Tom Piippo: Case of ‘the needle in the haystack’

March 20, 2019

Tom Piippo,

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The customer complaint was one of the usual.

“The ‘Service Airbag’ message comes and goes on my dash display.

“Sometimes it’s on for a mile then goes out.

“Sometimes it doesn’t come on until I’m a mile down the road, then stays on all day.

“Can you fix it? The light is on now!”

I headed out to the 2006 Sierra 2500 with my Tech2 scanner and retrieved a code B0103, Electronic Front End Sensor (Crash sensor) 2 Performance.

Tom Piippo

The simple diagnosis would be to swap sensor #1 with sensor #2 and see if the code follows the sensor or stays with the position. This simple and low cost “Swapnostics” would help determine if the sensor was bad or something else, like wiring, connectors or a bad Sensing and Diagnostic Module. That would be the simple thing to do BUT this truck has RPO (Regular Production Order) of HVY as indicated by the tag in the glovebox. Trucks with RPO HVY only have one Electronic Front End Sensor, and it is labeled #2. No simple swapping is going to happen here.

Some common issues with these sensors is corrosion at the connector. After all, it’s mounted up front and down low, under the radiator support where all the mud, water and salt spray from the northern Michigan roads can attack all year long.

The connector was clean & tight with the silicone gaskets and CPA (connector protection assurance) tab in place.

I inspected the body of the sensor as sometimes they crack, and corrosion makes the body swell and bulge like a frozen beer can in February. This one was nice and square!

Lets take a look at how this system works so we can better understand how to test the components. According to ALLDATA…

“DTC B0103

CIRCUIT DESCRIPTION

The inflatable restraint front end sensor utilizes a unidirectional 2-wire circuit. The front-end sensor modulates current on the interface to send ID, State of Health, and deployment commands to the inflatable restraint sensing and diagnostic module (SDM). The SDM serves as a power source and a ground for the front-end sensor.

When the ignition is turned on and input power from the SDM is first detected, the front-end sensor responds by performing internal diagnostics and sending an ID to the SDM. The SDM considers the ID to be valid if the response time is less than 5 seconds.

The front-end sensor continually communicates status messages to the SDM, which determines if a fault is present in the front-end sensor circuit. When a fault is detected, the SDM resets the front-end sensor twice by removing and reapplying power.

If the fault is still present, the SDM will set a DTC.”

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So, the front end sensor is more than just an inertia switch that will blow the airbag if you smack it with a hammer. It can send an ID signal, state of health signal, and other commands on this 2 wire circuit to the SDM.

So these circuits need to be handled like any other network circuits. There can be no erroneous signals, spikes, or grounds on this circuit, in fact GM twists this pair of wires all the way back to the SDM just to be sure that RFI (radio frequency interference) does not mess with it.

Be sure to read and heed all the service precautions before working on airbag circuits. This usually includes disconnecting the battery and waiting a specified period before disconnecting ANYTHING with these delicate circuits.

Again, read the service information for the model you are working on.

The Airbag Module (SDM) for this truck is located under the drivers seat, which must be unbolted for access. This truck has power bucket seats, so just unbolting the 4 corners was no big job and tilted it back to reveal the slit in the carpet to gain access to the SDM. Liberating the connector from the module I could locate the two wires that should eventually connect to the front end sensor #2. I measured the resistance of each circuit from the SDM to the front-end sensor and each measured less than .4 ohms. Then I “jumpered” the circuits together at the front-end sensor and read the resistance of both circuits together, reading at the SDM connector. I accessed the wire harness and began manipulating and flexing the harness while watching the meter. I noticed that this truck had recently been fitted with a snow plow, so I paid special attention to where the plow brackets were attached.

No problem found.

I was about to order a new front-end sensor when I realized that I had forgotten to test the other two sides of the circuit triangle. Opens, Grounds, Short to Power.

I clipped on lead of the ohmmeter to the empty seat stud and bingo! 4.32 meg-ohm short to ground! You might not think that this is really a short to ground, after all, how much current can 4.32 meg-ohms support?

Apparently enough! Now, how to find it?

I found that manipulating the harness under the door sill panel would make the meter fluctuate wildly, but never less that 4.32 meg-ohms. There was quite a bit of sand and gravel that had accumulated in this area over the past 11 years and I had an idea that this gravel had abraded the harness and insulation, but everything looked quite intact!

I stripped away the taped harness and inspected the twisted pair, wiping everything clean & dry. I knew that water (salted water) on a bare wire was causing my issue and I was looking right at the problem, but I just couldn’t see it. With the wires dried off, the short to ground went away!

They should make a tool for that, I thought. Not wanting to cut out chunks of wire harness and replace wires needlessly, I devised a plan. I soaked a rag in the salted drippings on the shop floor. I clipped a jumper wire to the rag and the other end to ground.

I then started wiping the rag along the wires until the meter started reading continuity again. There! The needle in the haystack. It looked like a gray speck on the yellow wire, but it was indeed a perforation in the insulation. This was just enough to skew the communication between the sensor and the module and tripped the service air bag message.

I inspected the wire carefully with a magnifier and saw NO corrosion, so I sealed the wire with liquid electrical tape. I vacuumed all the gravel from under the sill plate, re-taped the harness, and for good measure stuck it all inside of a corrugated loom. After reinstalling all the connectors, interior panels and seat, I hooked up the battery and retested the system… It worked flawlessly.

The success of your repairs depends that you follow a system; Confirm the complaint.

I would have no direction for this repair if the problem had not been present. I would have been tempted to “poll the audience” (as one game show would have you do) to research one of the websites that post known fixes for a B0103 DTC, which most often has you replace the front end crash sensor.

Determine the cause, in this case, perforated insulation.

Preform the proper correction, in this case, just a dab of sealer and re-wrapping the harness so it wouldn’t happen again.

I have banners hanging in my shop to remind me: Complaint, Cause, Correction, Confirmation, Communication.