SOLVED! Parasitic power draw from DC-DC TRANS fuse

[Mooseman]

This morning, the Sierra wouldn't crank with a weak battery, and this was just in -14c. It was last run just a day ago after it got its rustproofing done. This is its second battery since new. The OEM ACDelco AGM was replaced with another under warranty by the dealer at about 1 year old. Boosted it with my new FlyLinkTech Booster pack, which it was able to do easily. Once I got to work, tested it and it showed only 495CCA out of 760CCA and said to charge and retest. Got it on the charger now.

Did some research and if this one is failing, I will likely replace it with an Odyssey AGM Performance battery. Expensive buggers but are rated as the best available. There's also Optima yellow top but the quality took a nosedive when they moved production to Mexico but latest info says they have improved since. Not quite as expensive as the Odyssey.

With the holidays coming, this extra expense is not welcomed

 

[Mooseman]

Well fook! Just to be sure, I did a parasitic draw test and it's continuously pulling .250-.260 A (260mA), which is above the acceptable norm of around .050A (50mA). Started pulling fuses and literally the before last fuse on the backside of the instrument panel, fuse, 40A #F57, it dropped to .005A (5mA). All it says is that it's DC DC TRANS 2. The AI Googler tells me:

The fuse you identified, F57, powers the "DC DC TRANS 2" circuit. The component it feeds is likely a DC-to-DC converter module that is part of the vehicle's complex power management network (likely a Local Interconnect Network or LIN bus).
In modern Euro 5 and 6 engines (which the Duramax LM2 architecture complies with), these converters are crucial because they manage voltage from the "intelligent" or variable-output alternator to ensure stable power for sensitive electronics, separate auxiliary batteries, or specific subsystems.

So either that module is pooched or something it powers is pulling power. I'll have to wait until I get home as I have limited access to stuff on the Internet from work. The truck is dead without that fuse in place.

Since this is evolving into something bigger and specific, moving into its own thread.

Edit: Just to specify that this is on my 2021 GMC Sierra with the LM2 3.0L diesel.

 

[Mooseman]

FOOK! FOOK! Looks like GM already knew about this and it's the AutoStop Eliminator that causes it. That means I have to remove it. But before I do, I'll contact the company and see if I can get a replacement or some solution.

And now I get errors saying that the remote tailgate and rear lighting are not available and that electric steering is at a degraded state. The interior lights and windshield washer are not working neither. When I got home, I disconnected the battery to let everything reset itself and put the charger on again. Until I get an answer or remove the eliminator, I'll have to use a trickle charger.

Oh, and before leaving work, I retested the battery and it showing like ~560CCA but the tester said it was good.

I'll reconnect the battery later and see what happens. Damned electronics in this thing is insane!

 

[mrrsm]

Ugh... What a Nightmare...and if you have to replace the battery...The Fun does not stop after sorting out the Parasitic Drain:

 

[Mooseman]

Last night I removed the Autostop Eliminator as the evidence that it was causing the draw was too compelling. Haven't had a chance to redo the parasitic draw yet. I still have the issues with the washer, tailgate, interior and cargo lights. Preliminary research tells me that the BCM is in some sort of failsafe or lockout. To help me diagnose this, and other stuff in the future, I ordered a better scanner as the one I have now only does the main modules (ECU, SRS, ABS, etc.). This one is supposed to do them all.

OTOFIX D1 Lite Bidirectional Scan Tool, Bluetooth OBD2 Scanner Diagnostic Tool, 2 Years Free Update, 38+ Services,2025 All System Automotive Diagnostic Scanner, FCA AutoAuth, Active Test, CANFD&DOIP, Code Readers & Scan Tools - Amazon Canada

OTOFIX D1 Lite Bidirectional Scan Tool, Bluetooth OBD2 Scanner Diagnostic Tool, 2 Years Free Update, 38+ Services,2025 All System Automotive Diagnostic Scanner, FCA AutoAuth, Active Test, CANFD&DOIP in Code Readers & Scan Tools.

www.amazon.ca

And as far as that pesky stop/start, I'm thinking of ordering this one. Literally a 2 minute install at the connector on the battery and easily removable. It's pricey though. It's almost $200CAD and I don't know if it'll be hit with tariffs or more taxes once it hits the border.

Chevy-Buick-Cadillac-GMC Products

2026 support newly added! --> Automatically OVERRIDES the engine Auto Stop feature <--

www.smartstopstart.com

 

[Mooseman]

Small update. Still having the issues but another scan after clearing errors with my current scanner did give me some recurring codes to look at:

U18D7: Serial Data Gateway Module lost comms with Trans Module
U18DC: Serial Data Gateway Module lost comms with Brake System Module 1
U18DE: Serial Data Gateway Module lost comms with Glow Plug Module

The SDGM could be the central point of loss of comms, however I also do have this one:
P0562: Brake System Control Module Voltage Low Voltage (or System voltage low)

Retested the battery and it's giving me a "Charge-Retest" with 12.85V and 685CCA (should be 900CCA). And doing another parasitic current draw test gave me 11mA so that is resolved with the removal of the AutoStop Eliminator. I have already contacted the company and waiting for a response. If I don't get one, then we know they know about this problem with their device.

