S650 Dark Horse – Heavy Rain → Dead Battery, Fogged Windows, Damp Carpet

[turbo707]

TL;DR: After two days of heavy rain, my Dark Horse had a dead battery, complete electrical shutdown, fogged interior windows, and damp passenger carpet. Dealer later found no active leak. Looking to see if others have experienced similar symptoms.


After two days of heavy rain, my Dark Horse experienced a complete electrical shutdown, including a dead battery, inoperative key fob, and a StarGuard “power disconnected” alert. Shortly after jump-starting and idling the car, the interior windows fogged heavily and the front passenger carpet felt damp, despite initially feeling dry.

Battery voltage initially measured approximately 3.0 V, indicating a severe discharge event rather than a marginal low-voltage condition.

The vehicle was jump-started, driven briefly around the neighborhood, then parked and allowed to idle for approximately 30 minutes. After this idle period, battery voltage recovered to approximately 12.5 V.

During the initial interior inspection, the front passenger-side carpet surface felt dry. However, as the vehicle reached operating temperature, heat transfer from the passenger-side exhaust caused moisture retained in the carpet underlayment/padding to evaporate, resulting in interior window fogging. At that point, the front passenger floorboard carpet became wet to the touch,
consistent with moisture retained beneath the carpet that was not immediately apparent due to absorption by the padding.”

This raises the question of whether water was in the padding pooling or sitting on the metal floor pan beneath the padding, remaining trapped between the metal and the underlayment until heat caused evaporation and moisture migration upward through the carpet.

During the short drive, the vehicle exhibited abnormal transmission behavior and engine surging, raising concerns about low-voltage effects, module instability, or moisture-related electrical interference. Due to these symptoms, the vehicle was towed to the dealership and is scheduled for the water intrusion 25C43 recall repair on Wednesday.

Diagnostic Questions for the Community:

• Has anyone experienced water intrusion where the carpet surface initially felt dry, but moisture was present in the padding or on the metal floor pan beneath it and only became evident after thermal evaporation?
• Has water intrusion led to rapid battery discharge (within ~24 hours of heavy rain), complete electrical shutdown, or StarGuard power-loss alerts?
• In confirmed cases, was the root cause related to moisture-induced parasitic draw, BCM/PCM involvement, CAN wake events, module communication faults, or corrosion at body or ground junctions?

Any insight from similar cases that had water intrusion , How did the dealer go about repairs, dealer findings, or post-repair diagnostics would be appreciated thank you.

Photos below show interior window fogging after idling and heat soak of passenger floor board.

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[Yamazuki]

Just a guess, but it sounds like you need the 25C43 Water Intrusion Recall done.

 

[turbo707]

Yes, that’s most likely the case. I’m aware of the water intrusion recall and the vehicle is already scheduled for the repair. I’m mainly trying to see if others who experienced the same intrusion also had similar secondary symptoms (battery drain, electrical shutdown, or hidden moisture under the carpet) so I can compare notes and better understand what to look for during diagnosis and repair.

 

[Deleted member 65426]

After two days of heavy rain, my Dark Horse triggered a StarGuard “power disconnected” alert. The key fob was inoperative, and the vehicle exhibited a complete loss of electrical power. Battery voltage initially measured approximately 3.0 V, indicating a severe discharge event rather than a marginal low-voltage condition.

The vehicle was jump-started, driven briefly around the neighborhood, then parked and allowed to idle for approximately 30 minutes. After this idle period, battery voltage recovered to approximately 12.5 V.

During the initial interior inspection, the front passenger-side carpet surface felt dry. However, as the vehicle reached operating temperature, heat transfer from the passenger-side exhaust caused moisture retained in the carpet underlayment/padding to evaporate, resulting in interior window fogging. At that point, the front passenger floorboard carpet became wet to the touch, confirming subsurface water intrusion that was not immediately apparent due to absorption by the padding.

This raises the question of whether water was in the padding pooling or sitting on the metal floor pan beneath the padding, remaining trapped between the metal and the underlayment until heat caused evaporation and moisture migration upward through the carpet.


During the short drive, the vehicle exhibited abnormal transmission behavior and engine surging, raising concerns about low-voltage effects, module instability, or moisture-related electrical interference. Due to these symptoms, the vehicle was towed to the dealership and is scheduled for the water intrusion 25C43 recall repair on Wednesday.



Diagnostic Questions for the Community:

• Has anyone experienced water intrusion where the carpet surface initially felt dry, but moisture was present in the padding or on the metal floor pan beneath it and only became evident after thermal evaporation?
• Has water intrusion led to rapid battery discharge (within ~24 hours of heavy rain), complete electrical shutdown, or StarGuard power-loss alerts?
• In confirmed cases, was the root cause related to moisture-induced parasitic draw, BCM/PCM involvement, CAN wake events, module communication faults, or corrosion at body or ground junctions?

Any insight from similar cases that had water intrusion , How did the dealer go about repairs, dealer findings, or post-repair diagnostics would be appreciated thank you.

View attachment 613061 View attachment 613062

I'd put your car on a battery charger immediately. It sounds like the 12.5V won't last long. If you have a good charger like a Battery Tender brand, it will fully charge the battery, then step down to a trickle charge rate for long-term storage. Try to keep it charged because letting the battery go down that much will shorten it's life. The dealer will remove the windshield, seal the front leak points, remove the inner kick-panel then remove and check the BCM. Same for the right-rear panel in the trunk. Both BCM's should be checked and tested and probably replaced due to your car's symptoms. Good luck and make sure the dealer checks those BCM's.

