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12v System Failure While Driving

RowdyMY

Member
Jun 21, 2021
151
315
Raleigh
Did a quick search and didn't find this in the Y forums. Sharing my experience in case anyone else encounters this problem.

Today while I was driving on I-95 (in the express lanes no less which didn't help) I received a warning that "Electrical System is Unable to Support All Features - Shutting down features to conserve energy".

After about 30" that turned into a mippling warning with "Electrical System Power Reduced - Vehicle May Shut Down Unexpectedly"

I got off the freeway at the next exit (thankfully) and was able to pull into a WaWa parking lot. As soon as I put the Model Y into park the warning changed to "Unable to Drive - Voltage Supply Too Low"

Attempted multiple MCU resets and the warnings came back the same immediately upon restart. Vehicle was immobile.

Contacted Tesla Emergency Roadside and they dispatched a tow truck. I had forced the vehicle to power-off completely from the Safety & Security menu but by the time the wrecker arrived the 12v system had completely died and we were unable to put the vehicle into tow mode. Had to use nylon skids to winch it onto the flatbed.

SoC was ~70% on the main battery when this happened. Car is an August 2021 build (Vin 258xxx).

Lessons Learned:
1. Get off the road ASAP. Once the 12v goes you're sitting in a 4k pound paperweight
2. I had ~15 minutes from first warning to total failure
3. I had my dog with me. I cracked the window enough so that my arm could fit and manually release the door latch.
4. The seats are unable to be folded without 12v. I had to climb through and help my border collie over the backseat (and finagle my bag through - I was traveling)
5. You can open the frunk to get the tow pin by pushing on the top right corner of the tow pin cover and then hooking up a 12v power supply
6. Don't enable tow mode early. You will have to sit with your foot applying pressure to the pedal (or chock the tires if you have something to do that with)

D96CB9EA-BA29-45F1-9348-339ED1ECFE84.jpeg
 

jcanoe

Active Member
Oct 2, 2020
4,374
4,546
Maryland
This failure appears to be other than a failed 12V battery. The 12V battery is maintained by the DC-to-DC inverter that supplies power for the 12V systems while the vehicle is powered on. If the DC-to-DC inverter fails the 12V would quickly lose its charge.
 

RowdyMY

Member
Jun 21, 2021
151
315
Raleigh
This failure appears to be other than a failed 12V battery. The 12V battery is maintained by the DC-to-DC inverter that supplies power for the 12V systems while the vehicle is powered on. If the DC-to-DC inverter fails the 12V would quickly lose its charge.
Tesla Service hasn't looked at it yet. I've only worked with the roadside team.

I will 100% update when they do. My initial inference is a bad cell in the 12v. It almost presents exactly as a failed alternator, no?

Not trying to argue with you here, just going with what I have at the moment.

Its at TYCO in the parking lot with the windows cracked so I hope they get to it soon. The 12v did lose its charge very quickly. The brake lights and foot lights worked when the wrecker showed but the MCU wouldn't power on. By the time he got us to the SC the 12v was COMPLETELY dead. No footwell, door, or brake lights.
 

jcanoe

Active Member
Oct 2, 2020
4,374
4,546
Maryland
Tesla Service hasn't looked at it yet. I've only worked with the roadside team.

I will 100% update when they do. My initial inference is a bad cell in the 12v. It almost presents exactly as a failed alternator, no?

Not trying to argue with you here, just going with what I have at the moment.

Its at TYCO in the parking lot with the windows cracked so I hope they get to it soon. The 12v did lose its charge very quickly. The brake lights and foot lights worked when the wrecker showed but the MCU wouldn't power on. By the time he got us to the SC the 12v was COMPLETELY dead. No footwell, door, or brake lights.
OK, interested in reading what Tesla finds is wrong.

The SOP for getting the Tesla vehicle unlocked, running when the 12V battery fails is to connect a 12V source such as a jump starter pack to the leads underneath the tow hook cover. The hood will unlatch and then you can open the hood and remove the shroud that is by the firewall to access the 12V battery. Once you attach the jump starter leads to the 12V battery terminals the Tesla vehicle doors can be unlocked and the Tesla vehicle will start up. As long as the Tesla vehicle is running the DC-to-DC inverter continues to provide power for the 12V systems even if the 12V battery has failed. If the Tesla shuts off, then you may need to use the jump starter to start up again. As with an alternator in an ICE vehicle once the Tesla vehicle has been successfully started you should be able to disconnect the negative lead from the 12V battery removing the 12V battery from the circuit and the vehicle should continue to run (possibly with error messages).
 

RowdyMY

Member
Jun 21, 2021
151
315
Raleigh
OK, interested in reading what Tesla finds is wrong.

The SOP for getting the Tesla vehicle unlocked, running when the 12V battery fails is to connect a 12V source such as a jump starter pack to the leads underneath the tow hook cover. The hood will unlatch and then you can open the hood and remove the shroud that is by the firewall to access the 12V battery. Once you attach the jump starter leads to the 12V battery terminals the Tesla vehicle doors can be unlocked and the Tesla vehicle will start up. As long as the Tesla vehicle is running the DC-to-DC inverter continues to provide power for the 12V systems even if the 12V battery has failed. If the Tesla shuts off, then you may need to use the jump starter to start up again. As with an alternator in an ICE vehicle once the Tesla vehicle has been successfully started you should be able to disconnect the negative lead from the 12V battery removing the 12V battery from the circuit and the vehicle should continue to run (possibly with error messages).
If it helps we connected a jumper pack to the actual battery terminals to try and enable tow mode. MCU never came online. Another reason I think it might be a bad cell.

