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Originally Posted by Imp Pwr Online
Simply put both NA and TT ECUs put out the same inj signal. On the NA it just goes directly to the Inj. On the TT that signal just gets put thru the resistor pack first then to the inj.
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Well not exactly.
1. The ECU does not put out any "signal" to the fuel injectors. It quite simply grounds a circuit for a prescribed length of time (in milliseconds). For the TT injectors, that circuit happens to include an external resistor, with a total resistance in that circuit of about 9.5 ohms. The NT injectors supply their own extra resistance (no external resistor) for a total resistance in the circuit of about 13.8 ohms. In both cases, resistance in the wiring and connectors has been ignored. The main consequence of this difference in circuit resistance is the amount of current (amps) in each circuit. Bringing us to item 2.
2. Because the amps are higher in the TT circuit, and because the TT injectors are designed to act faster, the ECM decreases the duration of the pulse (that is, the amount of time the current is applied or the circuit is grounded) to compensate for the faster solenoid response. The ECM for NT injectors knows to use a slightly longer pulse duration. This is most evident if monitoring IPW on TT vs NT engines.
So, if the ECU is not changed (TT into a NT car after switching to TT injectors) then the ECU will be using a longer IPW than it should. An AFC should be able to compensate for this.