How to Use a BMW Motronic 55 Pin Adapter Board

Assembling the TBI Adapter Board

Bill of materials:

The first step to assembling the Megasquirt to Motronic 55 pin board will be to source an old ECU. It won’t matter if it has water damage or not, so you can usually source one from a junk yard for less than $75. Just make sure that the plastic connector is in good shape and the mounting plate fits your vehicle. The single board design such as the 173/179 ECUs are more desirable, as they will solder straight to the new board with little work, but the older dual board design will work if need be.

Next, you need to disassemble the ECU. First, remove the top cover by unbending all 10 of the tabs on the back side. The cover should lift free. Remove all 6 screws from the bottom and the connector and board will come free from the aluminum backing. Now that the board is exposed, we will remove the connector. Keep in mind the connector and pins are all we are trying to save here. If you look on the bottom of the board, there will be what looks like 2 black rivets on either side of the 55 pins that go to the connector. These are the structural mounting points between the PCB and 55pin connector. The easiest and most effective way to remove them is to grind off the posterior surface with an angle grinder, being careful not to go past the first layer of the rivet and avoid any of the 55 pins.

Next you will separate the solder joints that join the PCB and 55pin connector. This can be accomplished in two ways. The preferred method is to use a solder pump to remove all of the solder form the joints and carefully remove the connector. This may not be the most practical method, as a good solder pump can run upwards of $1,000 (It is also possible to use a soldering iron and a squeeze bulb, although this is not nearly as easy as a solder pump – ed.). Another method is to secure the plastic connector in a vice or similar clamp so that almost none of the plastic is showing, and use a heat gun on the back of the board to heat all of the joints to pry the PCB from the connector. You must be very careful not to bend the pins too much and be sure to use a good, high temp heat gun. My first attempt with this method was with a cheaper hobby grade gun that couldn’t quite heat the joints enough and bent several of the pins severely. I later found a good industrial heat gun that did a much better job at liquefying the solder. After you have heated the solder on one side to the point that you can see it liquefy, gently start to rock the board back and forth and side to side. If need be, you can use a screw driver to get some leverage from the back side. Just remember to take your time. The board should also start to get soft and pliable. This is very advantageous, as it will allow you to remove the pins in one small section at a time. Start at one side and move down the row pulling the board free of the pins as you go.

After the board has been freed of the connector, some debris from the board may still be on the pins. You can continue to use your heat gun and a wire brush to remove this or you can use your soldering iron and solder wick. Once this has been done, examine the pins and make sure none of them are bent out of place too far. Remember that you have to get all 55 of these to line up at the same time.

Now that the hard part it done you can get down to business. Install the connector to the MS adapter board and make sure it is fully seated. If you can’t get it to stay lined up, you can reinstall the bolts the go through the rivets you partially ground off or have another set of hands around. Either way, make sure the connector is at 90 degrees with the board and solder all 55 joints on the adapter board.

Next you will install all of the electrical components. I like to start with the most difficult part first. Remove all 5 2n2222a transistors (Q1-5) from the bag and mount them to the board. Make sure they are slightly raised from the board. There should be notches on the legs of the transistors to hold them at the proper height.

Next you will need to install all of the diodes (D1-8). They are all the same but are one way components. You must pay attention to the orientation when installing them or the circuit will not function properly and the Megasquirt could be damaged. There will be a banded end on the diode and a banded end of the silkscreen image of the component. Simply line them up.

Next, install all of the resistors. Note that not all of them are necessary if you don’t want some of the add-on options. Ign B and C and R13-14 are for using wasted spark coils and are not needed if you intend to use the stock coil and distributor. The boost driver and R11-12 are for use with the boost control circuit and are not necessary otherwise. Some of the other components may not be absolutely necessary but are cheap enough that it would be pointless to exclude them.

Now install the DB connectors U1 and U3. They should snap into place and are easily soldered from the back side.

Before you continue, you may need a few vehicle specific jumpers. Above the proto are, there are several numbered holes. Each one of those numbers corresponds to the pin on the Motronic connector. They are for various uses in different cars but I will do my best to cover all the bases.

