What does the GEM module control? | Ford Explorer Forums - Serious Explorations

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What does the GEM module control?


Elite In Memoriam
January 17, 2001
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Year, Model & Trim Level
1999 XLT
What are ALL OF the systems controlled by the GEM module?

From the Ford Service CD:

"The remote anti-theft personality (RAP) module controls the perimeter anti-theft, the keyless entry, and computer operated locks systems. For further information on the perimeter anti-theft, refer to «Section 419-01A». For further information on the keyless entry and computer operated locks, refer to «Section 501-14B».

The generic electronic module (GEM) is equipped on four-wheel drive (control trac system) or two-wheel drive vehicles with power windows. The central timer module (CTM) is equipped only on the two-wheel drive vehicles without power windows.

GEM/CTM diagnostics are compatible with New Generation STAR (NGS) Tester.

The GEM/CTM incorporates the functions of several different modules into one and offers diagnostics to easily locate and repair concerns affecting the subsystems that it controls.

The GEM/CTM constantly monitors the systems under its control and reports a concern in the form of a diagnostic trouble code (DTC). A DTC can be retrieved with NGS Tester through the communication link.

NGS Tester is a menu-driven tester that allows the user to run specific diagnostic tests. It can isolate faults in the GEM/CTM subsystems through a symptom-driven diagnostic procedure. Connecting NGS Tester to the GEM/CTM communication link will not alert the GEM/CTM. If the GEM/CTM is in sleep mode when the diagnostic connector is plugged in, the GEM/CTM must be activated. If the communication link cannot be established, it will advise to turn the ignition to RUN.

The GEM/CTM control system has two modes of operation, the normal operating mode and the tester dependent diagnostics mode.

The GEM/CTM can only detect open circuit/short to ground (GND) faults when the GEM/CTM is not energizing a load, and short to battery faults when the GEM/CTM is attempting to energize a load.

The normal operation of the GEM/CTM can detect errors. An integral part of the normal GEM/CTM operation is the continuous diagnostic capability. Continuous diagnostics detects errors and changes the GEM/CTM control strategy. There is no warning light for the GEM/CTM; therefore, a DTC concern will direct the technician to the GEM/CTM. Examples of faults that may be detected during normal operation include:

l GEM/CTM continuous self-test (RAM, ROM, EEPROM, A/D checking).

l Output driver monitoring (open circuit/short to GND, short to battery).

l Illogical combinations of input signals.


The GEM/CTM controls the heated backlite and mirror timer. When the rear window defrost switch is pressed, a ten-minute timer is started and the GEM/CTM energizes a relay that provides battery voltage to the heated backlite (and the heated outside rear view mirrors, if equipped).

The GEM/CTM de-energizes the relay when either ten minutes have elapsed, the rear window defrost switch is pressed again, or the ignition switch (11572) is turned to OFF or ACC. The ten-minute timer is temporarily halted if the ignition switch is turned to START, and resumes after the ignition switch is returned to RUN.

The GEM/CTM controls five modes of operation for the front wiper: MIST, INTERVAL (speed-dependent GEM only), LOW, HIGH and WASH.

These modes are active only when the ignition switch is in the RUN or ACC position.

The GEM responds to the windshield wiper control commands by interpreting inputs from the steering column seven-position delay/wash and four-position mode select switches. Except when the seven position switch is in interval 1, the speed dependent wipers will compensate for the extra moisture that accumulates on the windshield at higher speeds.

The GEM/CTM relay drivers control three external relays: wiper high/low speed relay, dynamic brake-run relay, and washer pump relay.

The GEM does not control the rear washer.

The GEM responds to the rear wiper control commands by interpreting inputs from the rear wiper mode wash switch. There are two modes of operation for the rear wiper: INTERVAL and LOW.

The GEM relay drivers control two external relays: rear wiper up relay, and rear wiper down relay.

The GEM/CTM controls all audible chimes and warning lights. The GEM/CTM performs a prove out on all GEM/CTM controlled warning light miniature bulbs (13466) (except the seat belt and door ajar) by turning on the miniature bulb for two seconds after the ignition switch is turned to RUN from START.

