I need some help on the engine electrical charging system. The system is only putting out about 12 volts (measured at the battery with a VOM, while engine is running). The alternator is a stock replacement, about 1.5 years old. I removed it from the vehicle and took it into the parts store for testing. According to the guy at the parts store, where I had it checked, it passed their test. When I reinstalled it, the problem remained (as it should, since nothing really changed). I cleaned the connections at the alternator and repaired a couple of questionable wires.

So, I started checking connections and voltages. I checked at the isolator and I’m not sure if I checked it correctly. As I understand it, when checking the voltage, the reading at Post “a” and “b” should be 12 volts with the engine NOT on and 13.5 volts (or so) when the engine IS running. So, I checked the voltage with a VOM by contacting the positive probe of the vom to post “a” and the negative probe to the frame as ground. Basically 0.5 volts was observed. Obviously, this is not the reading expected.

Two questions:
1. Did I check the isolator correctly?
2. Any suggestions on where to start for the charging issue?

You should have gotten at least 12.5V at the battery posts, and 13.3-13.5V at the alternator connection. (I don't know what posts are "a" and "b", since isolators are different. In any event, isolators act as splitters, dividing the charging power between the two batteries (House and chassis).

First the check is to the alternator output. If that reads 13.5[u]+[/u] (engine running), the alt is OK; then the isolator outputs need to be checked - 12.3-12.7V should appear there.

Instead of chassis ground, check voltages at the battery posts when testing the isolator.

It sounds like you may not have had a good ground when testing the alternator, and you may need to make a jumper wire to connect to a known good ground. The chassis battery negative terminal should be reliable, however.

0.5V is way out-of-line.

Rusty,
Thanks for the feedback. I have never checked an alternator for output while on the unit. How do I do that other than for checking voltage at the chassis battery?

Basically 0.5 volts was observed

Sounds like one (or both) of your voltmeter probes was not making a good connection. ORrrr your voltmeter was set to AC instead of DC??
Are your batteries not charging? If you don't have an actual problem, consider leaving it be.

It's always good to look for the easy and obvious, first. I do question the quality of the ground connection. I'm going to try something different, later today, when I get a chance. The settings were on the DC. I checked that twice. But, it was a good idea.

quote:

Originally posted by Mike Cropper:
Rusty,
Thanks for the feedback. I have never checked an alternator for output while on the unit. How do I do that other than for checking voltage at the chassis battery?

\

The voltage at the chassis battery will be after the isolator output, and if it's low, won't tell you if the isolator or the alternator's bad. There should be three terminals on the isolator - alternator ("input"), and house and chassis battery terminals. They use a common ground.

You want to check the voltage at the alternator input terminal.

Also, make sure you're checking the alternator isolator and not the converter isolator - there are usually two...the converter (120 VAC to 12 VDC) runs only when there's 120 V supplied.

If they aren't marked, measure the voltage with shore power on, then with it off. The one that drops is the converter's, the other is the engine's alternator's.

In no case should the engine be run with the alternator lead disconnected, unless you want to install a new alternator. It could fry the diodes).

But as Gunner mentioned, if it works, don't fix it...

I’d like to share an update in this little under charging mystery. To day, I took the Barth to a different auto parts store and asked them to “test” the system with their charging testing system. (I have not been able to take it to any kind of shop. Some of us Barthers still need to work for a living.) The battery tested low, but OK, which is what I expected. The ammeter probe was placed over the positive line at the battery, not at the battery connection on the alternator. The system stated that the system was charging. I’ll admit that I have no experience with this test system. However, my gut feel is that this test was OK. Meaning that it was looking at one side after the isolator

Now, the dash gauge and my VOM indicate that the system is not charging. The gauge is below the neutral line. The VOM reads 12.0 volts. I’m hoping to have the calibration checked on my VOM, tomorrow.

I have not checked the isolator check suggested by Rusty. That will need to wait till Tuesday evening.

I don't mean to disparage Parts Guys (OK, maybe I do), but they have a standard test they employ, mainly, "What's the voltage at the battery?". They're totally ignorant about motorhomes, and clueless about isolators, as there's no need for them to be qualified.

