All of a sudden, the chasis battery is not charging. When going down the road, nothing, and in fact I had to switch to the house batteries to get home. The son-in-law and I spent a good bit of time trying to figure it out--he's an airplane mechanic/jet pilot--and concluded the alternator went bad. There's 6 volts coming out of it when the motor is running. We think we understand how the aux battery switch functions and bypassed it to see if there was anything at the battery and nothing showed. Any comments on how big an alternator we should get to replace it? When hooked up to shore power and the converter working, and aux battery switch off, the two coach batteries are charging but not the chasis battery. Switch the aux battery to on and all three are being charged.

Your alternator might just need a regulator. There is a D shaped hole on the back into which you stick a screwdriver when running. If the voltage rises, it needs a regulator.

On the connector on your alternator, the big red wire should always be hot. The smaller brown wire on my 84 is hot (but with a lower voltage) when the ignition is on. It needs either a lamp or a resistance of about 15 ohms in the circuit.

What does your brown wire read with the ignition on?

My 84 does not have "GEN" light, as it has a voltmeter, but I put in my own idiot light because I might have an idiot day. Those connectors are prone to corrosion and loss of tension, so a new one might be a good idea.

May I suggest that you install an ammeter to watch things? You can never have too many gages.


A big alternator is not much help when charging both chassis and coach batteries, as it reads both, not just the one you want to charge. Save your money.

[This message has been edited by bill h (edited October 23, 2005).]

I had similar problems on my P-30 chasis and finally located a loose connection at the starter solenoid where battery cable connects with alternator wire (and ignition). Seems the stud streaches with heat and eventually the nut lostens.

------------------
Gary & Edie
North Idaho
1988 28' P-30 454

quote:

Originally posted by Grizzlygiant:
I had similar problems on my P-30 chasis and finally located a loose connection at the starter solenoid where battery cable connects with alternator wire (and ignition). Seems the stud streaches with heat and eventually the nut lostens.

A star or Belleville lock washer should keep tension on the terminals. The split lock washers don't do as well, as they only provide pressure on one area instead of all the way around.

That said, all electric troubleshooting should begin with terminal inspection and cleaning.

Check your isolator -I had an issue with my SOB - the alternator did indeed go bad, but a few months later, I had charging problems. The terminal on the exciter was bad (the wire pulled loose).

Then, after that was fixed, I had a problem emerge again, with the gauge. That turned out to be an undersized fuse in the instrument panel circuit...the battery was charging, but the instruments lied...

quote:

May I suggest that you install an ammeter to watch things? You can never have too many gages.

One DOES NOT want an ammeter with an alternator - live with a voltmeter....

Can you expand on your remark, Rusty?

I would like to keep an eye on things and was planning to use and AMP meter, it sounds like I have a bit of learning to do.

Thanks:

Timothy

I probably overstated the case, but ammeters are generally used only in unusual cases (such as testing) with auto alternators.

Actually, most DC ammeters are voltmeters that run the current through a small shunt and measure the voltage drop across the shunt, reading out in equivalent amps.

The typical auto ammeter read -20/0/+20 amps. The alternator in your RV is likely rated at 170 amps!

Detroit switched from ammeters to voltmeters very early on after adopting alternators due to a possible fire hazard. There were instances where short circuits or opens routed the alternator's full power through the dash, overheated the ammeter wiring and burned the vehicle. (Remember that an alternator is a device that can produce a lot of current; a generator's output is somewhat limited by the resistance in the armature coils - OTOH, an alternator's stator produces the output, and has significantly less internal resistance).*

So, an ammeter is not actually prohibited with an alternator, but it requires careful forethought and heavyweight components.

Further diminishing the utility of an ammeter is that a voltmeter is much more useful in determining what's going on in the vehicle's electrical system....idiot lights, well, that's another matter...


* For those not familiar with a major difference between generators and alternators, a major one (other than the obvious AC rectified to DC in the alternator) is how the power is produced and extracted from the device.

A generator has field coils (magnets) in the housing. The armature rotates inside this electrical field, and electicity is produced. The power is transmitted through carbon brushes contacting a segmented commutator. The current transmitted is limited by, among other factors, the size of the wire in the armature and the capacity of the brushes. (BTW, if it weren't for the segments in the commutator, the generator would produce AC)

An alternator flips this arrangement. In the alternator, the magnetic field is produced on the "armature" (rotor), and this rotating field induces electricity in the winding on the case, the stator. Because the power generated is in a fixed coil, which is directly wired to the electical system, it can be far more robust that a generator of equivalent physical size. While the alternator has slip rings, they aren't equivalent to the commutator ina generator; they carry 12v power to the rotor to energize the magnetic field in the rotor.

I did find one website that has an excellent description of an alternator and how it works: http://www.autoshop101.com/trainmodules/alternator/alt101.html

[This message has been edited by Rusty (edited October 24, 2005).]

