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

  1. #21
    Join Date
    Aug 2017
    Posts
    42

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    Yes, the gauge reads fine. It shows various AF readings as i change throttle positions. The car never dies once it's running, whether with the gauge, the o2 or both are plugged in, but they have to be connected after the engine is running or we get a no-start.

  2. #22
    Join Date
    Aug 2017
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    42

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    The O2 is the device that came with the gauge. It is a 6 core cable between the gauge and the O2 and that carries, live, earth so i cannot easily separate the gauge earth from the O2 earth. The 6 cores come together in a small plug which attaches to the gauge.

    I had suspected the PATs and moved the gauge, but that was before I noticed the unplugged O2 improves things. The excess O2-to-gauge cabling is under the dash so if that's what's making EMI, then it might be a factor. I'll go move it later and get it as far from the PATS halo as i can to see if that changes anything and report back.

    The gauge and ECU are about half the cockpit apart. The gauge is behind the gear stick, and the ECU is at the bottom of the passenger footwell

    Disconnecting the gauge will allow me to start. Currently i just pull the connectors apart, then reconnect, but i need a switch would be a more good version of that crudeness. I did try attaching the gauge to switched live, but of course switched live is on before the engine starts so it had no positive effect other than meaning the gauge goes off when i turn the ignition off.

    thanks for your input, everyone

  3. #23
    Join Date
    Jan 2006
    Location
    Seattle-ish
    Posts
    1,043

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    Have you contacted AEM yet to discuss? They may have some unpublished workarounds you can try.

    -John
    Westfield SEiW
    2.0L Duratec
    Throttle Steer

  4. #24
    Join Date
    Aug 2017
    Posts
    42

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    I haven't contacted them, but that is a good idea. Mainly I didn't because this used to happen with my USB in the cigarette lighter, too. This was before i had the gauge so i thought it was something more broad. I'm going to play with it some more and see if it is EMI from the wiring and device. I'll update you soon!

  5. #25
    Join Date
    Aug 2017
    Posts
    42

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    The plot thickens, folks.

    I have an SCT X4 for tuning. As you probably know, they plug in to the OBD port and nowhere else. They don't have separate power. I use the X4 in conjunction with the AF gauge to report the AF ratio for logging. Turbo Tom is tuning the car for me.

    Anyway. I had suspected my no-start was also happening with the X4 attached, and i just confirmed it by reading the voltage on the output side of the Fuel Pump Driver Module. When trouble comes, the voltage reads 0V on firing and the engine does not run. It reads 8V (ish) normally at idle. I don't think this is the only ECU symptom as it's not consistent, but it's the only easy way i have to measure if that ECU is acting up and a 0V definitely. My findings are:

    - X4 connected to OBD and AF disconnected and O2 disconnected from AF - no start
    - gauge attached but no O2 - start
    - gauge attached with O2 - no start
    - gauge and O2 connected but with wires completely pulled away from the dash and the PATS sensor - no start

    Apart from current draw, the only differences I am aware of are that the gauge alone uses an earth to the chassis. If the O2 is earthing through the exhaust, then it might be going through the motor and back to the battery.

    Today I have been under the car to clean up and ensure a good connection for the engine to frame earth. The battery to engine earth is new. I also made a new earth strap and connected to the body, but nope. Symptoms remain as above. I even ran a jump lead from the battery negative to the exhaust to give the O2 a different earth. Same

    I can get around all the above my unplugging/turning off everything before starting, but it's really doing my head in and i'd love to know why it's happening. Perhaps something's messing with the ECU's earth, but why don't the headlights or radiator fan cause trouble.

  6. #26
    Join Date
    Dec 2014
    Posts
    110

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    Not sure if it would help but on the advice of Tom Carlin, I decided to attend to the ground system with great care. I have a ground strip behind the dash that is connected directly to the battery ground. Everything behind the dash grounds there. I also have one at the rear of the car for the fuel pumps, lights etc. That is also wired directly to the battery. I also have a #4 cable running from the battery ground to the aft bolt on the exhaust manifold. I might have other stuff happen but I DO NOT have any ground issues. I am on my third O2 sensor (???) but other than that, I have had no problems. I used 14g wire for the connections between the ground strips to the battery with all connections crimped, soldered and heat shrinked. Once the individual grounds are screwed to the ground strips, hit the screws with green Loktight. You can then measure the continuity with an ohmmeter from each device to the battery, switch off and switch on. if there is any resistance, at least you will know where to start. You can't rust any grounds to the chassis due to the varying density of welds, brazes and riveted structures. I have restored several old race cars over the years and this strategy has always worked.

  7. #27
    Join Date
    Sep 2010
    Location
    Duxbury MA
    Posts
    14

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    Ummmm...why are you on your third O2 sensor?
    OK. Two things.
    One thing- you need to eliminate voltage as the issue. Somehow, you will need to run a pigtail out of the ECU power line and get a meter on it. If the voltage sags below 10V, I would chase that.
    Second thing- How does your ECU sense crank timing? Do you have access to an oscilloscope?

  8. #28
    Join Date
    Aug 2017
    Posts
    42

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    Hi,

    I inserted a pin in to the back of the permanent +12v cable on the ECU connector and measure the voltage at cranking both with the AF connected, AND without. In both cases it dipped to about 8.8V. Without the AF on, it started ok.

    When i tested the battery during cranking, it dipped to about 10.5 V. Not sure where my 2V are getting lost.

    I'd like to look up where my ECU permanent live comes from, but no wiring diagram. The next step is to start trying to trace it so if i get anywhere, i'll update you.

  9. #29
    Join Date
    Aug 2017
    Posts
    42

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    Looks like the 12v is coming a fused connection directly from the battery. The cable goes into the loom, but the permanent 12v on the ECU definitely ends up at this cable according to my continuity test. Next test is to backprobe the earth(s) on the ECU and see if i can see a voltage drop or some other poor connection.

  10. #30
    Join Date
    Sep 2010
    Location
    Duxbury MA
    Posts
    14

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    As long as you have the wires hooked up, measure the resistance in that line.

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