Monday, April 10, 2006

Tip #4: How does BCM module works?

Generally speaking, there are 2 types of engine computers used in your vehicle: one for engine management (called ecm, ecu, pcm or eca) and a second one for the vehicle body that controls the dash panel and all the gages; security like alarm system which monitors all openings like doors and tailgates; audio and sound system; wipers and cruise system…in short almost all the functions of your vehicle except the engine!

Sometimes you will experience your dash gages not working accompanied by both radio and wipers quitting at the same time. What to do? Check your BCM.

Here are some quick checks you can do to test your BCM:

Find the location of the BCM module usually located under your dash panel depending on the vehicle type and model. can assist you in finding the correct location/wiring diagram for most vehicles up to 2003-2004 models).

Once you find the BCM, you can do the following:

Wiggle test the wiring terminals going to it while the problem is acting up. If there is a bad connection, you might find a change in the symptom.

Undo the ecm terminal to isolate the bcm (remember to remove the battery connection every time you touch the bcm or your ecm). Why isolate the ecm? Because if there is a short in the ecm (engine computer) circuit, it will affect the bcm too. Sometimes after you disconnect the ecm, the dash gages return which shows the problem was the ecm system.


If unplugging the ecm does not work, try unplugging each component of your BCM. Example: remove the radio plug-in, security alarm module plug-in, wiper module, door module, cruise control module etc (do the removal one at a time and watch your dash to see the effect). The last module you touched where the dash gages start to work is the culprit. If none of these work, try measuring the voltage as shown below.

Measure the BCM pin ground supply provided in the wiring diagram. Any measurement higher than 0.250 volt is a red flag which shows something is shorted in the circuit. Again these ground pins or connections are shown to you in a wiring diagram if you are a member of ATS.

Measure the bus pin terminal, they are usually shown as positive or negative bus connector. Unlike the ecm where you check the reference voltage at 5 volts, measure the bcm bus connector pin and you should get 2.4-2.5 volt for normal operation. If the reading is normal (2.4 to 2.5 volts), it shows the bcm is OK. You can then replace the instrument panel first. If after replacing the instrument panel and still the same problem exist, replace the bcm.


Before replacing the instrument panel, check with your dealer or ATS to confirm the programming of your odometer. On most models, the odometer program is located in the bcm not in the instrument panel. So if you are replacing the bcm, you have to remove the old one so you can take it to the dealer who will need it to program your new bcm.


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