It is a go-between for the ECU and coil packs. I believe, but not 100% positive, it controls dwell. It amplifies voltage from the low-level ECU signal to a higher voltage to charge the coils.

the power transistor or also known as the igniter switch...it controls your ignition system notice if it isnt plugged in you wont get power to the ignition system? thats because that transistor allows for the circuit to be completed in the ignition system...basically creates the proper voltage for the coil packs to ignite without surge

bsbllfit7 said:

I have searched but found nothing. I have a simple question of what does the power transistor do and control. simple question. thanks in advance.

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Short answer quoted from another post:

set3422 said:

...It sends GRND or NO GRND signal to the coil pack to determine which coil to fire. The coil pack gets 12V power from MPI Ignition switch...

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Long answer for the technically inclined...

Operation description from the Service manual:

•Turn ignition switch to “ON” position, and battery positive voltage will be applied to primary winding of ignition coil

•When crankshaft position sensor signal is input to engine control module, engine control module makes ON-OFF control of ignition power transistor one by one.

•When ignition power transistor is turned on, current flows from ignition coil (primary winding) to ground through ignition power transistor.

•When ignition power transistor A is turned from ON to OFF, the spark plugs of No. 1 and No. 4 cylinders spark. Turning off ignition power transistor B from ON to OFF will produce sparking in spark plugs of No 2 and No.3 cylinders.
Hope that answers your question!

1gdsmftw said:

it controls your ignition system notice if it isnt plugged in you wont get power to the ignition system?

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I don't follow what you mean by this. Explain.

No one knows 100% about the power transistor. There is a lot of circuitry inside of it. It is not just simply 2 transistors and a circuit to make a tachometer signal. Someone would have to reverse engineer it.

bsbllfit7 said:

I have a simple question of what does the power transistor do and control.

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All DSM owners should read the 1G Technical Manual. dtm

alright that helps alot thanks guys

I'm an electrical engineer and psiaddict's long answer is correct. But in simple terms for the layman, each power transistor is just an on/off switch controlling current flow to the coil (to it's primary winding). When on, the current flows and builds up energy (in a magnetic field inside the coil). When turned off (at plug firing time) the magnetic field collapses inducing very high voltage (around 50kv) in the secondary winding (the coil is just a voltage step up transformer). This voltage is then sent to the plugs (2 plugs at the same time per coil).

Okay, outside topic but I was looking at my IM today(head out of car) and I noticed the power transistor doesnt plug into anything else (besides the harness wires). Why does this power transistor need to be bolted on anywhere (aka Intake manifold or firewall), cant you just unbolt it and sit it on the harness and tuck the wire away?

Edit found my answer, must be grounded.

Pin 3 is the ground but the power transistor uses the mounting as a heatsink.

I am goin to run a megasquirt on a 95 gst, i need the original ecu to run the power transistor or just by conecting the ign2,3 and ign 1,4 and ground? Please need help i not have the original ecu.

A wiring diagram from the power transistor and coils? White is ign signal, blue and brown are coil packs what are the others grounds?

What if the power transistor isn't grounded how would it react mine isn't grounded car was function fire order like trying to start but it took a lot to get going but finally started but then turned it off turn it over by hand because the misfire I'm guessing sounded no good but everything's good and to cancel out a timing slip before I fire it over again it reved up fine but seem like it lacked power now it just turns and turns like no spark could my power transitor not being grounded cause all this ?

Start a new post. Your post is also nearly unreadable.

Like Paul said, it's nearly unreadable. As for the first part of your statement, your car would not start if the PTU wasn't grounded somewhere, that's how the unit works - by grounding a constant +12V signal from the battery to the primary winding of the coils and then releasing the circuit (ground) to collapse the field then allowing energy to discharge via the secondary windings and then cause your spark... so your PTU is grounded.

The mechanical attachment of the PTM to the mounting bracket isn't for electrical grounding but instead acts as a heatsink for the PTM and to keep it from bouncing around. The coils draw lots of current when charging and switching that current generates heat. Overheating the PTM can cause it to fail.