Tundra's Voltmeter Reading Up and Down Intermittently

A little background...2006 DC 92K miles, as far as I know the alternator is original and it has a Toyota TrueStart battery with a 5/18 purchase date on it.

I have barely been driving my Truck during the lockdowns (10 weeks or so) because I'm working from home. Until today, I think I drove it 5 times and none for more than 10 miles. One day, when I had to run to the office I notice my voltmeter was running low, just above the 12 hash mark and my HU which shows the voltage as a number was in the 12.3-12.6 range....like this...



The needle is normally dead center and points right at the middle hash mark exactly and reads 13.7-13.8 on my HU.

I didn't think that was right but didn't pay too much attention because I was in a hurry and the truck had started right up and run fine. Today. I went to start it and the battery was dead...but not completely dead...all interior lights worked but it did not have enough juice to turn the motor over. I jumped it from our Odyssey, but it still did not have enough juice to turn over, so I let it charge up for about 10 minutes and jumped it again and it started up. Before jumping it I noticed the water level in the battery was a little low but not bone dry, so I topped it off with distilled water. I just figured I'd been using it so infrequently that the battery had lost charge, so we headed up to the North Ga mountains. On the way there the gauge would intermittently drop down like the picture above and show 12.3-12.6 volts while cruising along. There was no noticeable difference in the way it ran at all. No stuttering, no lugging, full power, all electrics worked. I noticed that if I came to a complete stop the voltmeter would return to normal and stay there for awhile while driving, but at some point would drop back down and would sometimes go back to regular when driving, but definitely would go backup every time I stopped. I could not determine if any change in driving drove it down, because it would just happen while cruising along...but every time I came to a stop, it would hop back up.

I had been frequently disconnecting and reconnecting my battery while installing my leather heated seats over the course of the last few weeks, so I thought maybe my battery connection was bad, so I stopped at an AutoZone after about an hour of 70 mph highway driving, disconnected the battery, cleaned the terminals up (even though they didn't look dirty and there was no noticeable corrosion) and reconnected it. I had AZ come out and run their test and their little machine said the battery, starter and alternator were all fine and that the battery was showing a 71% charge. We decided to continue on the other hour up to the mountains, but in the next 5 miles, it started doing it again...now on back roads with intermittent stop signs where it would pop backup. I stopped and started it 3 more times today and it started every time. No running issues at all, no electrical issues at all...but it had the same issue all the way home for 110 miles of going back forth between a normal and low voltmeter reading.

I just put a multi-meter on the battery and got the following.

12.57 with no key
12.40 with key in with ACC on
13.92 running (Truck's voltmeter was in it's normal position).

I'm planning on checking the battery readings tomorrow AM to see if there is any change.

I don't have a ton of electrical knowledge, so to the folks out there with more smarts than me...does this sound like a batter dying early or an alternator on it's way out? Any suggestions on other tests to determine if it's one or the other or something else entirely?

I considered that it could be some sort of parasitic drain from my seat heater install...but that was pretty straightforward and while it's possible it could drain the battery when the truck is off, I don't see how it could cause the significant fluctuation in the voltmeter while driving...but maybe I'm wrong there? Any suggestions are appreciated. If it's a bad battery I want to find out ASAP so I can get in on that Northstar group buy starting on Tuesday...

 

My guess would be battery. It's been sitting around with no long trips to charge it up. Maybe put a tender on it to top it off and check again. Highly unlikely to be alternator failure.

 

Here my Off and ON readings with a 1.5 year old battery. Hope this helps.

 

Updated multimeter readings this AM before I used the truck...

12.30 with no key
12.05 with key in with ACC on
14.2 running (Truck's voltmeter was is just a c-hair above the center hash-mark).

I'm going for a drive and will check it later today and report the readings back.

 

After taking it or a mixed driving (15 miles highway 15 stop and go), it never dropped on the stop and go but did drop on on the highway. Readings when I got home.

12.56 with no key
12.32 with key in with ACC on
13.8 running (Truck's voltmeter was at the 1/2 hash mark)

 

My Tundra did that when the alternator was about to go. It would flicker and come back, but after a while it would drop and not come back. Then I was stranded on the side of the road.

replaced the alternator with a denso reman and I was good to go.

 

TLDR

but it looks to be pretty normal maybe a touch low. Tap the alt with a hammer and see if it jumps up.

 

Voltage regulator in the alternator is failing.

Easy test.

Get on the highway and engage cruise control. When the voltage fluctuates and drops, the cruise will have a hard time maintaining your steady speed. It's gonna feel like a miss fire.

 

That's the weird thing....I drove on the highway for an hour at 70 mph and at least 1/2 the time was with CC on and there was no issue with driveability...no missing I could detect....but I'll try it again, thanks.

 

I forgot to mention, I was driving at night. The headlights added an additional load.

 

Saw similar on my truck after I bought it last year as an'06 w/65k original miles. Didn't notice any needle fluttering on test drive, noticed it when I hit surface streets after my 2 hour drive home from the stealership. Stealership forced me to sign 'as-is' disclaimer - which I was OK with based on how immaculate the truck was, results of the full inspection the selling stealership did, plus my own inspection - so I just chalked it up to a GenI thing.

