“Diagnosing the system can be surprisingly easy once you have a basic understanding of its operation and have a wiring schematic to follow. ”

Ford Motor Co. introduced a “new” electronic speed control system on its Crown Victoria, Grand Marquis and Lincoln Town Car the latter part of 1992.

Since then, this system was added to additional models until 1997, when most gas-powered Ford vehicles equipped with speed control were using it. While there are subtle differences between vehicles, such as an occasional wire color change, location of the brake pressure switch (BPS), how the vehicle speed information is generated, and in the Town Car and most vehicles produced since the late 1990s, the addition of a terminal to illuminate a “speed control light,” the systems are the same. For this article, I am using a 1996 F-150 as an example.

Diagnosing the system can be surprisingly easy once you have a basic understanding of its operation and have a wiring schematic to follow. The system consists of:

• Speed control switch assembly
• Speed control servo with integrated module assembly
• Vehicle speed input provided by the programmable speedometer/odometer module (PSOM)
• Stop lamp input
• Cruise control cable
• Brake pressure switch.

If a fault code is flashing on the airbag restraint system light and/or if there is an inoperative horn, address these problems first since they are often related to an inoperative speed control system caused by a faulty clockspring.

To diagnose, first verify that all stop lamps, including the third eye, are operational. Then verify power and ground at the servo assembly. Power is supplied to pin 7 of the servo (purple/orange on this vehicle). The ground is pin 10 (red/light blue). If OK, check the speed control switch operation. The speed control switches provide input to the module to request a specific function, using a series of resistors incorporated into the switch assembly. This produces a different voltage at the input terminal of the module, depending on which switch is depressed. The different voltages at this terminal tell the module which function is selected.

To test the switches, backprobe pin 5 of the servo connector (light blue/black wire) with a digital volt/ohm meter (DVOM). Reference voltage will be approximately 6.4 volts with key on and with no switches depressed.

Note:

• While pushing the ON switch, voltage should increase to about 11.2 volts.
• While pushing the Coast switch, the value should change to about 1.5 volts.
• The Set/Accel switch will bring it around 4.0 volts.
• The Resume switch will change it to about 5.4 volts.
• The OFF switch should drop it to about 0.0 volts.
• These values, with the exception of the OFF switch, may vary plus or minus 0.4 volts due to actual battery voltage at the time of testing.

To test for a sufficient vehicle speed input, backprobe pin 3 of the module (grey/ black wire on this vehicle). Remember, this signal is created by the PSOM based on the input it receives from the rear differential speed sensor, so using a labscope is the preferred method. A reading of 25 Hertz equals about 10 mph. The signal generated is a 5-volt AC square wave that increases in frequency at the rate of approximately 20 Hertz for each 10 mph increase in vehicle speed. If a labscope is not available, any automotive meter capable of reading Hertz will work.

The brake pressure switch is a safety device that provides the power to operate the electric clutch inside the servo that drives the cable. This normally closed switch replaces the vacuum dump valve used on prior speed control systems. Depending on the model, the location in the hydraulic system will vary, but the operation is the same. When the brakes are applied, the increase in hydraulic pressure opens the switch and current flow to the electric clutch is interrupted.

A common problem found on the electronic speed control system is a voltage drop across the BPS. Using a 12-volt test light or DVOM will not prove there is sufficient current flow across this switch to energize the electric clutch. IDENTIFIX has documented many cases of correct voltage using a DVOM or bright testlights on this circuit, yet replacing the BPS has restored speed control operation.

The most reliable way to verify this circuit is to remove the 10-pin connector from the servo assembly. Pin 9 of this connector is an orange wire. It comes directly from the output side of the BPS.

• Using a high beam headlamp, run a jumper wire from pin 9 to the headlamp.
• Use another jumper wire, attached to the headlamp ground terminal, and connect it to the battery negative post.
• If the headlamp is not illuminated to full intensity, the switch is defective and needs replacement.

If all previous tests pass, two possibilities remain: a broken speed control cable, which can be eliminated by a visual inspection, or a defective servo/module assembly.

Jeff Boskowitz is an IDENTIFIX Ford team leader. He is ASE master and L1 certified, with 29 years of diagnostic repair experience.

This information is provided by IDENTIFIX®. IDENTIFIX® resources cut diagnostic time and provide repair solutions that increase the shop's bottom line. From Repair-Trac pattern failure quick fixes, to Diagram-Online wiring diagrams by fax, to the Repair Hotline staffed by 32 master techs who specialize in diagnosing complex problems by phone or fax, IDENTIFIX® helps techicians fix more cars in less time. For more information on IDENTIFIX, call (800) 288-6210, 8 a.m. - 6 p.m. Central Time. www.identifix.com. © 2008 IDENTIFIX. All Rights Reserved. © 2002 IDENTIFIX. All Rights Reserved.

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