4.8 Fuel pump relay 4 WIRING
4.7.3 On/off IAC valves
Rare but supported. The MS3Pro’s PWM / Idle Out 1 can drive an on/off idle solenoid directly if the solenoid draws
3 amps or less.
4.7.4 Thermal IAC valves
These valves were used on some early Bosch L-Jetronic systems and are not controlled by the ECU. They use a
thermal element that warms up and reduces idle speed as they heat up. If you have such a valve, power it from
the fuel pump relay.
4.7.5 DC servo IAC valves (currently not supported)
This design incorporates a two wire DC servo with a three wire position sensor. We’ve only seen them on a few
Ford and Mitsubishi throttle body injected engines from the 1980s. The MS3Pro does not currently support this
idle valve type.
4.8 Fuel pump relay
The MS3Pro fuel pump relay output is a low side driver that supplies ground, not power. It can only handle a
maximum of 3 amps, so you will need to use this to turn on a relay to provide the fuel pump with enough power.
We recommend supplying the fuel pump relay with a separate source of 12 volt power from the ECU main relay to
minimize electrical noise. Do not wire a fuel pump directly to the MS3Pro. Also, do not wire the fuel pump relay
coil in such a way that it receives 12 volts with the key off, as this power can backfeed into the MS3Pro and cause
it to stay powered up when the ignition is turned off.
4.9 Tach output
The tach output supplies a 0 to 12 volt square wave that is compatible with many factory tachometers and nearly
all aftermarket units. Just wire its output to the tach signal wire.
4.10 High current outputs
The MS3Pro’s three high current outputs can be used to control devices that draw up to 5 amps. These outputs
are what are known as low side drivers; they provide a switched connection to ground. The other side receives 12
volts, ideally only when the ignition key is on. Recommended uses besides on/off output include nitrous control
and running boost control solenoids, which pulse open and shut.
Note that any unused injector outputs can be used for the same on/off or low frequency functions as the high
current outputs.
4.11 PWM medium current outputs
These outputs are low side drivers like the high current outputs, but can operate at a higher frequency and have
a maximum current of 3 amps. Devices connected to these outputs absolutely must not receive power when the
key is off, as the circuit used to clamp high frequency voltage spikes can feed power back into the MS3Pro’s power
supply and prevent the ECU from turning on. Maximum frequency is 1021 Hz. The high frequency outputs work
best with valves that can stay in the middle of their travel, like variable valve timing control valves and idle air control
valves.
4.11.1 Solid state relay control
Use a solid state relay to control devices that require a pulsed signal with more current than the MS3Pro’s outputs
are rated for. Any high current, PWM, or injector output can be used to run a solid state relay. One readily available
solid state relay is the Standard Motor Products RY330K (ask your local auto parts store for the radiator fan relay
for a 2001 Jeep Grand Cherokee with a 4.0 inline six). The part number given above include a handy color-coded
connector, and is fused for 40 amps in its factory application. Here is what the wires on this relay do:
AMP EFI MS3Pro manual version 1.202, firmware 1.5.0, 4/21/2017 Page 49
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