If a vehicle battery goes dead after a number of hours, perhaps overnight, and there is no apparent and obvious source of electrical consumption in the vehicle (no lights on, no radio or other electronic devices operating, no doors left ajar), the common name for this condition is “excessive parasitic draw.”
Depending on year, make and model, car manufacturers specify a maximum draw upon the battery when everything is OFF. In modern cars there is often a minimum waiting period of up to 15 minutes when all lights have been shut off, the engine switched off and the vehicle closed up. Some components, especially in the bus system(s) may remain active for that long, but then they are expected to quiet down and drop below the minimum draw of about 50 mA. A known good battery should be able to supply that much electrical current (to the clock, for example) for many hours and still retain enough power to start the vehicle, even on a cold morning.
Recently, our Honda Technical Support Operator line fielded a call from a repair shop which was struggling to diagnose a 2007 Honda Ridgeline pickup truck with dead battery symptoms. Our help line technician made sure that the first order of business was to charge and test the battery itself to make sure that it was capable of holding a charge. Once that was established, it was necessary to test the charging system to make sure that the battery was being kept in adequate state of charge. Also, the shop technician was instructed to question the owner of the vehicle as to their driving habits. After all, if the vehicle was parked for long periods followed by extremely short drives, that could cause depletion of battery charge over time.
Finally, we instructed the shop technician to test the parasitic draw on the battery. He found it to be approximately 350 mA, clearly much higher than any standard allowed.
Measuring and diagnosing parasitic draw is a separate topic and covered in a related article. In this article we will cover the practical difficulties often encountered when an electrical problem is encountered in an aging vehicle. One tip for this particular vehicle, locking the vehicle using the key will put the vehicle to sleep immediately.
In this case, we asked the technician to use standard electrical-draw diagnostic technique to narrow the draw down to Fuse 7 in the under-dashboard power distribution center. This fuse is known to power the following:
- Instrument cluster
- Power windows
- Security and body module
By disconnecting these components one at a time, the tech discovered that disconnecting the cluster reduced the draw greatly. Accordingly, his shop obtained and installed a used cluster. Upon rechecking, it turned out that the draw was still there.
Using additional tests, the tech established that disconnecting the cluster reduced the parasitic draw to 80 mA, higher than allowed by specifications.
We instructed the shop tech to look at the wiring diagrams for the vehicle very carefully. By do doing so, he found that Fuse 7 also powers B-CAN network.
We instructed him to back probe the B-CAN network wire at the back of the cluster. Doing so, he found that network activity was present. While registering parasitic draw of 350 mA, he removed the connector from the cluster and again the draw dropped to about 80 mA, but network activity was still present, even after a lengthy timeout. Something on the B-CAN network was evidently staying awake.
We told him to refer to the wiring diagram and detach every component on the B-CAN network except the body module. The draw remained. The body module is integrated into the fuse panel. Using the wiring diagram, he checked inputs to the body module to see if a stray input was keeping the module awake. There is no scan tool access to this network to see data parameters. After checking what he thought to be every input, the shop installed a new body module, but this step also did not fix the problem.
We asked him to retrace his steps and he came to same conclusion: that the body module was staying active.
By rechecking all the inputs, he discovered something he had missed: The Ridgeline has a two-way tailgate. It can be lowered conventionally like any pickup, but it also opens like a barn door. The barn door feature was broken, but after fixing it he found another input. This trim level of the truck also has a tailgate electric release with wiring hidden in the pickup bed. With the tailgate lowered conventionally this was difficult to see. Testing at the switch indicated the power to it was shorted to ground. This short is the equivalent to a release request; a release request that never stops. This was why the body module was always awake.
Tracing the wiring, the tech found a section of wiring that was melted together, causing the short.
A replacement harness for the trunk lid cured the draw, allowing the modules on the B-CAN network finally to rest.