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Microsquirt Conversion
Harnesses Info for 240 or 740

UPDATED: September 21, 2025 CONTACT

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240 MS Main Engine Harness

Detailed Description

240 Conversion Harness USER GUIDE

740 MS Main Engine Harness

Sub-Harnesses

740 Conversion Harness USER GUIDE

VE5710 LS Coil Harness

VE5729 COP Harness

VE5715 Bosch Ign Module Harness

VE5725 Hall Distributor Harness

VE5727 CAS Harness

VE5723 MAF Pigtail Harness

Amphenol AT Connector Info

MS Resources (PDF files)

https://www.youtube.com/watch?v=XZM7n9MXciQ

Questions? CONTACT ME

Update September 2024: MS harness parts are now out of stock - no longer available.

240

VE5701 Main Engine Harness Microsquirt for 240.
This set will come with all connectors and components needed for MS-V3. The 35 pole Ampseal comes fully assembled with cables (photos below). Cables that require twisting are properly twisted and cables that require shielding are properly shielded. You can buy a universal harness cheaper somewhere else (one with really short wires), but you cannot get one anywhere else that is correct with colors, lengths and shielding and ready to assemble for your Microsquirt V3 fitted 240, unless you build it yourself. The wire colors and lengths in this set will be the same as shown in my harness assembly guide below for the 240 conversion harnesses.
NOT IN STOCK - NO LONGER AVAILABLE

740
VE5701 Main Engine Harness Microsquirt for 740.
This set will come with all connectors and components needed for MS-V3. The 35 pole Ampseal comes fully assembled with cables (photos below). Cables that require twisting are properly twisted and cables that require shielding are properly shielded. You can buy a universal harness cheaper somewhere else (one with really short wires), but you cannot get one anywhere else that is correct with colors, lengths and shielding and ready to assemble for your Microsquirt V3 fitted 740, unless you build it yourself. The wire colors and lengths in this set will be the same as shown in my harness assembly guide below for the 740 conversion harnesses.
NOT IN STOCK - NO LONGER AVAILABLE

Sub-Harness Options.

35-pole Ampseal plug fully assembled with all cables made for the Microsquirt V3 CONTROL MODULE.

This connector will come fully assembled with all needed cables for MS-V3. Cables that require twisting are properly twisted and cables that require shielding are properly shielded.
You can buy a universal connector cheaper somewhere else (even one with really short wires), but you cannot get one anywhere else that is fully assembled with correct colors, lengths and shielding and ready to install for your Microsquirt V3 fitted 240 or 740, unless you build it yourself. The wire colors and lengths in this set will be the same as shown in my harness assembly guides below for the above 240 or 740 conversion harnesses.
NOT IN STOCK - NO LONGER AVAILABLE

VE5710 LS Coil Harness
MORE INFO CLICK HERE
NOT IN STOCK - NO LONGER AVAILABLE

VE5729 Coil-on-Plug Pencil Coil Harness (16V)
MORE INFO CLICK HERE
NOT IN STOCK - NO LONGER AVAILABLE

VE5715 Bosch Ignition Module Harness
MORE INFO CLICK HERE
NOT IN STOCK - NO LONGER AVAILABLE

VE5725 Hall Distributor Resistor Harness
MORE INFO CLICK HERE
NOT IN STOCK - NO LONGER AVAILABLE

VE5727 CAS Resistor Harness
MORE INFO CLICK HERE
NOT IN STOCK - NO LONGER AVAILABLE

VE5723 MAF Sensor Harness Pigtail
MORE INFO CLICK HERE
NOT IN STOCK - NO LONGER AVAILABLE

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VE5701 240 Microsquirt V3 Main Engine Harness:

Completed harnesses are no longer available.
Yes, these harnesses are no longer being made, however this information will remain here for anyone wishing to use it to create their own harness builds.
The harness PDF assembly guide linked below has been enhanced and offers substantial detail for anyone wishing to use it for your own project.
All parts, connectors and terminals are detailed with dimensions, sizes and part numbers needed to complete a harness build on your own.