First off, on a typical vehicle, low voltage at the positive battery terminal can be caused by a bad alternator, a bad battery, or a bad ground.

What you should see (typically) at the alternator output is 13.6-14.4 V. An isolator will drop that about 0.7 V.

It sounds like you're having problems charging the batteries, so here's the procedure:

1. Voltage at the alternator terminal of the isolator should be 13.3 - 14.6 V

2. If 1., above is on spec, then both battery terminals on the isolator should read 12.6 V to 13.9 V.

3. If 1. is on spec, but there's a differential between the outputs on the isolator, there's either a bad battery or a bad isolator. Switch the chassis and house battery connections at the isolator. If the same isolator terminal shows low voltage, it's a bad isolator. If the low voltage follows the battery (i.e., changes terminals), bad battery.

The dash voltmeter isn't much of a precision instrument (which is why very few have actual voltage markings, just a band that's "OK"), and its reading may be off due to a bad ground.

Rely on your voltmeter. They rarely, rarely, get "out of calibration" as they're very simple, robust devices, but checking it against a known source is never a bad idea.

I have the same problem while the engine is idling and even up to about 45-50mph. I had an auto/truck electronics expert check out my problem with this solution, slightly reduce the size of the alternator pully so it turns just a little faster. I did not do this as yet and don't know if that is the answer.

Jim

A better solution would be to replace the alternator, because it sounds like it's bad.

Speeding it up with a smaller pulley merely delays the inevitable - or accelerates it due to extra bearing stress.

My org. alt. did the same thing and this was the new alt. that he advised going to a slightly smaller pully on.

I never went to the smaller pully and at present I just keep an eye on it.

At night time I have to be doing 60-65 with the lights on to keep it at about 13volts.

Jim

I'm afraid I don't agree at all with your tech (you probably already guessed that) . There's something amiss with your system - if you're up my way (north of Gainesville), there is an excellent electrical shop in Alachua (about 1.5 miles from I-75 Exit 399), or I'd be happy to troubleshoot it for you. It's not rocket science, it's just a logical progression of tests.

For a quick-and-dirty test: If you have enough slack to remove the alternator cable from the isolator and connect it to the battery positive terminal, I'd try that and see what the voltage is; that would pinpoint the isolator if voltage is normal; otherwise, the alternator or ecitation is suspect (see below).

Also, I would use a hand-held voltmeter, since the one on the dash may not be accurate.

Alternators (and hence dash voltmeters) will show low readings if the alternator is bad (not enough voltage generated - a bad diode maybe), the excitation circuit has a bad connection (doesn't push the output voltage up high enough), or the battery is bad. Occasionally, the reading will be low with a bad battery ground, but that condition usually results in a high reading.

It's not uncommon, either, for a replacement alternator to be defective off-the-shelf.

Rusty, speaking of logical things to do, my next place to look will be grounding connections and wire connections. I remember a post a while back that discussed the affect of poor grounding connections. Since almost all of these coaches are at least 8 years old, there could be a few loose, broken, or corroded ground connections.

Great suggestion - the alternator ground could be bad, and the excitation too low.

It's test lead and ammeter time. Run a wire from an alternator bolt to the batt negative. Run a lead with an ammeter from the alt large connector to the battery positive. It should be the only wire there. This will eliminate your MH wiring. Fire it up and read the ammeter and a good voltmeter. If your battery is good, the amps should start out high and decline as the battery is charged and the alternator senses heat. The volts should slowly rise. Then turn on all the lights. The amps should rise and the volts will fall a little, but still be above 12.5 volts.

If things are not right, pull the flat connector off the alternator and check voltage with the ignition on. You should have full batt voltage at the big wire and a little less at the small wire, owing to the resistance of the idiot light or resistance wire. Be sure the connections are tight and clean. Those connectors take a lot of heat, dirt and vibration stress. Shops sell new ones for a pittance. I don't think you have an idiot light, but there is a resistance wire in the harness that takes its place for field excitation. There are some bootleg one-wire conversions that eliminate the small wire circuit, and don't work well until rpm are up