Draining rear battery

[chief1983]

I'm having a strange problem with my rear battery. It was at the mechanic's and when I got it back the charge on it was about 8 volts. The truck would still start, barely, and the mechanic said he had started it probably 20 times over a few days. When the truck starts cold, Gen 2 has had a habit of not 'coming online' until a little gas is given. I have a Battery Bug on my batteries so that's how I knew immediately which battery was going haywire. The belts seem to squeal more than before cold, but it still quits and the battery bugs then read 14.4 to 14.7 V on both batteries, but the rear one seems to climb to that over 10 to 20 seconds after the alternator kicks on. Not sure if that's strange or not. Ran it for a while, shut it off, the battery bug says it's at 12.1 volts. Next day, down to 10. Didn't check much in between so I don't know if it was just undercharged and had a surface charge or not. I pulled the rear battery and hooked it up to a battery maintainer I linked below, that includes auto-desulfation and bad battery detection, and it charged it over a period of about 8 hours at 2A without a hiccup. Haven't load tested it yet, haven't had time since Sunday. I also checked the resistance across the disconnected terminals (so at the +12V and +24V poles), without disconnecting the rear battery. It was reading 40 ohm. By my math, if there was a 40 ohm resistance across the battery that's about a constant 3.6W drain. Seems a little high to me, like I might have a short or something. I don't think these trucks are supposed to have any always on devices, I know I haven't installed a car alarm or anything.

So, I'm going to do more testing as soon as I get more evening time again, but does this sound more like a bad battery, given that my charger/maintainer didn't complain, or more like a short in my electrical system? And if so, isn't Gen2 about the only device, with its isolated ground, that could drain only the rear battery? The mechanic was mostly under the car, fiddling with the bellhousing/dust cover and might have bumped something around the starter, and I've been messing with my vacuum valve which is positioned right over the alternator, so it might have even been something one of us messed up. If anyone's ever seen something like this before I'd love some advice, because I'd hate to shell out $220 for a rebuilt Gen2 on a whim, and there's a lot of wires to trace for continuity, but it looks like that's my next plan of action barring any tips.

SC-600A:

 

[Warthog]

Sometimes the rectifier in the alternator will go bad and cause a battery drain. Still works but causes a drain.

Have you rebuilt the alternators?

1stSarge - CUCV Electric is selling kits now for ~$40.

Or the Group out of florida is where I have purchased many kits in the past.
High output Alternators - Alternator Parts - Starter Parts - Repair Kits - DELCO 27SI HEAVY DUTY REPAIR KIT

 

[chief1983]

Haven't rebuilt one before, probably something I could tackle. I'll be sure to probe the alternator first for continuity. Thanks for the links.

 

[idM1028]

Sometimes the rectifier in the alternator will go bad and cause a battery drain. Still works but causes a drain.

Have you rebuilt the alternators?

1stSarge - CUCV Electric is selling kits now for ~$40.

Or the Group out of florida is where I have purchased many kits in the past.
High output Alternators - Alternator Parts - Starter Parts - Repair Kits - DELCO 27SI HEAVY DUTY REPAIR KIT

Happened to me once. Dropped a fresh set of batteries into the truck. Somewhere between half an hour to an hour with the truck just sitting the voltmeter was in the yellow . On another note, one of the things belt squeal can be indicative of is a heavy load being placed on a component. I once tried jumping a Ford (go figure) with the truck and sure enough, the belt for GEN1 started squealing even though it was good and tight.

 

[chief1983]

Yeah, I was wondering if the alternator could be failing causing the belt to squeal because it's offering more resistance than it should. That's why I mentioned that. But since it seems to smooth out fairly quickly I haven't worried about it too much. Doesn't seem like a little resistance at startup would be related to a shorted component, at first glance at least.

 

[M1008driver]

And if so, isn't Gen2 about the only device, with its isolated ground, that could drain only the rear battery?

Do you have a radio power cable? Mine had a short and when I removed it my battery was stronger. Just and idea...

EDIT- Tug on those battery cables and see if they are coming loose. I once had a bad connector and my battery never charged right. 8 bucks at the battery shop for a new military grade connector.

 

[paramedic7831]

Do you have any after market 12v stuff on the "low battery"? Check to see if the pulled out battery looses charge over a day or so out of the truck not hooked to anything. There are also battery cut of switches.

 

[SGT Estum]

If you have Fleet Farm stores in your area, you can pick up alternator rebuild kits there. I picked one up for 100 am 27SI rebuild for $16 or so (I don't remember the exact price, but it was quite reasonable). I haven't used the rebuild kit yet, though. I know it said "Made in USA" so that enticed me into buying it.