I'm leaning towards the battery being the culprit. Maybe the voltage drops too much during cranking but the charging system is working fine. Just to have a clear conscience, I'm probably going to replace it.

 

[mrrsm]

An additional resource for Technical Service Bulletins that might offer other solution angles:

2021 GMC SIERRA 1500 TECHNICAL SERVICE BULLETINS

Read all technical service bulletins filed for the 2021 GMC SIERRA 1500. Information from the National Highway Traffic Safety Administration (NHTSA).

www.arfc.org

 

[Mooseman]

I got an AllDataDIY subscription so I have access to all TSBs and full schematics, which for only the affected circuits, was printed into a small book! Everything is BCM controlled.

Minor update. New message this morning "Assisted steering reduced", no new codes that my current scanner can detect. Voltages are fine.

In a holding pattern until the new scanner arrives. Looking up the SDGM, although there are two, it's actually a single module. If it needs to be replaced, it's around $200CAD new but I can find a used one for $100CAD. It would need to be programmed. Same for a BCM but it's actually cheaper, $131CAD new.

Looking at this J2534 pass-through so I can use SPS if I need it. I cancelled the scanner and will get this pass-through instead and find the software out there.

TOPDON RLink X3 OBD2 Adapter for GM/Ford/Chrysler, J2534 Pass-Thru Programmer, OBD USB FORScan Adapter, OEM Programming and Diagnostic Tool, Fit for 8+ OE Software, Supports J2534/CAN-FD, Auto Switch, Code Readers & Scan Tools - Amazon Canada

TOPDON RLink X3 OBD2 Adapter for GM/Ford/Chrysler, J2534 Pass-Thru Programmer, OBD USB FORScan Adapter, OEM Programming and Diagnostic Tool, Fit for 8+ OE Software, Supports J2534/CAN-FD, Auto Switch in Code Readers & Scan Tools.

www.amazon.ca

 

[mrrsm]

Best Diagnostic & Calibration Programming Tool Find of THE DECADE! Thanks for playing our Automotive Advice ***Santa Claus*** ...

Merry Christmas, @Mooseman ...!!!

 

[Mooseman]

Be forewarned, you MUST use a Windows computer to work with GDS2 and SPS so you better take some antihistamines for your allergies . I'll probably start a thread about it in the Scanners/Tech 2 section.

Just picked up a new Costco Energizer H8 AGM battery made by Clarios (formerly Johnson Controls). The Odyssey was simply too expensive but even Costco wasn't cheap neither (total $378CAD). Either this evening or tomorrow I'll install it and go deep with the Topdon X3.

 

[mrrsm]

Roger THAT.... and to add to THAT New Thread ...and ... For ALL Who Dwell at GMT Nation:

Here are 3 PDFs that cover (1) The RLINK X3 Overview and (2) ALL RLINK X3 Links to the How -To - Download - Install -Activate the Software and FAQ Links for the Big Three NAO Markets OEM X3 Software (GM, FORD, CHRYSLER) and (3) How to Hook Up the RLINK and "Windows" Laptop Computer Hardware:

TOPDON RLink X3 | J2534 Pass-Thru for GM, Ford & Chrysler

Built for North America’s Big Three, the TOPDON RLink X3 delivers dealer-level diagnostics and programming for 2006–2025 GM, Ford, and Chrysler vehicles.

www.topdon.us

 

[Mooseman]

Bad news. I can't get the version of GDS2 I downloaded to work as it was hacked specifically for the VCXNano. So I'll probably have to pay to use it for 3 days at $84CAD. I guess it's still cheaper than going to the stealership.

I did receive the TopDon X3 and that software installed fine. I'm not working on it tonight and will wait till tomorrow to pay and install the proper GDS2 so I'll have the full 3 days to use it. And I also disconnected the battery in the truck so it can really reset itself and I'll replace the battery tomorrow as well.

Another thing that might not go well is that I don't have any computer with Windows 11, which they specify as a requirement.

Sure hope this works

 

[Mooseman]

Was able to use GDS2 enough that I'm ready to call the BCM for sure, the SDGM and possibly the T19 Power Supply Transformer. I even checked the infamous body to frame ground strap that's behind the RF wheel and it's still solid. The BCM absolutely refused to obey any commands I would send to it (i.e command the wipers or windows) and would say "command is out of acceptable range".

But now I have another problem...



