Twice in the last year I have left a hood open for a few days and come back to a dead battery. 2001 Golf and 2010 Fusion, both with no underhood light, both running fine immediately prior and again after charging the battery. Does anyone have any insight into why that happens?
Alarm? Factory anti-theft?
Gotta be a switch somewhere keeping stuff awake. Don't argue with new cars. Its just tilting at windmills.
Knurled
MegaDork
11/18/15 12:25 p.m.
If you want fun, try to use your wipers with the hood popped on the Golf. It won't let you.
Another +1 for hood popped possibly keeping a module or two awake.
I had a C4 Corvette with an underhood light that would come on when the hood was open.
I should comment that I don't intend to do anything about it, I just dislike not understanding what's happening.
I'm pretty sure the Golf doesn't have any sort of alarm. The Fusion might have some factory anti-theft functionality.
A guy at work wondered if it has anything to do with preventing the radiator fan from running when the hood is up. I didn't know there was any logic around that.
even if the golf does not have an alarm, I bet there is a switch to tell the car the hood is up.
In reply to mad_machine:
I'm sure you're right about the switch. What I don't understand is how that would lead to a drain on the battery. What does that circuit look like?
DrBoost
UltimaDork
11/18/15 2:03 p.m.
The hood open switch is telling the body control module awake, that's keeping all the modules awake more than likely. That'll do it
In reply to DrBoost:
Ah! So it's not that the switch is passing any real current, rather it an input signal to a control module that consumes power because it's kept awake. That could certainly do it.
But if there is no key in the ignition, why would the body control module care if the hood is up? Or is that related to the radiator fan as mentioned above?
The Fusion hood switch is an input for the SJB(smart junction box... AKA fuse box under the dash) and will keep it awake.
Same with the VW except it inputs to the CCM(central control module) and the radio too.
You can check for modules going to sleep by using a DVOM to check current draw on the battery. Do it with hood up first check after 15+min then close it and check again.
Some modules will go to sleep but wake up repeatedly every X min or hours to recheck that the hood is closed(or other inputs) so its best to graph the draw out over time. A $2000 lab scope helps :)
pkingham wrote:
In reply to DrBoost:
But if there is no key in the ignition, why would the body control module care if the hood is up?
Please do your mental health a big favor and stop trying to decipher the logic used in modern automotive electronics. That way lies madness.
Why does my Volvo have only one button for the power locks, so if I want to unlock the doors from the inside, after they have locked without my permission, I have to push the button to lock, then push it again to unlock? If I dwell on that too long, I start looking for cheap flights to Goteborg and wondering where I can buy an automatic weapon once I get there.
Per chance, is this sitting outside where condensation and dew can get onto things? That can create a surface short tbat drains a battery.
In reply to foxtrapper:
Nope, both times the cars were inside my shop.
I'm going with the theory of the switch keeping a module alive and consuming power.
Thanks all for the input.
Open hood = telling the ECU it is open. Keeps computer awake and uses power. It may use quite a bit of power as it will probably keep the entire computer system in the car active. The computers in modern cars use quite a bit of power. One of the reasons car manufactures have been pondering upping the voltage in cars from 12 to 48 volts.
logdog
SuperDork
11/19/15 7:29 a.m.
DrBoost wrote:
The hood open switch is telling the body control module awake, that's keeping all the modules awake more than likely. That'll do it
This.
Thats why you have to shut the hood and door latches with a screwdriver when diagnosing a parasitic draw.
I hate stupid power draws like this. My 911 had the dome light stop working. And then the battery would die after three days. Turns out the switch in the door died. That convinced the car that the driver never got out and might need to open the windows, so the computer should stay awake. It was maddening sorting that out.
it actually makes a small amount of sense. If you open the door to your car, chances are you are getting in and are going to start it up. Same with opening the hood. This way the computer "awakes" and is ready for the twist of the key
wspohn
HalfDork
11/19/15 9:36 a.m.
Damn modern cars!
I like my old MGC - leave the battery hooked up all year even when it is stored. Crank it once a month (to make sure the clutch doesn't stick - darned ferrous disc linings!) and still just fire it up the next Spring. No radio or other parasitic draws.
Had a bunch of modern cars go flat just sitting from draws on things like OnStar, and I even disconnect my V6 Jamaican MGA as the GPS speedo has a very small draw just sitting there.
I had that happen on a Corvair many years ago.
It had an under hood light with a mercury switch, Parking on sloped driveway turn the light on. Killed the battery over night.