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Excessive Parasitic Draw? Check If The B-Can System Is Awake (A08070K)

Publication date: 2008-07-01
Reference number: A08070K

EXCESSIVE PARASITIC DRAW? CHECK IF THE B-CAN SYSTEM IS AWAKE

EXCESSIVE PARASITIC DRAW? CHECK IF THE B-CAN SYSTEM IS AWAKE

TECHNICAL SERVICE BULLETIN

Reference Number(s): A08070K, Date of Issue:  July 2008
HONDA: All models equipped with the B-CAN system
CURRENTLY APPLIES TO:: All models equipped with the B-CAN system

Service Information

When it comes to parasitic draw, the body controller area network (B-CAN) system-first introduced in the '2003 Accord-is a major contributor. With the ignition switch turned to LOCK (0), all B-CAN-related components still receive battery power. This gives the various control units such as the gauge control module, the multiplex integrated control unit (MICU), the door multiplex control unit, the climate control unit, the power seat control unit, etc., a means of communicating when, for instance, a power door lock switch input signal changes or there's a signal received from a keyless remote transmitter. Knowing what the B-CAN system does when you turn the ignition switch to LOCK (0) will help you to recognize if it's causing an excessive parasitic draw problem. Normal parasitic draw on a battery varies from model to model, depending on the number of control units involved. Just remember if the draw is less than 50 mA,  it's normal;  if it's 50 mA or more  , it's excessive  .

Wake-up Mode and Sleep Mode 

The B-CAN system has two operating modes to reduce parasitic draw: the wake-up mode and the sleep mode. With the ignition switch turned to ON (II), the system is in the wake-up mode. During this time, the system has both +B power (HOT AT ALL TIMES)  and IG1 power (HOT WITH IGNITION SWITCH ON  ) and there's a parasitic draw on the battery of about 200 mA or more. When you turn the ignition switch to LOCK (0), the system stays awake as long as the key-off timer (part of the MICU) is running. The key-off timer shuts off when the B-CAN system receives a door switch open input signal; otherwise, it shuts off right at 10 minutes  . With the system now in the sleep mode, the parasitic draw on the battery drops to less than 50 mA  . It still has +B power (HOT AT ALL TIMES)  , though, and there are a number of signal inputs that can wake it up to do some work. A good example is the power door lock switch. When you push the switch to its lock or unlock position, the system wakes up, does whatever it needs to do, and then goes back to sleep 5 seconds  later.

Don't Just Rely on Component Replacement 

Many service techs replace B-CAN-related components because they think those components are causing the excessive parasitic draw. When checking for excessive draw, they find that by pulling the backup fuse to the MICU, the parasitic draw drops to normal. What they don't realize, though, is that the real cause for the B-CAN system staying awake could be a stuck door lock switch. If that door lock switch sticks in either its lock or unlock position (the input signal stays on), the system won't return to the sleep mode. This will keep an excessive parasitic draw on the battery that will kill it in short order. Just how long that actually takes depends on the battery's state of charge and other variables. A parasitic draw of about 200 mA  will usually kill a battery in about 2 days. So the bottom line is this: When it comes to excessive parasitic draw, you can't rely on component replacement to always fix things. Even though the suspected component was replaced, there's still an excessive parasitic draw problem that will probably wind up in a comeback . . . not to mention one very unhappy service customer.

Narrowing Things Down 

An easy way to find out if the B-CAN system is staying awake-and causing an excessive parasitic draw problem-is to momentarily short one B-CAN communication wire to ground. There are two types of B-CAN systems: a slower (33.33 kbps)  single-wire system, and a faster (125 kbps)  two-wire system, which was first introduced in the '2008 Accord. Shorting the single wire to ground (single-wire system) or the CAN_H wire to ground (two-wire system) at any of the B-CAN-related components stops all B-CAN communication and puts all of the control units into the sleep mode. If doing this gets rid of the excessive parasitic draw, then an input signal to one of the control units or a control unit itself is keeping the system awake.

Finding the Culprit 

There are two ways to find out which control unit is keeping the B-CAN system awake. One way is to use the All Data List  in the HDS to check for any abnormal switch inputs such as a door lock switch that stays on. Such inputs will keep the system awake. The other way is to start unplugging the components on the B-CAN circuit one at a time. When you unplug the suspected control unit, the rest of the B-CAN-related components will fall sleep. Make sure there are no abnormal switch inputs to that control unit, though, such as a door lock switch that stays on, otherwise the B-CAN system will stay awake.

Some Handy Tips 

Without a doubt, chasing down an excessive parasitic draw problem can be time-consuming, even frustrating work. Here are some handy tips to help make that job go just a little easier:

Meet the LH41A Clamp Meter! 

Most clamp meters can't  check for excessive parasitic draw because they won't detect the small electrical fields that parasitic draw generates. Also, the jaws on many of these clamp meters are just too small to fit over battery cables, making them unusable for this job. This is where the LH41A clamp meter comes in. This required special tool has several important operating features:

Fig 1: LH41A Clamp Meter
G05788845
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