I have to apologize for giving some wrong information that pertains to a later model year where the programming is necessary. On the 1995 Ranger you have, the Vehicle Speed Sensor (VSS) is located on the output shaft area of the transmission (or transfer case) housing. I think you may have a problem with the sensor, or the wiring from the sensor. You have to make sure the wiring is not shorted. The ground wire must be good in order for the sensor to create a signal. The sensor creates a pulsing voltage that the PCM and speedometer can use. If the wiring is bad, the voltage can jump around or drop out completely. The speedometer needle is evidence that the VSS voltage is erratic. Since the PCM depends on the VSS for proper shifting, VSS issues are probably the reason for the problems.
Diagnostic Trouble Codes: 639, P0715. (May or may not have codes stored -- Check Engine light will be on if codes are stored.)
Vehicle Speed Sensor (VSS) 9E731:
Description: The Vehicle Speed Sensor (VSS) is a magnetic pickup that sends a signal to the processor assembly. This VSS signal tells the processor assembly the vehicle speed.
The PCM uses this information to control shift scheduling and EPC pressure.
Symptoms: Harsh engagements, firm shift feel, abnormal shift schedule, unexpected downshifts may occur at closed throttle. Torque converter clutch will not engage, elevated EPC pressures.
NOTE: EPC shorted to ground or failed low may result in a false VSS code. Diagnose EPC concerns first. Then recheck VSS.
Please disregard what I was describing in my previous post about the programming needed. The Vehicle Speed Sensor's signal is probably causing your issue. Since the speedometer shares the signal with the PCM, you can guess how confused the PCM will be if the speedometer is jumping around. There may not be a code set, and the check engine light may not illuminate in all cases. But the symptoms warrant diagnosis of the VSS circuit. Here is a wiring diagram:
Below I have attached some info about the GEM, it is for your reference, and shows that the GEM has nothing to do with the transmission shifting control as the shop had advised you before.
GEM Controlled Functions
The Generic Electronic Module (GEM) 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 constantly monitors the system and reports concerns in the form of diagnostic trouble codes (DTCs). The DTCs can be retrieved with the Rotunda New Generation Star (NGS) Tester 007-00500 or equivalent through the communication link.
The NGS is a menu-driven tester, allowing the user to run specific diagnostic tests. The NGS can isolate faults in the GEM subsystems through a symptom-driven diagnostic procedure. Connecting the tester to the GEM communication link will NOT alert the GEM. If the GEM is "asleep" when the diagnostic connector is plugged in, the GEM must be "awakened." If the communication link cannot be established, it is advised to turn the ignition to RUN.
The GEM has a sleep function to minimize battery consumption. During the sleep mode, the GEM turns off all outputs and monitors only the following:
l Door ajar (driver's door, passenger's door, liftgate, rear right and rear left doors)
l Key in ignition
l Key in RUN or ACCY
l Door handles (driver's or passenger's)
l Two-step unlock switch
l Neutral safety switch
l Illuminated entry request -- RAP module
If a change of status occurs at one of these inputs while the GEM is asleep, the GEM will "wake up." When the GEM awakens, it performs an internal self test, and begins normal operation. At this point, the GEM will have all of its functions operational.
The GEM will go into the sleep mode immediately after both of the following conditions have been met:
l Key in OFF or key not in ignition
l After 45 minutes with no change on any wake-up input
The GEM control system has two modes of operation, the normal operating mode and tester dependent diagnostics mode.
The normal operation of the GEM can detect errors. An integral part of the normal GEM operation is the continuous diagnostics capability. Continuous diagnostics detects errors and changes the GEM control strategy. There is no warning light for the GEM; therefore, a DTC concern will direct you to the GEM. Examples of faults that may be detected during normal operation include:
l GEM continuous self test (RAM, ROM, EEPROM, A/D checking)
l Output driver monitoring (open circuit/short to GND, short to battery)
The GEM can only detect open circuit/short to GND faults when the GEM is not energizing a load, and short to battery faults when the GEM is attempting to energize a load.
l Illogical combinations of input signals
A description of the subsystems controlled by the GEM follows.
Heated Backlite and Mirror Timer
When the rear window defrost switch is pressed, a 10-minute timer is started and the GEM energizes a relay that provides battery voltage to the heated backlite (and the outside rear view mirrors if installed). The rear window defrost heats up to defog the rear window or melt accumulated snow or ice.
