No Start, ECU (apparently) Shuts down while cranking. | AEM
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39CAMC
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Joined: 12/10/2018 - 09:29
No Start, ECU (apparently) Shuts down while cranking.

Car is a Miata with a Honda J32 V6 conversion.  I did not build the car originally, but one of my customers bought it from the shop who came up with the conversion kit.

We replaced the engine, the old one expired due to overheating and spinning all 6(!) rod bearings on track.   Installed the new engine, and the very first time cranking it didn't want to start, it just cranked.     

We checked connections, grounds, etc and hit it again and it fired right up.   We did all of our various things to get it ready to return to the customer, starting and stopping many times.

Sure enough, the customer comes to pick up and crank, no start.   It then started the next day to drive into our shop and then as we messed with things, stopped firing altogether. 

At one point, I suspected the Miata main relay, but I have both replaced and jumpered that to no avail.

As far as I can tell, it is lacking spark.   I used a spare coil pack/spark plug and it does not spark.  I can hear fuel pump prime when key is first turned on and the old school test of spraying starting fluid does not make it do anything.  It doesn't have a shrader for testing fuel pressure.

Anyway, a safe/only assumption here is that we screwed up a wire, or left something unplugged, missed a ground, etc.

I figured the next best step was to hook up the ECU and see if I could get any live trace data or a log on sensors and trace the problem down.   

I cannot get the ECU to hook up with USB, but it will hook up to the laptop via serial.  As soon as I crank, it loses connection.  If I have PC logging enabled, it tries to save the log immediately.  I cannot reconnect to the ECU until I power cycle the key.

I have checked the power inputs on B1, B9 and B21.  All have 13.2v static and 10.2-10.5 cranking.

B2, B10, B20 and B22 grounds show continuity to ground.

In my various searching, I think I saw something about shorted sensors causing the ECU's to shut down...could that be what is happening here?  Or is there other places I should be checking for power?

Thanks for any input!

DaveW

 

 

AEM_SB
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Joined: 06/07/2014 - 14:29
Even if a multimeter shows

Even if a multimeter shows the battery voltage around 10V when cranking, it's pretty common for it to briefly drop lower when the starter first engages, which will cause the ECU power to drop out. A new engine with better compression will put more load on the starter, which may cause the battery voltage to be dropping lower than the previous engine. Losing connection with the laptop while cranking is a bad sign, that should never happen. Add the channel 'Run Time' to the channels list and watch for it to continue counting while the power is on. Run time should not reset back to zero or low numbers when cranking, that would indicate the 12V power to the ECU dropped too low and the ECU reset.  Some production cars that were designed to have a rear-mounted battery run a separate power wire from the battery to the ECU, so the ECU power is not affected by the starter engaging.

 

 

Some OEM harnesses will connect the ECU's signal ground wire to the engine or intake manifold, if your harness does this it's very important that the engine is grounded to the chassis & battery with a very good connection to avoid drawing excess current through the ECU's signal ground wire. Ideally the ECU's signal ground wire should only connect to sensors, it's better that it doesn't connect to the engine or intake manifold at all. 

 

Many of the Lowside output pins have a circuit that will prevent the ECU from powering down or resetting properly if they are receiving 12V power when the ECU is off, for instance if they are triggering a relay that is getting its 12V from permanent battery power (not via the same ignition source that powers the ECU). My notes say the LS4-LS12 outputs are the most likely to cause problems. Solution is to change the wiring so the relay's coil (pins 85 or 86 using Bosch relay pin labeling) is only receiving 12V power when the ignition key is on. I realize it's pretty common to run a big power straight from the battery to the relay, you can keep that big power wire connected to pin 30 (the high-current switched power source) but make sure it's not connected to pins 85 or 86 (the coil that triggers the switched power). 

 

Hope that helps,

SB

     

    39CAMC
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    Last seen: 1 year 4 months ago
    Joined: 12/10/2018 - 09:29
    This has nothing to do with

    This has nothing to do with ECU, but just in case you have a thought about what is going on.....

    I have done a ton of testing on the voltage drop issue....to no avail.   Tried new cables, new grounds, shorter cables, new battery, , new starter, etc.   Every once in a while, the car will start off the battery but not consistantly and never twice in a row.

    I even hooked up  B1, B9 and B21 direct to battery and B2, B10, B20 and B22 either to direct to chassis ground or directly to the battery ground via 4ga cable.  No start.

    However, if I hook B1, b9 and b21 directly to a battery charger (isolated from battery power) the car starts.  Every. Single. Time.
     

    When hooked to the battery, I am showing .2 voltage drop between battery pos terminal and my b1-b9-b21 bundle.   Ditto on the negative side.

    My thinking is that I have a ground problem between the ECU and chassis and or sensors and the 14 volts from the battery charger over powers it.  I can't figure that anything could possibly fail in the ECU that would only work when it had 13-14 volts applied...

    Any thoughts?

    DaveW

    AEM_SB
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    Joined: 06/07/2014 - 14:29
    Interesting. Will it restart

    Interesting. Will it restart if you stall the engine and crank without turning the key to the 'off' position? 

    39CAMC
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    AEM_SB wrote:

    AEM_SB wrote:

    Interesting. Will it restart if you stall the engine and crank without turning the key to the 'off' position? 

    Yes, I just tested it.   Started with ECU hooked to battery charger and stalled by putting in gear and popping clutch with e-brake on.   Immediately restarts.

    EDIT - I did some more voltage drop testing on the ground side.  Between the battery neg terminal and my temporary chassis ground point for the ECU, I am showing almost 7/10 volt drop which is problematic.   However, even when I hook those ground points between directly to the battery, it still won't start.

    DaveW
     

    AngelEK9
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    Any update on this post? I

    Any update on this post? I have the exact same problem and i got word that it might just be an ECU problem. Some people say AEM's typically have these kind of startup issues

    Angel Vasiliou