Body Electrical Malfunctions

In the days before the onboard computer, simple electrical devices, such as headlamps, were wired directly to the battery in a basic circuit that included a switch and a fuse. Pulling the switch out, if it were of that design, closed the circuit allowing current to flow from the battery to the headlamps—it was that simple, then.

On modern vehicles, that is no longer the case. Added to this circuit is a branch that goes to the onboard computer. When the switch is pulled out, a signal is sent to the computer, and if the right conditions have been met, current is allowed to flow through the circuit, illuminating the headlamps. If that sounds a bit complex, well, it is. But that is how the modern automobile is wired.

Years ago, if the headlamps didn’t work, the fuse was the first thing to inspect. If that was good, then the headlamp was checked (it was usually this item). Diagnosing the problem was that easy. But with complexity in design comes more complexity in diagnosis. And the diagnosis of body electrical problems will be the focus of this article. Let’s review the operation of the Body Control Module and look at a sample problem.

THE BODY CONTROL MODULE

The Body Control Module (BCM) is located in the passenger compartment, mounted to the bulkhead underneath the left side instrument panel (Figure 1). It utilizes integrated circuitry and information carried on the Programmable Communications Interface (PCI) data bus network, along with several hard-wired inputs, to monitor sensor and switch inputs throughout the vehicle. Using this input information, the BCM controls and integrates a large number of electronic functions and features of the vehicle. This is done through hard-wired outputs and electronic message outputs, over the PCI data bus, to other control modules in the vehicle.

These outputs are visual and audible information (like a chime) to the vehicle passengers and control of various vehicle functions (such as power door locks). Actually, the BCM provides a whole range of features that are associated with the control of the vehicle body. These include headlamp time delay, automatic door locks, interior lighting, exterior lighting, automatic headlamps and illuminated entry. There are a few more, but you get the idea.

Let’s look at a specific function. In this case, it will be headlamp function since this component was described in general terms in the introductory paragraphs. The headlamp, dome and door ajar switches provide signals to the BCM (direct inputs). These inputs are seen by the BCM as changes in the switch status. To the driver it is the action of turning the particular switch to the on position. The BCM then sends a PCI message to the Front Control Module (FCM) when the switch status is changed. This enables the necessary drivers to set the required illumination configuration. The configuration of the headlamp system (headlamps, parking lamps, fog lamps) is determined by the BCM.

Four wires are connected between the headlamp switch and the BCM. Wire #1 contains information regarding the position of the headlamp switch (6 possible positions). Wire #2 contains information regarding the position of the dimmer switch (4 possible positions). Wire #3 is a dedicated signal return (ground) wire. Wire #4 provides power to the front fog lamp indicator.

Automatic headlamps are controlled by the BCM. With the headlamp switch in the Auto position, the BCM controls all the exterior lights on the vehicle, plus the instrument panel lamps. The level of light will be based on the ambient light (the lights will be brighter as the ambient, or daylight, becomes darker). This is done by a sensor that is located on the front windshield in front of the rear-view mirror.

So, when the driver gets in the car and turns the key on, the exterior lights, including the headlights are turned on. As many of you recall, in the old days, the use of the headlamps was independent of the operation of the car. That is no longer true.

DIAGNOSING PROBLEMS

When a problem occurs with the headlamps, or another body component, what do you do? Where do you start to look for the source of the problem? Well, these are valid questions.

Begin your body electrical system failure diagnosis by testing the related fuses in the fuse block and the intelligent power module. Verify that all bulbs are in good condition. Also, test continuity of the circuit ground. Quite often, the source of an electrical problem can be found with the connectors and wires. Wire connectors that are loose can become corroded (corrosion will have a chalky or green color). If corrosion is found, replace the bad terminals to avoid a recurrence of the same problem.

If a more difficult body system problem exists, following the Six Step Troubleshooting Procedure is recommended.

• Verify the complaint
• Verify any related symptoms
• Analyze the symptoms
• Isolate the problem
• Repair the isolated problem
• Verify proper operation of the system

To aid you in troubleshooting a body electrical problem, remember that the BCM has an internal diagnostic capability. When an open or short circuit exists, the DRBIII®, or equivalent scan tool, can be used to read the BCM faults. These faults are very descriptive in identifying the appropriate feature that has faulted. Remember the following points when using a scan tool:

• First, be sure that the DRBIII®, or equivalent scan tool, is communicating with the body control module; for example, if the scan tool displays a No Response or a Bus +/- Signals Open condition, this condition must be diagnosed first
• Read the Diagnostic Trouble Codes (DTCs) with the DRBIII®, or equivalent scan tool
• Once the DTC or customer complaint is identified, locate the matching test in the Table of Contents in the Body Repair Manual and diagnose the symptom

It is important that appropriate service and diagnostic manuals for the vehicle being repaired are being used. Diagnostic procedures change every year. New diagnostic systems might be added from the previous year and carryover systems might be enhanced.

A HEADLAMP PROBLEM

We have been focusing on headlamps in this article, so let’s take a look at a headlamp problem. We will diagnose and solve it using the diagnostic procedures outlined in the specific diagnostic manual. Let’s assume the vehicle with the problem is a 2007 Dodge Caravan.

The problem is that the headlamp switch is open. Possible causes can be bad connectors, the headlamp switch multiplexer (MUX) circuit is open or the return circuit is open, or the headlamp switch is open.

The first step is to connect the DRBIII®, or equivalent scan tool, and turn the ignition on. Be sure all previous codes have been erased. Turn the headlamps to the on position and read the DTCs. If the DRBIII® display reads, Headlamp Switch Open?, turn the ignition off.

Next, disconnect the headlamp switch harness connector. Connect a jumper between the headlamp switch MUX circuit and the headlamp switch MUX return circuit in the headlamp harness connector (Figure 2). Turn the ignition on. If the headlamp switch sensor voltage displayed on the DRBIII® reads less than 0.5 volts, replace the headlamp switch.

If the displayed voltage was greater than 0.5 volts, the next step would be to check the resistance of the headlamp switch MUX circuit. If the resistance displayed on the DRBIII® reads greater than 5.0 ohms, repair the MUX circuit for an open or high-resistance circuit. If the reading is less than 5.0 ohms, check the resistance of the headlamp switch MUX return circuit.