The GEM de-energizes the relay either when 10 minutes have elapsed, the rear window defrost switch is pressed again, or the ignition switch (11572) is turned to OFF or ACCESSORY. The 10-minute timer is temporarily halted if the ignition switch is turned to START and resumes after the ignition switch is returned to RUN.
Windshield Wiper Control and Interval Timer
There are five modes of operation for the front wipers: MIST, INTERVAL (standard and speed-dependent), LOW, HIGH and WASH.
These modes are only active with the ignition switch in the RUN or ACCY position.
Front Windshield Wiper Control (with Speed Dependent Interval Wiper)
Modes: OFF, Interval, LOW, HIGH, and WASH.
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. Speed dependent wipers compensate for the extra moisture that accumulates on the windshield at higher speeds. At higher speeds, the speed dependent feature shortens the delay between wipes when using the variable interval wipers. Delays will automatically adjust at speeds between 16 and 105 km/h (10 and 65 mph).
The GEM relay drivers control three external relays: wiper high/low speed relay, dynamic brake-run relay, and the washer pump relay. When the washer switch is activated longer than 300 ms, the wash and wiper (low speed) functions are provided until the switch is released. The release of the switch will be followed by three low speed wipes. If the switch is pressed between 100 ms and 300 ms (with the mode in the OFF position), the wiper system will provide one wipe at low speed with no wash.
For standard interval, the time between low-speed wipes will range from 1.5 to 20 seconds. These delays will shorten as vehicle speed increases up to 105 km/h (65 mph).
This is the fastest wiper speed with no dwell. It is selected when the multi-function switch (13K359) is in the HI speed position. If the wash button is pressed for 0.3-10 seconds, the washer pump will energize until the wash button is released.
This is the slowest wiper speed with no dwell. It is selected when the multi-function switch is in the LO speed position. If the wash button is pressed for 0.3-10 seconds, the washer pump will energize until the wash button is released.
If the wash button is pressed for 0.3-10 seconds, the washer pump will energize until the wash button is released.
Rear Interval Wiper Control
Modes: OFF, Interval, LOW and WASH.
The GEM responds to the rear wiper control commands by interpreting inputs from the rear wiper mode wash switch. There will be two interval wiper settings to provide the following rear wiper dwell times.
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The GEM relay drivers control two external relays: rear wiper up relay, and rear wiper down relay. When the washer input on the rear wiper mode/wash switch is activated longer than 300 ms, the wash and wiper functions are provided until the switch is released. The release of the switch will be followed by three low speed wipes. In the OFF position, if the switch is pressed between 100 ms and 300 ms, the wiper system will provide one wipe at low speed with no wash.
There are six audible warning tones and four warning lights that the GEM controls. The GEM performs a prove out on all warning light miniature bulbs (13466) (except the seat belt, 4x4 and door ajar) by turning on the miniature bulb for two seconds after the ignition switch is turned to RUN from START. The warning chimes and lamps are described below.
A single tone will be provided each time a ground signal is received from the message center. A one-second tone will sound for warning indications from the message center. A 100-millisecond tone will sound for each message center button depressed.
Headlamp On Warning
A chime sounds when the exterior lamps are on (parking, license plate and headlamps (13008)) and:
l There is no key in the ignition switch 20 seconds after ignition is turned off.
l The driver's front door (20124) is ajar (or open).
l Vehicle speed is below 24 km/h (15 mph).
Door Ajar Warning
When at least one door is ajar and the key is in the RUN position, the door ajar cluster lamp is activated and remains on until all doors are closed. A warning tone will sound once any time the key is in the ignition switch and any door is open. The GEM's flexibility allows customizing of different warnings (i.e., a steady tone for a driver's door ajar).
Driver Seat Belt Warning
If the driver's seat belt is unbuckled when the ignition switch is placed in RUN, a chime sounds for 6 seconds (or until the driver's seat belt is buckled) and a warning light is turned on for 61 seconds (or until the driver's seat belt is buckled).
Key In Ignition
Any time the key is in the ignition switch and the driver's door is open, the module sounds a chime (120 chimes per minute) until either the key is removed from the ignition or the driver's door is closed.
The exterior lamps are not part of the battery saver protection, and will remain on even if the GEM turns the interior lamps off.
