Learning doesn’t stop once you’re out of school. If you can learn more about a problem, you can work your way through to a solution.
That’s what Brian Todd did with a 2003 Nissan. The car presented problems that rendered the headlamps inoperative. Let’s hear about the problem and how he fixed it straight from him.
I recently had a 2003 Nissan Altima 3.5 that woke me up to the importance of understanding the Controller Area Network (CAN) and lab scope diagnosis.
The customer called me with the complaint of his headlamps not working on cold mornings. He coupled this with an odd observation. He told me the rear defroster would act funny at the same time. The symptoms would last for about 10 minutes, after which time everything would return to normal.
Because he had an upcoming business trip, we made arrangements for him to leave the car for the week. On the first frigid morning, I went out, turned the key on, turned the headlamp switch and rear defrost switch on, and sure enough, no lights. The rear defrost lamp would flicker, and I could hear the defrost relay clicking on and off.
By that time I got a sense of what was going on, so I opened the hood and looked for any obvious causes (battery connections, harnesses, etc.). The headlamps and rear defrost came on in what was probably less than two minutes. I am not going to dispute his claim of 10 minutes, but on a dark December morning, when you get in your car, turn the lights on and start your commute to work I guess two minutes could feel like 10. Nevertheless, the fault was confirmed.
Everything worked normally for the remainder of the day, until the next cold morning. Because this was an unusual problem, the first step I took in my diagnosis was to check my resources (TSBs, Identifix, iATN), but none of them helped. Next, I decided to look for the link between the headlamps and rear defroster, and spend some time studying and understanding the circuit. I am a firm believer that if know how it is supposed to work, you can fix it.
I found that both the headlamp switch and rear defrost switch are inputs into the Body Control Module (BCM). The BCM then sends its message via the CAN bus to the Integrated Power Distribution Module (IPDM), which in turn controls the primary circuits of the headlamp and defroster relays, amongst others. Whew! That’s a lot of things involved for headlights.
Where Do I Start?
I checked all the modules on the network for any trouble codes, but none were present. The IPDM is a receive-only module, and is not capable of self-diagnosis, nor can it transmit information. It only responds to messages on the CAN bus that are meant for it. The BCM has bi-directional controls, so I tried commanding the headlamps and rear defroster on via my scan tool functional test on the next cold morning, but this yielded me the same no lights/no defroster results. At least now I knew my problem did not concern any BCM inputs.
The possibilities were starting to narrow. Was the message bad from the BCM? Was the IPDM getting the message and not responding correctly? Was there some other module or network problem corrupting the message? It was time to hook my labscope to the CAN bus and see what’s going on.
Now I had a baseline and good clean CAN A and CAN B waveforms. I then prepared for the next morning by accessing the BCM and IPDM modules and their respective CAN terminals. I could hardly sleep that night, anxious to see what I might find.
Powering up my labscope and turning the key on yielded a very different looking CAN message. Definitely not like the first snapshot I took. The waveform was definitely corrupt.
That good, clean CAN waveform had lost all it's uniformity. I then started by disconnecting the CAN Terminals at the BCM, but the waveforms did not change. Then I went to the IPDM and disconnected it and voila, the CAN waveform cleaned up instantaneously.
I ordered and replaced the IPDM. The headlamps and defroster now work flawlessly even on the coldest of mornings.
Interestingly enough, I found when the CAN message to the IPDM is interrupted by disconnecting either the BCM or the IPDM, the IPDM defaults to headlamps ON. When I started my disconnect procedure, I started at the BCM. If I had not been looking at the CAN waveform to see that CAN was still corrupt, or If I didn't have a lab scope, I might have been fooled into thinking that the IPDM was capable of turning the lights on with the BCM disconnected, and that the BCM was at fault.
It pays to dot your I’s, cross your T’s, and keep your lab scope at the ready.
TST Big Event Comes To California
Mark your calendars for Sept. 29, which is when Technicians Service Training (TST) will hosts its first West Coast Big Event.
This one-day training event will be at the Ontario Convention Center in Ontario, Calif., from 8 a.m. to 5 p.m. Registration includes all handout materials, food and refreshments. G. “Jerry” Truglia and Dave Hobbs are scheduled to present.
Truglia’s “P0420s—Keeping the Light Off” covers diagnosing catalytic converter efficiency DTCs the smart way by utilizing a comprehensive diagnostic process that includes looking at fuel trim, oxygen sensors, PCM reflashing and much more.
Hobbs’s “Network Diagnostics—CAN Data” covers everything you need to know to diagnose and repair network and communications problems (those pesky U-codes!) on domestic and foreign vehicles. Hobbs covers both pre-CAN and CAN.
For complete information, contact TST at [email protected] or call (845) 628-6928. Space is limited.
