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Chevy volt BECM IMD (insulation monitoring device) and battery insulation informations

3076 Views 50 Replies 5 Participants Last post by  remy_martian

I would like to know if someone already deep studied or reverse engineered the insulation monitoring device integrated on the BECM module of the chevy volt Gen1 ?
I know that it's not the most appropriated forum category but I know that's is probably where I can potentially find this kind of informations.
At the same time I'm also looking for someone that already measured insulation values of a complete chevy volt without and with official GM coolant inside with a megohmeter ?

I'm currently fighting with capricious Chevy volt on which I've tested insulation of all the HV parts independently, with very good values on all of them including the battery pack, but the car's IMD still measuring a low insulation value.

So I started to reverse engineer the IMD to find any problem on it and I'm looking for references values of battery/modules insulation.
For the moment I get >2000MOhms @500Vdc on the total battery pack without coolant and >30MOhms when it's full of official GM coolant. I can't imagine that this value is considered too low by the IMD...

So i'm interrested by any reliable informations on this topic.



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The ceramic chip capacitors would be high on my list of culprits.

i see several long resistor chains making up voltage dividers to drop HV down to lower levels.

Nissan laef does a test similar to this using a low frequency AC.
Are you sure that the 6M resistor strings are connected to chassis ground? Or is that tied to some other ground plane.
It's nice that there are reference designators for all the components so the board could be easily traced out.

Any manufacture logo on the wide leg spacing devices, is marking code W216SC04 ?

Maybe some sort of isolation or opto coupler device?
If you are reading a voltage or 6Mohms to chassis, then inspect/replace any shorted little ceramic caps marked in the photo

Passive circuit component Circuit component Hardware programmer Capacitor Electronic engineering
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What is on the bottom side of the board--there may be other components and circuits?
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..?
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?
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So those 4 mounting pads are not connected to chassis (12V negative terminal)?

OP: that is a good find on the pdf document, helps explain much about the system,

Font Parallel Slope Rectangle Schematic
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Are there 3 CAN busses talking to the BECM on the gray connector? Or just 2 populated on the board
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