When the circuit is short, the airbag light is on
Then there's the red ENGINE light on some '80s vintage Ford cars that was wired to alert the driver if there was an oil pressure or coolant problem. But the driver typically had no idea what the problem was when that light came on, and I knew of one shop that had it confused with the yellow CHECK ENGINE light that had frustrated them on so many GM cars from that era. The shop told one girl who owned one of these cars not to be all that concerned when that ENGINE light came on, even though the car would quit and wouldn't re-start until she had let it cool a bit.
When it finally came to me (she actually drove it into the write-up area), I found that somebody had filled the cooling system with straight tap water a year or two earlier, and the radiator and heater core were totally clogged with rust. That tough little 2.3L had been so hot it had quit running probably a dozen times or more. The thermostat had turned blue, like it would if you heated it with a torch. I removed the radiator to have it cleaned, flushed the cooling system and replaced the thermostat, and that near-bulletproof little engine ran like a sewing machine. Who knows how close she was to a terminal meltdown because somebody told her to ignore an important warning light.Many of today's drivers are still confused by the CHECK ENGINE or SERVICE ENGINE SOON lights, and there are GM cars that have SERVICE VEHICLE SOON lights that can be troublesome and quite mysterious to the uninformed.
Then there are those funky ISO symbols that look kind of like a small radiator, but is that what it is? One lady poured her engine full of water because the oil filler cap was so near the cooling system fill point on her SUV. There are warning lights that look like wrenches and gears and a number of other symbols, and every light is either driven by a sensor or computer acting on inputs
One particularly debilitating warning light, even on older vehicles, is the battery light that is connected to the alternator. If that light doesn't work due to a severed wire or damaged printed circuit on older systems (Fords particularly), the alternator won't work because the voltage regulator gets its initial trigger through the warning lamp bulb. With that in mind, the first thing you should check is whether that battery warning light is operational with the key on and the engine off. If it's not, you need to find out why. The warning lamp has a parallel resistor to provide a current path if the bulb blows, but that resistor can't help it if the wire to the regulator is cut. Furthermore, if the circuit that feeds the warning lamp is shorted to ground, the battery light will be illuminated, but the alternator won't work at all. That scenario can lead to an unnecessary alternator replacement. Check your wiring schematics to determine how the light fits into the circuit for troubleshooting purposes..Airbag Lights
Airbag lights can be fun, especially since the voltage supplemental restraint systems provided for diagnostics is only a whisper. Even a tiny amount of oxidation can cause the black box to fire up an airbag light. And while creative repairs might be OK on other electrical/electronic systems, any thinking technician will realize just how dicey it is to patch an airbag harness. Unsanctioned airbag harness repairs are a discreet no-no and more troublesome than repairing somebody's redundant cruise control cancellation circuits with butt splices and Scotch Locks.
Most turn of the century Explorers came without the airbag in the seat that prevents the driver's head from crashing into the B pillar in a T-bone collision. But all the wiring was in place for that option, and a problem-prone yellow dummy plug was installed at the side airbag connector under the seat. Consequently, those models were fraught with Driver Side Airbag codes that were all the more confusing because a less than savvy tech might be looking for the problem in the circuit feeding the airbag on the steering wheel. After all, isn't that the Driver Side Airbag? No, it isn't. That one is referred to simply as the Driver Airbag in the scan tool's lexicon.As repair orders with this irritating anomaly continued to surface, flawed but contemporary 20th century wisdom might dictate that a splice-and-solder job right at the module connector between the two wires feeding that long useless loop might be a viable choice. After all, what could go wrong? Well, maybe nothing, and maybe a whole heck of a lot of everything, considering the potential liability of such unauthorized modifications, whether they be real or imagined by a judge and jury. As it was, Ford came out with a TSB that provided a factory authorized wiring overlay to alleviate that concern
The repair on this month's Motor Age subject vehicle was one I didn't expect, because nobody was complaining about it. One of the college maintenance department pickup trucks was in my shop for a simple oil change, which was performed by a student. After he and the other automotive trainees had gone, I was finishing the vehicle inspection when I noticed that the airbag light was on (GM calls its system SIR for Supplemental Inflatable Restraint). I mentally chastised the student for neglecting to mention that fact.
