[piwhy]

Those resistors are locally grounded on the board - "ground" is a common return for the circuits on that board.

The four caps KB circled are the only connection to chassis on that board - the schematic is wrong.

Yes my draft schematic is wrong, I've been distrubed by these likely shorted or partially shorted capacitor apparently. I was very surprised that these voltage reference created by the resistor chain on the IMD was permanently connected to the car frame !

Yes it can probably works well without this cap, I will replace those properly and verify again the value measured by the car. Do you have any idea or at least an approximate value on this kind of ceramic cap ?

I will keep you informed soon. 🙏

 

[piwhy]

If you are reading a voltage or 6Mohms to chassis, then inspect/replace any shorted little ceramic caps marked in the photo

View attachment 134919

I removed the fourth capacitor you mentioned, by the way they are 11nF capacitor. But I still have 0.05Mohms between board gnd and chassis gnd and around +/-180V between ground and both HV polarity when HV is connected. So I have to find and other shorted part on the board...

 

[kennybobby]

What is on the bottom side of the board--there may be other components and circuits?

 

[piwhy]

What is on the bottom side of the board--there may be other components and circuits?

Not that much as you can see :

I will try to identify component connected to frame ground reference.

 

[piwhy]

R0042 is the 51kOhms resistor that connect board ground to car ground.
So apparently this mid-point HV reference is connected permanently to the car ground reference !?

 

[piotrsko]

On mine that resistor is tied to chassis however I do not know whether mine is functional since (oh horrors) I don't use the BMS

 

[kennybobby]

That explains your measurements; makes you wonder how they use that circuit to determine that insulation has degraded.

All the circuits on the HV side will need to be traced and drawn up to get some idea of what is the purpose.

Need to know the pinout of that orange connector too, looks like 4 inputs with 2 blank pins between each one..?

 

[piwhy]

That explains your measurements; makes you wonder how they use that circuit to determine that insulation has degraded.

All the circuits on the HV side will need to be traced and drawn up to get some idea of what is the purpose.

Need to know the pinout of that orange connector too, looks like 4 inputs with 2 blank pins between each one..?

Yes it makes sense now. So my first draft of the circuit diagram is not totally wrong except that there is this 51kohms resistor and some 11nF cap between PCB ground and car ground.
I know that on cars equiped with DC charging interface, the insulation monitoring device must be disconnected at least during charging session because insulation is tested and monitored by the charging infrastructure. So apparently on Volt Gen1, which was not equiped with CCS or Chademo, IMD is always connected...

Yes on the orange connector you get x4 inputs separated by two NC pins as follow :

I didn't detailed the "after HV relays" inputs in my diagram because I supposed these are only used to check a welded contactor or a similar feature. This part of the circuit is not used for isolation measurement. The three right AQW216 (supposed) photomos are used for this part of the circuit. (@piotrsko what is the year of BECM you have ? because this part of the circuit whas apparently removed after 2013)
The three left AQW216 (supposed) photomos are used with the components described in my draft schematic and linked with HV inputs directly coming from battery pack (before HV relays)

I suspect this pcb to be at least a x4 layers board, so it makes reverse engineering a bit more tricky.

 

[piotrsko]

Mine is an October 2011

@kennybobby: insulation degrading? Perhaps to the point of leakage, but that typically isn't an insulation thing, it's a connection corrosion cause. I cant see where an insulation could cause a leak without a big failure immediately later.
Edit: everything this stuff is attached to is generally plastic

 

[piwhy]

The point is that actually I didn't found any measurable degradation of isolation.
I've checked all internal parts of the battery pack with a megohmeter @500V (I get over 2000Mohms on all the HV interfaces of the contactor board including battery heater, >25MOhms on the whole battery pack filled with official GM coolant inside). I also checked all the external parts (inverter, cabin heater, AC, harness, charger, ...) I measure isolation values between 1000 and 2000MOhms or above on these parts.
Then, even when the car ignition is OFF (all HV relays open in the battery pack) the IMD measure an insulation value between 500 and 800kohms which is a way too low. It's supposed to be >3000kOhms when ignition is off.
So that's why after several days of insulation measurement i'm almost convinced the problem comes from the IMD itself...

 

[piotrsko]

In that case, disconnect the IMD and remeasure isolation. While being a royal pain, it will determine whether or not the IMD is faulty. Otoh, other than my unit, where do you get a new one? I am impressed you are pulling this pack apart and re assembling.

 

[remy_martian]

You cannot get megs of isolation with a 51k resistor connecting to a board worth of circuitry.

I'm not buying that resistor via goes to chassis ground - did you verify that via connection by eye or with a meter?

 

[remy_martian]

One thought - did you replace any coolant lines or do any work on it at all?

 

[piwhy]

You cannot get megs of isolation with a 51k resistor connecting to a board worth of circuitry.

I'm not buying that resistor via goes to chassis ground - did you verify that via connection by eye or with a meter?

I've measured this value with ohmmeter.

 

[remy_martian]

Yeah - you better look. There could be something else making that connection.

 

[piwhy]

One thought - did you replace any coolant lines or do any work on it at all?

No, I didn't replaced any coolant line. I only replaced the battery heater with a new one which was the first suspicious part when I dropped the battery pack.

 

[piwhy]

Yeah - you better look. There could be something else making that connection.

But that's coherent I have 0ohms value in relation to board gnd on one side of this resistor and 0ohms value on the other side of the resistor in relation to the frame gnd. And the value measured accross is coherent as it's indicate "513" on this resistor (51x10^3 ohms).
So apparently insulation measurement is based on this design on this car...

 

[kennybobby]

Are there any DTCs being thrown for the BECM related to the IMD, if so what are they and where can we find the factory workshop manual to read about this?

Where does this board reside--internal to the pack or external?

Is there a separate BMS board or is that the function of this board? Or put another way, what are all of the functions of this board?

In the nissan laef the 'IMD' function is done on a section of the LBC (BMS) board inside the pack.

In the mitsubishi miev an insulation and ground fault check function is done in a separate little box inside the pack, independent of the cell monitoring boards and the BMS box located under the rear seat.

Most of the EVSE units that i have seen have some sort of ground check and ground fault isolation check circuit that must pass or charging will not be allowed.

What do the 4 threaded mountings pads contact when the unit is bolted together--do they attach to vehicle chassis or to something else?

 

[piwhy]

I found an interesting document here : https://www.seattleautomotive.com/files/SAP0201SM.pdf from page 82 where they describe two different isolation circuit (one passive and one active).
This document is not focused exclusively on chevy volt gen1. But what we spoke about here is the passive isolation circuit measurement.
I can't say for the moment is there are two different circuits (passive + active) on chevy volt gen1 and if my suspicious error of measurement comes from the passive one or active.

 

[piotrsko]

On my pack everything was clipped to a plastic housing then has an isolation cover of plastic so in the parlance double isolated. Only the harnesses interconnect to anything grounded to chassis . A coolant leak, defective unit, the internal heater, or a frayed harness are pretty much your only choices for high voltage leaks. The way it's built exactly 1/2 battery voltage isn't easily done because there wasn't an external connection at that point. But hey, it's been 8 years since I got mine, so maybe I forgot something