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Noel, on the liklihood of your TPS and IAC plugs getting swapped-- well I figure you'd know for sure on that one if it happened.
Regardless, if vroom needed a new transistor, could be you do too. The LED test should tell you for sure; I know that it works because I had both the 10 and 23 signal monitored on a breakout box with LEDs for LH2.0 cars. The IAC is the same circuit.
On the 544 I have made a few notes, precious few as I have never owned any LH2.2 cars, only an ECU from one. The two transistors are arranged such that T301 drives pin 10 and the base of T302, which in turn drives pin 23. This ensures 180 degrees phase difference for the IAC motor armature.
The collectors are protected against reverse spikes with 4004 type diodes, and the base to emitter resistors are 330 and 100 ohms respectively, so your in-circuit forward bias tests with a DMM will be less clear than you might be used to, but still capable of seeing a short or wide open. A Simpson 260 on low ohms, which runs a bit more current through a junction, will see through those resistors like they are not there.
T301 is the lonesome one to the side; mine is an SGS type BDX77. I'd look at T302, but that is under the heat sink clip and I'd have to drill the rivets to see it. I'd bet it is the same type, TO-220 package NPN bipolar. If I needed one I'd use a TIP-120, because that is what I have in the drawer.
Key thing to remember is one transistor should always be in the opposite state as the other, and that the ECU uses a separate isolated ground (pin 25) for these and the injector outputs. I'm sure you want to know what killed the transistor before trying another ECU, if that is indeed what happened.
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Art Benstein near Baltimore
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