Opening the System Window
First I honked the horn to see if maybe the clockspring had been compromised, and was rewarded with a nice crisp two-note response. That generally means the problem is somewhere besides the clockspring, but not always. Let me briefly digress at this point to say that I've seen at least one clockspring fail internally on a 2002 Dodge Dakota and cause an airbag deployment loop code and found that the horn circuit in that clockspring was just fine. The horn honk test was inaccurate on that Dodge.
With the Genisys connected and the 2001 C1500 selected, I found a B0022 code, which basically indicates a short in the airbag deployment loop feeding the driver airbag on the steering wheel. The criteria are that ignition voltage is within the normal operating range and the airbag module detects a steering wheel deployment loop resistance less than 1.3 ohms for at least 500 milliseconds. These deployment loop tests are done continuously as long as the ignition is on..So what's included in the loop? Well, there's the steering wheel airbag, the clockspring and the two wires that form the sides of the loop – they go directly to the airbag module. Oh, and don't forget the shorting bars built into every connector in the circuit to mitigate inadvertent airbag deployment during service, which is a nasty event that can ruin your whole day, if not your whole life. And gold plating is the order of the day on airbag circuit pins as well as the shorting bars on a late model Chevy.
My idea came down to three distinct possibilities. My first and most probable prognosis was that the deployment loop wires were touching each other somewhere (or moisture in a connector, maybe?). Possibility number two would be that the steering wheel airbag was shorted. The third possibility was a faulty module. Since the module and the airbag weren't likely to be the cause of the concern, my focus became the deployment loop wires.
Disarming the Bomb
Most of us have heard stories of airbags that exploded while somebody was working on the car, and some of those stories are more tragic than they are funny. Disarming the bag is a must, and we all know that, but familiarity breeds carelessness, sometimes enough to flirt with disaster. It was time to disconnect the battery and wait a few minutes for the backup capacitors in the module to bleed off. There are a variety of dummy plugs (system specific) available that can be inserted to fool the system into believing an airbag is there, but I didn't have one handy for this particular vehicle. So with the system disarmed, I removed the airbag from the steering wheel, an adventure in itself if you've never done one. I have a medium size cheap flathead screwdriver I've modified for the job, and after a time or two, it's quick and easy to unlatch the bag and disengage it from the wheel.
The airbag connector has a little locking tab that is supposed to be snapped out of place, and the tab swings from a plastic tether so it won't be lost. After that tab is disengaged, the tiny airbag connector can be unplugged from its socket. This particular airbag connector came loose from the squib as I was trying to remove the locking tab. "Interesting," I thought, and I stored that thought for future consideration.
Reconnecting the battery, I rechecked the SIR system codes, and this time I got a B0026 code, which is the polar opposite of my previous code. What that meant to me was that the problem was right there at the airbag: with the connector plugged into the bag, I got a shorted loop, and with it disconnected, I got an open loop. Shorted airbag squib? Maybe, but, as I thought the situation through, I reflected on the fact that while the connector was plugged into the bag, it had in fact come loose while the locking tab was still in place. There was a distinct possibility that the connector had been making contact with the pins, but the connector hadn't gone deep enough into the bag's pin socket to disengage the shorting bar. In that case, the airbag connector shorting bar's connection across the pins would be the cause of the B022 code. It made perfect sense.
Seemingly insignificant details like this connector situation can be totally overlooked by students during a disassembly operation, effectively muddying the water for an instructor who's trying to teach and fix vehicles at the same time.
Disarming the system again, I unseated the locking tab, seated the connector securely and snapped the locking tab back in place to find that the connector was firmly rooted in its socket this time. Reconnecting the battery cable, I found that all the DTCs were now gone – blue skies and no warning lights! What didn't make sense was how it had happened. That airbag light hadn't been on before, and I knew for a fact that none of the maintenance guys had taken the airbag off the steering wheel and caused this problem. I was certain that they didn't know how to perform that operation.
Conclusion
The airbag light was off and the bag was re-armed. It was a fun little exercise, one that gave this mechanic-turned-instructor a simple dose of diagnostic pleasure in the final hour of that day.
Warning Lights
Richard McCuistian is an ASE-certified Master Auto Technician and was a professional mechanic for more than 25 years. He is an auto mechanics instructor at LBW Community College/MacArthur Campus in Opp, Ala. E-mail him at [email protected]