One review of the X3 did say something about a weak USB plug and I got bit by it too. I could return it and order another one or fix this one by replacing it with a USB-A connector. I think the added length with the adapter just puts too much strain on the USB-C plug.

Decided to just fix it by cutting and splicing in an old printer cable. And added some extra length to help prevent this again. Looks like it works and connects to the computer but will have to confirm with the truck connected.

 

[AmpOverload]

Was able to use GDS2 enough that I'm ready to call the BCM for sure, [...]. The BCM absolutely refused to obey any commands I would send to it (i.e command the wipers or windows) and would say "command is out of acceptable range".

I've never seen a GM module reject a control command, so this piqued my curiosity enough to run some tests with GDS2 and a random VIN from a 2021 GMC Sierra 3.0L Duramax.

I modified my (CANbus) simulator to send a failure reply and then used GDS2 to to send a 'driver-side window down' command and got this:



Is that the message/screen you saw?

 

[Mooseman]

Yes it is. So you were able to replicate it? What would cause this? A borked BCM? At first I though it could have been that plug that was shorting out or something.

 

[AmpOverload]

Yes it is.

Thanks for confirming that!

If you already have adequate evidence to condemn the BCM, then it may not matter, but I'll mention this in case it helps in any way (for you or any future thread readers)...

I asked about the message you saw because, in addition to satisfying my own curiosity, this is what the GMLAN document says about a negative response to a device-control command (GMLAN Service $AE for those into the technical stuff and to assist future searchers of such info) with a "response code" of $31 ("Request Out Of Range"), which is what I'd simulated for that screenshot:

• This code shall be returned if the CPID requested is not supported by the device.
• This code shall be returned if a tester sends a request with a valid secure CPID and the device's security feature is currently active.

("CPID" would be the command being sent to the BCM, e.g. 'lock all doors').

Frankly, it doesn't seem like either of those possibilities should have been in play with your BCM, but who knows?

 

[Mooseman]

Thanks for the info. I'm definitely replacing it. Just have to find one.

So I got an SPS subscription and the X3 adapter is working correctly with my hacked wire. Tried to update some modules but none needed to be updated. Didn't want to risk reloading the software on the BCM as that requires a lot of re-configuring and if it's defective, make the truck totally unusable.

Gonna look for a BCM today. And because I have other modules reporting that they can't talk to the SDGM, I'll look for that too. I think when I pulled the fuse for the DC-DC Trans fuse (aka: Multifunction Power Supply Converter or T19 Power Supply Transformer), it send a surge to its connected systems as it is basically a capacitor for when the Stop/Start system restarts the engine to maintain the 12V systems. I might replace it too.

While I was using SPS and GDS2, the truck kept giving me a "Low battery voltage, start engine" warning message despite me having a charger connected, the DMM showing over 14V and the dash gauge going all over the place. I think this points more towards the BCM. And while running, the gauge and my ScanGauge II will go above 15V but I think it's not reporting the proper voltage.

 

[AmpOverload]

Yes it is. So you were able to replicate it? What would cause this? A borked BCM? At first I though it could have been that plug that was shorting out or something.

(Sorry for the delayed reply. I just now noticed your edit to expand yesterday's reply.) This may not be something you want to pursue if you've already decided to replace the BCM, but I'll mention it anyway, in case it provides any insight whatsoever....

Having never seen a module fail to honor a well-formed command from me (or from the software I'm using -- GDS2 in this case), I can only speculate. But...

Bearing in mind that my GDS2 tests were done with a "random" VIN, it looks like the BCM on a 2021 Sierra 3.0L Duramax is on the normal CAN bus (pins 6 & 14 of the DLC).

So, focusing on just the BCM for the moment, is it correctly reporting status information in GDS2? For example, does it show something like the "Driver Window Main Control Down Switch" as "Active" when you manually activate the switch? If so, then you know that the BCM's connection to the CAN bus is working, so the BCM should be honoring any of the commands that GDS2 is sending, assuming that GDS2 is sending the proper, well-formed commands. I saw control commands (Service $AE, as mentioned previously) going out on the CAN bus when I tested this, so unless your GDS2 is behaving badly, I don't think there's any issue from the GDS2 side.

Also, with your hardware+software setup, can you send any commands to any other module on the vehicle's normal CAN bus and have it work as expected, (i.e. just to rule out the possibility of something more broken than just the BCM communication)?

If the random VIN I used is appropriate enough, you should have the following modules on the normal (pins 6 & 14) CAN bus:

1. Engine Control Module
2. Glow Plug Control Module
3. Transmission Control Module
4. Trailer Tire Pressure Indicator Module
5. Brake System Control Module
6. Body Control Module
7. Serial Data Gateway Module - Processor 1
8. Instrument Cluster
9. Telematics Communication Interface Control Module

Picking something safe and appropriate, can you send control commands to any of them without GDS2 reporting some sort of failure?

