I just spent some time with the electrical portion of the '89 FSM. I haven't had a similar problem but it's a good time to attempt to understand the fuel circuits. There's a lot that happens within the ECU that I can't see without an circuit board diagram but the associated wiring schematic shows two (2) circuits within the control relay:

Current flows from one set of contacts on the MPI control relay to the MAF sensor, the ISC, the purge control solenoid valve, the EGR control solenoid valve, the injectors and the ISC. The trigger for that circuit appears to be the "light green" wire from the ECU to the control relay.

The other set of contacts powers the fuel pump drive terminal with supply voltage from the ignition switch. The trigger appears to be the "white/red" wire from the ECU to the control relay.

You can check voltage on those wires before and after the engine quits to see if there's a change. However I'm very suspicious of the ground connection on the control relay. Check continuity on the "black" wire coming from the control relay to ground. If you read anything other than zero, you may have found the problem. If you're using an analog meter, be sure to first "zero" the meter.

If the control relay harness/connection passes the ground continuity test and there's voltage on the 2 identified relay trigger wires - if it were my truck it would then be time to get out the dynamite. <img src="/forums/images/graemlins/lol.gif" alt="" />

I know there are differences in the '89 ECU and your own, but I don't know what they might be. The setup could be quite different and the wire color markings certainly could be so it would be better to use the manual for your model year. The ground wire on the control relay would be the same "black" color, though. Maybe someone with a '91 manual can verify the wiring as identical or not.... in any event, it won't hurt to measure voltage on those wires.

Good luck,
Frank

Frank
It is not uncommon for the IAC motors to take out the drivers in Mitsubishi vehicles. Here is a trick that i use to test a transistor for any ECM controlled circuit. Remove the ignition fuse to the ECM. Place an ammeter in place of the fuse. disconnect the componenent (relay, motor etc.) Turn the key on. The meter should read between 4mA to 40mA depending on the transistor and its circuit.
Forget the six pack. I am still waiting on that armed chopper you showed me.
Conner
<img src="/forums/images/graemlins/lol.gif" alt="" /> <img src="/forums/images/graemlins/lol.gif" alt="" />

So if the ECU is blown, the ammeter would read zero? Is that the idea? If so, you wouldn't know which component was blown, though, would you?

Also, couldn't there still be current draw if one section of an ECU was open and another working correctly?

I've never seen the circuit diagram of an ECU, but did have my spare one apart to look at the board - didn't count the components, though.

Oh - the chopper's back-ordered. <img src="/forums/images/graemlins/lol.gif" alt="" />

Frank

Conner -

If you have a minute or so, take a look at the 3.0L MPI circuit and see if you agree with my thought in the above post... I don't know what else to suggest.

Frank

Frank Yes, if that paticular driver is bad you will see zero. Later vehicles allow you to active a circuit through the scan tool so you know what you are dealing with. In earlier vehicles, i just look at a wiring schematic and identify what else may be activated at the same time and disconnect those components, Actually a lab scope is the best tool as i can even find a working driver thats not pulling the component all the way to ground.
Hope that chopper makes it on time for hunting season.
Conner

Another update for this 91 monty. Took ECU to local Mits dealer and had them send it to their preferred repair tech for diag. Comes back with repair needed on fuel pump circ and idle circ. <img src="/forums/images/graemlins/angry.gif" alt="" /> So for no reply from Avpro who was suppose to fix it. Didn't mind taking my $150 though. I know the ISC is bad and I am going to replace it. I have checked the TPS and it seems to be OK. Is there a way to check the idle switch that is in the TPS? <img src="/forums/images/graemlins/scared.gif" alt="" /> I may have another 4th of July in August <img src="/forums/images/graemlins/patriot.gif" alt="" />