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H03008 - My RAC Plus I/II 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 is complete, return the Rotary Switch to the previous position.
H03008 - My RAC Plus I/II 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 is complete, return the Rotary Switch to the previous position.