How To Test BCM With Multimeter

Today, we’re tackling a task essential in automotive troubleshooting: testing a BCM with a multimeter. When things start acting up, it’s often the BCM that’s throwing a tantrum.

So, how do you get to the bottom of it? With your tasrusty multimeter, that’s how. Let’s break it down, step by step, electrician-style.

What is a BCM?

A Body Control Module (BCM) is essentially a vehicle’s electronic system’s central nervous system. Think of it as the brain behind the scenes that controls and monitors various electronic accessories in a car, truck, or SUV.

This includes various functions, from the power windows and door locks to the interior lights, exterior lighting, and sometimes even the climate control and alarm systems.

The BCM interacts with other modules and systems in the vehicle, receiving and sending signals to ensure everything operates as intended. For example, when you use the remote to unlock your car, the signal is received by the BCM, which then processes this information and sends commands to unlock the doors and perhaps even turn on the interior lights.

What is a BCM

Beyond convenience features, the BCM plays a crucial role in the vehicle’s security and diagnostic systems. It can detect malfunctions or issues within the systems it controls and may store trouble codes that can be read with a diagnostic scanner, helping technicians identify and fix problems.

The BCM is a critical component that ensures the smooth operation of a vehicle’s electronic systems, enhancing both convenience and safety for the driver and passengers. Its ability to communicate with other systems and modules within the vehicle makes it a key player in the overall functionality and performance of modern vehicles.

How To Test a BCM With a Multimeter

What You’ll Need

  • Multimeter
  • Vehicle’s wiring diagram
  • Screwdrivers
  • Pliers
  • Socket set

Preparing for the Test

First things first, safety. Disconnect the battery before you poke around. We’re not looking to get zapped today. Once that’s done, locate the BCM.

It’s usually under the dash, but it could play hide and seek elsewhere, depending on the vehicle. Consult the wiring diagram if it’s playing hard to get.

Setting Up Your Multimeter

Let’s zero in on setting up your multimeter because this step is where the magic starts. If you’ve been in the game as long as I have, you know a multimeter is as essential as your morning coffee.

But if you’re new to this or have just been using it to check batteries, let’s kick it up. Here’s how to prepare your multimeter for some serious BCM testing action.

Choosing the Right Setting

Multimeters can measure voltage, resistance (ohms), and current (amps), but for BCM testing, we’re mainly concerned with voltage and resistance. Here’s the lowdown:

  • Voltage (V): Use this setting to check if the BCM is receiving or sending power to a component. Your multimeter likely has settings for AC (Alternating Current) and DC (Direct Current). You’re almost always dealing with DC for automotive work, so make sure it’s set to DC voltage. Look for the symbol that’s a solid line above a dashed line.
  • Resistance (Ω): This setting is your go-to for checking continuity, which is a fancy way of saying you’re ensuring electricity can flow through a wire or component without any breaks. It’s also handy for checking if there’s a good ground connection. The symbol for this setting is an omega (Ω). When checking continuity, you want to see a low resistance value. Some multimeters even have a continuity setting that beeps when there’s a continuous path, which is music to your ears when you’re in a tight spot.

Getting to Know Your Probes

Your multimeter comes with a pair of red and black probes. The standard procedure is red for positive and black for ground or negative. When you’re setting up:

  • Plug the black probe into the COM (common) port. This is your ground.
  • Plug the red probe into the port marked with a V, Ω, or sometimes a symbol that looks like a diode. This measures voltage, resistance, and sometimes other functions like diode testing.

Checking for Power and Ground

When diving into the heart of the matter with our BCM, checking for power and ground isn’t just a step; it’s the cornerstone of our troubleshooting. Without proper power and ground, our BCM is just a fancy brick taking up space.

So, let’s break this down further, electrician-style, and ensure we’re doing everything by the book.

Why Power and Ground Checks are Crucial

The BCM, like any other electronic device in your vehicle, needs two critical things to function: power (+12V typically) and a good ground connection. Think of it like a heart needing oxygen and blood to keep beating.

Power gets the BCM up and running, while the ground is the return path for electrical current. If either is missing or inadequate, the BCM won’t function correctly, leading to all sorts of electrical gremlins in your vehicle.

Tools and Setup

You’ve got your multimeter in hand, but let’s ensure we’re setting it up correctly. You’ll want to set your multimeter to the DC voltage setting to check power. Most vehicles operate on a 12V system, so you look for readings around this value.

For checking the ground, switch your multimeter to its continuity setting (symbolized by a sound wave or ohms symbol if you’re also checking resistance).

Checking for Power

  1. Identify the Power Wire: Using the vehicle’s wiring diagram, locate the power wire(s) for the BCM. This is typically a constant power wire that should have voltage regardless of the ignition switch position.
  2. Test for Voltage: Connect the multimeter’s negative lead to a known good ground – this could be a metal part of the vehicle’s frame or the negative battery terminal. Touch the positive lead to the power wire terminal on the BCM connector. You’re looking for a reading of around 12V (give or take a little, depending on the state of the vehicle’s battery). If you’re getting significantly less than this or no voltage, you’ve found a problem area.

Checking for Ground

  1. Identify the Ground Wire: Refer to the wiring diagram to find the BCM’s ground wire. This wire should be connected to the vehicle’s chassis or a ground distribution point.
  2. Test for Continuity: Switch your multimeter to the continuity setting for a quick check. Connect one lead to the ground wire terminal on the BCM connector and the other to a known good ground. You’ve got good ground if you hear a beep (on continuity mode) or see a very low resistance value (almost zero ohms). If not, an issue with the ground path needs addressing.

