openinverter forum

So I finally did some more diag and found that my 100W 50ohm precharge resistor was blown. This was after many, many starts. I initially replaced with 100W 100ohm resistors and realized I was going the wrong way as they blew as soon as I turned the system on (again never got out of precharge).
I swapped with a 100W 30ohm resistor and it worked several times. However, when I turned the system off, and then turned it back on but just let the precharge run out while I was checking some parameters on the inverter, I went to turn it on again and it looks like the resistor died.

I haven't changed anything else since it was working fine last before the first resistor blew.

Am I using the right resistor value? Could there be anything else I'm missing? Could the parameter settings have anything to do with this?

Thanks in advance!

spiff wrote: ↑Tue Nov 04, 2025 1:51 am So I finally did some more diag and found that my 100W 50ohm precharge resistor was blown. This was after many, many starts. I initially replaced with 100W 100ohm resistors and realized I was going the wrong way as they blew as soon as I turned the system on (again never got out of precharge).
I swapped with a 100W 30ohm resistor and it worked several times. However, when I turned the system off, and then turned it back on but just let the precharge run out while I was checking some parameters on the inverter, I went to turn it on again and it looks like the resistor died.

I haven't changed anything else since it was working fine last before the first resistor blew.

Am I using the right resistor value? Could there be anything else I'm missing? Could the parameter settings have anything to do with this?

Thanks in advance!

There is something on the line that is always draining into precharge. Can you list your devices that consume current and go from there? Is it possible your precharge relay stuck closed? That would explain resistor death.
You need to find what component is drawing power in the 1s - 2s time precharge should happen and make that stop.

I have Eltek charger always connected on the line because it doesnt like going through precharge. But this means i need one extra 30A fuse to cover any incidents and servicing actions...

Thank you! The devices I have connected are:
- DC-DC converter (Tesla)
- Cabin Heater (but connected via contractor triggered by HVAC system and not on)
- LDU
- AC (Tesla, but not connected currently)
- OBC (Tesla Gen 2) - 12V disconnected for now when I’m not charging the battery to help diagnose.

I wonder if the DC-DC being connected is doing this. This is the one thing that I think has changed recently, but already in place when it last worked.
It’s turned on with 12V ignition so would definitely be on during precharge. Would that make a difference? I’m not super happy that every time I try turning it on, I blow another $5 on a resistor……

I'm pretty sure this is the resistor I have. I know it was recommended by someone on the forum, but I don't recall where or whom. I would also note that this is only precharging my drive unit, the charger/dc-dc are precharged separately, which was a personal choice most don't do.

EDIT: Wrong resistor, see post below.

Wow. 0.5ohm 100W is 14amps peak vs 1.8amps for the one I was using (100W, 30ohm).

What is your HV battery voltage? Not sure how much of an impact the DC-DC on the same curcuit would affect things but I’ll try changing the resistor to what you’re using if it doesn’t.

spiff wrote: ↑Tue Nov 04, 2025 6:28 pm Wow. 0.5ohm 100W is 14amps peak vs 1.8amps for the one I was using (100W, 30ohm).

What is your HV battery voltage? Not sure how much of an impact the DC-DC on the same curcuit would affect things but I’ll try changing the resistor to what you’re using if it doesn’t.

Wait wait wait, I went back and double checked, and found the original invoice. THIS is the resistor in my car. 50 Ohm, 100W

Also my fully charged pack voltage is 398V.

Thanks. That would make more sense…. So next step for me is to disconnect the DC-DC and see if it helps pre-charge. Will let you all know what happens.

So I swapped in a 100W 40ohm resistor, but found the original params file I was using before. After I kept thinking about it, I realized that the car started blowing pre-charge resistors after I updated the parameters. So I uploaded the original params file and it started working without blowing resistors! One other question I have is that now it feels like regen is on, but it gives such a howl when regen is happening. Is this normal?? Take off, cruising and acceleration feel great!! Almost there, but I don't want to take it too far without confirming the sound is okay. Previously, when it felt like regen wasn't on, it wasn't making any significant noise when coasting or slowing down.


