I this is a long shot……………
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Topic: I this is a long shot……………
Posted By: Lars(wi)
Subject: I this is a long shot……………
Date Posted: 27 Nov 2025 at 10:44am
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but, is anyone here familiar with BMW’s? Son has a 2015 335i, when the weather turns cooler(under 50 deg F., the battery drains dead. Several battery swaps over the years, has not solved the issue, numerous trip to the stealership temporarily fixes the issue. Last week when I was at his place it happened again, cool day, jump started the car, did his route for the day, came home, shut it off, couple hours later, battery dead as a door nail. Hooked up the charger, after a few minutes, the electric water pump turned on. I know the electric water pumps on these cars are problematic, has anyone conquered this issue. I know BMW’s are an electrical nightmare. Any help/suggestions, or some tanerite would be greatly appreciated. ------------- I tried to follow the science, but it was not there. I then followed the money, and that’s where I found the science. |
Replies:
Posted By: jaybmiller
Date Posted: 27 Nov 2025 at 11:37am
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no ,but it sounds like 'something' is turning on,draining the battery when <50*F. Maybe a seat heater,windshield heater or some other 'hightech' item ?? I'd put an ammeter between battery and 'the rest of the car' to see how much current is being drawn. Any lights on ( under the hood ?). Newer cars have 'CANbus' ballast resistors and they WILL kill a battery. Maybe the alternator is 'kinda' bad ? If ammeter say power is being used, start pulling fuses, one at a time, to locate WHICH circuit it is. Please report back with any new information ! ------------- 3 D-14s,A-C forklift, B-112 Kubota BX23S lil' TOOT( The Other Orange Tractor) Never burn your bridges, unless you can walk on water |
Posted By: AC7060IL
Date Posted: 27 Nov 2025 at 12:03pm
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Not familiar with particular model. Google it? Search yr model issues & watch internet light up about issues. Good luck. Below is a link of my find from ; “2015 335i battery issues” search Could be it has “on demand” alternator charging?? If so, it’s battery’s ground terminal sensor may be defective? A temp fix ~ delete that sensor for a direct ground. That will produce a constant charge alternator to assist the moment. Get new sensor for long term fix. https://www.reddit.com/r/BmwTech/comments/11x4enb/2015_bmw_335i_battery_discharging_while_stopped_i/" rel="nofollow - https://www.reddit.com/r/BmwTech/comments/11x4enb/2015_bmw_335i_battery_discharging_while_stopped_i/ |
Posted By: Lars(wi)
Date Posted: 27 Nov 2025 at 1:38pm
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We have noticed the electric water pump will continue to run after turning off the engine, even after the engine cools. Internet has posted different solutions, we put a new battery a hour ago. Water pump turned on. Some solutions include; unhook battery for several hours to allow ECU to reset, the water pump is faulty ECU is sending signal to w. p. to stay from faulty reading from temp sensor ------------- I tried to follow the science, but it was not there. I then followed the money, and that’s where I found the science. |
Posted By: jaybmiller
Date Posted: 27 Nov 2025 at 3:31pm
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OK, I understand why an electric fan to cool the rad down makes sense BUT keeping the waterpump on ?? Query... any idea how LONG the pump runs ? If more than say 5 minutes there HAS to be a problem ! ------------- 3 D-14s,A-C forklift, B-112 Kubota BX23S lil' TOOT( The Other Orange Tractor) Never burn your bridges, unless you can walk on water |
Posted By: Lars(wi)
Date Posted: 27 Nov 2025 at 4:20pm
Sometimes the pump will continue, until the battery is dead. Sometimes it shuts off right away. The dead battery issue happens every autumn, and will be an on-again, off-again challenge until spring warm-up. ------------- I tried to follow the science, but it was not there. I then followed the money, and that’s where I found the science. |
jaybmiller wrote:Posted By: Hubert (Ga)engine7
Date Posted: 27 Nov 2025 at 4:47pm
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Since it happens during the cooler months I would suspect either a faulty temperature sensor or a faulty ecu. For some reason the ecu senses that there’s too much difference between ambient temperature and engine temperature so it tells the water pump to run. After resetting the ecu it may take a while for it to sense the “problem” again. Wish you luck on finding where the gremlin is hiding. ------------- Just an old country boy saved by the grace of God. |
Posted By: Les Kerf
Date Posted: 27 Nov 2025 at 10:27pm
Yup. If practical, I would try to use a non-contact inductive clamp ammeter to measure total current draw at the Negative cable, then use the meter to measure the electric water pump current to see if that is the only source of unwanted draw. If they match, then you have narrowed it down. I know nothing about BMW's, but the older American vehicles with which I am familiar used some variation of a thermistor to send temperature data to the CPU. They were/are susceptible to poor connectivity and/or grounding. The CPU in those systems typically completed the ground circuit for a relay which in turn runs the fan/pump/whatever.
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Posted By: DaveKamp
Date Posted: 29 Nov 2025 at 11:21pm
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Jay, the reason the water pump stays running, is to prevent heat-soak as a result of the engine shutting down when hot. Heat absorbed into the castings is still equalizing, flowing from the combustion chamber elements (piston, wrist pin, rod, cylinder head, and top 3" of engine block, exhaust ports, etc) towards the water jackets just as if the engine is still running, but with the engine stopped, that heat still has to go SOMEWHERE, and while the radiator may still be a viable cooling path, coolant can't exchange from the engine to the cooling medium once the coolant is too hot to accept it anymore. Keeping it circulating for some period of time helps alleviate that... so they run the electric pump for a few minutes as part of the 'cool down' cycle. ------------- Ten Amendments, Ten Commandments, and one Golden Rule solve most every problem. Citrus hand-cleaner with Pumice does the rest. |
Posted By: Les Kerf
Date Posted: 30 Nov 2025 at 8:03am
But it sometimes continues to run so long that it drains the battery, and it only does it during the cooler months. Weird. I'm wondering if there is some sort of programming glitch in the computer code rather than a hardware problem. Nigh unto impossible to prove though.
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