I brought them to a proto area so that these wires could be used to bring in or out signals without running new wires. If you wanted, you could use the old AFM wire for a MAF or external MAP sensor. Next to all of the numbered holes is one labeled “Spr”. This on goes back to the DB37 and in to Megasquirt. This would be the location for bringing out the MAP or MAF signal.

Once this is done, prepare to mount the adapter board to the aluminum base plate that the Motronic connector came from. You will not need the larger screws, but the small ones that went through the rivets on the connector / Motronic board are reusable. Before you mount the board though, you should prepare the TO220 components (Q6-7, boost driver, Ign B & C). In my eyes, the best place to mount them is right on the back plate. To do this you will need to bend the leads at a 90 degree angle upwards. That is, so that if the BIP373 for example is sitting face up on the desk, the pins point straight up. If you don’t feel comfortable with them there, you can mount them vertically with a heat sink.

Place all of your transistors on the aluminum plate and align them with the holes on the board. Make sure you apply thermal paste before you set them on the aluminum plate. Install the board and secure it with the two mounting screws. If you are happy with the placement, solder all of the transistors (Q6-7, boost, and Ign B&C) from the top side and clip off excess material. The transistors should be very secure from the board alone, but if you want, you can also drill mounting holes for them.

Building and Configuring the MS-II

If you’re assembling your MegaSquirt-II from a kit (Part# MS230-K):

• For the most part, stick with the standard assembly documentation at http://www.msextra.com/manuals/ms2manuals/.  I’ll just be covering any departures from that doc here.
• Install all of the components except for R57.
• Install the optional BIP373 ignition driver Q16. Jumper IGN to IGBTOUT.
• Step 52:  Set the jumpers for VR input.  Jumper VRIN in to TACHSELECT and TSEL to VROUTINV.
• If you are using MS2 Bowling & Grippo code, jumper JS10 to IGBTIN. If using MS2/Extra code, put a 330 resistor in the proto area connected to the top side of R26 on one side and IGBTIN on the other.

For any of the add on circuits to be used with the MS2extra code we will bring the signals out to the IAC ports and SPR ports near the DB37. None of these are necessary to run the engine.

The pin usage for these setups will vary from code to code and is for the most part user selectable. So the pin you use for each of these functions is up to you. For my (Ryan’s) setup in MS2/Extra, I use these:

(Note: Some of these features are only available in beta code. -ed.)

Fitting It in the Case

The TBI adapter board is designed to squeeze both the adapter and the Megasquirt into the stock case. The Megasquirt board is supported by the DB37 connector at one end and the other end is supported by the side rails.

Megatune Ignition Configuration:

• Trigger offset = 88 (this will vary, depending on the distributor orientation, see notes at the end of the article)
• Ignition Input Capture to ‘Falling Edge’
• Cranking Trigger to ‘Calculated’
• Coil Charging Scheme to ‘Standard Coil Charging’
• Spark Output to ‘Going High (Inverted)’

Dwell will need to be adjusted depending on your coil type.

More information on setting Trigger Offset:

You must also set the initial position of the trigger (called the ‘trigger offset’), then check it using the Trigger Wizard in TunerStudio  (Tools Menu).  The trigger offset setting will vary according to your distributor position (where it is in rotation) but you’ll need to set it properly…  Basically you use the Trigger Wizard and adjust the ‘trigger offset’ and/or twist your distributor until the advance number in the Trigger Wizard matches what you’re reading with your timing light.  The +/- buttons on the trigger wizard will adjust your trigger offset.  You’ll need to use these buttons and a timing light to make the number on your light, and the big number on the left in the Trigger Wizard, match up.

Here’s the information on this direct from the MegaTune Manual:

Before tuning your advance table, be sure to use a timing light to verify that your ‘trigger offset‘ is calibrated. Changing the Trigger Offset in MegaTune will not change the displayed advance, instead, it changes the actual advance as seen with a timing light. Your goal is to make these two match.

To do this, get your engine warmed-up (otherwise the timing moves as the temperature increases) and idling, then use a timing light to verify to be certain your actual advance as shown by a timing light equals your the advance display on the advance gauge in MegaTune. (8, in this case). (Note that positive numbers denote BTDC, and negative numbers denote after TDC.)

Image courtesy of Bowling and Grippo