Alternator Not Charging Chassis Battery

This is a Chevy chassis by the way but I'm sure the principles are the same. There are two frame sizes for the Chevy alternators - I just can't tell if they are interchangeable but I think that they are not based on some esoteric wording I saw.

Can anyone confirm if they are interchangeable so I can just get the bigger one next time?

On my motorhome's dash I have a voltmeter monitoring the coach battery bank and another voltmeter monitoring whatever is feeding the always hot stock cigarette lighter receptacle on the dash - I assume that the dash receptacle directly tracks the engine battery voltage because it's always hot. I also have an ammeter on the dash monitoring current into and out-of the coach battery bank (two 115 amp hour 12V deep cycle AGMs).

Both voltmeters read within around 0.01 to 0.04 volts of each other all the time (if the automatic coach/engine battery interconnect solenoid is actuating fully and if it's contacts are in good condition). Both meters read well above 14 volts when the engine is started whether we start down the road or merely sit with the engine idling. Both meters show a gradual decline in voltage as the engine heats up, ending up averaging in the mid-13 volt range as we travel in moderate outside ambient temperatures. Both meters decline lower than the mid-13 volt range as we travel in high outside ambient temperatures. I've seen readings as low as 12.9X volts on both meters with a hot engine while traveling in scorching summer temperatures. I've seen the ammeter spike as high as 70 amps with 50% discharged coach batteries when the engine is idling. As the generator runs long enough, or the engine idles long enough, or we travel for 3-4 hours, the ammeter declines to show zero current flow - indicating that the coach batteries are fully charged.

Either the Ford alternator regulator or some Winnebago-installed device seems to be acting exactly as it should in caring for our coach batteries under all conditions. Again, there is no voltage difference to speak of (including isolation diode voltage drops) between the two voltmeter readings, so whenever the ignition key is on the coach batteries and engine battery seem to be all hooked together in direct parallel with each other.

For what it's worth for information, the link below shows the atlernator performance curves for three different size alternators as used in Ford vans back in 2005. Note that the alternator speeds (rpms) shown, are alternator shaft speeds ... not engine speeds. For our chassis, the alternator shaft is over-driven by a ratio of 2.72:1 times faster than engine rpm. Take some typical alternator shaft speed of say, 2000 rpm, and then compare the current output capability of the 120 amp (page 7), 140 amp (page 7), and 130 amp (page 6) Ford alternators. The difference in output current capabilities - at whatever the batteries will accept at whatever voltage the voltage regulator sets in any particular set of conditions - in my opinion is enough to make it a good practice to always install as large an RV alternator as will fit and one can afford:
https://www.fleet.ford.com/truckbbas/non-html/2005/vs_pdf/05elect.pdf

A higher rated alternator sounds great always, but the fact is bigger ratings do not always translate to higher amperages at the RPM they are likely to see.

many examples of that here:

http://www.balmar.net/?page_id=15141

Also, the voltage regulator plays a bigger part in battery charging when idling or driving, than the numerical rating of the alternator. While a single battery depleted to 65% state of charge at 14.7v might accept 40 amps, at 13.7v it might only accept 15 amps, so a higher rated alternator will do precisely nothing when the vehicles voltage regulator chooses a 'safe and timid' voltage to aim for, thinking all it has to do is recharge or maintain a fully charged engine starting battery.

Even stepping this up for multiple depleted batteries, there is NO guarantee the 130 amp alternator will outperform a 105 amp alternator. Not when 13.7 is the voltage chosen by the regulator, and not when the amperage curve shows the max rpms are only available at the top end of the rpms where the alternator is likely to not spend any time.

My 120 amp alternator, well it can accomplish that at 2400rpm. My cruising rpms rarely exceed 1900. but I can choose voltage, and have an Ammeter on my dashboard, and can easily see what the alternator is returning to batteries. The voltage chosen plays a HUGE part. I can spin a dial at any rpm and repeat this time and again.

The higher rated alternator should in theory handle the heat it generates better, but theory and reality ore often well opposed from each other, especially if one chooses to test theory with measurement tools and influencing the variables, rather than rely on internet folklore.

I considered a larger amperage model but the 105 was the only one available locally in the right frame. I do see they are available online at higher amps if this one goes. I saw references to problems with the higher amp models.

Hopefully with this one and the shorter battery cable runs it will be fine. The only things that will be running are the fridge (usually propane anyway) and the batts charging.

It's too late now, but did you install a larger capacity alternator than the original?

It's always a good idea to have plenty of atlernator capacity in any RV that relies on the engine alternator to help with charging of the coach batteries ... just in case. As a reference point, our Itasca Class C E450 motorhome's alternator is rated at 130 amps and many late model Class C motorhomes come stock with even larger capacity alternators.

Nothing to do with being 20 years old?

Anyway glad to hear all is good again :)

To take this full circle - the problem was....

And the winner is!

The alternator!

I pulled out the alternator, took it to the local O'Reilly to confirm on their machine - it was smoked.

So I found a replacement and installed it. Voltage is good! Let's keep it that way.

I think the issue was caused by a combination of prior owner running house batts low (gigantic Trojans) and the system being overloaded or something.

Anyhow, fun project for a Sunday. Thanks for everyone's help.

If you are not getting voltage at the output of the alternator and you have an external regulator you need to see what is on the field connector. If it has an internal regulator the alternator is kaput. It should be putting out plenty of voltage.

Another setup uses an external sense wire that has a fuse. You should be able to look up the alternator replacement based on the chassis.

May auto part stores will check your alternator for free.

Yes ..The truck/engine battery is the chassis battery

Most often if and when a diode based isolate fails
It fails going to the house/aux battery
Because this is the one that gets cycled and pulled down to dead
Then the engine is started and revved up to charge the house battery
Putting a high amp strain on that diode

Just saw your post Mr. Wizard. I will check.