4.4 Other sensors 4 WIRING
appropriate. Do not use CMP- or CKP- with Hall effect or optical sensors. Leave this wire disconnected and
taped off; do not connect to ground.
Some sensors put out a square wave signal on their own, like the tabbed body ones sold by AMP EFI. Others
switch from a ground output to a floating (undefined) output. The ones that behave this way require running a 1K
resistor from their power terminal to their signal terminal (called a pull-up resistor) to work with MS3Pro. If you’re
not sure which sensor type you have, supply it with power and ground, and check the voltage on its output wire
when the sensor is both far away from any metal and touching a piece of steel (note - do not use stainless steel for
this test; some grades of stainless steel aren’t magnetic.) If the output voltage stays at zero the whole time, you’ll
need the pull up resistor; connect the 1K resistor from the signal wire to the power wire. If it is at zero volts in one
state and bounces up to at least 5 volts in the other state, it doesn’t need a pull up resistor. The MS3Pro package
includes two 1K resistors in blue sealed tubing with blue wire leads.
In some cases where a Hall effect sensor puts out 12 volts and the wiring harness has severe noise issues,
tying the CKP- or CMP- wires to the VREF output can reduce noise.
4.3.3 Points triggering and points replacement devices
If you wish to lock out a points distributor mechanism and let the MS3Pro control the ignition, you can treat the
points output as a Hall sensor with a ground to floating output. You will need to connect a 1K resistor from a 12
volt source to the points, and also wire the points to the CKP+ wire. Do not use the CKP- wire; tape it out of the
way safely.
Using a Pertronix Ignitor or equivalent is similar. With a Pertronix, connect the Ignitor’s red and black wires with
a 1K resistor, and leave the red wire connected to switched 12 volt power. Connect the black wire to CKP+ and
leave CKP- disconnected. Other modules may use different wire colors.
4.3.4 Fuel only installations
CDI ignitions send a high voltage pulsed signal, from 300 to 600 volts, to the coil. You do not want to try to get a
tach signal from the coil by any means when using this sort of ignition, including external adapter boxes. Instead,
use the ignition box’s tach output wire to the CKP+ instead if you are using a distributor advance mechanism to
control timing and just want the MS3Pro to control fuel. Do not use the CKP- wire or the CMP+ and CMP- wires.
4.4 Other sensors
4.4.1 Temperature sensors
The MS3Pro is designed to work with the type of sensor called a NTC thermistor. NTC stands for negative
temperature coefficient, and simply means that the resistance goes down as the temperature goes up. This
is how most automotive temperature sensors are set up, including the GM type sensors available from AMP EFI
distributors. Normal automotive temperature sensors have two terminals. One connects to the coolant temperature
sensor or intake air temperature sensor input of the MS3Pro. The other one connects to the sensor ground wire.
These sensors have no polarity and can be wired either way.
There are two common variations on automotive temperature sensors. Closed element type sensors are resis-
tant to water, oil, and other liquids. We recommend using this type for coolant. They are not a good idea to use
with air temperature, as they react slowly and are prone to absorbing heat from the intake manifold. Open element
sensors are better to use for air temperature. We recommend mounting them upstream of the throttle body to avoid
absorbing heat from the intake manifold, unless you have a plastic intake manifold. If you are using a turbocharger
or supercharger, the IAT sensor must be mounted downstream of the compressor and intercooler (if used).
Sometimes you will find cars with one wire temperature sensors. We do not recommend using this style as
they are grounded through the engine block, and this will make for a noisy signal. A few cars employ a three wire
coolant temperature sensor. This sensor is typically two sensors in one housing with a common ground, with one
sensor driving a gauge and one sensor for the ECU.
TunerStudio has the calibration data for many common temperature sensors in its sensor calibration tool, in-
cluding GM, Ford, and Bosch. You’ll find those under Tools -> Calibrate Thermistor Tables.
AMP EFI MS3Pro manual version 1.202, firmware 1.5.0, 4/21/2017 Page 37
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