TMC is an independent, primarily volunteer organization that relies on ad revenue to cover its operating costs. Please consider whitelisting TMC on your ad blocker or making a Paypal contribution here: paypal.me/SupportTMC

Accessing the 12V feed to PEM (1.5)

Discussion in 'Roadster: Technical' started by nick, Mar 19, 2016.

  1. nick

    nick Member

    Joined:
    May 22, 2012
    Messages:
    55
    Is there an easily accessible way to probe the 12V power feed going to the PEM on a 1.5?

    My Roadster sometimes glitches on key ON requiring the 60A cabin fuse to be pulled to reset things.
    Measuring at the cabin fuse box the 12V rail is stable and clean (13.6V, ~100mVp-p) under steady load.

    When the key is turned to the ON position there is a very short-duration sag of about 4.5V followed by an overshoot of about 3.5V. The APS starts to recover the sag in under 80ns and crosses the 13.6V line in under 200ns (sag is very very short).

    Pulling each cabin fuse (one out at a time), including pulling the 60A fuse doesn't prevent the sag, suggesting it isn't caused by something in the cabin. Interestingly pulling the 60A fuse created a delay between the key ON and the CLICK from the PEM - the sag shows on the CLICK, not on the key ON.

    If I can measure the sag close to the PEM that should definitively confirm it is the PEM turning on that is causing the transient load. If I need to add capacitance it would be best added before but close to the transient load.

    TLDR: is there an easily accessible way to probe the 12V power feed going to the PEM on a 1.5?

    Nick
     
  2. nick

    nick Member

    Joined:
    May 22, 2012
    Messages:
    55
    Oh, it just clicked... the CLICK will be the electronic park brake.

    New question... is there an easy way to unplug the electronic parking brake or disable by pulling a fuse? I don't need it to mechanically disengage, just electrically.

    Nick
     
  3. dhrivnak

    dhrivnak Active Member

    Joined:
    Jan 8, 2011
    Messages:
    2,150
    Location:
    NE Tennessee
    I do not think there is an electronic parking brake as it is manual. But there is a mechanical pawl that engages in the transmission that locks it like park on a normal transmission. You should be able to test by engaging/disengaging tow mode with the VDS
     
  4. wiztecy

    wiztecy Active Member

    Joined:
    Apr 29, 2012
    Messages:
    2,714
    Location:
    Santa Cruz, California, United States
    The CLICK you're hearing is most likely the high voltage contactor (located in the ESS box, believe driver's side) engaging from its disengaged state. Same contactor can be heard when you charge.
     
  5. nick

    nick Member

    Joined:
    May 22, 2012
    Messages:
    55
    Tow mode was a great idea. Result - it isn't causing the sag (and isn't the same click sound).

    Not the transmission lock, not any of the cabin-fuse-box paths, and not the 60A cabin circuit.

    PEM is next on my list... need to find a 12V point to probe.
     
  6. dhrivnak

    dhrivnak Active Member

    Joined:
    Jan 8, 2011
    Messages:
    2,150
    Location:
    NE Tennessee
    Under the hood is a high voltage controller. A shoe box sized box with Al fins. Mine went out and when it did I heard a clicking behind the drivers seat like a contractor closing. Mine was a single click. My unit needed replaced at $1200. My eyes you'd is going bad.
     
  7. nick

    nick Member

    Joined:
    May 22, 2012
    Messages:
    55
    Just ordered the parts needed at add capacitance to an easily accessible circuit.

    Plan is to remove the 60A fuse in the cabin, simple plug-in circuit of a small 50 gram 120,000uF super-capacitor (the size of a small flat cellphone battery) + 60A fuse. 10k resistor to GND for initial charging and a heavy path to GND connected once charged.
     
  8. gregd

    gregd Member

    Joined:
    Dec 31, 2014
    Messages:
    573
    Be careful with the supercaps... if they fail, they tend to fail shorted, so should be fused somehow.
     
  9. nick

    nick Member

    Joined:
    May 22, 2012
    Messages:
    55
    Failure mode - good thinking. Will look at adding a 10A fuse in the capacitor path.

    Of course if it fails the car will never smell the same :)
     
  10. MLAUTO

    MLAUTO Member

    Joined:
    Mar 8, 2014
    Messages:
    314
    Location:
    SW Florida
    Not sure you are headed in the right direction. What exactly is the "glitch" you are talking about?
     
  11. nick

    nick Member

    Joined:
    May 22, 2012
    Messages:
    55
    #11 nick, Mar 22, 2016
    Last edited: Mar 22, 2016
    Voltage sag and overshoot on the 12V rail on key ON:
    Dropbox - 2016-03-19 13.50.48.jpg

    -4.32V sag, 3.68V overshoot. These numbers are typical, sometimes worse.
    The 12V power supply is otherwise working well - under 100mVp-p noise, handles step change due to headlights and other high current accessories ok.

    Time reference - start of sag to lowest point is 80ns, total time below nominal is under 200ns:
    Dropbox - 2016-03-19 13.42.26.jpg

    Nick
     
  12. hcsharp

    hcsharp Active Member

    Joined:
    Jun 7, 2011
    Messages:
    2,532
    Location:
    Vermont
    Where are you probing it? I'd like to test my car for comparison but it's a 2.5 so it won't provide a good reference.
     
  13. nick

    nick Member

    Joined:
    May 22, 2012
    Messages:
    55
    From the cabin fuse panel - I pulled the fuse for the horn and used the hot side for measurements.

    The 2.5 has a 12V battery doesn't it? (1.5 doesn't)
     
  14. MLAUTO

    MLAUTO Member

    Joined:
    Mar 8, 2014
    Messages:
    314
    Location:
    SW Florida
    So what happens that you need to pull the cabin fuse and reset?
     
  15. nick

    nick Member

    Joined:
    May 22, 2012
    Messages:
    55
    Supercap will be on the hot side before the 60A fuse (with its own 10A fuse).... no problem pulling the 60A fuse or any other fuse like normal.

    Will need to be careful disconnecting/reconnecting the main battery pack but easy enough to remove and discharge before taking it in for a battery pack replacement.
     
  16. gregd

    gregd Member

    Joined:
    Dec 31, 2014
    Messages:
    573
    Another thing to consider is what you are measuring the 12v against (what is "ground"). The apparent sag in +12 could just as easily be a rise in the point of reference. Not all grounds are necessarily equal, if large currents are being drawn. 60A is definitely on the "large" side...

    Just a thought.
     

Share This Page