CLICK IMAGE FOR LARGER ONE

FULL 240 MS HARNESS GUIDE (41 pages, 8 mb PDF)
https://www.davebarton.com/pdf/Harness_Microsquirt240-2024.pdf

SHIELDING
Microsquirt is particularly sensitive to EMI (electromechanical interference). This harness uses noise suppression techniques to reduce interference. One method is to use SHIELDING on sensitive sensor signal wires. If you don't understand how shielding works, here's a short video.
https://youtube.com/shorts/chbmHJWtE-0?si=5e0PH73N2B_2r3TC

FULL DESCRIPTION
Microsquirt V3 engine harness for Volvo 240 (or 740) 4 Cylinder (Microsquirt ECU not included)
This main engine harness has been specifically designed for the use of Microsquirt V3 with a Volvo 4 cylinder in a 240. The 740 version harness may be found HERE.

COMPATIBLE WITH ANY 240 or 740
This harness will work well on any year 240 or 740, however if you have a 1993 240, you'll need to make some changes to the circuits for the 8-pole firewall connector, since a 1993 240 doesn't have that mating firewall connector.
Installing this harness and using Microsquirt will require that you have a understanding of how Microsquirt works and how it needs to be programmed.

SHIELDED WIRES:
It’s well known that low voltage EFI sensor or ignition signals can be affected by electrical interference (EMI). Megasquirt and Microsquirt are particularly vulnerable to EMI. Universal harnesses are available for Microsquirt, however they have been found to lack sufficient shielded wires. This harness was specifically designed to use an abundance of high-performance shielding on vulnerable circuits. In many circumstances, it can also be acceptable to twist lengths of a harness, which will also provide a level of protection. The user guide offers info on both.

AMPHENOL SEALED CONNECTORS:
Microsquirt V3 uses a 35-pin AMPSEAL connector and a sealed case to make a fully sealed unit. This harness uses a 35-pin AMPSEAL connector for this purpose.
This harnesses also uses a number of Amphenol AT multi-pin connectors. Amphenol AT connectors are very high-quality sealed connectors that are very easy to use and just happen to be 100% interchangeable with Deutsch DT connectors. More detailed information about these Amphenol AT connectors (and Deutsch DT connectors) may be found in my Connector Crimping Page.

Engine Speed, Ignition triggering OPTIONS (crank or cam position sensing).
Direct plug-in options for this harness include:
A DSM CAS (Cam Angle Sensor) may be used. This is a custom CAS (made by Yoshifab), which is adapted to a Volvo distributor bottom half. It's then mounted in the normal engine block distributor location. This CAS can plug directly into this main engine harness (using a small adapter sub-harness). It will provide engine timing for Microsquirt. The CAS is generally used with a distributorless multi-coil ignition.

An LH 2.4 60-2 flywheel with Volvo crank position sensor may be used. This is the same as a standard Volvo CPS for an LH 2.4 car. It will provide engine timing for Microsquirt and is generally used with an LH 2.4 style ignition distributor and with a Bosch ignition amplifier. This may also be used with a distributorless multi-coil ignition.

A Volvo LH 2.2 Hall sensor style block-mounted 240 distributor (or 740 distributor). This is a standard ignition distributor from an LH 2.2 240 or 740. This distributor has a Hall sensor. It may be directly plugged into this harness (using a small adapter sub-harness). The Hall sensor in the distributor can provide engine timing for Microsquirt. A single factory coil may be used. With this option, there are multiple choices available for ignition amplification, but the simple choice is a Bosch ignition module (740 EZK type).

Other options, such an EDIS module and 36-1 front crank trigger wheel can be adapted if preferred, but these are not yet available as a direct plug-in with this harness.

SUB-HARNESS OPTIONS:
An optional available coil sub-harness may be added for a Coil-Near-Plug configuration using four GM LS style coils (PN D514a). This configuration works well with an 8 valve head. LS coils have their own logic level drivers, which receive trigger signals directly from Microsquirt and will work without the need for a separate ignition amplifier. LS coild would be wired for Wasted Spark Configuration using the two Ignition Outputs from the Microsquirt ECU. IgnOut1 is connected to and controls coils 1 and 4. IgnOut2 is connected to coils 2 and 3. When an ignition is configured for Wasted Spark, two coils are fired at once. So in this case coils 1 and 4 will fire together when either cylinder 1 or 4 is ready for spark. Then coil 2 and 3 fire at together when either of those cylinders need spark.

