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Thread: MAF voltage error, car running rich

  1. #1
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    MAF voltage error, car running rich

    This particular issue is solved, but I thought I'd share it with the forum, might save someone some time/agro in the future.

    I've wired in a fuel piggyback, that adjusts the voltage from the MAF, thereby causing the ECU to adjust the mixture. Anyway, I couldn't get the mixture to do what I wanted, it was all over the place, but generally too rich. We had it out at the track last weekend, and couldn't solve it, so disconnected the MAF entirely. The car actually ran pretty well, albeit a bit rich, and both me and Junior new lap PRs. New tyres helped.

    Long story short, I had bunched the new wiring into the adjacent injector wiring, and the MAF signal wire was getting interference from the injector wires, and signalling roughly an 0.2 volts more than it should have to the ECU, about 1.5 volts at idle where it should be around 1.2 volts. The ECU was responding by adding more fuel. It appears that the interference may have been through the heat shrink where I spliced in the new wires running to the piggyback. I didn't manage to figure all this out until last night, but once the new wiring and the splices were clear of the injector wires, it ran fine.

    Once it was sorted, it was cool watching the ECU retrim the fuel on the wide band, took about 3 minutes at idle for it to be corrected back to 1 lambda.

  2. #2
    Purist, whats that? Jason Broadhurst's Avatar
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    Good pickup bro.

    Do you mean the injectors, or ignition leads? Injectors run about 1 A max each, more briefly but not a heap. Induction from 1 A adjacent to a MAF/MAP signal would be very difficult to achieve. A high resistance fault via a stray strand or pointy solder blob would have a huge impact though.

    It's not common practice to shield MAP or MAF signals so it's an interesting problem you had!
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    Quote Originally Posted by Jason Broadhurst View Post
    Good pickup bro.

    Do you mean the injectors, or ignition leads? Injectors run about 1 A max each, more briefly but not a heap. Induction from 1 A adjacent to a MAF/MAP signal would be very difficult to achieve. A high resistance fault via a stray strand or pointy solder blob would have a huge impact though.

    It's not common practice to shield MAP or MAF signals so it's an interesting problem you had!
    Well, I thought it was the injector wires, but now that you mention it, the junction of the ignition wires (It's coil on plug) was close by too, I'll have a closer look and report back. And thanks for the pointer about crappy soldering, I thought it was OK but I'll double check that too.

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    Here's a pic of the splices and adjacent area. The black goes from the MAF (which is out of shot above) to the piggy, and blue from the piggy to the ECU. They were taped to the existing bundle on the left side, which has the injector wires plus some others such as MAP, possibly crank angle, plus maybe a couple more. The ignition coil wires branch off below out of shot, so they were maybe 80mm away from the splices.



    And here's a close up of the splice from the MAF side. Not sure if the soldering is up to standard, it's a bit lumpy, but solid and I couldn't feel any wires hanging out of it. If you have a close look at the bracket you can see some blue residue, I think this has rubbed off from the heat shrink, which may have created a weak spot in the insulation.

    Last edited by Sprinkles; 18-05-17 at 08:29 AM.

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    Not quite solved......

    The car ran great at it's last outing..... for 5 laps, then the MAF signal went crazy again, so we disconnected it again, and pressed on with no MAF. I am thinking that heat soak or vibration are triggering the issue.

    A bit more research has suggested that extending the length of the wiring running between the MAF and the ECU adds vulnerability to picking up stray signals. I've added probably 2.5 metres of additional cable run, maybe more. So I have picked up some shielded cable and intend on earthing the shield which I understand helps in this situation, and will run it via a more direct route to shorten the additional length.

    Is it preferable to earth the shield on the chassis or on the engine, or does it make no difference?

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    Registered User AlexinPerth's Avatar
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    Either end, but just one. Don't earth it at both ends.

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    Quote Originally Posted by AlexinPerth View Post
    Either end, but just one. Don't earth it at both ends.
    Thanks Alex. My plan to earth the shielded cable is to strip off about 20mm of the outer insulation, strip about 50mm off a regular bit of cable to run to ground, and wrap that around the braided shielding, then hold it in place with heat shrink. Am I making sense?

    Here's a pic of the cable:


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    arboreal bukkake briney's Avatar
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    3m makes shielding tubing. you can slip it over the outside of a bunch of wiring. maybe try to get hold of some of that? then heatshrink over the top?
    Quote Originally Posted by Skompa View Post
    The throttle linkages jammed on the CDs in my old Triumph 2000 and stuck it at WOT.

    I didn't realise for about 15 mins as it never broke the speed limit.

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    Registered User AlexinPerth's Avatar
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    Quote Originally Posted by Sprinkles View Post
    Thanks Alex. My plan to earth the shielded cable is to strip off about 20mm of the outer insulation, strip about 50mm off a regular bit of cable to run to ground, and wrap that around the braided shielding, then hold it in place with heat shrink. Am I making sense?

    Here's a pic of the cable:
    What I normally do with that is use a multimeter probe (or similar) to unbraid the sheath along a line, and then gather all the braid on one side of the cable, and twist it all together.

    Sort of like this, but twisted together into one group

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    Quote Originally Posted by AlexinPerth View Post
    What I normally do with that is use a multimeter probe (or similar) to unbraid the sheath along a line, and then gather all the braid on one side of the cable, and twist it all together.

    Sort of like this, but twisted together into one group
    Like this? Obviously with solder and heat shrink as well..


  11. #11
    Registered User AlexinPerth's Avatar
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    Yep

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    Quote Originally Posted by AlexinPerth View Post
    Yep
    Cheers Alex. Will provide further updates in this thread

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    Purist, whats that? Jason Broadhurst's Avatar
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    If you want to get really legit - once the conductor is connected and heat shrunk, you can wrap it in aluminium foil to continue the shielding across the join.

    That's only if you are joining two shielded cables. if you are just terminating it to it's plug then leave the shield open at the engine side (but insulated to stop shorts to the conductor or chassis) and earth the shield on the ECU and in the cabin. This doesn't and shouldn't run to the ECU sensor earth, it's fine to take it all the other shield earths and then to the chassis or body earth.
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    Finally got it solved

    The car went on the dyno today, and verified that the interference issue is finally solved. Here's what I did, with some progress pics.

    Earthed the shielding on the body, at the piggyyback end, one wire complete, the other soldered and about have heatshrink applied:




    I shielded the spliced end as well, by exposing some of the braided shielding.....





    then wrapped the braid and soldered splice in aluminium foil (thanks for this tip Jason), and covered the joint in another layer of heatshrink:




    So it's been a bit of a mission getting this issue sorted, but hopefully we will finally see the benefit of it at the next track day in a couple of weeks.
    Last edited by Sprinkles; 12-07-17 at 09:52 PM.

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    Purist, whats that? Jason Broadhurst's Avatar
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