I can't take credit for this but here is how to make your own for a couple bucks. It will work on MEFI 1-4 I believe.
DIY Code Reader
Materials needed:
One 12volt resistor type LED light - Standard LED nothing special make sure it is a 12 VOLT
One small paper clip cut in half
That's it!
This home made code tool works every bit as well as the one I paid $50.00 for. It will work on most 1993 to 2000 marine EFI systems with the ten pin Data Link Connector. This includes MerCruiser, Volvo, Crusader, PCM, Indmar, and a few others.
How to hook it up:
1) Ignition key 'OFF"
2) Remove the cap from the Data Link Connector (DLC)
3) Slide the LED into the female terminals of position E and F on the DLC, making sure that the positive side goes into terminal F and the negative side into terminal E. No damage will occur if you get it backwards, it just won't work.
4) Turn the ignition key to the 'ON' position. The LED should come on steady.
5) Insert the half paper clip into terminals A and B. This puts the engine in 'Service Mode' and codes will begin to flash on the LED. On 1996 and earlier engines you may hear the fuel pump come on. If not you should be able to hear the Idle Air Control (IAC) motor move. Use care in this step, do not insert in the wrong terminals or ECM damage could result! Look closely, the terminals are clearly marked.
6) Read codes by observing LED flashs. If the self diagnostic system is working it will flash code 12 (one flash, pause, two flashes, long pause) it will repeat three times. If other codes are present they will flash in order of lowest to highest. Continue to read codes until the code 12 sequence is repeated.
You can also be clear codes (by moving the throttle to 100% and back) and set base timing while in Service Mode.
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