6 days and less than 200 miles later watching 1-3v fluctuation while driving, low battery light came on while driving to work. Assumed old crappy battery was the issue (3~4yrs old) and NAPA was a few blocks away, so I popped in. Installed new battery, started up, before I could idle for a minute and pull out of the parking lot, smoke was billowing out from the grill/hood area.

Whipped back around, parked, flung the hood open, electric burning smell, smoke pouring from front/lower passenger side of engine. Alternator apparently caught fire, I could see glowing and flickering. Whipped the cables back off the new battery and USAA had it towed to the local stealership, they wanted $3k to repair/replace so I had USAA tow to my preferred shop who charged $105 plus my choice of parts.

I still see periodic 1v-ish fluctuations randomly. Other than one turn-key-and-everything-went-dead moment months later, haven't noticed any oddities.

tl;dr - "new truck", my needle was fluctuating 1-3v, swapped in new battery, alternator caught fire, fun ensued.

 

Maybe check your grounding wires or looseness or corrosion?

 

That would be my thought.

 

Good call, we're on the same page. I meant to probe for loose cables more than corrosion*, especially after the one-time turn-key-forward-and-everything-shut-off a few weeks after the alternator was replaced in November. It reeks of loose cable or something arcing out. First thing I did after that was jump out of the truck and check for loose batt terminals. No dice, everything tight. I need to dig for a diagram of all grounds to inspect.

Haven't found any corrosion* anywhere on the body/frame/wiring. The truck stayed in the Birmingham, AL area most of its life as a "business vehicle" for someone working at a bank, they bought it fully loaded, serviced it religiously, then traded in, next person had around Auburn AL for a few months and traded it in.

Other than the single-stage paint being oxidized to hell, some general sun fading, and a little wear on the driver's seat leather bolsters, it was pretty mint when I got it. It's been one of the best trucks I've owned, amazing it's 14 years old.

 

Took a video of my situation. This is after a new Denso alternator and new battery. @speedtre is this the same fluctuation you're seeing?

This was taken at a stoplight, fully braked while in drive, A/C OFF, with headlights and wipers on. Worth noting I saw no changes with headlights and wipers off.

Video is here.

 

So, my truck just started doing this last night, exactly the same as in your video. No drivability issues, but does seem just a little sluggish to start, like the battery is a little weak.

 

In my case, battery is high quality, tests out fine, and not even a year-old. I can't tell if this is something that just started, or if it's something I just noticed but has been going on since I've had the truck. I'd love to hear if other Gen1 owners are seeing the same volt fluctuations or not.

 

Holy Fluctuations Batman! Alternator? Still the original? My readings suggest 200k miles is the average replacement time on these.

 

What ended up being the fix? Im having the same issue with battery gauge fluctuating. With truck off battery is at 12.6 v, same at alternator pos lead. With truck on im getting 13.9v at battery and at alternator...after doing a load test and running rpm's up to 2k voltage drops at alternator pos lead to 12.7v. Ive taken battery cables and connectors off, cleaned...didnt have hardly any corrosion from the start but just to be sure. Have a feeling its the alternator but would like confirmation to be sure.

 

Mine was a mixture of a bad battery and the alternator belt. Replaced both and it's perfect.

 

Well I bought an agm duralast platinum battery from auto zone about three weeks ago and hadnt noticed any issues until the last few days...also today while doing load test when i turned off my ac it sounded like the truck was getting weaker and voltage went down. When i turned the AC back on voltage came back up and sounded stronger.

 

Most all I read about these AGM Batteries they are high priced problems. Might be your problem. Or, alternator or belt as suggested above.

 

You might be right...would hate to replace the alternator and still have this issue

 

Though if it was the battery my voltage wouldnt be dropping down to 12.7v AT the alternator, no?

 

Bad Batteries give our trucks weird Mojo. We see lots of people come and go with a New Battery that is bad. These AGM Types sounded cool a few years ago, but the more I read they require trick chargers and special diode fuses etc.

Hard to say what your problem is without being there. Report back once you find out.

 

@DavidWooderson21

If you have another vehicle with a regular battery you can swap batteries around temporarily to test to see if that schmamcy battery is your culprit without buying a new battery until you know.

I've done that before a few times.

 

Honestly contemplating taking the agm battery back to AZ and telling them i just want a regular lead-acid...didnt have this problem til i swapped to the agm...if still having problems after that then im definitley thinking the alternator

 

Good call. Roll the savings $$$ into a Reman’d Denso if needed.

 

Had no problems returning the agm battery to AutoZone, installed and drove around for about an hour, no fluctuations in voltage on my voltage meter or on battery gauge on dash...gonna drive around next few days do a bit more testing but so far it looks like it may have been the battery.

 

Could still be the alternator or belt. Weird that it would improve when turning the AC on. Unless turning the AC on increased the tension on the belt and improved the belt contact with the alternator pulley. Could point to a bad belt or tensioner? How old is the belt?
We seem to chase some of these weird gremlins around a lot and it usually ends up being battery or ground related. You've replaced the battery, so I'm back to belt issue? Good luck sorting it out, let us know what you find.