What is STI - self test input

[WmWatt]

Would anyone know what the STI "self test input" is for, what it does, or how it works? I've searched the Internet and my collection of automotive books and manuals. All I can find is it has to be connected or grounded or something to set timing, read check engine codes, and perhaps other procedures. I've also looked at the wiring diagrams in the Haynes Festiva manual but don't see it there so don't know what it's connected to. Thanks for any enlightenment.

[ryanprins13]

When grounded it prevents the computer from adjusting ignition timing so you can set the base timing. How i dont know.

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[WmWatt]

That sounds right. On the carburetted model timing is advanced by the distributor (vacuum or mechanical). When checking the timing on a carburetted engine Haynes says to pull the two vacuum lines from the distributor and plug (I use golf tees which I keep in the toolbox). I've read here in the forum that on the carburetted model the computer just changes the air/fuel ratio based on the signal from the oxygen sensor once it has warmed up enough to send a signal, but the computer doesn't alter ignition timing. The carburetted model has a "one-pin check terminal" (green/red wire) which has to be grounded to read the check engine light codes. The wiring diagram has three wires labelled "check connector" going into the computer, one of which is green/red but Haynes says nothing about computer internals or what that wire does. It also has one wire going from the distributor to the computer (blue/oragne or possibly blue/white on my car) with no stated function. And there is the rpm wire from the negative terminal of the coil to the computer, as well as a matching wire to the optional tachometer.

The fuel injected models have the STI "self test input" wire which has to be grounded both to check the timing and to read the codes. The STO "self test output" wire is where the codes are read with the voltmeter (or light flashes or a code reader plugged into the connector).

I think the term "self test" might refer to testing circuitry in the computer which peridiodically checks the emission system while the engine is running. My friend's Honda with the OBDII system had that. (EFI Festivas prior to 1996 had propietary or OBDI systems according to Haynes.) Her check engine light would go off and on but mostly on. The emission test centre would not even hook up her car because the light was on. It was an automatic fail. So I suppose grounding the STI wire might also turn off the "self test" procedure in the computer. I'm guessing there.

I have a couple of early Haynes emissions system manuals I should check to see if they have any info on how the computer works.

[tooldude]

"Self Test" refers to the program that checks input and output values against tables of values in permanent memory, and also the act of performing the tests. Grounding the STI connector forces the testing and, as we know, reveals codes for values outside set parameters. These are hard faults, but intermittent faults are also set in memory.-- The BL/O wire from ECA to ignition module (carb models) triggers the ign module to advance timing 10* when above 1000m on signal from the altitude compensator.