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Thread: Ongoing electrical problem - anyone want a challenge! ;-)

  1. #1
    Join Date
    Aug 2017
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    42

    Default Ongoing electrical problem - anyone want a challenge! ;-)

    Hi, all

    This is the ongoing issue I've mentioned in the hard starting thread, and a few others, but I wanted to share what's up in case anyone who's good at electrics can help. It's confusing the hell out of me, but I'm no auto electrician.

    The headline is that when I connect my AF gauge or something like a USB socket in the manner described in the diagram, it seems to upset the ECU and the car won't start. From measuring and logging, the RPM data disappears from the logs using my SCT device, and the Fuel pump driver module stops sending 8v to the pump, as normally commanded by the ECU.

    The problem comes when the AF is powered and the earth is in any way connected to the chassis earth. As soon as I disconnect the device, the car starts and runs well. The rewiring I've done to see what happens is


    • You'll see from the diagram that the wiring is about as simple as it gets. +ve from the battery, -ve from the chassis but this setup results in no start.
    • If i run an earth from the battery to the earth bolt (in diagram) or even directly to the AF gauge negative wire - no start
    • if i get a spare battery and connect - and + to the gauge with the O2 sensor connected - no start
    • Connect a spare battery to the AF gauge and DISCONNECT the O2 sensor - START
    • Connect the car's battery to the AF and DISCONNECT the O2 sensor - START



    So... to me it seems like an earth issue, but I don't know what or how to test. I can tell that the ECU's case is earthed, that the engine, chassis and battery have very low resistance between any of them. The Battery to engine wire is new, the battery is new, but now I'm stuck. I don't have a wiring diagram and without taking out the dash and un-bundling the loom, i can't easily trace wires. I do have the Focus green book which shows me ECU pins and their connections.

    If anyone has any thoughts on what's going on here, I'd love to hear them!

    Otherwise the car is great. It goes like a scolded cat and has made me into a minor celebrity in the neighborhood

    Thanks in advance

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  2. #2
    Join Date
    Sep 2010
    Location
    Duxbury MA
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    14

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    I don't think you can say that the exhaust system is at ground potential. What with gaskets and rubber hangars and all...
    What brand gauge and sensor? Forgetting starting for a moment, does the A/F read (full lean since the car is not running) and does the O2 sensor get hot? In your scenario above, its not really clear that you COMPLETELY disconnected the A/F from the car electrics when you did your battery test.
    You are clearly making the ECU unhappy. Time to look for voltage drops. How is your ECU grounded?

  3. #3
    Join Date
    Aug 2017
    Posts
    42

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    Hi,

    It's an AEM 30-4110 UEGO gauge.

    You're right about the exhaust, but all i can tell you is that the resistance between the pipe and the battery -ve is 00.2 ohms on my multimeter, that's basically the same as between the battery and chassis, or battery and engine.

    The above diagram shows how it's usually wired. In other tests, I have disconnected the wires from the car, + and -, then run new temporary wires to the gauge to ensure they're separate. It'll start if i do that AND the O2 sensor is unplugged from the gauge. If the sensor is plugged in, we're back to no start.

    The 02 does get hot when it's plugged in.

    The ECU appears to be grounded through its plug, and the ECU case seems also to be ground. I can't easily see where the plug wires then go as they're all bound up and I have no diagram, so I'm hoping someone knows of tests I can do such as the voltage drop you mention. From speaking to others, I hear that a big enough drop will upset the ECU, but that was in the context of a bad starter pulling lots of current.

    When i read the battery voltage during cranking, it goes from 12.3 down to 10 and a bit. Then back up to 13.4 ish when running. With the AF connected, it does the same, except it won't start.

    Thanks

  4. #4
    Join Date
    Nov 2017
    Location
    Deming WA.
    Posts
    7

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    Hi, you might find this link helpfull
    https://www.americanmuscle.com/autom...l-install.html

  5. #5
    Join Date
    Nov 2017
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    Deming WA.
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    7

  6. #6
    Join Date
    Sep 2010
    Location
    Duxbury MA
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    Here is another thought or 2.
    Is your O2 sensor 3 wire or 4 wire? I am guessing 3. My stuff is all 4, but I do recall seeing a ring with a wire under it for a 3 wire which went off to ground on a car which I was working on. That ground is typically for sensor ground, not the heater. I know that sounds bass ackwards, but that is what I have seen. Heater ground is back through the controller...in your case the gauge.
    Two of my cars run Megasquirt. At 10 volts the ECU gets sleepy and the cars wont start.
    Here is another experiment. Start your car, then plug in the O2 sensor once already started. Does it keep running?

  7. #7
    Join Date
    Jan 2006
    Location
    Seattle-ish
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    Quote Originally Posted by hairball29 View Post
    or something like a USB socket in the manner described in the diagram, it seems to upset the ECU and the car won't start.
    Can you expand on the above? The current conversation is centered around the WBO2 set up, but this indicates that might be a red herring. What USB setup creates the same issue?

    Thanks,
    John
    Westfield SEiW
    2.0L Duratec
    Throttle Steer

  8. #8
    Join Date
    Aug 2017
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    Hi, all

    thanks for the input. The 02 sensor has a 6 core cable that plugs in one end at the gauge, and terminates at the O2 sensor

    JohnCh, i just went out to test again. When i first started working on this, i just had a cigarette lighter socket with a dual USB plug in it which seemed to break it. Just now i tested and that seems to not be the case now. At least today! this is what i found:

    - Cigarette lighter with USB plug connected to my phone and charging (so drawing current from the battery) and wired in using the same connections as the AF above - start
    - AF gauge plugged in as above NOT connected to O2 - Start
    - AF gauge plugged in as above and connected to O2 - NO start

    So, it seems that the car will withstand a small amount of juice being drawn through the battery and back into the chassis, such as the USB charger, but perhaps when the O2 sensor heater draw is added, the no-start comes back.

    now, i should say that it failed to start pretty consistently before with just the USB plugged in, but i couldn't consistently recreate that just now. there were a couple of non-starts

    Do you think it's possible that the extra draw from the AF is lowering the voltage too much for the ECU to stand? I thought stock ECUs could stand a drop to below 10v? does anyone know what their battery drops to when cranking?

  9. #9
    Join Date
    Sep 2010
    Location
    Duxbury MA
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    That is exactly what I am thinking.
    My battery drops less than a volt when cranking with my Seven and Europa...never bothered to check any of the other cars. Did you get a chance to try start without O2 plugged in then plugging it in while running? That will settle once and for all whether low voltage is your ECU issue as the alternator will keep the voltage up. Did you check voltage at the ECU while cranking? I don't think any of them will run below 9 volts and as I mentioned earlier, I have a couple where its "Game Over" at 10V.
    Go get em!

  10. #10
    Join Date
    Jan 2006
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    Seattle-ish
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    Another thing to try -- and someone please correct me if this is a bad idea -- is to jump start the car. Hook up another running vehicle like you would do with a dead battery on the se7en and try to start it. I imagine with the other car running and likely a much larger battery in place, a big voltage hit would be far less likely to occur under these conditions. If it starts easily, you have likely confirmed this is the issue.

    -John
    Westfield SEiW
    2.0L Duratec
    Throttle Steer

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