Troubleshooting Tips

  • No Power? Check the fuse box first. A blown fuse is often the culprit. Also, inspect the relay if applicable. Don’t forget to check the integrity of the wire itself for any breaks or corrosion.
  • Bad Ground? Look for loose or corroded ground connections. Ground points can sometimes lose their integrity due to rust or wear. Clean the connection points and ensure they’re tightly secured. If the ground wire is damaged, it may need replacing.

Testing Communication Lines

Let’s dive deeper into testing communication lines on a BCM because this part can get tricky, and it’s where many gremlins like to hide. Communication lines are the highways of information between the BCM and other modules in the vehicle.

Depending on the car’s make and model, these lines can use different protocols, like CAN (Controller Area Network), LIN (Local Interconnect Network), etc. Each has its own way of talking, and if there’s a communication breakdown, it’s like a traffic jam in rush hour – nothing moves.

inside of a car

Understanding Communication Protocols

First up, you have to know what you’re dealing with. CAN is the most common because it’s robust and can handle much data traffic. LIN is simpler and cheaper, often used for window controls or mirrors.

There might be others, but these two are the main players. Your vehicle’s wiring diagram should tell you which system you’re working with and where the communication lines are.

Setting Up Your Multimeter for Communication Line Testing

You’ll set your multimeter to the DC voltage setting for basic checks. However, remember that while a multimeter can tell you if there’s activity or voltage present, it won’t decode the data or tell you if the communication is healthy – it’s not an oscilloscope.

You’re looking for the presence or absence of voltage that indicates activity or checking for shorts to ground or power.

Testing CAN Lines

CAN lines usually come in pairs – CAN High and CAN Low. They should have a specific voltage when the system is idle (usually around 2.5V for CAN Low and around 2.5V for CAN High, but this can vary).

When the system is active, these voltages fluctuate as data is transmitted. If you see 0V or a constant voltage with no fluctuation, there’s likely a problem. Also, check for continuity between the CAN lines and ground or power – there shouldn’t be any.

Testing LIN Lines

LIN lines are simpler. You’re generally looking for a steady baseline voltage that spikes during communication. Again, use your multimeter to check for the presence of this voltage and for any shorts to ground or power.

Common Issues and Troubleshooting

  1. No Voltage or Fluctuation: If you do not see any voltage or the expected fluctuations, the communication line might be cut or disconnected, or there could be a fault in one of the modules on the network. Check for physical damage to the wires and connectors.
  2. Shorts to Ground or Power: A short can shut down communication entirely. Use your multimeter to check for continuity between the communication lines and ground or power. If you find continuity, trace the wiring to find and repair the short.
  3. Intermittent Issues: These are the trickiest. They might be caused by loose connections, corroded terminals, or damaged wires that only make/break contact under certain conditions. A good wiggle test while monitoring the voltage can help pinpoint these.

Checking Outputs

When you’re dealing with a BCM, you’re essentially dealing with the central nervous system of the vehicle’s electronic functions. This part of the test is crucial because it tells you whether the BCM is doing its job by controlling various components as it should.

Let’s break it down into detailed steps to follow along without missing a beat.

Identifying the Output Circuits

First up, you need to know what you’re testing. The BCM controls various functions – from your power windows and door locks to the headlights and interior lights. Pick a function to test, and then consult your wiring diagram to identify the output circuit from the BCM to that component. This is your target for testing.

Preparing for the Test

Before you dive in, make sure everything is set up correctly. You’ve got your multimeter and wiring diagram and identified the output wire at the BCM for the function you’re testing.

Ensure the vehicle’s battery is disconnected before you start poking around to avoid any accidental shorts or sparks.

Continuity Check

With the battery still disconnected, it’s time to check for continuity. This means you’re making sure the path from the BCM to the component is intact, with no breaks.

Set your multimeter to the continuity setting (usually represented by a sound wave or ohm symbol). Place one probe on the BCM’s output terminal for the function you’re testing and the other on the corresponding terminal at the component end (like the interior light, if that’s what you’re testing).

If your multimeter beeps or shows a low resistance value, congrats, you’ve got continuity. If not, a break in the line somewhere needs fixing.

Voltage Check

Now, reconnect the battery because it’s time to see if the BCM is sending power as it should. Set your multimeter to measure DC voltage.

Place one probe on good ground (metal chassis works great) and the other on the BCM’s output terminal for your testing function. Activate the function via the vehicle’s controls (like turning on the interior light).

You should see a voltage reading close to the battery voltage (around 12V). If you do, the BCM is sending power. If you don’t, either the BCM isn’t sending power, or there’s an issue in the circuit preventing power from flowing.

Load Testing

For a more thorough check, you can perform a load test. This involves using a test light or a specifically designed load tool instead of your multimeter.

Connect the tool in place of the component (like the light bulb) and activate the function. If the test light illuminates or the loading tool shows proper operation, the circuit can handle the required current, and the BCM functions correctly.

If not, you might have a weak BCM output or a resistance issue in the circuit.


Testing a BCM with a multimeter isn’t rocket science, but it does require a systematic approach and a bit of know-how. If you’ve followed these steps and still can’t pinpoint the problem, it might be time to call in a favor from a buddy with a diagnostic scanner specifically for BCMs.

Remember, half of being a good tech is knowing when to call in reinforcements.

And there you have it, straight from the garage floor. Testing a BCM might seem daunting initially, but with the right tools and patience, it’s just another day at the office for our techs.

Keep your multimeter handy and your wiring diagram closer, and never be afraid to tackle the tough jobs. That’s what separates the pros from the weekend warriors.

Alex Klein Author


Alex Klein is an electrical engineer with more than 15 years of expertise. He is the host of the Electro University YouTube channel, which has thousands of subscribers.