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MJLJ V3 kit assembly guide

Assemble Power Supply

Begin by mounting and soldering the following components:

Power Supply Components
Part Number Description
X1 Right angle DB9 connector. Snap the connector in so it is flat against the board.
X2 Molex Mini-Fit 14 position right angle receptacle. Mount the connector so it rests perfectly flat against the board. This is important for proper fitment into the case.
C1,C2 33uf Electrolytic capacitors
U3 Voltage Regulator- LM2937ET-50 5V or STMicro LF50 5V . Bend the legs at right angle so the metal back of the device is flat against the board.
D2,D3,D4 1N4004 1A 400V Diode. Note band orientation!
D5 1N4742 12V Zener Diode. Note band orientation!


Mjlj v3 build step1.jpg
Power supply components installed

Verify power supply operation

Now that you have built the power supply, you should verify it's proper operation before you move on to the next assembly step.

If you haven't already built your MJLJ wiring harness using the 14 pin Molex plug, now would be a good time. To perform this test you will need to provide at least GND and +12v to the MJLJ.

If you need to, refer to the MJLJ connector diagram:

Mjlj 3.x pinout.png


Power up the MJLJ using a power supply that supplies at least 6v. A 9v battery works great for this.

You will need to verify that your board has the proper voltages in the right locations. Check the following:

Device U1 - the Motorola MCHC08KX8 microcontroller. Verify +5v on the pads shown below.

Mjlj v3 U1 vcheck.jpg


Device U2 - the MAX232 RS232 Transceiver. Verify +5v on the pads shown below.

Mjlj v3 U2 vcheck.jpg

Device PS1 - the MPX4250 Pressure sensor. Verify +5v on the pads shown below.


Mjlj v3 PS1 vcheck.jpg

Device QG1 - the 16MHz Oscillator. Verify +5v on the pads shown below.


Mjlj v3 QG1 vcheck.jpg

Device IC1 - the ULN2003 transistor array. Verify +12v (or whatever your supply voltage is).


Mjlj v3 IC1 vcheck.jpg

If all check pass, great! Move on to the next step. If you are not reading proper voltages at the locations specified above, DO NOT proceed to the next step until you resolve the issue.

If you have problems, double check solder joints on all of the connections. You will have likely missed a joint or you might have one that does not have a solid connection. Refer back to the information on how to make a good solder joint if you need a refresher.

Install discrete components

Now you can install the discrete components!

Mount and solder the following parts:

Discrete components
Part number Description
C3,C4,C5,C6,C7,C10 0.1uF ceramic capacitor. Bend the leads close to the body of the capacitor.
D1 1N4733 5V Zener Diode. Be careful installing this device since you will likely have the glass version, and it's a bit fragile. Bend the leads accurately so the device slides into the board and mounts flush to the surface. Note band orientation!</td>
T2 2N2222 NPN switching transistor. Install with the flat side facing the upper right hand corner, with the molex connector to your right. (See picture) There is no T1, by the way. :)
R1 100 ohm 1/4W 5% tolerance resistor. Markings: brown-black-brown-gold
R2 680 ohm 1/4W 5% tolerance resistor. Markings: blue-silver-brown-gold
R5 1K ohm 1/4W 5% tolerance resistor. Markings: brown-black-red-gold

Mjlj v3 build step2.jpg
Discrete components installed

Re-verify power supply operation

After adding the last set of parts, it's strongly recommended that you re-verify proper power supply voltages. Power up the board the same way you did previously and check the following with a volt meter:


Device U1 - the Motorola MCHC08KX8 microcontroller. Verify +5v on the pads shown below.

Mjlj v3 U1 vcheck.jpg


Device U2 - the MAX232 RS232 Transceiver. Verify +5v on the pads shown below.

Mjlj v3 U2 vcheck.jpg


Device PS1 - the MPX4250 Pressure sensor. Verify +5v on the pads shown below.

Mjlj v3 PS1 vcheck.jpg


Device QG1 - the 16MHz Oscillator. Verify +5v on the pads shown below.

Mjlj v3 QG1 vcheck.jpg


Device IC1 - the ULN2003 transistor array. Verify that it is close to +12v (or whatever your supply voltage is).

Mjlj v3 IC1 vcheck.jpg

If all check pass, great! Move on to the next step. If you are not reading proper voltages at the locations specified above, DO NOT proceed to the next step until you resolve the issue.

Again, if you have problems, double check solder joints on all of the connections, verifying you used the proper soldering technique.