An optional available coil sub-harness may be used to support a Coil-On-Plug configuration using four Denso pencil style coils (PN 129700-3881). These work well with a B234F 16 valve head. Since these pencil coils don’t have internal drivers, this configuration requires an optional external ignition amplifier (i.e.: such as Yoshifab). If using a Yoshifab amplifier, it will be a direct plug-in to this optional sub-harness.

This main engine harness can support an optional available sub-harness for a MAF sensor, if desired. Microsquirt gives you the choice of using MAP only (which is the most common use), however if needed you may also configure MAF only or MAF/MAP blending. Several MAF sensor types are supported and that info is detailed in the below guide.

INJECTORS:
Microsquirt V3 has two injector driver outputs. Up to four high-impedance (saturated type) injectors may be used per driver (for up to eight injectors). This main engine harness is configured for FOUR injectors. The injectors are considered to be Batch Fire, which means all injectors fire at the same time (same as Bosch LH). For this four cylinder harness, each Microsquirt driver output is assigned two injectors. Each driver limits injector current to ~5 amps and will drive high-impedance injectors without any changes. If you opt for low-impedance (peak and hold type) injectors, they will require the addition of a resistor pack (resistors in series) to limit current. Instructions for building and adding a resistor pack, which can be a direct plug-in, are included in the below guide.

MAP:
Microsquirt V3 has no internal MAP sensor (due to the compact case), so an external MAP sensor is required if you will be using a MAP sensor. A good choice is a General Motors type, which may be found with 1, 2, 3 bar, etc. capacity. A 2-bar sensor will support boost up to 14.7 PSI (1 bar over normal atmospheric pressure). This harness uses a General Motors MAP sensor connector for direct plug-in support.

ALTERNATOR:
This main harness for a 240 incorporates two separate leads for the alternator D+ (charge warning light) connection so that you may have your choice of mounting the alternator on the right (normal 240) side of the engine or on left side (740 style). And the fat red B+ cable is provided separately at a length for the alternator on the right side. The separate cable will make it easier if you decide to move the alternator to the other side.
The main engine harness for a 740 uses leads for mounting the alternator in the normal left side 740 location.

IAC:
This harness provides direct support for a Bosch 2-wire PWM style idle valve (typical LH 2.4 style IAC valve).
If a 4-wire stepper motor idle valve is desired instead, an optional Microsquirt Stepper Adapter Module is available from other sources and can be adapted. Those circuits are detailed in the below guide.

POWER:
Power to the Microsquirt ECU, relays and fuel injectors will go through a relay/fuse block (included with the main harness). Fuses are included, but relays are not included. You will need three relays, which will control Main Power, Fuel Pumps, and Ignition. You should choose 12 volt Automotive 4 or 5 pin Micro Relays. These relays will have two 1/4 inch (6.3 mm) pins and two or three 3/16 inch (4.8 mm) pins. 15A or higher capacity is recommended.

It is strongly recommended that the 12v main battery power source to the relay/fuse block for this harness should have a clean, direct path to battery positive. Do not use power from just any 12v source you find in your dash. Using a dedicated power cable from the battery to the relay block is the best choice and will provide the cleanest power for the ECU.

Microsquirt V3 will not support a knock sensor directly and knock sensing is not directly supported by this harness. While it’s not here, it’s possible to adapter or add an external interface module for a knock sensor if that's needed.

MicroSquirt V3 comes with preloaded embedded code (unlike Megasquirt -II). You may upgrade to newer code as new versions are released, but you will not have to load the code initially to get MicroSquirt running.

The bootloader function in Microsquirt is externally accessible. Serial RS-232 signals are directed through the 35-pin AMPSEAL connector to an external 3.5 mm data jack provided near the ECU for convenient connection to your computer.

Microsquirt controllers and harnesses are not offered or approved for use on emission controlled vehicles.
Check the laws that apply in your location to determine if using Microsquirt is legal for your application.

RELAY AND FUSE BLOCK