Central Timer Module, body control module, gem module

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

The Central Timer Module (CTM) is a crucial component in modern vehicles that often goes unnoticed by the average driver. Some carmakers call this small computer a Central Timer Module, while others call it a body control module or Generic Electronic Module (GEM module).

Every car maker is switching to digital controls for

Central Timer Module for a Dodge Dakota

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.

A central timer module acts like a smart switch in your home

In a digital vehicle, switches are only signal inputs to a computer. They don’t actually switch power. Want to turn on your headlights? The switch gets low voltage reference a body control module (BCM). When you turn the switch, it completes the circuit to ground causing a voltage drop. The central timer module/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 its version 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, 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 kick panel on the left side. 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-to-voltage and place voltage onto the bus. That screws up all digital communications and 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