The battery saver feature removes extended interior lamp power drains to the battery (10655). The GEM keeps power to these loads for 40 minutes after the ignition switch is moved to OFF.
The GEM will keep power available for all interior lights, key ring lights, cargo lamp, and hood lamp circuits for 40 minutes after the ignition switch is turned off. The battery saver timer is reset by lifting a door handle (receiving an illuminated entry request), activating a door ajar switch, changing the ignition to START, or moving the key to the RUN or ACCY position. No lamp or chime indication is provided.
A WAKE-UP command is sent when:
l any door becomes ajar or is opened or closed.
l the front door handle is lifted (if equipped with illuminated entry).
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 RAP module.
The GEM 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 Door handle switches
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 25 seconds have elapsed after all the doors are closed.
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.
The interior courtesy lamps will be turned on if any front door handle is lifted or if any door is ajar. If the lamps are turned on by lifting the front door handle or by illuminated entry request, the lamps stay on for 25 seconds.
The interior courtesy lamps are turned off when the ignition switch is turned to RUN.
The module provides temporary power to the power windows, the rear wiper (for parking of the rear wiper off the liftgate glass), the power moon roof, 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. If either door is opened during a one-touch operation, the module will wait until the one-touch operation is completed before de-energizing the delayed accessory relay.
When the power switch for driver's 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's window will automatically move to its full down position.
Only the driver's window has this feature. Holding the driver's switch down longer allows the operator to stop the window as soon as the switch is released.
The one-touch window feature can be enabled or disabled by the operator.
1. Close both front doors.
2. Key ON.
3. Key OFF.
4. Remove key from ignition switch.
5. Press and hold the down window switch to enable/disable the one-touch window feature.
6. Reinsert the key into the ignition switch within two seconds after pressing the down window switch.
7. Continue to hold the switch until a two-second tone is heard.
8. Release the switch while the tone is active.
9. After the two-second tone, the following confirmation beep sequences will be heard:
l One beep -- feature disabled.
l Two beeps -- feature enabled.
Repeat the procedure again to toggle back and forth.
Upon receiving a two-step unlock command, the GEM 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.
Four-Wheel Drive Electronic Shift-on-the-Fly (Ranger)
This feature electronically shifts the vehicle transfer case between 2WD, 4WD, and 4WD LOW. The system mode is selected by the operator through a three-position rotary switch on the instrument panel. The operator is informed which mode the system is in by two cluster indicator lamps, one for 4WD HIGH, and one for 4WD LOW (in 4WD LOW, both lamps are on). The GEM accomplishes shifts between different system modes by interpreting inputs from 4x4 selector switch, the vehicle speed signal, and the transfer case contract plates. Based on these inputs, two relays (L2H and H2L directional control), the transfer case motor, and an output for the transfer case electric clutch, are used to shift into 2WD, 4WD HIGH, or 4WD LOW. The GEM will not allow shifts into 4WD LOW unless the brake is depressed, the vehicle is in NEUTRAL (with automatic transmission vehicles), the clutch is pressed (in manual transmission vehicles), and the speed is less than 5 km/h (3 mph).
This feature electronically shifts the vehicle transfer case between 2WD, automatic 4WD, and 4WD LOW. The system mode is selected by the operator through a three-position rotary switch on the instrument panel. The operator is informed which mode the system is in by two cluster indicator lamps, one for A4WD and one for 4WD LOW. The GEM accomplishes shifts between different system modes by interpreting inputs from the 4x4 selector switch, vehicle speed signal, transfer case rear and front shaft Hall effect speed sensors, and the transfer case contact plates. Based on these inputs, the shifts are controlled through two outputs to the center axle engage and disengage solenoids, two relays (L2H and H2L directional control) to control the transfer case shift motor, and an output for the transfer case electric clutch. While in the automatic four-wheel drive mode, the electric clutch engagement is automatically controlled based on front and rear shaft speed differences (wheel slippage), transferring increased traction to the front wheels. The electric clutch is fully engaged when in the 4WD LOW mode.
A transfer case NEUTRAL capability is also available for vehicle towing needs. This feature will be available and programmed at the dealership. The transfer case can be put in NEUTRAL by placing the transmission in NEUTRAL, the key in the OFF position, and the speed is 0 km/h (0 mph). The transfer case will automatically be shifted out of NEUTRAL if the vehicle ignition switch is moved to RUN or ACCY, or the transmission is moved out of NEUTRAL.