The exterior lamps are not part of the battery saver protection, and will remain on even if the GEM/CTM turns the interior lamps off.

The GEM/CTM will keep power available for all interior lights including the dome lamp, map lamp, outside rear view mirror illumination lamp, door lamps, glove box lamp (if equipped), ashtray lamp, key ring lights, cargo lamp, for 45 minutes after the ignition switch is turned off. The battery saver time is reset by receiving an illuminated entry request from the RAP module (if equipped), activating a door ajar switch, changing the ignition to START, or moving the key to the RUN or ACC position.

The battery saver feature removes extended interior lamp power drains to the battery (10655). The GEM/CTM keeps power to these loads for 45 minutes after the ignition switch is moved to OFF. After the ignition switch is moved to OFF and 45 minutes have elapsed with no change of state on any wake-up inputs, the GEM/CTM will enter a sleep mode. During the sleep mode, the GEM/CTM maintains all of its outputs in a deactivated state until a wake-up-command is received. For additional information, refer to «Section 417-02».

A wake-up command is sent when:

l A change in door state from open to closed, or closed to open occurs.

l The key is inserted or removed.

l The ignition switch is turned from OFF to RUN.

l The ignition switch is turned from OFF to ACCY.

l A wake-up request is received from the remote anti-theft personality (RAP) module.

The GEM/CTM illuminates the interior courtesy lamps when any door is set ajar or is opened. The following are inputs used for controlling the courtesy lamps:

l All door ajar switches (including liftgate).

l Illuminated entry request with RAP module.

l Two-step unlock door disarm switches.

All interior courtesy lamps are turned off when any of the following occurs:

l All the doors are closed and the illuminated entry feature is not active.

l The ignition switch is turned to RUN.

l Vehicle speed is over 8 km/h (5 mph).

l A power door lock command is received before the five seconds have elapsed for two-step unlock.

The GEM/CTM controls the interior lamps illuminated entry function with the interior lamp relay. The relay is activated by applying ground to the relay coil through C283-12, circuit 1008 (LG/O), and is deactivated by removing the ground.

The RAP module sends a signal to the GEM on the illuminated entry request circuit to indicate a request to turn the illuminated entry feature ON or OFF.

The GEM illuminates the interior lamps when an unlock signal is received from the RAP module. The illuminated entry feature will be canceled when any of the following conditions are met:

l 25 seconds have elapsed since the illuminated entry feature was activated and the interior lamp control is not activated.

l The GEM receives a request from the RAP module (remote transmitter lock button pressed) and the courtesy lamp control is not activated.

l The ignition is in the RUN or START position and the interior lamp control is not activated.

The illuminated entry inputs are:

l Door open warning lamp switch (grounded when the handle is lifted; an open circuit when the handle is released).

l Ignition switch in the key-in, ACCY and RUN position.

l RAP illuminated entry request.

The GEM/CTM sleep mode outputs are:

l None (All circuits are deactivated during sleep mode).

If a change in any of the above inputs occur while the GEM/CTM is asleep, the GEM/CTM will wake up and monitor all of its inputs, and will have the ability to activate any of its outputs.

The GEM module provides temporary power to the power windows, the rear wiper (for parking of the rear wiper off the liftgate glass), and the message center for ten minutes after the ignition switch is moved from the RUN to OFF position. If, during this ten-minute period, either front door is opened, the delayed accessory relay will be de-energized.

The GEM controls the one-touch down window feature. When the power switch for the driver window control is held down (between 0.04 and 0.4 second) and then released with the ignition switch in RUN OR ACC, the driver window will automatically move to its full down position.

Only the driver window has this feature. Holding the driver window switch down longer allows the operator to stop the window as soon as the switch is released. For additional power window information, refer to «Section 501-11».

To activate the two-step unlock system the GEM requires that there must be two transitions of the unlock switch. After the GEM senses the first transition, it unlocks that door where the transition occurs and illuminates the courtesy lamps. There is a five-second window that begins from the first transition of the unlock switch. If within that window, a second unlock command is sensed by the GEM, the GEM will energize a relay to unlock all the doors after the second transition of the unlock switch returns to the vertical position in the door lock cylinder."