Model S late 2016 75D isolation fault story

[yeukfung]

Hi all,

I am newbies here, would like to share a nightmare isolation fault story on my friend's Tesla Model S, he purchased as second hand and have been running good over last year for over 20k km..

It's the first time I mess with the electric car, while I have been DIYing petrol car for over 10 years. Thanks for this forum providing huge amount of valuable information on electric vechicle.

The story started with 2 months ago, my friend called me that his tesla is NO START symptom, it was isolation faults, then he towed to service center. what the service center said there are some error code stored in AC module and Battery Heater module.

Interestingly the service center is unable to confirm it is the root cause, and asking for additional $750 for in-depth checking by removal of battery.

Then this car is towed to my DIY playground, and then I eventually encountered the term: ISOLATION, as it will never exist in 12V petrol car world.


I purchased a fluke isolation meter and perform all the testing for
- PTC heater (+ve -ve are in good isolation)
- battery heater (+ve -ve are in good isolation)
- DC-DC Invertor (+ve -ve are in good isolation)
- AC pump (+ve -ve sometimes good, but one time bad, only got 6Mohm for both HV cable with 250V testing voltage)


Then I was stucked at that moment, and then we attempted to recall what the service center say, we purchased a brand new AC PUMP and Battery Heater to give a shot.

Unfortunately the replacement is done, and the car is still NO start, same ISOATION faults occurs.

Then I purchase 1 day access of toolbox 3 to see what is going on actually.

Some interesting finding is, I entered service mode via toolbox 3, and then select some data stream to monitor
most of the time the information is as below (which indicate the battery is healthy)
- BMS_state FAULT
- BMS_isolationResistance 3500kOhm
- BMS_contactorState OPEN

while the car will peroidically attempt to close the contactor (the isolationResistance drops to 0 kOhm during PreCharging state!)
- BMS_isolationResistance 0kOhm
- BMS_state PRECHARGING
- BMS_contactorState OPEN


So, it defintely got some isolation fault, as the number should not be lower than 10k Ohm, and now it's 0k Ohm.

As the car is pushed to a location which is far away from the jack, it would be quickest to check from the Charging Module located at the back seat.

Taking off the seat and test the isolation, Vola!
+ve 0Mohm, -ve 0Mohm

So happy that at least confirmed there must be some short circuit to ground along the HV path, now then I detach the HV cables connected to the Charging module and retest the charging module isoation, got around +ve 25Mohm and -ve 25Mohm

and then testing the cable, it was 0Mohm for both +ve and -ve...

which indicates there must be some other parts which causing the error, and then I have no choice, but to roll the car up the jack and perform the rear motor isolation test

luckily rear motor seems OK.

I have detached the cable from rear motor and both HV cables still got 0Mohm reading. which leads me to test the front of the car.

I take off the left wheel and the trim, then able to access to the front motor connector, I detached it, and test again

The result is interesting
+ve is having 1.2MOhm reading
-ve is having 0MOhm reading


I replug the HV cable to the front motor, I got
+ve 0MOhm
-ve 0MOhm
again...


There must be some faults between battery, rapid splitter, FJB...

I scheduled a day to do the job, so need to prepare a stand for temporary holding the battery, and then do the troubleshooting.

Once I got the battery removed, I started to test at the rear motor HV cable
got the same reading
+ve 1.2Mohm
-ve 0Mohm
which is good news, as this isolation is NOT related to battery


Then I detach the connector on FJB which links to rapid splitter, then perform test on the HV cables
+ve 550Mohm
-ve 550Mohm

wow, which can conclude that there must be something wrong with FJB!!!

but wait, I then try to disconnect the HV cable linked between FJB -> Front motor, and do some isolation testing on FJB
+ve 550Mohm
-ve 550Mohm


hmm, not the FJB module, then it must be the HV cable between FJB -> Front motor, and I tested,
+ve 550Mohm
-ve 550Mohm


It's a nightmare, and I am stucked at this point.

Suddenly, I remember during the detach of the negative HV cable on between FJB -> Front motor, this cable is tightly touching the 12V +ve cable,

There is a scratch mark on this 12V +ve cable, and this 12V +ve cable is also touching the ground wire, although the ground wire is covered with insulation.

What my thinking is, the HV cable is connected to the front motor, which is secured by motor mount. which means there might be some minor up and down movement for the cable or pressures applied to the cable, that in the long run, it will eventually make the insulation weaker and weaker, that allow the high voltage to pass through.

I covered the affected area with insulation tape, and then seperated the 12V +ve and HV -ve by rubber and then reattach the HV cables to FJB, retest the isolation
+ve 550Mohm
-ve 550Mohm

what the hack, it's magical!!!

then assemble all the connections and attach the battery, retest at the HV cable in rear motor, I got
+ve 25MOhm
-ve 25MOhm

which is excellent.

after connecting all those connector back, and the car is able to MOVE!

the entire journey makes me think that, it's kind of a design flaw, the concept of isolation is easy to get, but to actually do the real isolation testing, it is not a small job...