 

[chief1983]

I am currently letting the battery sit after having been charged, unplugged from anything, until I can get back to it. So I'll see if it's losing charge internally somehow this weekend probably. Haven't added any aftermarket stuff yet.

Don't know of any Fleet Farms, we have Rural King which is where I got a replacement Group 31 for $85. 1100 CA, 925 CCA, 180 RC.

I found loose cables in the 24 bus on the firewall the other day and tightened them up, this was a week or two before this trouble happened, when I was diagnosing the glow plug problems. The post terminals on the batteries are all tight, I just installed the Battery Bugs this spring, and the cables on the terminals all seem very sturdy. I even found the missing fourth terminal boot the other day, slid way up the cable to the firewall.

 

[chief1983]

Just got a $200 gift card for Amazon from work, what a coincidence. Thinking about not bothering trying to rebuild it myself now and just getting this alternator from Amazon, and wondered if anyone has heard of Rare Electrical before. They don't seem to list a location on their website but they seem to at least ship from the East Coast. Don't know if they're selling good stuff or Chinese clones or something. Same warranty as the Wilson rebuild from NAPA, and about $13 cheaper, and I get to keep the core to maybe rebuild at my leisure later, and have a spare on hand.

Amazon.com: New M1008 M1009 M1028 CUCV HUMVEE Alternator 12v 1105500 OE 100amp: Automotive

 

[SGT Estum]

Two thumbs down for any company that spells HMMWV as "HUMVEE"

 

[chief1983]

That might not be entirely fair, it _is_ a registered trademark of AM General, albeit not in all caps. But then, their description below is also in all caps, that might be reason enough to be worried

 

[idM1028]

They're ok. I added a new pair into the truck once, but now I've got the Caddy alt in the GEN1 position (lifetime warranty=) I've still got the one I got from Rare Electrical in the GEN2 position (albeit with one rebuild on it). Last time I had it taken out and tested it was putting out 14.6 volts and 90 amps or so. Only problem I've had with it is the bearing's kinda loud and gritty, but as long as it works and puts out juice I'm not that worried about it.

 

[chief1983]

For reference, I ended up taking the battery to Autozone and they confirmed it had a dead cell. So no need for an alternator rebuild (yet). I thought 40 ohms was a low resistance for idle drain, but the other alternator also had 40 ohms across it, so it looks like that's not really the problem I thought it was.

 

[wallew]

OK, let me also suggest you folks consider changing out your battery cables.

I cannot tell you how BAD a shape most battery cables are. They rot and rust and sulfate inside the whole length of the cable.

If you have NOT replaced ALL your cables, it is cheap insurance. You can swap out battery and alternators, but if the cables that connect them to the starter or the rest of the system are rotten, you WILL have problems.

For around $100 worth of cables, your truck will spin up and you will know that the cables are NOT the problem, if you still have any electrical starting issues.

Just food for thought.

 

[lindyp38]

i need to learn to use my volt meter............good luck with ur truck........electrical gremlins arent my favorite....

 

[chief1983]

I think I've got the newer cables, where the small 12v wire comes off the front positive instead of the rear negative. Or whichever way meant they've been replaced at least once. Still, no way to tell how long ago that was, but I imagine they're less than 15 years old at least. Terminals are still in great shape.

 

[M1008driver]

When I had the battery connectors changed on my M1008 the cable under the battery clamp was very rusted. The battery shop had the heavy duty wire cutter and crimper and the new clamps only cost me $8 each. The rest of the battery cable was good after cutting off the ends.

I would say it was worth it and made a difference, even if small.

 

[topgun217]

Exactly! Battery cables are cheaper than a headache. Had the headache for 2 days, then went to NAPA and got new cables and not one problem since. I also sprayed the connections with the protectant spray

 

[Matt1031]

Pull the battery, charge (again) and load test. Could be a bad cell.

12.1v is only about 50% state of charge. It's not even testable.
See chart about 1/2 way down page here

Also while it's out look for a approx 1" round white sticker on the battery that reads something like A-9 or G-01. Letter is the month. number(s) is last digit of year batt was mfg. Many of the major auto parts stores use the same sticker now. If batt is over 5 years old - probably not worth trying to save.

To check for drains you can use a DC amp clamp on the main ground or NAPA sells a small ammeter that plugs directly into fuse box (allows you to narrow down which circuit the amp draw is coming from).

Nice thing about a DC amp clamp is that it allows you to test alts without pulling them and is an absolute godsend for troubleshooting glow plug problems. I paid $80 for mine off Ebay.