A Perfectly Short Case Study
Just before our spring break started, one of our collision students asked us to check his 2006 RX-8 because the cruise control wasn't working. Instead of having my auto tech students do the work, I told the collision student he could try to figure it out with my guidance.
I wasn't able to test drive the car at that time so I took his word about the CC not working. Since it was a newer vehicle, I had him connect a scan tool, which was a new experience for him, and check for DTCs. The only DTC (P0571) was for a brake switch circuit problem. The DTC didn't ring any bells for the student, but I asked him to look up what Mazda had on diagnosing the DTC using AllData. Once he found the procedure, we performed the first couple of steps in the DTC diagnostic, and I left him to check for power at the brake light switch as part of Step 5 of the diagnostic process for the DTC.
I explained how to use an unpowered test light to check the connector for power, and in a few minutes he reported there was no power to the connector. The DTC diagnostic says that if no power is present on the B+ terminal "Repair or replace the wiring harness for possible open circuit or short to ground, then go to Step 11." This had the student slightly alarmed to think he had to replace the wiring harness. Once I confirmed he had correctly used the test light and that there was no power to the brake light switch, I plugged the connector back in, pressed the brake pedal and told the student to observe the brake lights.To his complete surprise, the brake lights didn't work.
Some Q&A took place, and then I asked him what his next step should be, to which he said "check the fuse." So I left him to find the brake light fuse. A little later he informs me there is no brake light fuse to be found. He only knew of the small interior fuse box in the driver's kick panel. So back to AllData he goes to find if there are any other fuse boxes lurking about. Again, surprise and even delight were apparent on his face when he located the massive under hood fuse/relay box.
Using his new friend the test light, he found power on only one side of the Stop LP fuse. After discussing how one does not simply "borrow" a fuse, a replacement 15A fuse was installed, the brake pedal pressed, and no illumination of the brake lights took place. He thought this quite strange. I told him to recheck his fuse, which was of course, blown. The look on his face when I told him fuses were around $48 apiece was priceless.
He now knew he had a bigger problem when he found out he was going to be introduced to "the wiring diagram."
In the interest of space and of reader attention spans, the second half of this saga will be in next month's newsletter.