I have a friend with a 2007 Mercury Mariner. Sometimes it is fine. Other times it won't start or the battery is dead. Dealerships have replaced battery & alternator. They've kept it for days and it has no issues starting. When she has it home, or at work, it won't start. They take it back and are unable to make it not start, or find any issue. It will start with a jump, but sometimes the battery is so dead that the key fob won't even unlock the vehicle.

Thoughts? This has been going on since a month after the alternator was replaced.

I have seen alternators draw power when not running. I would assume the dealer would have found a draw on the system by now. Possibly an intermittent issue with the voltage regulator.

Does she leave anything plugged into the aux. ports? Are there any aftermarket accessories?

Two things come to mind, either it's a bad ground so you don't get enough electricity to the starter. OR, it's a short somewhere in the vehicles electrical system that's drawing the battery down. Both are relatively not too bad to trouble shoot usually.

Well, my initial thoughts went to those possibilities as well. But this has been in and out of two dealerships since February and she's about to lose her mind.

If the dealerships can't find the issue, then it is something your friend is doing. Is she absolutely sure that she is not leaving an interior light on, or glove box open. Is there a switchable light in the rear cargo area?

Another thing to check is the main battery connection to the alternator, make sure it has a clean and tight connection.

My wife has a tendency, and my kids, to leave an interior light on. I know it sounds to simple but it happens. They seem to push button "on" an interior light. Which they don't turn off when the vehicle is finally parked. So as they get out of vehicle, which turns all interior lights on when doors are opened, they forget/don't notice they left a light actually "on". Then when vehicle is locked as they walk away the "on" light stays on instead of going off.

It's happened enough times that when I double check lockup at night I always look at the wife vehicle sitting in garage to make sure no interior lights are still on. Otherwise she will have a dead battery in the morning. Many times I have had to turn off a light.

Otherwise as one has mentioned, sounds like a bad ground or short. And you would be surprised at how many "techs" are not very good at finding bad grounds or shorts. I've ran shops in the past and have first hand experience on this.

Oh, and I've had to close up a sun visor vanity mirror several times to turn off the light.

Good luck.

This year had relays that stuck closed. I have seen A/C relays stuck closed and drained the battery. The easy way I check is to try and turn the A/C compressor clutch while the engine is off, of course. If the relay is engaged the clutch would be energized and you won't be able to turn the clutch.

The sticking relay can be intermittent as well.

The dealership apparently placed a monitor on the vehicle for several weeks. It showed no unusual current draws.

So is this the Rhyme of the Ancient Mariner?
It's just cursed, man. Damn albatross.

Sorry couldn't resist.

Well, she just emailed me

"I think it is something that gets wiggled loose. It doesn't happen when the car is parked at the shop. They all hop in die weeks turn the key and it starts so they get out avd try it again later. I drive home and 3 hours later my car won't start."

Started happening after the alternator was replaced? Maybe there's something wrong with the alternator. Could it have an internal short that is position sensitive?

This message has been edited. Last edited by: arfmel,

Stab in the dark, but is the battery fully charged to begin with? Is it possible she makes a lot of short trips such that the altenator doesn't have sufficient time to charge the battery?

My Crown Vic. does the same thing. The interior lights come on, on their own, no one can figure out why. It happens enough that it will kill the battery.

Ford dealer can't find out what is doing it. Replaced some relays and switches but still does it.

ARman

I have an old pickup I keep around for occasional trips to the home or garden store.

It would give bouts of no crankitis. it will not start, as if a bad battery. Usually it would start just fine. You'd try to start it, no start, come back later and it'd start. Sort of mimics your mechanic shop experience. Long story short, it was not a battery, charging or short issue. It was the starter with a bad spot on the armature. It acted up as the OP described. A new starter fixed it. That was weird.


Good luck to you.

Bad ignition switch intermittently not completely disconnecting. I have seen it before on other vehicles.

this may be helpful

https://www.yourmechanic.com/a...-car-battery-drain_2

you may have a parasitic drain of the battery

We had one on our 2009 Acura awhile back. Turned out to be the BlueTooth module.

We disconnected it and resolved the issue.

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The classic diagnostic on this is to confirm there is a current draw and then pull fuses till you find the culprit. I am assuming that the obvious bad battery, ground, starter is confirmed as no.
If the the dealer has no problem and doesn't see a current draw (assuming they are not idiots) and the owner does that fits the bill of something they are doing different. Maybe she uses the radio and leaves it on, they shut it off for example.
But in any case what's needed is a simply diagnostic when its dead.

Would think that the dealers would catch an obvious drain.

Since the issue started after the alternator replacement, this could be an issue with the three wire connector to the alternator. These connectors are known to cause intermittent charging issues. There is a kit to replace this connector. Could be that the dealers didn't spend too much time really examining the charging system. I would really start with the alternator.

quote:

Originally posted by Shifferbrains:
Would think that the dealers would catch an obvious drain.

Since the issue started after the alternator replacement, this could be an issue with the three wire connector to the alternator. These connectors are known to cause intermittent charging issues. There is a kit to replace this connector. Could be that the dealers didn't spend too much time really examining the charging system. I would really start with the alternator.

I had a problem with the connector on one of my Corvettes. A gummy substance formed in the connector and I cleaned it off with a rag and carb cleaner. I used a small screw driver/rag to clean inside the connector contacts.

Some vehicles in the past had a reverse polarity diode across the terminals to short out any reverse voltage spikes. Maybe the diode is bad which means it could be an open circuit or a high resistance which would be your source of battery drain.

The best thing to do is get a circuit diagram of the vehicle and go from there.

Why was the alternator replaced in the first place?

Does she have extra weight hanging on her keychain?

Maybe wires rubbing, connections disconnecting, parts heating up, when she drives it (that doesn't happen with the shop mechanic only starting it, but not driving)?

I would start by checking the voltage on the battery when it won't start. Is the battery really dead, or is the car just dead?