For the record, here are the two files for your records, (incase anyone wants them). Anyone know what the specific parameter that would have caused the pre-charge resistor to blow?
Here is the original one (with boost, fweak, fslipmin and fslipmax copied from Boxster EV's file): Here is Boxster EV's file that kept blowing the precharge resistor: As far as I could tell, these are the parameter differences between the original one ("Orig") and the Boxster EV one ("New"):

New: "sincosofs": 2048,
Orig: "syncofs": 0,

New: N/A
Orig: "minpulse": 1000,

New: "udclim": 410,
Orig: "udclim": 490,

New: "throtramp": 15,
Orig: "throtramp": 5.84,

New: "ampmin": 10,
Orig: "ampmin": 0.34,

New: "slipstart": 55,
Orig: "slipstart": 35,

New: "ifltfall": 3,
Orig: "ifltfall": 0.06,

spiff wrote: ↑Sun Nov 09, 2025 3:23 am One other question I have is that now it feels like regen is on, but it gives such a howl when regen is happening. Is this normal?? Take off, cruising and acceleration feel great!! Almost there, but I don't want to take it too far without confirming the sound is okay. Previously, when it felt like regen wasn't on, it wasn't making any significant noise when coasting or slowing down.

I have a decent amount of gear noise from mine, but I've not noticed a significant change when in regen. I assume you checked the fluid level? Also check your CV joints and axles. I've had issues with mine.

Also for reference, my "street" tune is attached if you want to take a look through. This is for a Sport LDU, but I've mostly only changed fslipmax and fweak when changing between base and sport.

So I was continuing to do some testing (topped up the oil) and the noise was still there. So I turned off regen for now to keep testing other aspects. Slowing down is fine without it, both in coasting and braking.

But unfortunately, I almost blew another resistor. After a 5 minute spin, it was pretty warm, almost hot. It still worked, but I could tell it would fail soon.

Is there a parameter that controls letting it stay on after the main contactor closes? Is there some reason the pre-charge contact would continue to stay open?

spiff wrote: ↑Mon Nov 17, 2025 2:09 am Is there some reason the pre-charge contact would continue to stay open?

Is it possible the precharge contactor is welded? Check for continuity with the car off. (be very careful. Checking contactors with a multimeter is how I damn near let the smoke out of... myself)

Hi Everyone, I'm still trying to diagnose the issue here. Now using Gemini AI to help and I have to say it is helpful. I'm still at the point now where the motor won't engage at all. It turn on the "ignition" (give the inverter 12V), but it never sees main voltage rise. The pre-charge resistor blows (doesn't even get warm). I've confirmed none of my contactors are welded and when I manually apply 12V, each one triggers. My spot values show udc value is 7V, so it's not seeing HV battery. voltage. (~370V).

Gemini is now saying that, after disconnecting the motor HV leads, I should be seeing continuity between the negative lead on the LDU compared to chassis ground (which should also be the logic 12V ground), or specifically that I should be connecting the HV- to ground. I don't see this in the wiring diagram or documentation. Just want to confirm if it should be this way.

holy crap, please do not take AI advise for power electronics!
The advice is outrageously dumb.

HV and LV are always separated.

You need to know what your doing if something is dangerous and not trust some dodgy cobbled together llm. Getting ideas is ok as long as you confirm here as you did.

Haven't followed this thread otherwise.

johu wrote: ↑Sat Feb 21, 2026 8:03 pm holy crap, please do not take AI advise for power electronics!
The advice is outrageously dumb.

I'll second this as someone who uses ai a lot.

Also Gemini is probably the least accurate that I've tried. Feels like their ai search results are incorrect at least 50% of the time too.

I'd guess your main contactor or the connections going to it are weak/ high ohm and the pre charge is getting power pulled through it because of the voltage drop when pulling a bunch of current.

Yes, I'm not trusting it which is why I'm asking here. It is good for helping to search for definitions of parameters and providing some basic background info of how systems work. But yes, anyone using AI should always be validating.

It is strange that the car worked great before and now it doesn't. It slowly started blowing pre-charger resistors after a short drive, and now blows them instantly. I've checked the contactors but will double check everything again. Just annoying that I haven't changed anything but now happening consistently....

then like jrbe said... measure resistance /continuity of the main current path. High resistance would also heat up, so you might be able to feel or smell the problem

I think this is a common issue if the main contactor cuts in too early. Pre charge is "slowly" charging the high voltage capacitors and before they're fully charged the main contactor closes prematurely resulting in a huge current surge that cooks / ruins the main contactor. They can still measure ok in a simple multimeter resistance test as a pin tip connection can make good continuity. Push some current through it and the fried contacts can't pass the current well and you get more and more of a voltage drop. This voltage drop starts pulling current through the pre charge hv circuit and cooks the pre charge resistor.