BTW, when doing GDS2 tests, it's always helpful for me to know the exact VIN, if you feel inclined to share that. Unlike with Tech2/Tech2Win (where it seems that the VIN is essentially irrelevant), GDS2 wisely uses the VIN to decipher some important information. Feel free to put zeros in the last several VIN characters to retain some anonymity (or just PM it to me). It's also useful to know how to answer any of the GDS2 questions (i.e. about various components present that are not determined from a VIN) that you would have encountered when first connecting to this vehicle, if you can remember any of that.

Gonna look for a BCM today. And because I have other modules reporting that they can't talk to the SDGM, I'll look for that too.

If your Sierra is like the one I "faked" to GDS2, then there are 2 "processor" components to the "SDGM" (Serial Data Gateway Module) on this vehicle -- one on the normal CAN bus and one on the SWCAN bus (pin #1 on the DLC). So it might be interesting to know which modules cannot talk to the SDGM and if they're all on the same bus.

While I was using SPS and GDS2, the truck kept giving me a "Low battery voltage, start engine" warning message despite me having a charger connected, the DMM showing over 14V and the dash gauge going all over the place. I think this points more towards the BCM. And while running, the gauge and my ScanGauge II will go above 15V but I think it's not reporting the proper voltage.

If you decide to dig any deeper on this issue, there should be at least one PID somewhere that reports the system voltage. I don't know about this vehicle, but oftentimes, several modules will report a value. It might be helpful to find and monitor one or more of those PIDs, just to see what they think the supply voltage is.

 

[AmpOverload]

If your Sierra is like the one I "faked" to GDS2, then there are 2 "processor" components to the "SDGM" (Serial Data Gateway Module) on this vehicle -- one on the normal CAN bus and one on the SWCAN bus (pin #1 on the DLC). So it might be interesting to know which modules cannot talk to the SDGM and if they're all on the same bus.

Replying to myself here...

Just realized that you already knew about the "dual-headed" SDGM module and already posted the DTCs showing the modules that cannot communicate with the SDGM and it looks like they're all on the normal CAN bus, FWIW.

 

[Mooseman]

Thanks @AmpOverload , lots of good info but it all turned out to be a wild goose chase for the BCM and an actual failure of the SDGM and possibly the Multifunction Power Supply Converter.

So I replaced the SDGM, the Converter and the BCM. The used SDGM had the exact same programming so I didn't need to reprogram it and the errors related to it disappeared (Victory!). The Converter was just to be sure it was working fine but I think the original was OK.

Tried to reprogram the BCM and it refused to let the truck power on at all. Followed the programming instructions exactly (power off, key fob in the programming pocket, etc). SPS tried multiple times and failed each time. I tried reprogramming something else, went with the steering module, and it went fine so it's not the tool or SPS. I reconnected the original BCM, tried to program it and it failed the same way.

So I was ready to throw in the towel and take it to the stealership. Before I did that, I wanted to put back the fuse for OnStar as I pulled it soon after buying the truck as I didn't want the dealer just putting it back in and reactivating OnStar and big brother so I pulled the antenna wires for it. I was looking for that extra fuse as I put it in an empty slot and I start looking. I notice that one empty slot was actually for the BCM! (labelled BCM2). I then realized that when I did the parasitic draw test, I put back the fuse in the wrong slot just above, which was labelled as empty.



I did check the fuses with a test light before and they all tested good minus the ones that weren't powered, which I dismissed.

So this explains why the BCM was only partly working, why it didn't obey commands and why it wouldn't reprogram. However, I did catch the bad SDGM.

Now I have to keep remembering to turn off that damned auto stop/start . Not sure if I'll get that other disabler. Pretty expensive for what it does.

 

[Mooseman]

Just a clarification on the problematic stop/start eliminator device. I contacted Autostop Eliminator and received a reply. They said it was one of their competitors that had the problem. I checked the original receipt from 2021 and it was actually another seller, 4D Tech Stop/Start Eliminator. Autostop Eliminator has graciously offered a 20% discount regardless. I may just go with them. I have sent an email to 4D Tech and will see if they will refund me. They don't even offer it anymore for my year truck so I wonder if they knew all along.

 

[mrrsm]

End-To-End ...an FAQ Worthy Thread!

 

[Mooseman]

Not quite for an FAQ because I am the only one that had this happen to, but still a good thread nonetheless.

I was contacted by TopDon after I posted a bad review of the RLink X3. At first they offered just a refund, which I replied I accepted, then they replied again and said they would refund AND send me a new tool! Nice! They did mention that they wanted me to send back the broken one. I'll just have to cut off my hack job.

And I'm probably going to accept AutoStop Eliminator's 20% offer. I just hope I don't get whacked by tariffs. I keep forgetting to turn it off manually and is extremely annoying!