Weird problem - no start, showing -40*F

[Crowz]

Originally Posted by oldskewel

Thanks for the ideas.

My daughter took it for an uneventful drive last night.

Figured I'd look into it today...

First thing, battery measured at 11.96V (as before, this at the jump ports in the engine bay)

Key in, to ON, shows 59*F. So unless someone snuck in and replaced my temp sensor last night ...

Carefully turned off all the HVAC and computer monitor nonsense that really wants to be wanted.

Try to start - click click click, instrument panel and computer monitor flashing, turning on, off, etc.

Repeated a few times, same result.

Battery read 11.5V after those attempts.

Hooked up a battery charger, ran it on 10A for a couple of minutes. No help. Got impatient, so I pushed the switch to the 50A engine start and it started just fine. Makes sense, if the battery is marginal.

So now it is parked in my driveway, getting a long, patient 10A charge.

=================
My thoughts so far:
That -40*F was probably the computer getting jammed up just like crowz thought. I had wondered if -40*F might instantly provide the answer, being such a specific clue. Guess not.

11.7V might be a weak battery in this case. I often have no problems with voltages in that range on cars, but this is more of a computer with wheels.

The battery is almost 4 years old, with a Feb 2014 sticker on it.

This car takes lots of pretty short trips, so it is entirely possible that the battery will live a little longer with some periodic full charges. I'll test this out and see how it goes.

Hopefully the solution will either be to keep the battery charged better, or to just replace it, and nothing more than those.

I'd replace the battery.

If the alternator isn't bad (test voltage while its running to find out) then it should be 12.5 or higher not running with the key off.

12.4 minimum fully charged.

[Fifty150hs]

Originally Posted by Crowz

I'd replace the battery.

If the alternator isn't bad (test voltage while its running to find out) then it should be 12.5 or higher not running with the key off.

12.4 minimum fully charged.

You should be seeing at least 13.7 volts while running. If you don't, you probably have a bad alternator. Also, how old is your battery? They're usually only good for about 5 years. Some can go longer, but on average that is max life for a lead-acid battery. In the X5, 11.5 volts is definitely NOT enough to start. That's why when you pumped up the charger to start, it did. It supplied the power, not your battery.

Check your battery in the morning after being on the charger all night. If you aren't showing 12.4 volts or better, you likely have a bad battery.

[oldskewel]

Battery was:
12.6V immediately after charging overnight.
14.0V while idling.
12.2V at end of day yesterday, after driving and waiting an hour.
12.1V this morning, started just fine.

Battery is less than 4 years old, but yes, I may need a new one some time in 2018.

Temp sensor area is still in perfectly good shape.

I expect something ran it down overnight when this problem surfaced. Not sure if it was a one-time event ... we'll see if there is some problem in the car that will cause it again.

Maybe there is a wiring issue with the temp sensor, which both causes the faulty temp reading and also draws down the battery??? I will keep that in mind if something like this happens again. If so, more likely to be rodents chewing on wires than shrapnel or tree stumps.

To summarize, my main question had been about the -40*F. If there were a known common failure (e.g., FSR) that showed that, it would have helped me diagnose. I did also learn the location of the temp sensor, which may be helpful one day. So at this point, the conclusions are:

1. No, the -40*F on the dash is not a clear smoking gun for a well known failure.

2. It is just that the computer is confused, probably by lowered voltage. But still odd, because it was not *that* low. Maybe low enough to prevent starting. Maybe low enough to warrant getting a new battery. But still surprising to me that 11.x V would confuse the computer.

[wpoll]

Originally Posted by oldskewel

But still surprising to me that 11.x V would confuse the computer.

Not really surprising at all. These suckers are VERY fussy when it comes to battery condition. I get the yellow DSC (4x4) warning light flashing during start up after just a few days of around town (short) running - warning me that the DSC module is unhappy about the voltage drop. Engine still starts etc. but i bang it on a charger to be certain.

I have seen similar issues on BMW bikes too - low voltages in BMW systems cause all manor of odd gremlins.

All bets are off until you either replace the failing battery or resolve the source of the drain (whichever is the real issue here). Attempts to address any apparent errors should also be put on hold until you have resolved the low voltage problem, as I'm betting they wil disappear.

[oldskewel]

Updating a week later ...

No problems. I just charged the battery a week ago, and it has been running fine since then with no issues at all. No repairs or debugging.

So some one-time event must have drained it. Not sure if it is a failing component that will make it happen again eventually. Waiting.

That -40*F still something of a mystery, but the temp readings have been accurate since then.

UPDATE - Dec 2018 - temp sensor, battery (Feb 2014), and kids are all still doing great. Also, I have noticed that -40*F once or twice since then. It came up in a different thread just now, which is why I'm updating here. Another feature of that issue is that when it gets corrected, the displayed temp ticks up gradually from -40 up to the correct temp, rather than switching instantly.