I just had a "replacement" 160 amp Alternator installed last week.

Went-on a 750 mile trip over the past few days, getting home late last night and parked in the street.

...everything seemed to work fine on the trip...

Went to move the vehicle into the driveway this morning and noticed the Tachometer "bouncing" up and down fairly rhythmically.

Clipped the VOM onto the output terminals of the alternator and I'm getting a rhythmic and fluctuating voltage output from 8 volts to 13.5 volts [needle moving rhythmically back and forth]...

Any suggestions as to "why" this is happening or "what" to do next???

I will be calling the truck repair place in the morning ...just thought I would search for a reason "why" [first].

Does the rhythm of the fluctuating voltage change with RPM?

What does the voltmeter do when the engine is off?

Was there any load cycling on and off?

Does this occur with coach and chassis batteries paralleled?

How does the chassis battery test?

How is the belt tension?

Are the connections clean and tight?

Is the alternator hotter than normal?

1. "YES" ...the rhythm increases with RPM.

2. The meter goes to ZERO when the truck is shut-off.

3. I'm not certain what "load cycling" is, but nothing was being operated on the vehicle requiring other than normal day-time electrical needs.

4. "YES" ...ALL batteries are in parallel.

5. Both chassis batteries read a little over 12 votls at rest.

6. The belt tension is questionable, as this was a potential problem in the past.

7. Connections appear to be clean and tight.

8. The previous alternator was running hotter than normal ...I have not checked this one ...I will go for a quick run a little later and check the temp. ...you couldn't even touch the old-one as it would get blazing HOT!

It sounds like a bad alternator (integral voltage regulator or a bad diode?); after a long trip, the batteries should show 13+ volts (13.6 V fully charged).

There's a possibility that a bad battery is causing excessive draw, which would have accounted for the high temp on the old alternator; the new one could have better temperature regulation (drops the output if the alternator gates too hot), and the fluctuation is caused by that kicking in - which could point back to the batteries...altough the temp regulation shouldn't show a short cycle like that.

I'm sure bill h will also have some suggestions, but my take is that when you take the coach in, have both the batteries and alternator tested.

"YES" ...it is a very short "cycle", as the needle on the meter waves back and forth between 8 and 13.5 like a winshield wiper.

This does not begin immediately ...about 3 minutes following a start-up.

Chassis batteries read 13.5 while engine is running.

I would disconnect all the coach loads, including the coach battery.

I would try a known good battery in the system. I suspect you may have heavy batteries, so this can be accomplished without any heavy lifting if jumper cables are used.

I would connect an ammeter in the alternator output lead. Now might be a good time to buy a clamp on DC ammeter. Sears has one that lots of folks like and is reasonable.

Warning, rant follows:

Every RV needs a digital volt meter and an ammeter on the dash. Even if you are not an electrician and have all your work done by others, these gages will help you recognize and report a problem. An simple example is high or low voltage. High voltage can destroy electronics and low voltage can leave you with a vehicle you can't start.

And gassers should have a fuel pressure indicator.

End of rant.

Further Investigation...


Drove 500 miles on Friday with NEW Alternator ...first time out ...everything seems fine ...daytime operation without lights

Drove another 10 miles during the day on Saturday ...in-town driving ...daytime operation without lights ...everything seems fine.

Left last night at midnight to drive home about 180 miles ...ALL night-time driving with headlights "ON" the entire time ...NO FLUCTUATION IN TACHOMETER ...all lights stayed BRIGHT and what appeared to be fully charged batteries at ALL times.

Got home at 0300 and parked in the street...

Went-out this morning to move to driveway ...NO HEADLIGHTS ON ...Tachometer is waving up and down as described in my first post.


Just went for a 10 mile run down the freeway and back...

1. Alternator seems hotter to the touch than it should-be ...you can only touch momentarily without burning your fingers.

2. Decided to turn-on headlights ...PROBLEM STOPS! Backed-in driveway and connected VOM, with engine running and headlights "ON" ...getting readings of 13.5 with ZERO wavering in voltage ...as soon as headlights are shut-off, the voltage fluctuation begins again.

3. I'm assuming that whatever this is, "began" or "happened" last night and was not noticed until today with headlights OFF.

4. If the A/C is turned-"ON", the voltage fluctuation stops as well.

Robin, you're not making this easy!

Seriously, it sounds like one of a couple of things to me:

1. Bad voltage regulator - only regulates smoothly with a certain amount of current. A bad diode in the alternator (there are usually three to six) might also cause this.

2. A bad ground on the alternator (or on the internal voltage regulator). Try running a jumper from the alternator casing to a known good chassis ground, then test again unloaded.

If I were the mechanic, if the alternator still bench-tested good (Normally they're most suspect under load, so that's how most are tested - but loading yours masks the issue), I'd remove the alternator, pull it apart, and check the internal connections and the individual diodes.

I still wouldn't rule out a possible bad battery.

Hopefully bill h, olroy, Bill N Y, MWrench, and/or others can chime in...

I tried the GROUND JUMPER with NO CHANGE in symptoms.

It's like something is "tripping" and "re-setting" in rapid succession.

At idle, it does-this 45 times per minute [cycles between 8 volts and 13.5 volts]
The needle on the VOM and the Tachometer wave back and forth like a wiper blade. It will do this faster as the RPM's are increased.

Well, that eliminates the main alternator ground (which is rarely a problem, as it's grounded through the mounting bracket - but I have encountered that issue in the past, especially since most alternator housings are aluminum and brackets are steel - but only in maritime climes, where corrosion is awful).

