Trailer Charging/Voltmeter Question

[duramax170]

Today while pulling a pop up camper home the voltmeter dropped to around 12-12.5 volts. Since I have had the truck it has always been at 14. The pop up has a deep cycle marine battery that was fairly drained after running the heater all night and we didn't charge it all before we left camp.

We have installed the fuse under the hood to allow charging, and the only very informal test we ran was to check the trailer voltage, and test it again with the truck hooked up and it showed higher. Normally we charge the battery before we leave for the trip and again the morning we are leaving camp so this is the first time it has actually been low and hooked up to the truck

The volt meter would return to near 14 for awhile and then go back down. No warning lights came on. When we got home bot batteries were reading 13.5 with the truck idleing and still hooked to the trailer.

So my question is, is it normal for the voltmeter to show a drop that large while hooked to a trailer or should I be concerned about the alternator.

Sorry this is so long
Sam

[DmaxMaverick]

That sounds completely normal for the conditions, with the OEM 105 amp CS130 alternator. Mine did the same during dry camping (no generator or shore power, which is 90% of my camping). I upgraded to a high output 244/DR alternator (www.alternatorparts.com), and now it sits on 13.8V (gage and confirmed actual) no matter what, charging, not charging, idle or revved up, hooked up to RV or solo. Recovery after a cold start is almost instantaneous now, compared to 30-60 seconds before. I highly recommend an alternator upgrade from them, and it's less expensive than the OEM replacement (and made in USA). Another option is dual alternators, but I decided it was too many moving parts and engine/belt load, with less gain than a single HD. I'm very pleased with mine.

[duramax170]

Today was the frist day I have driven the truck since I first posted. With out the trailer the gauge staye low. Didn't have a volt meter so i plugged in the predator and it showed battery voltage at 12.0. When I would let off the accelerator it would drop to 11.9 or 11.8. I'm guessing it is time for a new alternator

[DmaxMaverick]

Do another check with your Predator, and do the same with a VOM (meter), if you can. Use them to check the voltage before a start, key on, then while the engine is running. The Predator won't let you read data with the key off. When voltage variations start to appear, most often it's the batteries that are suspect. If your alternator was on the way out, you will normally get a day or two of use before it won't start, or the voltage drops so low you have other electrical/electronic issues. If the alternator indicator (red battery picture) isn't showing on the message center while the engine is running, the alternator is most likely OK.

[duramax170]

With the VOM after the truck had been sitting while i was at work both batteries were 12.35. Truck started fine but the batteries were 11.8, they slowly crept up to 12.05. Went to drive home and the battery light aluminated.

[DmaxMaverick]

How old and what brand/series are your batteries? Also, it wouldn't hurt to clean the battery terminals, and check closely for any sign of excess battery venting and/or leaks. I'm still not convinced you have an alternator problem.

If you do a lot of dry camping and use the truck to charge RV batteries, it is never a bad time for an alternator upgrade. The difference is night/day. In my case, the alternator upgrade cut my charge time by more than 1/2, and all at idle. I don't use a generator (too much weight and fuel consumption), but have an integrated inverter in the RV. 2 deep cycle batteries in the RV, and 2 Optima Yellow Tops in the truck and we always have power for everything but A/C and microwave (don't need either of them). Replacing the alternator now would eliminate it as a cause of your problem.

[duramax170]

You nailed this one.

Batteries are about 18 months old Interstate MTP78 800CCA 1000CA.

Replaced the alternator yesterday with no change. Had the batteries tested today the passenger side has 220CCA, driver 130. Saw some corrosion on passenger side positive cable. The cable were put back on so I could move the truck for the Interstate guy to in stall new ones, and all of a sudden VOM was back to 13.8, batteries were replaced for free either way.

At this point I look at the alternator as preventiative maintanence, but out of curiousity I had the old one tested. With 175,000 miles, It passed all tests twice.

Thanks a lot for your help, after you first post I went ahead and ordered the 244 alternator so it will go on at some point and I will have a spare I suppose.

Thanks Again
Sam