From the "DUH!" department...on humbojb's original post - thanks to timnlana's post which made me overcome my brain cramp.

humbojb, your symptoms match what would be expected if the excitation current in the rotor were reduced or eliminated. This could result from a broken wire to the rotor (or a loose connection) or worn slip rings (unlikely). The reduced (6V) output would be the result of residual magnetism in the rotor.

The usual check is to remove the connector, and check the IG terminal with the ignition turned on and with it off. You should get 12V on, OV off. (This is the brown wire bill h referred to)

I'd do that before replacing the alternator; any good auto electrical shop can make the test in a few minutes.

Anyway, I'd stick to the OEM-specified alternator capacity if you do have to replace. Remember that alternators are rated on amperage, and the vehicle's wiring is designed for that and no more. Installing a larger-capacity alternator just might blow the fusible links (and few are easy to get to...)

[This message has been edited by Rusty (edited October 24, 2005).]

Yes, ammeters are primarily used in unusual cases, such as testing. That is why I recommended one. You don't need an ammeter unless something goes wrong.

And the ammeter does not know or care whether the power is rectified electronically or mechanically.

The typical auto ammeter is -60/0/+60 amps. A quick check of Summit revealed a slew of 60's and one 100. No 20's.

Very few, if any, P30s have 165 amp alternators, or even half of that. A single belt will not drive much over 95 amps very well.

Good info as always - I dated myself - the ammeters I recall were -20/0/+20 - but then, I can't remember what I had for breakfast...

The one strange issue about the ammeter circuits causing fires is why they weren't fused...makes no sense.

I would think the best place for ammeters would be "downstream" of the isolator, so the charging current (or not) to both the house and chassis batteries could be monitored - maybe one ammeter with a selector switch. That would reveal if there were a bad battery if the charge rate were consistently high.

quote:

Originally posted by Rusty:

I would think the best place for ammeters would be "downstream" of the isolator, so the charging current (or not) to both the house and chassis batteries could be monitored - maybe one ammeter with a selector switch. That would reveal if there were a bad battery if the charge rate were consistently high.

My preference is to place the ammeter in the traditional location between the starter solenoid stud and the rest of the system. However, a copout is to put it in the alternator hot lead. This is easier and will suffice for most trouble shooting scenarios.

I suspect a switch in the charging line would add unnecessary complication. I am not a fan of isolators, preferring instead to use a single pole, double throw, center off momentary-on switch to control the coil of the solenoid. The momentary side of the switch is hooked to the coach battery, the on side is hooked to the ignition, and the third terminal is out to the coil of the solenoid.. This way one solenoid will- in the momentary position-parallel the batteries to allow the coach batteries to help a low chassis battery start the engine, and in the on position connect the alternator to the house battery when the ignition is on). The momentary position is for emergency start when the chassis batteries are low, the on position is dual for charging the coach batteries when driving, and the middle is off for when you don’t want to charge while driving. I like to start with the switch in the middle position, then switch to the dual position after everything is settled down. But with solar, I often don’t use the alternator to charge the coach batteries at all unless we are running the fridge on 12 volts while driving without sun. The existing parallel solenoid can be used for this, with only the control circuit modified. Diagram on request.

This eliminates the voltage drop of the isolator and allows switching the coach batts in and out of the charging circuit for troubleshooting, in addition to the normal paralleling for charging.

Hi all,

Now that we in an electrical frame of mind, I have a question about battery shut off switches. My 1981 Euro does not have a shut off for the coach batteries. I purchased a battery cut off switch with a large plastic key that is used to open or close the circut. Can I use this between the battery ground pole and the ground wire for the coach batteries to disconnect power when the motorhome is not in use?

Barry

quote:

Originally posted by Barry Rechtorovich:
Hi all,

Now that we in an electrical frame of mind, I have a question about battery shut off switches. My 1981 Euro does not have a shut off for the coach batteries. I purchased a battery cut off switch with a large plastic key that is used to open or close the circut. Can I use this between the battery ground pole and the ground wire for the coach batteries to disconnect power when the motorhome is not in use?

Barry

You can use the switch in either the ground or hot circuit, whichever is easier.

quote:

I suspect a switch in the charging line would add unnecessary complication. I am not a fan of isolators, preferring instead to use a single pole, double throw, center off momentary-on switch to control the coil of the solenoid.

My question was whether installing two shunts, with the readouts on one ammeter selected by a switch (rather than having two ammeters).

I, too, would rather have a manual selector but my SOB has an isolator, which has been trouble-free. The only issues were, on two separate occasions, wires pulled out of improperly crimped terminals...fortunately, neither shorted. It also has a separate momentary-on emergency start switch.

The thing I like about your idea as opposed to isolators is that one never knows what an isolator is doing.