Accessing the 12V feed to PEM (1.5)

[nick]

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

 

[nick]

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

 

[dhrivnak]

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

 

[wiztecy]

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.

 

[nick]

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.

 

[dhrivnak]

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.

 

[nick]

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.

 

[gregd]

Be careful with the supercaps... if they fail, they tend to fail shorted, so should be fused somehow.

 

[nick]

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

 

[ML Auto]

Not sure you are headed in the right direction. What exactly is the "glitch" you are talking about?

 

[nick]

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

 

[hcsharp]

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.

 

[nick]

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)

 

[ML Auto]

So what happens that you need to pull the cabin fuse and reset?

 

[nick]

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.

 

[gregd]

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

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.