I'm back to the internals of the alternator itself.

The isolator (splitting the charge between the house and chassis batteries) is solid-state, so it wouldn't normally act that way (like a mechanical voltage regulator*), especially since the batteries are holding a decent charge once you load the alternator with the headlights or the A/C clutch.

So it's off to the shop to have the batteries and alternator tested. Sorry I can't be of more worth; electrical problems are hard enough to diagnose when I'm crawling around under my coach (and I'm an EE) - it's hard to give you useful suggestions from 1000 miles away....but do be sure the alternator is tested under no-load conditions.


* I'm assuming you don't have a mechanical voltage regulator - it would be rare on a coach that new - but that would be a classic symptom of a sticky regulator....

Thanks Rusty and Bill H.


I will attempt isolating the batteries in the morning, as I have had some battery problems this year and was hoping to get-by one more season without adding any further expense to the [overly inflated] budget.

Was told the Batteries and Tires were "NEW" when I bought the coach ...My research and knowledge has become much better in the past 2 years [thanks to ALL of you guys].

The "NEW" 10-year-old Tires have been replaced ...now it's time to deal-with the "NEW" 8 -to- 10-year-old Batteries.

I will check all Batteries; looking for anything to stop the current symptom.

I do believe the Isolator IS "solid-state", so I doubt there would be a purging problem there.

Everything-else points to the alternator...

Seems-like the symptoms are backwards and that there would be a problem while under load and not the opposite(???)

I took the HOUSE LOOP out of the circuit, so the only batteries that were connected are the chassis batteries.

The sypmtoms STOPPED with things hooked-up this way.

So now it seems to be a matter of determining if it is one of 3 house batteries or the SOLID STATE Isolator.

...does the ISOLATOR have some sort of protection circuit in-it that would cause the constant voltage fluctuation; as-if it were continuously attempting to "protect" something???

quote:

I do believe the Isolator IS "solid-state", so I doubt there would be a purging problem there.

quote:

4. "YES" ...ALL batteries are in parallel.

If all batteries are in parallel, there need not be an isolator.

I have an abiding dislike of diode isolators, but that is another subject.

Again, I would substitute a known good battery into the chassis system and see what the alternator does with only that battery connected. That would mean using a jumper to bypass the isolator and eliminating whatever paralleling device your breakaway has. I have no Breakaway experience, so I am unable to be precise, here.

Sometimes a situation defies normal battery testing. Substitution (and elimination) can show a problem that testing does not.

quote:

Was told the Batteries and Tires were "NEW" when I bought the coach. Now it's time to deal-with the "NEW" 8 -to- 10-year-old Batteries.

Ummm...........those are pretty old batteries, particularly since you don't know what kind of care and use they have had. How do they test with a hydrometer?

Added on edit: I was watching a sailboat race while making this post, so it does not respond to your 8:32 post, but was in response to your 2:28 post. I am going to leave it anyway, just for general information.

quote:

So now it seems to be a matter of determining if it is one of 3 house batteries or the SOLID STATE Isolator.

...does the ISOLATOR have some sort of protection circuit in-it that would cause the constant voltage fluctuation; as-if it were continuously attempting to "protect" something???

Do you have a solid state isolator? Does it look like this?

If so, it probably has one lead from the alternator, one to the chassis batteries and one lead to the coach batteries. If this is so, remove the alternator lead and the chassis battery lead and connect them together. Wrap with tape. Fire it up and see what happens.

Now, if the connections are as I described, remove the lead to the house batteries and connect it to the alternator lead. Wrap with tape and fire it up.

If things are good, the isolator, which has been eliminated, is bad. If things are bad, look at the coach batteries and their wiring. Or anything connected to the coach batteries. Be sure all coach loads are disconnected while testing. Is there an emergency start relay? They can screw up, too. One drove me crazy once.

What kind of house batteries do you have?

If one of the house batteries is bad, and needs replacement, you will then have an unbalanced bank if all three are in parallel. In a parallel bank, batteries should be identical, or at least close. If you find one bad battery, I would replace all three. At their age, they have earned a place in Valhalla.

Isolator:

Solid State

Interstate Industries PowerLine Multiole Battery Isolator

160 AMP Maxium Alternator

...ans "YES", it has the terminals as you have described

Main -and- ALT. connected:

Tachometer still bounces up and down, but VOM connected to ALT. reads 13.25 volts with no fluctuation.

AUX. -and- ALT. connected:

Tachometer is stabile and "normal", VOM reads 14 volts with no flucuation.

I also purchased a Hydrometer today...

Chasis batteries are: Interstate 8D-XHD 1155CCA

Hydrometer reads and average of 1.275 in ALL 6 Cells [x2 batteries].

VOM reads 12.25 after charge.

[unsure of "AGE" of starter batteries.



The House Batteries are Exide Marine GEL batteries.

2 of these batteries are in parallel

The Chassis batteries are in parellel

The 3rd House GEL Battery is in series located between the 2 house batteries in parallel and the 2 chassis batteries in parallel.

This is how I bought the coach.

...not sure if this is a proper or "normal" sequence of "battery arrangement".

I think the 2 chassis batteries are good enough for now and will last till next spring ...it's the house batteries that are in question.

Should I scrap the 3rd "in-series" house battery and replace the 2 "in-parallel" house batteries with Golf Cart Batteries; completly eliminating the 3rd HOUSE BATTERY???