2007 Mercury Mountaineer: Voltage drops to 4 volts while driving

[TedL]

2007 Mercury Mountaineer Premier V8 AWD, 199,700+ miles. Background: Have had occasional, unpredictable situations where the battery is drained and needs a jump to start. (Similar problem in this Mounty's predecessor, a 2008 Mountaineer V6) such that I routinely monitor Control Module Voltage using a BlueDriver ODBII dongle and iPhone app. More frequent low battery/no start calls to AAA prompted the purchase of a rechargeable battery jumper kit. Had to use it twice Saturday.

SATURDAY:
Driving about a mile from the second jump-start, I noted the Control Module Voltage varying on infrequent occasions of slightly over 14 volts, to 13, 12 and upper 11 volt range and occasional swings to 9 volts. ON ONE OF THOSE SWINGS... I noticed that the transmission seemed to be in THIRD, which allowed the vehicle to move forward, but not shifting. At a stoplight, I put it into Neutral, and heard and felt various transfer case systems clunking. Again when I put it in drive.

I found a safe place to stop, shut the car down and restarted. Voltages were in the 13+ range. Resumed driving. It behaved normally UNTIL... I SPOTTED THE VOLTAGE DROP TO 4+ VOLTS. The transmission went to Third... and I could hear/feel the drivetrain transfer case systems changing. INSTRUMENT CLUSTER LIGHTS LIGHT UP, and THE WRENCH Icon appeared in the cluster.

Again, found a safe place to stop. Shut down. Couldn't get the engine to turn over. Just ZZZZZT. I have AAA and a AAA battery. So, a call (and wait) for AAA's battery guy ensued. He jumped, observed charging voltage fluctuations and said what I described indicates the alternator is going out, likely bad diodes.

I've HAD the alternator replaced in this vehicle since acquisition in July 2019. Replacing the alternator didn't stop the battery drain/need a jump/things are fine for a while... issue. And I've observed voltage drops such as described, but NEVER to 4 volts, and never with the Third gear/drivetrain changes Saturday.

QUESTIONS:
1: Is any of this familiar?
2: What was diagnosed as problems?
3: What fixed the problem?
4: Did identifying one issue and fixing THAT lead to discovering other issues that contributed to the overall condition?
While this might indeed be an alternator going out, the fact that similar voltage issues have been on two similar vintage Mountaineers (and possibly a 1996 Explorer XLT owned before the 2008 Mounty)—has me wanting to avoid replacing the alternator if that's NOT the problem.

Especially since I see other reports of similar voltage issues here and in other on-line threads.

I'm wondering whether this is connector or wiring related, possibly affecting computer-to-computer or device to computer communication.

Would be helpful to know where to look if this IS familiar to someone, as there are SOOO many possibilities.
I haven't found anything online describing quite what I experienced Saturday.

Thanks much for reporting your experiences.
Ted

 

[94Eddie]

Were any codes tripped during the time the problems occurred?

 

[Turdle]

Found my ghosts in the machine

Hey guys, If you've got unexplained, and intermittent, problems, here's a possible cause. My Ex has developed what is best described as the "hiccups" at idle. It doesn't happen all the time, and feels like a misfire. No codes, though. So I thought I'd just inspect the engine for (esp the...

www.explorerforum.com

Check for chaffed wiring battery harness to starter

 

[TedL]

Were any codes tripped during the time the problems occurred? These are codes that I’ve seen when we’ve had other power related issues.

A variety of codes were tripped as a result of the power drop. All seem to have been the ones I’ve seen earlier relating to loss of power by different systems, not necessarily one’s related to cause of the power drop. I can post the list.