 

[MAT1955]

@turbo707 ..... brilliant idea to document and take pictures. Document, document, document and then document some more. When discussing this with service (I 'd like the manager involved) I'd have my phone discreetly recording the conversation.....just in case. You have a lot of money in that rig and electrical issues can be a nightmare later on.

 

[turbo707]

Update – Dealer Findings & Technical Questions

Dealer reports no confirmed water intrusion in the passenger carpet or underlayment and no corrosion at the PCM or wiring connectors. Windshield/cowl area was resealed and a controlled water test passed with no leaks detected.

From a technical standpoint, I’m still trying to understand:

• What would cause interior window fogging/steam once the passenger floor pan heated up if no active leak was present?
• Could elevated cabin humidity from prolonged heavy rain, combined with floor pan/exhaust heat, cause moisture to off-gas from the insulation and make the carpet feel damp without measurable standing water?
• Is it possible the battery discharge event was unrelated/coincidental?

 

[Deleted member 65426]

I have a nerd suggestion. Get an inexpensive weather station on Amazon (or similar). Turn it ion, then place it on the passenger floor carpet. Check the humidity every few hours when the car is parked inside the garage. If it's higher than outside humidity (or the garage interior humidity) you have evaporation going on inside, therefore moisture under the carpeting. Do the same in the trunk passenger area. Just an idea. Retired engineer overthinking.

 

[Deleted member 65426]

Turn it "on".

 

[smurfslayer]

Is it possible one of the windows was not fully in the 'up' position?
a lot of us have had low battery, parasitic draw symptoms. I haven't personally, but it's been pretty common here.

If your battery was so discharged that trying to start resulted in nothing, or clicking and nothing, that's probably enough to cause a 'reset' like pulling the cables. This would wipe out the transmission adaptive learning tables. There's a procedure to follow after a reset that will hasten the onset of normal behavior from the transmission.

 

[LouG]

Turn it "on".

I preferred ion, it sounds more engineery.

 

[broncoboy22]

Hope everything is figured out for you man

 

[Ewags81]

i don't know your circumstances so please don't take offense. i think given the amount of electronics on our cars, i'd definitely be more inclined to run the battery through a full charge cycle before running the car, to avoid any damage to charging system and/or electronics maybe due to under or overcharge. especially if the battery had gone to zero. i hope all goes as expected from here out.

 

[turbo707]

I have a nerd suggestion. Get an inexpensive weather station on Amazon (or similar). Turn it ion, then place it on the passenger floor carpet. Check the humidity every few hours when the car is parked inside the garage. If it's higher than outside humidity (or the garage interior humidity) you have evaporation going on inside, therefore moisture under the carpeting. Do the same in the trunk passenger area. Just an idea. Retired engineer overthinking.

That’s a great suggestion—very smart way to quantify what’s going on instead of guessing. I actually have a digital temperature/humidity monitor already, so I’m planning to try something similar.

My car isn’t garage-kept, but I still think it could be useful to document baseline humidity levels over time, then compare them after starting the car and allowing the exhaust/floor pan area to heat soak. Watching how quickly and how much the interior humidity rises should help determine whether moisture is off-gassing from the carpet padding or insulation versus just ambient conditions.

I’ll also consider placing one in the trunk area for comparison. Appreciate the idea—good data beats speculation.

 

[Ciulster]

My car has been kept outside uncovered it's whole life, through heavy rains and snow, and is currently awaiting parts for the water intrusion recall and it has never fogged up that badly even with me sleeping in the vehicle off overnight! That seems like a LOT of moisture. Is it possible some of the rain was able to leak through the cowl and soak the cabin air filter? I'd maybe check the condition of that to see if it looks like it got wet.

On a side note these cars fog up really badly at the slightest bit of moisture. Driving into a carwash without recirc on is enough to get half my windshield and my rearview mirror to fog up. Fogging of a rearview mirror is not really something I've had issues with in the past I need to find something to coat it with as it sometimes does it in the mornings too.

 

[IFFV68]

I'd put your car on a battery charger immediately. It sounds like the 12.5V won't last long. If you have a good charger like a Battery Tender brand, it will fully charge the battery, then step down to a trickle charge rate for long-term storage. Try to keep it charged because letting the battery go down that much will shorten it's life. The dealer will remove the windshield, seal the front leak points, remove the inner kick-panel then remove and check the BCM. Same for the right-rear panel in the trunk. Both BCM's should be checked and tested and probably replaced due to your car's symptoms. Good luck and make sure the dealer checks those BCM's.

Wouldn’t it be easier just to replace the battery and quit messing around with the one you have?
I have a 2024 GT premium 401 a package.
I get the Mustang wash twice a week at a car wash. It’s sitting in the snow. It’s sitting in the rain.
I’ve had no issues with any water anywhere. Everything works.
I know about the recall but I’m afraid to take it to my Ford dealer because if it’s not broke, don’t fix it.
I’m just going to leave it alone.
All the lights work. CarPlay works. Everything works.
If it goes bad, I’ll get the recall done.
if not, I’m going leave it alone.
Good Luck with your dilemma.

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