It's also common when people snug down hardware but don't tighten it thinking they'll remember 3 months + later. It looks tight... Leave it obviously loose or tighten it to spec, no in between.

Check everything.

Paint pens, markers, heat indicating paint, tamper evident paint (loosened) can all help with quick visuals.

And most importantly don't forget you're working with high voltage that won't hesitate to kill you.

Yeah I'd second everything here. Never, ever, ever, connect either pack voltage terminal to chassis ground. Never trust Gemini, it does a bad job of scraping bad info from the first few search results (On a production EV forum I had to try to explain how regen braking could not be happening/dragging while the car was trying to accelerate. But since an AI said it could...)

jrbe has a good point on a potential bad main contactor. Try replacing the main contactors, and obviously be exceptionally careful on any work there. A stray movement while checking a contactor with a multimeter came damn close to ending me, even when I thought I knew what I was doing.

spiff wrote: ↑Sat Feb 21, 2026 8:49 pm Yes, I'm not trusting it which is why I'm asking here. It is good for helping to search for definitions of parameters and providing some basic background info of how systems work. But yes, anyone using AI should always be validating.

It is strange that the car worked great before and now it doesn't. It slowly started blowing pre-charger resistors after a short drive, and now blows them instantly. I've checked the contactors but will double check everything again. Just annoying that I haven't changed anything but now happening consistently....

Hm... It seems like you might have changed the order of components turn-on when precharge happens.
Correct order is like this:

1. 1st key position 12V live contact to wake up your VCU
2. 2nd key position to signal START and Turn ON Negative DC contactor if you use one
3. Turn ON Precharge relay
4. After 2s (or on condition of voltage) turn ON Positive DC contactor and turn OFF Precharge relay
5. When everything is connected correctly only then turn ON the loads like DCDC, Brake pumps and Heater...

You obviously drain power BEFORE you firmly pull DC contactor in. And if you use voltage monitoring (as you should) voltage is allways a lower than it should be for successfull precharge. And this gives load the time to blow the resistor.

Thanks. I’ve re-wired somewhat to turn on the inverter interface first, then followed by the negative and pre-charge relay. I’ve disconnected all the accessories for now to debug and I’ll figure out later how to properly sequence them to come on a after the main contactor.
I also upgraded to sine-5.35 and finally stopped blowing pre-charge resistors. However the throttle was off as I hadn’t gotten around to doing the 10k resistor retrofit. The next day I did that and calibrated the pedal, then re-started and the potnom error went away, but now udc voltage isn’t reading. I’ve had issues over the last while where, during testing phases, I’d leave the car for quite awhile, say over the winter season (everything disconnected) then had to reload the software or parameters to get it working again without changing any wiring.

Here, when I was originally working on the system the other day, it was a little warmer outside (+3-4 deg C). Although I’m inside the garage, there is a minor difference in temps. The interface seemed to be responding to parameter changes and commands. Yesterday, it was closer to -10-15 deg C. The web interface seemed to be really sluggish and spotty.
Specifically:
1. It would connect to the inverter wifi.
2. It would load the page but sometimes wouldn’t show parameters or spot values.
3. When it did show the full page (after power cycle), it would sometimes accept commands and parameter changes, but sometimes not.

Is this expected behaviour? I’d estimate parts of the garage were below 0 deg C (perhaps -5). The error I’m getting now when trying to start is still udcbelowudcsw, last error: Pre-charge which makes sense because the udc value doesn’t change at all when I’m starting and watching the plot. In fact, even before start, when I plot, the value (~2V) doesn’t even show in the graph where as the previous day when it was warm, it did. I also measured the contactors and the negative and pre-charge are closing and opening as they should be.

Again thanks in advance.

Just an update. The motor is finally running again.
Note for closure: Seemed as though the web interface was "sluggish" and didn't seem to be taking the updates or accepting commands intermittently. Even after cycling the 12V power, it didn't seem to help. The main issue I had was the system didn't seem to register any HV at the udc spot value so it would never meet pre-charge conditions to allow start. I ended up checking all the connections, but also completely disconnected the HV battery when I was confirming each of the contactors was working properly on independent signals. I read somewhere that the system can get "stuck" and not easily clear memory so I'm assuming that's what happened here.

As to the original issue of the pre-charge resistor continuing to blow, I was on old paramaters that were incorrect.

Thanks all!!