Install active components

Next install the active components and IC sockets.

Active components
Part Number Description
QG1 16MHz oscillator. Make sure the sharp corner on the device matches the sharp corner on the board's silkscreen
U2 Maxim MAX232 RS232A transceiver (or equivalent). Be sure to match the notch of the chip with the notch depicted by the silkscreen!
IC sockets Install both 16 pin IC sockets. Be sure to match the notch of the socket with the notch depicted by the silkscreen!

Mjlj v3 build step3.jpg
IC sockets and active components installed

Optioning the board for MAP or TPS

MJLJ V3 offers the ability to configure engine load sensing with either a Manifold pressure sensor (MAP) or a Throttle Position Sensor (TPS).

It is up to your application to determine which is best for you; if you have multiple carburetor/throttle bodies and a very lumpy cam, it may be difficult to get a good MAP sensor reading, so TPS might be your route.

If you can get a good MAP sensor reading, this would be your best option, since it more directly measures engine load, and compensates for changes in Volumetric Efficiency (how well the engine breathes), such as engine modifications or different altitudes.

IMPORTANT! DO NOT configure the MJLJ with both MAP AND TPS!! You must choose one or the other

MAP Sensor Option

(Skip below if installing TPS)

Mount and solder the following components:

MAP Sensor components
Part Number Description
C8,C9 0.1uF ceramic capacitor
R4 270 ohm 1/4W 5% tolerance resistor. Markings: red-violet-brown-gold
PS1 Motorola MPX4250AP 2.5 Bar pressure sensor. Bend the leads at a 90 degree angle, so when it is mounted, the vent on the case points down, and the part identification points up (see picture). Also, the pin with the notch should face the DB9 serial connector. The pins should be bent right where they become wider, before they go into the case of the pressure sensor.

Mjlj v3 build step4 map.jpg
Provisioned with MAP sensor


TPS sensor option

To install support for TPS, mount and solder the following components:

TPS Sensor components
Part Number Description
R3 1K ohm 1/4W 5% tolerance resistor. Markings: brown-black-red-gold
C11 0.1uF ceramic capacitor
C12 0.001uF ceramic capacitor
C13 0.22uF ceramic capacitor
C14 4.7uF Tantalum Capacitor. This capacitor is polarized! Match the "+" marking on the body of the capacitor with the "+" on the circuit board's silkscreen.
D7 1N4733 5V Zener Diode. Be careful installing this device since you will likely have the glass version, and it's a bit fragile. Bend the leads accurately so the device slides into the board and mounts flush to the surface. Note band orientation!</td>
L1 1uH Ferrite core inductor

Mjlj v3 build step4 tps.jpg
Provisioned with external TPS input

Final board assembly

To complete the assembly, install the programmed MC68HC908XK8 microcontroller and ULN2003 IC into their respective sockets.

The microcontroller plugs into the socket near the middle of the board; be sure the notch on the IC matches the socket- it should point up when viewing the board with the Molex connector facing the right.

The ULN2003 Transistor array plugs into the socket between the microcontroller and the molex connector. As with the microcontroller, ensure the notch on the device matches the socket. It should point in the same direction as the microcontroller.

It would be a good idea to do a final power-up test, checking voltages and ensuring nothing 'smokes' :)

It's also recommended that you secure the MAP sensor to the board- A small nylon "zip tie" is a quick and convenient way of doing this. For high vibration environments, you may also want to fasten the voltage regulator (U3) to the board with a small screw and nut.

Mjlj v3 build step5 map.jpg
Completed board optioned for a MAP sensor

You are now complete with the board assembly!

Case installation

MAP Sensor

Install the plastic bulkhead fitting in the end plate which has the large, notched square hole. Install the fitting so the plastic nut faces the inside of the case. Be sure not to over torque the plastic nut.

If needed, wet the end of the short section of hose with a small amount of water and press it onto the MAP sensor. Repeat the same by pushing the hose onto the bulkhead fitting.

You can now slide the board into the case. If provisioned with a MAP sensor, the board slides into the case from one direction only, due to interference with the edge of the sensor and the case.

TPS Sensor

Slide the board into the case so the respective end plates align with the board connectors. You can install the two plates on either side of the case with two screws each, which allows you a final inspection before installing the top portion of the case.

Mjlj v3 build step6 map.jpg
MAP sensor version installed in case.

When you are satisfied with your work, install the top of the case with the remaining screws.

Mjlj v3 build step7 map.jpg
Complete assembly, MAP sensor version