? About Power Supply to ECU

Posted by silverbulletAWD, May 16, 2005
Tuning & Engine Management - 4G63 EPROMS, ECU, MAF, knock, EGT, wideband, datalogging, fuel trims, etc. Read this Forum's Strict Guidelines.

  1. silverbulletAWD

    silverbulletAWD Proven Member

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    Holland, Michigan
    I could use some help here guys. I've got a car here that will crank over but there seems to be no power to the ECU. No spark, no injector pulse. I'm getting the switched power through the MPI relay. And on the plug into the ECU I'm getting power to what the diagram calls the 'Power Backup" and the "MPI relay". What I'm not getting is power to pins 102 and 107 which are both labeled "Power Supply" I dont get it at any key position and I can't seem to find the source of this power yet either. I checked the MPI fuse off the battery but that looks like the same color wire as the "Power Backup" and for that matter the fuse is fine.

    Does anybody know where the power source for the two dead pins comes from so I can trace it or when or if or how those are supposed to get power? Or if it's safe to try to jump power into them? Any other ideas or common experiences? I'm pulling my hair out at this point.

    Thanks,
    Andy Prins
    #1
  2. steve

    steve DSM Wiseman

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    The MPI relay when energized provides power to pin 102 and 107 from pins 4 and 5 of the MPI relay. Pin 63 on the ECU is the control line the ECU uses to turn the MPI relay. It's the ECU side of the MPI relay coil and will be at 12v until the ECU turns the relay on by pulling that pin to ground.

    Sounds like it's time to pull the ECU and check for capacitor leakage. Not powering up in response to the ignition switch is one of the common failure modes of advanced corrosion damage.

    Steve
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  3. silverbulletAWD

    silverbulletAWD Proven Member

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    Hmm, I guess I will pull the cover off this ECU and take a look. The only thing is that I already tried another ECU that was believed to be OK and nothing changed. I will check again though.

    I also, out of curiousity, went out and checked the MPI Relay plug of a running car and got all of the same results w/ a test light as what I've been getting on the problem car. It seems like it could be the relay but I've already tried 2 different ones of those too. And I know those were good. Do you have any other ideas if it's not the ECU or the relay?

    You've given me a few other things to try so I get at it first thing tomorrow and see what happens. Thanks for your help. I'm certainly no electrical guru. Although with the plague I've been cursed w/ here I should be an expert by now I guess...

    Andy
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  4. steve

    steve DSM Wiseman

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    Andy,

    I just going on what you've told us so far. SO let me explain how the system works and perhaps somewhere along the line you'll read something that gives you some ideas.

    The ECU has two different power inputs.
    Backup power comes to ECU pin 103 from fuse 19 (room) by the drivers left foot. Backup power is used to retain the contents of RAM in the CPU and to power the circuit that controls the activation of the MPI circuit. You should always have 12v at pin 103 or the ECU will never power up.
    The MPI power come from the 30A MPI fuse on the battery + terminal to pin 10 on the MPI relay. Inside the MPI relay it goes the contacts and through a diode to one side of the coil so you'll see 12v at pin 8 on the relay because that's the other end of the coil winding and runs to ECU pins 63 (Control) and 66 (Sense).

    There is one other input to the ECU involved in powering up and that's the signal from the ignition switch RUN position (IG1) that goes to ECU pin 110.

    When the ECU sees +12v on pin 110 it pulls it's side of the MPI relay coil to ground (ECU pin 63) causing current to flow in the coil and close the contacts connecting relay pin 10 with relay pins 4 and 5. They feed power to ECU pins 102 and 107 as well as the MAF, CAS, BCS, ISC, Injectors, purge solenoid, fuel pressure solenoid, and EGR solenoids.

    With the arrival of +12 to pins 102 and 107 the main +5v regulator starts and logic resets the CPU. At that point the ECU should turn on the CEL, set the boost gauge to read 0 and prepare for you to start the car.

    You can check the MPI relay by grounding pin 8. if the contacts close and you see 12v on pins 4 and 5, it's good. If you don't have 12v at pin 10 on the relay all the time then the MPI fuse is blown or the wiring is bad. If you don't have 12v at relay pin 8 then the coil in the relay is bad.

    If ECU pin 110 doesn't have 12v when the ignition switch is in RUN then there is a problem in the ignition circuit or the ECU is really fried.
    If ECU pin 63 doesn't go from 12v to 0v (or within a half volt of 0) when 12v is applied by the ignition switch the ECU is bad.

    Once the ECU power up there are still a whole tree of problems that can keep the car from starting.

    I hope that helps you track down your problem.

    Steve
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  5. silverbulletAWD

    silverbulletAWD Proven Member

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    That is some very good information. Thank you. I'm going to go out and start going through these things.

    I'm definately aware there can be many other problems as well. I started by swaping out every other part in the entire spark ignition system to no avail. Once I realized I wasn't getting any injector pulse while cranking over I figured it must be back at the ECU. I'll keep trying things here. I feel bad because it's my younger brothers car and he wants to go racing so bad and I've been so busy with my business and my own car I can hardly touch his car. Thanks.

    Andy
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  6. Turbo Talon DL

    Turbo Talon DL Proven Member

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    Im having ECU issues now, I swear, its ALWAYS something with my car :mad: I dont get it!! :cry:

    Here's whats going on (lack there of): the car sat for a month with a new battery. The car started last time I was home to put the new stuff in it, I actually drove it with the new battery. I get home today, put the key in, the boost gauge doesnt move, the AFC doesnt turn on, no spark, no fuel, no nothing. The car just cranks over. It used to fire right up. It has been having problems with TPS codes, but the car still DROVE. It doesnt perform as well as a trouble code free ecu would, but it got me around town. It had a TPS code for the longest time. Would that TPS code be a clue as to my ECU getting ready to take a sh!t on me or what? The car sat for a month, and now does NOTHING. No CEL either to pull a trouble code. Its still under warranty from Foreign Auto Computer, so they're gonna play with it now!! Let me know what you guys know. Send me a PM if you want to help me. Thanks...
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  7. timloomis

    timloomis Proven Member

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    im saying its you caps.mine did almost the same thing.the mpi relay would sound like morse code then eventually work right.turned out to be leaky caps.
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