I hope my experience would help someone when they are struggling, and sorry for the long post.

 

[Gtech]

Hell of a first post dude, thanks for sharing your experience.

As far as I know all HV cables are double shielded (like a coax cable) and that shield is connected to vehicle ground so when a HV cable is scraped open to any kind of material HV inner will make contact with shielding and boom iso drops to 0Mohm.

Anyway, I would advise to replace that HV cable

 

[maximizese]

Sounds to me that you are a wonderful friend. Thanks for sharing the experience and shedding some light for some of us DIYers who want to do our own due diligence before committing to Tesla's Service Center diagnostic fees and repair expenses.

 

[yeukfung]

Hell of a first post dude, thanks for sharing your experience.

As far as I know all HV cables are double shielded (like a coax cable) and that shield is connected to vehicle ground so when a HV cable is scraped open to any kind of material HV inner will make contact with shielding and boom iso drops to 0Mohm.

Anyway, I would advise to replace that HV cable

Hi Gtech,

Thanks for your tips, after I read your post, and then I visit the internet to search for the cable section, the HV cable is shield.

Then I had a feeling that the reading was good after I assemble everything back, is just the luck that the cable was moved around and the direct contact maybe just temporary altered, it is not related to the insulation tape I wrap over the cable for sure.


The story resume as follow:

The car is able to move around my DIY workshop slowly, everything can start the car and no error message.

Then my friend assemble everything back, charge the Air Conditioning and then before the test drive, we have ensure the isolation is good before we go out for real road test.



and then everything is nice, acceleration good and then after around 10km drive...

Vechicle May Not Start, BMS_w035 warning...

Since we are in the middle of road, we cannot stop the car, so I drive back to workshop directly, without opening driver door, climbing out from window to ensure the car is not turn off, it's important step, or else I will be end up pushing the car again.

Then turn on the service mode, and isolation = 0Mohm...



So, we are now searching for the HV cable.

Unfortunately, we cannot buy any NEW cable from tesla, as it's a restricted part... so we have to look from ebay, or some scratchyard.

And also, I found that last time I try to diag the HV cable, my method was totally wrong, I should put the -ve touching the grounding shield, and +ve in the middle of cable to check on the isolation value of the HV cable.

Anyway, after the first time battery removal, it should be much quicker for the 2nd time.

With the above real experience, I can confirm that, if HV cable has isolation fault, there is no luck that it will work again, as Gtech suggested, replacement is the only option.

 

[Gtech]

Hi Gtech,

Thanks for your tips, after I read your post, and then I visit the internet to search for the cable section, the HV cable is shield.
View attachment 980129
Then I had a feeling that the reading was good after I assemble everything back, is just the luck that the cable was moved around and the direct contact maybe just temporary altered, it is not related to the insulation tape I wrap over the cable for sure.


The story resume as follow:

The car is able to move around my DIY workshop slowly, everything can start the car and no error message.

Then my friend assemble everything back, charge the Air Conditioning and then before the test drive, we have ensure the isolation is good before we go out for real road test.


View attachment 980131
and then everything is nice, acceleration good and then after around 10km drive...

Vechicle May Not Start, BMS_w035 warning...

Since we are in the middle of road, we cannot stop the car, so I drive back to workshop directly, without opening driver door, climbing out from window to ensure the car is not turn off, it's important step, or else I will be end up pushing the car again.

Then turn on the service mode, and isolation = 0Mohm...
View attachment 980130


So, we are now searching for the HV cable.

Unfortunately, we cannot buy any NEW cable from tesla, as it's a restricted part... so we have to look from ebay, or some scratchyard.

And also, I found that last time I try to diag the HV cable, my method was totally wrong, I should put the -ve touching the grounding shield, and +ve in the middle of cable to check on the isolation value of the HV cable.

Anyway, after the first time battery removal, it should be much quicker for the 2nd time.

With the above real experience, I can confirm that, if HV cable has isolation fault, there is no luck that it will work again, as Gtech suggested, replacement is the only option.

Yeah it's a bummer Tesla won't sell us certain parts.. Not sure why they think junkyard parts would be safer then buying new from them... idk about scratchyards that sounds nasty.

If you remove the damaged cable don't forget to show us a picture of the wear mark.

 

[yeukfung]

Hi All,

I would like to follow-up the story, the issue is finally addressed by replacement of the HV cable between FJB -> Front Motor.

Since I have the experience of detaching the battery, this time just taking 45 minutes of time to detach the battery, I am amazed this is pretty fast comparing to the petrol car, which taking off core component takes much longer time.

