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Frequently Asked Questions


My RAC Plus won't count. What's wrong?

In most cases, when a RAC Plus won't count, it is not the unit itself that has the problem. It is usually a problem with the interface sensor or wiring.

Check the following:


1. Check that there is a calibration number stored in memory. The unit will not count if there is no calibration number. To check this, when you switch on the unit the display will show the four digit calibration number in the upper display. If this number is all zeroes, the unit will not count. The factory uses .900 as the default number.


2. Be sure that you are attached to the speed sensor output. It is generally at the transmission or the rear differential. If you are unsure about being attached to the correct output, disconnect the plug and move the vehicle. If the speedometer does not function, you have chosen the correct plug wires.


3. Perform a Sensor Test. Locate the Sensor Test button on the front upper right of the Modular Distance Sensor (MDS). When pressed, this will generate an internal low-level signal that is fed directly into the VSS Input circuit. First, unplug the VSS Input connector from the right side of the MDS. Second, turn on the RAC and press the CH key just like you would prior to starting a measurement. Next, using a small pointed object (pen, pencil, screwdriver, etc.) or your finger press the Sensor Test button for a few seconds. The RAC should count when the button is pushed. What number it counted doesn’t matter as long as it did count.


If the RAC did count, everything from the MDS up to the RAC is okay and the problem is most likely a poor connection at the vehicle’s speed sensor.  Make sure you have a good electrical connection at the tap in point. Once you are sure you tapped into the correct location, it is always better to wire solder the connection.

After checking the connection, plug the VSS Input connector back into the MDS and try the RAC again.


4. Perform a Tap Test. The Tap Test will determine if the distance pulses being sent from the MDS are getting to, and being processed by, the RAC.  The Tap Test is performed using the rotary switch on the MDS. First make sure you note the current position of the rotary switch (1, 2, 4, 8,16, 32 or 64), as you will have to return the slot back to this same position after the test is completed.


Next, turn on the RAC. Press the CH key just like you were beginning to measure. Using a small screwdriver, rotate the switch between the Tap and Test positions four or five times. (Note that when the switch is turned clockwise until it stops, it is at the Test position.) The RAC should register. The count shown does not matter, just as long as the RAC did register a count. If it did count, the cable from the MDS to the RAC and the instrument itself are OK.  If the RAC did not count, the problem is most likely a bad cable to the RAC or the RAC itself is bad. If available, try another RAC and repeat the Tap Test. If the second RAC doesn’t count, the problem has to be the cable between the MDS and the RAC.


Once the test in complete, return the Rotary Switch to the previous position.


My RAC Plus counts while the vehicle is not moving. What's wrong?

There are three possible causes for this:

1. Your wiring may be picking up stray pulses from the vehicle. This can occur if the wiring is to close to the alternator, spark plugs, distributor cap or engine coil.

2. Your ground wire may not be connected properly. Double check its connections.

3. Your Modular Distance Sensor may not be working correctly. Contact us for information on getting replacement parts.


My RAC Plus counts in increments of 2, 5, 10, etc. What's wrong?

Adjust the rotary switch on the modular distance sensor and then recalibrate. Refer below to How do I adjust the Vehicle Speed Sensor pulse rate? for more information.


How do I calibrate my RAC Plus?

Follow the seven steps of the calibration procedure to properly calibrate your instrument.  More Details.


My calibration number is outside the recommended range of .500 to 1.200. What should I do?

Adjust the rotary switch on the modular distance sensor and then recalibrate. Refer below to How do I adjust the Vehicle Speed Sensor pulse rate? for more information.




How do I adjust the Vehicle Speed Sensor pulse rate?

There is a rotary switch located on the modular sensor that should be changed.

The signal pulses coming from the vehicle speed sensor are generated for use by

the vehicle’s computer, engine/transmission control, fuel management, ABS brakes, etc.

The pulse rate can vary from 4,000 to in excess of 100,000 pulses per mile.

The Modular Distance Sensor (MDS) will condition and amplify these pulses for use by

the RAC. Since the higher pulse rates are not required for accurate distance

measurements, the MDS incorporates a divider circuit to reduce the pulse rate.

This is done by adjusting the rotary switch on the front of the MDS, as shown here:

Although your particular vehicle may vary, generally Chrysler and Ford vehicles use a 4 to 1 ratio (position 4 on the switch) while General Motors vehicles use a 16 to 1 ratio (position 16 on the switch). To adjust the ratio, use a small screwdriver to rotate the switch until the slot in the switch points to the desired ratio. Note that when the switch is turned counter-clockwise until it stops, it is at the 1 to 1 ratio.


The adjustments go from 1 to 1 (1 pulse into the sensor, 1 pulse out) through 64 to 1 (64 pulses into the sensor, 1 pulse out). You may need to adjust the ratio again based on the results of the calibration procedure.  Any time you change the pulse ratio, you will need to re-calibrate the DMI.


How do I check the firmware version of the RAC Plus?

To see what firmware version is in your RAC Plus, turn the unit on while holding down the CLR button. The top display will show the firmware version and the bottom display will show the model version.


Where is the correct location to connect the Modular Distance Sensor to the Vehicle's Speed Sensor?

Most vehicles should have a 2-wire output from the VSS.

On most Ford and Chrysler pick-up trucks, the best place is on the rear end differential housing using the rear ABS signal.

On General Motors pick-up trucks, the best place is the VSS at the transmission.

On many vehicles (both trucks and autos) you can go to the cruise control signal or to the vehicle ECU which is often located under the dashboard.


Note: Vehicle wiring changes from year to year and model to model. For vehicle-specific color code information, submit a Support Request Form. Your local Auto/Truck dealer can also usually tell you where the VSS can be located on your vehicle.


Connect the red wire of the long 20’ VSS cable to the high speed signal wire from the vehicle’s speed sensor. (The black wire is for optional use with vehicles made before 2002. In these vehicles, it should be connected to the low speed signal wire of the vehicle’s speed sensor or chassis ground.) Crimp on tap splice connectors are supplied for convenience on initial installation. However, we recommend removing the tap splice connectors and reconnecting using wire to wire solder once you are sure all connections to the VSS are proper. Insulate with sealant/electrical tape as needed.

What is the difference between the RAC-100 and the RAC-200?

The RAC-200 contains all of the features of the RAC-100 with the addition of 1000-event memory storage. This allows for saving of surveys for later download to a computer using the RACPro program. The RS-232 port used for downloading can also be used to control the unit in a real-time mode from a lap-top computer.

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