Open to your interpretation though. It gave enough juice that the brake cylinder would "pulse" when I stepped on it but nothing else powered on. Tried the card key as well.
 

jcanoe

Active Member
Oct 2, 2020
4,374
4,546
Maryland
If it helps we connected a jumper pack to the actual battery terminals to try and enable tow mode. MCU never came online. Another reason I think it might be a bad cell.

Open to your interpretation though. It gave enough juice that the brake cylinder would "pulse" when I stepped on it but nothing else powered on. Tried the card key as well.
If the 12V battery has a bad cell with a dead short (as opposed to the more common open short) then any power from the jump starter pack would have been shunted to the dead short within the bad cell. One work around would have been to disconnect the cable from the negative terminal on the 12V battery and connect the jump starter positive lead to the positive side of the 12V battery and the negative lead to the disconnected negative battery cable. This would remove the 12V battery from the circuit, might have enabled the MCU to power up using the jumper starter.

Whatever it turns out to be that failed it should be covered under the Tesla new vehicle 4 year warranty.
 

RowdyMY

Member
Jun 21, 2021
151
315
Raleigh
If it helps we connected a jumper pack to the actual battery terminals to try and enable tow mode. MCU never came online. Another reason I think it might be a bad cell.

Open to your interpretation though. It gave enough juice that the brake cylinder would "pulse" when I stepped on it but nothing else powered on. Tried the card key as well.
Also I had been driving for ~40 minutes already. Wouldn't this DC-to-DC inverter issue have already been apparent?

I'm really only familiar with the onboarding of electricity, not how the system distributes and maintains.
 

RowdyMY

Member
Jun 21, 2021
151
315
Raleigh
If the 12V battery has a bad cell with a dead short (as opposed to the more common open short) then any power from the jump starter pack would have been shunted to the dead short within the bad cell. One work around would have been to disconnect the cable from the negative terminal on the 12V battery and connect the jump starter positive lead to the positive side of the 12V battery and the negative lead to the disconnected negative battery cable. This would remove the 12V battery from the circuit, might have enabled the MCU to power up using the jumper starter.
We tried battery only first. Then we actually connected the + to the battery and the - to a hood mount (it really looked like a motor mount). No Change.
 

jcanoe

Active Member
Oct 2, 2020
4,374
4,546
Maryland
We tried battery only first. Then we actually connected the + to the battery and the - to a hood mount (it really looked like a motor mount). No Change.
If the negative battery cable was still connected to the 12V battery when you connected the jump starter to the hood mount then the 12V battery was still in the circuit. If there is a dead short in the 12V battery it would shunt any power from the jump starter, preventing the MCU from powering up.
 
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RowdyMY

Member
Jun 21, 2021
151
315
Raleigh
If the negative battery cable was still connected to the 12V battery when you connected the jump starter to the hood mount then the 12V battery was still in the circuit. If there is a dead short in the 12V battery it would shunt any power from the jump starter, preventing the MCU from powering up.
Didn't have the right bit to take out the HEPA filter TBH which hindered our WaWa parking lot troubleshooting. I'll let you know what Tesla Service says when they get to it.
 

Madsen203

May 26, MYLR, White ext, Black int, Tow, 19”
Jun 1, 2021
554
411
Bay Area
Even the tow truck driver went "woah" when he popped it.

There are about 12 Torx screws that hold it in place. Might need to add a bit to my flat kit. I had to help hold the plastic back so he could get a good contact.
There are only two 10mm holding the HEPA filter in. One on each side. The whole unit pops out. No electrics or anything else connected as it’s press fitted on the air intake. The 12 Torx is only to replace the filters. It takes only a few seconds to remove the whole unit. I can’t recall if you have to remove the frunk bin but that’s only another 4 10mm bolts and popping the cover where the button is which is two clips.
 
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RowdyMY

Member
Jun 21, 2021
151
315
Raleigh
Any update on this?
They have no idea yet. "It's more than just the 12v system" is all they've told me. I got an update today before they closed that the technician was still working to diagnose and the root cause is yet to be determined.

I will say that the service advisor has been great. I was concerned when I was dealing with just the emergency roadside team. They closed my ticket twice on Friday, once before the tow-truck was dispatched and then they closed the actual service center ticket after the driver unloaded the car! I had to re-open an emergency ticket to get the tow-truck sent and then re-open the service center ticket when they opened for the week on Monday. The service advisor got uber credits into my account immediately and then secured a loaner (3SR+).

I imagine there are a few interested in the issue if you're inquisitive like I am. I'll keep you posted. @boingolover
 
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Similar issue that my vehicle had after 6 months of ownership (from new) and it ended up being the 12V battery.

Vehicle totally died while performing software update (failed twice then error messages galore) and then towed to SC. Tesla installed a new 12V battery and reinstalled software and all has been good. Keep us updated.
 

RowdyMY

Member
Jun 21, 2021
151
315
Raleigh
Update.

@jcanoe this is going to be one of those rare circumstances where we're truly both right.

The power conversion system was faulting and was the main failure. The manner in which it failed also caused a bad cell in the 12v battery.

The PCS and 12v battery were both replaced.

Everything was covered under warranty (2,400 miles/< 2 most old)
 

acatwith12

Member
Jul 27, 2021
721
610
Sunnyvale
Update.

@jcanoe this is going to be one of those rare circumstances where we're truly both right.

The power conversion system was faulting and was the main failure. The manner in which it failed also caused a bad cell in the 12v battery.

The PCS and 12v battery were both replaced.

Everything was covered under warranty (2,400 miles/< 2 most old)
Dam that's crazy - glad you got it fixed!
 
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