measuring voltage drop during cranking. Connect the voltmeter's negative lead to the negative battery terminal and the positive lead to the axle shaft on front-wheel-drive vehicles, or to the transmission case on rear-wheel-drive vehicles. Crank the engine. You should see no more than 0.1-volt drop. If you see more than that, (1.0-2.0 volts is not uncommon), the transmission's bushings may be acting as the ground cable.
A signal only tells part of the story
When diagnosing a misfire condition on a fuel-injected engine, and you’ve used a signal checker of some sort to confirm whether the injector is actually receiving a signal, remember this: If an injector signal seems to be evident, the injector itself may be faulty in an electrical sense, or the injector's spray pattern may be altered, triggering the misfire. Food for thought: Injector circuitry varies widely. Some systems trigger injectors in groups; others trigger them individually, which is the case with sequential injection. Keep this in mind because an engine with several "dead" cylinders may be the result of a loss of signals to a group of injectors.
Thermostat thesis
The technical sophistication of modern onboard electronics often causes us to lose perspective on other real factors that are interrelated. One such example is a car with a fuel mileage problem. It could be something as simple as the lowly thermostat. If the coolant temperature is too low, the coolant sensor will tell the PCM to turn on more fuel to compensate for the lower temperature. Since many areas of the country are into the winter months, proper coolant temperature, and thermostat operation, is something to keep in mind.
Short report
Here's an effective way to track down an electrical short in main parts of the electrical system: Connect a 5-amp circuit breaker across the fuse block terminals of the affected circuit. With the circuit activated, probe the various parts of the circuit and connectors with your test light. If the light blinks on and off, continue to the next portion of the circuit. When you get to a point where the test light doesn't come on, the short will be between that point and the last point where the light flashed.
Defroster diagnosis
To check the power supply, connect the clip end of a test light to one end of the grid buss bar and touch the other buss bar with the probe end. (You can also do this with your multimeter). If the light comes on, power and ground are OK. If the light doesn't come on, connect the ground clip to another ground. If the light now comes on, there's an open ground circuit. If the light still doesn't come on, check the fuse and connections.
To check the defroster's heating ability, connect the clip end of the test light to a good ground. Now, touch each horizontal bar at the supply end with the probe. The test light should glow brightly. Continue moving across the bar to the ground end and touch the bar gain. If the bar is working correctly, the light should dim as you move towards the ground end. If the test light brightness stays the same as you move across the bar, you've found one with an open circuit.
As with most things, the best approach is usually the simplest. If you remember this, you can keep your productivity at its highest, with the lowest amount of lost diagnostic time and comebacks.
