4 WIRING
4 Wiring
First, some general advice about wiring.
We use automotive grade, high temperature TXL wiring in our harness (it’s rated for up to 125°C), but you
should still do your best to keep it away from heat sources. Also take any potential sources of mechanical damage
into account - flip up headlight mechanisms, the side to side movements of the engine on its mounts, etc.
Usually the biggest source of electrical noise isn’t RFI or other radio noise - it’s poor grounding or having noise
injected into the power wires by another device. The MS3Pro works best when grounded to the battery negative
terminal for maximum noise resistance. We do not recommend grounding the MS3Pro to the chassis or sheet
metal under any circumstances. You may either crimp all ground wires to a single lug, or crimp the ground wires
on the gray connector to one lug and the single ground on the white one to a separate lug. The sensors need to
be grounded to the sensor return wire, which grounds them to the MS3Pro. Grounding sensors to the engine or
body will create jitter from ground offsets.
You will also want to make sure the 12 volt power wire is clean. We recommend using one power wire from
the battery for the MS3Pro EFI main relay, and a separate power wire to power the relay for the injectors, fuel
pump, and ignition system for maximum noise reduction. Fuel pumps and ignition coils in particular tend to inject
significant amounts of noise into their 12 volt supply wire. Also, installing a capacitor (the points condenser out of
an old fashioned points distributor will work) on the supply line to the ignition coils can reduce the amount of noise
they feed into the 12 volt supply.
You can start off a debate that goes on for hundreds of posts on an Internet forum over whether it’s better to
solder or crimp wires to join them. Either one can work very well if done right, and either one can be unreliable if
done poorly. It’s our experience that getting a good solder joint is more reliant on good technique, which can still be
done with cheap tooling, while a good crimp joint is more reliant on good tooling. Either way, the joint needs to be
sealed (such as with heat shrink tubing) to keep out moisture and contaminants. Heat shrink tubing also provides
some measure of protection against vibration.
We designed the MS3Pro’s power supply to run on a wide range of supply voltage. It can operate on voltages
as low as 6 volts to cover voltage dips during cranking, although if your voltage drops that far, we’re not sure your
battery is going to crank it over. The MS3Pro has an input voltage limiting circuit that clamps input voltage at 22
volts, and feeding it 22 volts for any sustained length of time will blow the input voltage protection fuse.
4.1 ECU pinout
The ECU uses two AMPSEAL connectors, where the pin numbers are molded into the connector housing.
AMP EFI MS3Pro manual version 1.202, firmware 1.5.0, 4/21/2017 Page 30
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