Procedure I have done:

0) benchtest the used cable, ensure the isolation is good for both +ve and -ve

1) detach the battery


2) at Rapid Splitter connection, isolation test the negative to ground, got 0 Ohm


3) detach the connection of front motor, retest at Rapid Splitter connection, isolation test the negative to ground, got 0 Ohm

4) detach the connection of FJB -> front motor, retest at Rapid Splitter connection, isolation test the negative to ground, got 250 MOhm
this gives me confident that the root cause is the cable.

5) detach the entire cable, test on the floor with isolation test, the negative cable has sometimes good value, and when I move around the connection, it has 0 MOhm, indicating the cable is short to ground, however, there is no sign of wear mark, it just looks fine... when I try to move around the cable, and I cannot get the 0MOhm value again... real strange...


6) comparing the parts number of existing 1055875-10-D, the one I received is with newer revision 1055875-10-E


7) installing it back to the car, the newer revision HV cables has much better clearance between the HV negative and the 12V +ve, unlike the old version, which was touching each other when I first saw.


8) assemble everything back, and start the car, got a solid 1680kOhm


9) enable to service mode, remove speed limit and perform tested drive over 60km, shutdown the car few time, perform super charged, everything seems back to normal

I am glad that the car finally is back on road again

Quite a long journey, most of the time is waiting for parts, but it is real happy when the root cause is finally identified and fixed.

Hope the experience may give some hints to someone who may be also struggling with similiar symptoms.

 

[Gtech]

Hi All,

I would like to follow-up the story, the issue is finally addressed by replacement of the HV cable between FJB -> Front Motor.

Since I have the experience of detaching the battery, this time just taking 45 minutes of time to detach the battery, I am amazed this is pretty fast comparing to the petrol car, which taking off core component takes much longer time.

Procedure I have done:

0) benchtest the used cable, ensure the isolation is good for both +ve and -ve

1) detach the battery
View attachment 987521

2) at Rapid Splitter connection, isolation test the negative to ground, got 0 Ohm
View attachment 987528

3) detach the connection of front motor, retest at Rapid Splitter connection, isolation test the negative to ground, got 0 Ohm

4) detach the connection of FJB -> front motor, retest at Rapid Splitter connection, isolation test the negative to ground, got 250 MOhm
this gives me confident that the root cause is the cable.

5) detach the entire cable, test on the floor with isolation test, the negative cable has sometimes good value, and when I move around the connection, it has 0 MOhm, indicating the cable is short to ground, however, there is no sign of wear mark, it just looks fine... when I try to move around the cable, and I cannot get the 0MOhm value again... real strange...
View attachment 987526

6) comparing the parts number of existing 1055875-10-D, the one I received is with newer revision 1055875-10-E
View attachment 987522View attachment 987523

7) installing it back to the car, the newer revision HV cables has much better clearance between the HV negative and the 12V +ve, unlike the old version, which was touching each other when I first saw.
View attachment 987524

8) assemble everything back, and start the car, got a solid 1680kOhm
View attachment 987525

9) enable to service mode, remove speed limit and perform tested drive over 60km, shutdown the car few time, perform super charged, everything seems back to normal

I am glad that the car finally is back on road again

Quite a long journey, most of the time is waiting for parts, but it is real happy when the root cause is finally identified and fixed.

Hope the experience may give some hints to someone who may be also struggling with similiar symptoms.

Great that it's fixed, intermittent issues are nasty

I think when you remove the hose+tape you put over the wear mark you can see ground touching the inner copper core?

 

[yeukfung]

Great that it's fixed, intermittent issues are nasty

I think when you remove the hose+tape you put over the wear mark you can see ground touching the inner copper core?

Hmm, I didn't check indepth..

Entire cable I take out looks just the same as the one I received, I didn't attempt to cut off to check, as moving the cable a bit and the isolation changed which probably indicate it's quite difficult to locate the defect area.

 

[Gtech]

Hmm, I didn't check indepth..

Entire cable I take out looks just the same as the one I received, I didn't attempt to cut off to check, as moving the cable a bit and the isolation changed which probably indicate it's quite difficult to locate the defect area.

I meant the piece of cable you repaired I assume under there are the 2 layers touching eachother

 

[yeukfung]

I meant the piece of cable you repaired I assume under there are the 2 layers touching eachother

View attachment 988326

Hi Gtech,

Sorry to get back you late, was quite occupied recently.

The car runs fine since the fixes, probably over 1.5 months now.

I just take a photo of the cable this morning, and you can see no obvious scratch mark on the negative cable area:



and I bench test again this morning, the ohm reading is good again, got 550Mohm...

So, if the owner is unlucky, this issue could be intermittent and very hard to troubleshoot..