Rick's Free Auto Repair Advice

Central Timer Module, body control module, gem module

What the heck is a Central Timer Module, body control module, gem module

As I read auto forums I constantly hear people asking what a body control module does, where it’s located and how to fix it. These computers are known by several different names. Central Timer Module, body control module, gem module. Every car maker is switching to digital controls for almost all vehicle functions. It’s an effort to reduce wiring costs and complexity, reduce weight, and have complete control over all systems to further increase MPG. To understand how the system works, take everything you know about switches and wiring and throw it out the window.

In a digital vehicle, switches are nothing more than signal inputs to a computer. They do NOT switch power. Want to turn on your headlights? The switch gets a low voltage reference voltage from a body control module (BCM). When you turn the switch, it grounds the reference signal. The BCM sees the voltage drop to 0 or near zero (depending on how much voltage drop is in the circuit). Depending on the level of digital complexity, the BCM can provide ground to the control coil on the headlight relay. Or, it can send a digital signal onto the digital bus. In the case of Chrysler, the digital bus connects to the Integrated Power Module (IPM) or Totally Integrated Power Module (TIPM). The TIPM receives the signal and activates either an electro-mechanical relay or a solid state relay.

GM calls their BCM a body control module. Ford calls theirs a Generic Electronic Control (GEM) module. Chrysler calls theirs a Central Timer Module (CTM). In the case of Chrysler, and depending on the vehicle and trim package, the CTM can control the following systems:

Battery Saver Functions For Exterior and Interior Lamps
Cargo Lamps
Chime Warning
Courtesy Lamps
Dome Lamps
Dome Defeat
Door Ajar Switch Status
Fog Lamps
Head Lamp Time Delay
Intermittent Wiper Controls
Low and High Beam Head Lamps
Optical Horn
Park Lamps
Central Locking (VTSS)
Door Lock Inhibit
Driver Door Unlock
Enhanced Accident Response
Power Door Locks
Remote Radio
Remote Keyless Entry
Vehicle Theft Alarm (VTSS)
Horn Chirp
Optical Chirp

The Central Timer Module is located behind the left side kick panel. It is a high failure item. However, before you replace a BCM, make sure the digital bus isn’t shorted. Sometimes a module on the bus can short and place voltage onto the bus. That screws up all digital communications results in all kinds of crazy behavior. Also, since the BCM relies on monitoring low voltage reference signals to the switches, any corrosion can also screw things up. So make sure you check the condition of the connectors before replacing an expensive module.

Finally, most BCMs must be programmed by the dealer or a shop equipped with the proper scan tool. You CANNOT take a BCM from a similar vehicle and slap it into yours. It must be programmed to your exact VIN. In most cases, you’ll also have to reprogram your remote key fobs and keys.

So whether your brand calls it a Central Timer Module, body control module, gem module, know that it’s the computer that controls various features that used to be directly controlled by switches.

© 2012 Rick Muscoplat

Posted on by Rick Muscoplat

Custom Wordpress Website created by Wizzy Wig Web Design, Minneapolis MN