How to use a GM TBI adapter board

Updated 1/23/2008

What does this board do?

GM TBI adapter board (currently available in bare PCB form) allows you to install MegaSquirt into GM trucks with the ECM part number 1227747 and cars with ECM part number 122746. Some examples include full size trucks from 1988 to 1992 with the 4.3 V6, 5.0 or 5.7 smallblock, or the 7.4 big block. It also works with many Camaros and Caprices with the TBI engine. This board is meant to be used with Megasquirt-II only.

Assembling the TBI Adapter Board

Bill of materials:

ComponentDescriptionDigi-Key Part Number
LS1, LS2SPDT relay255-2080-ND
R1, R41.0K resistor1.0KQBK-ND
R247K resistor47KEBK-ND
R3, R610K resistor10KEBK-ND
R5560R560QBK-ND
D14.7V 500mW zener1N5230BDICT-ND
D21N4001 diode1N4001DICT-ND
Q1TIP121TIP121GOS-ND
Q22N39042N3904FS-ND
J1DB37 female connectorA32129-ND or 182-837FE-ND
  • Disassemble the donor ECM and remove its harness connector using a “solder sucker” or solder wick.
  • Install R1 (1.0K resistor) and D1 (4.7 volt zener diode), taking care to install D1 with the banded end pointed towards the band on the PCB.
  • Install R2, the 47K resistor, if you are installing this on an engine that lacks an IAT sensor. If you have an IAT sensor installed, omit R2.
  • Install R3 (10K resistor) and R4 (1K resistor) if you are using this with an automatic transmission.
  • Install R5 (560 ohm resistor), and R6 (10K resistor).
  • Install D2 (1N4001 diode).
  • Install Q2, the 2N3904 transistor, if you are using this with an automatic transmission.
  • Install Q1, the TIP121 transistor. This one goes straight up and will bolt to case rail.
  • Install the two relays LS1 and LS2.
  • Install the DB37 connector.
  • Install the connector removed from the donor ECU.

 

 

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 in the MSExtra MS2 Manual.  I’ll just be covering any departures from that doc here.
  • Step 22: You will want all of the IAC jumpers.
  • Step 50:  You’ll be installing the ‘Hall/Optical Input Circuit’  (currently all steps under 50a).  If you see any steps that say ‘only do this if you are triggering from the negative terminal of the coil’, you aren’t, so don’t.
  • Step 51:  You don’t need these components, but I’d recommend installing them anyways.
  • Step 52:  Set the jumpers for Hall/Optical/Points.  That would be XG1 to XG2, OPTOIN to TACHSELECT, and TSEL to OPTOOUT.
  • Step 53: The adapter board gives you the option of using the GM MAP sensor instead of the one built into the Megasquirt. To do this, omit the MAP sensor and wire pin 1 of the MAP sensor header to the SPR3 pin.
  • Alternatively, you can use the Megasquirt’s MAP sensor for real time barometric correction. To do this, install the MAP sensor, but bend pin 1 so that it does not go into the hole (the hole will be jumpered to SPR3). Solder a 1K resistor to pin 1 of the MAP sensor, and solder a wire to the other end of the resistor. Cover this with heat shrink tubing, and run the other end of the wire to the JS5 pin. If you are using the sensor for barometric correction, do not connect any tubing to it.
  • Step 65:  You will not be using the IGBT high current driver. Jumper JS10 to IGN to bring the signal out on pin 36.
  • To use the knock sensor input, run a jumper wire from JS4 to SPR4.
  • If you are using the automatic transmission lock-up control or shift light feature, run a jumper wire from SPR1 to pin 8 on the U1 40 pin socket. This will let you use the acceleration enrichment LED to control this.

That’s it– you’ve got your Megasquirt-II Engine Management System fully prepared to work with the adapter board.

 

If you’re using a DIYAutoTune.com MS230-C or MS2357-C MegaSquirt-II Assembled ECU:

  • Our assembled units are already set up for fuel and ignition control. There are several optional features you may want to use.
  • To use the stock GM MAP sensor and switch the built in MAP sensor to a barometric correction sensor, you’ll need to carefully desolder pin 1 of the MAP sensor. Jumper the resulting hole to SPR3. Solder a 1K resistor to pin 1 of the MAP sensor, and solder a wire to the other end of the resistor. Cover this with heat shrink tubing, and run the other end of the wire to the JS5 pin. If you are using the sensor for barometric correction, do not connect any tubing to it.
  • To use the knock sensor input, run a jumper wire from JS4 to SPR4.
  • If you are using the automatic transmission lock-up control or shift light feature, run a jumper wire from SPR1 to pin 8 on the U1 40 pin socket. This will let you use the acceleration enrichment LED to control this.

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 = 10 (this will vary, depending on the distributor orientation, see notes at the end of the article)
  • Ignition Input Capture to ‘Rising Edge’
  • Cranking Trigger to ‘Trigger Rise’
  • Coil Charging Scheme to ‘Standard Coil Charging’
  • Spark Output to ‘Going High (Inverted)’

Dwell is very low; try around 1.5 ms and adjust as needed.

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 MegaTune  (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.)

 

Spare outputs and inputs

The adapter board lets you control several features besides fuel and ignition. Two of these can be accessed through the spare port configuration screen in MegaTune. The acceleration LED (PM4) now controls either the shift light or the torque converter lock-up.  While the factory torque converter lock-up is controlled by the vehicle speed sensor (among other things), the MS-II does not have a vehicle speed sensor input. Instead, you can control it so that it is active above a certain RPM, and you can also set MAP reading or throttle position as a secondary condition. The FIDLE port can control a relay on pin A2 of the connector. This is not connected to anything in a stock harness, but you can use this for controlling a cooling fan or other basic on/off devices.

For knock sensing, you can use the factory knock sensor on the engine.

The board does not control emissions devices like the EGR or canister purge.

 

Click here to view the TBI adapter board schematics in PDF format.