Alternator not charging.

bigskymt said:

You didn't mention the engine type. If its GM one of the top plug in small wires needs 12 VDC that comes from ignition to "excite" the alternator and turn it on. If the cruise fails also you might be missing some ignition output voltage.  JC Whitney used to sell a 100 amp GM altwernator with one post only, the 12 volt hot lead. Good Luck

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That's a good point, meaning the cruise control could be related to this problem, if that's where it gets its brown wire voltage. But if that's the case, it should still be easy to fix the alternator by a simple design change, and just leave the cruise control broken for now. And then the cruise control will no longer be able to cause an alternator failure.

If a single wire alternator can be mounted, that's a simple way to cheat and not fix it. ;D

But I hear the single wire alternators put out a lower voltage at idle (but why?) and this can cause minor problems in some rigs.

Don: all alternators have lower output at low idle. Police and taxi use a high output low rpm model and change pulley size. The single wire alternator is designed for marine use with no charge light and voltmeter.
Oh by the way, i noticed your 11-B, 4th Div a few posts back.
WELCOME HOME!!
terry, 11-B LRRP, Americal 70-71

bigskymt said:

Don: all alternators have lower output at low idle. Police and taxi use a high output low rpm model and change pulley size. The single wire alternator is designed for marine use with no charge light and voltmeter.
Oh by the way, i noticed your 11-B, 4th Div a few posts back.
WELCOME HOME!!
terry, 11-B LRRP, Americal 70-71

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But what I was saying is that the single wire alternators are even lower voltage output at the same low RPM when compared to other alternators, but I have no idea why. And yes, I know they are very common on boats. In fact, I think even my Volvo in/out  has one, but it has a voltmeter, and there's quite a swing from low to mid RPM's.

BTW, thanks, and same to you! Today is very close to the anniversary day of my DEROS from RVN in 1970.  SHORT!! My DEROS date was September 3, but I was a day or two late in returning. When I arrived in Vietnam on September 3, 1969, Ho Chi Minh had a heart attack (yes, he really did die on that date!). I wonder who told him Big Bad Don arrived.

BTW, IMO, those LRRP meals were the best food in Vietnam, so you didn't have it all that bad as a LRRP. ;D  Seriously, some of my favorite war stories are from the LRRPS.

Wow...Thank you for all your input and ideas!  You've all been a great help (and support!   )

I MAY have found the problem.....I think it is the batt. isolator as suggested.  I put my meter on one battery post and get approx 12 volts,  then on the other batt. post and get the same.  When I put my meter on the alt. post I get just over 13 volts.  But still showing a discharge on the voltage gage on my dash (dont have alt. warning light).  So the alt. appears to be sending a charge, but its not getting to the batteries.  The confusing thing is, when I run the alt. wire directly to the battery post, there is still no indication of charge.....But the volt gage is working because it does show a drop when I turn on lights, or ac etc.....

My isolator has four posts.  The center is for the alt. wire, and the top and bottom are for the chassis battery and house battery.  I assume the fourth (smaller post closer to the alt. post) is to supply power to excite the alt.

I had to order a new isolator, so it will be a few days before I know for sure.......Does this sound like a reasonable deduction to you all????  ???

PS....US ARMY, Second Signal Gp., Aviation Det.  Long Than North, Viet nam 68-69
Welcome home guys!!

You may want to re read reply #3!

Mark R. said:

You may want to re read reply #3!

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You may want to re-read the OP's second entry....

trailblazer50 said:

Wow...Thank you for all your input and ideas!  You've all been a great help (and support!   )
So the alt. appears to be sending a charge, but its not getting to the batteries.  The confusing thing is, when I run the alt. wire directly to the battery post, there is still no indication of charge

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Did you ever check the voltage on the brown wire? I don't think your alternator is fully turned on and the 13 volts you're seeing is just adding to the confusion. It's probably at a very low current and won't charge anything, which will explain why no charge when you connect direct. Besides, I would expect higher than 13 volts. 13 VDC there leaves almost nothing to charge the batteries with after the normal voltage drop  of the diodes in the isolator. My guess is your isolator is NOT your problem.

At least say something about the small brown wire on the plug of the alternator! Do you see it? Did you measure the voltage there in respect to the engine case? 

Diode Isolator: I had simillar problems with my previous Southwind and 454. I took the alternator output and connected direct to the diode output for the battery...........in other words the center of the diode isolater was alternator input, top connector was house batts, bottom was engine batt., I moved input from center connector to lower connector. I then got full alt. output to engine batt and the diodes still protected from house current draw.
As for the 13 vdc alt. output: I replaced alternators at least twice each year,(Pep boys lifetime warranty) I was trying to design a quick release velcro alt. mount system when I bought a pusher.
My altenator always checked out on the bench but no one ever checked it for max output. I finally found that a new alt. would produce about 14.5 vdc when new. After a couple months the max output would drop to about 13.5. I pretty sharp GM tech thought I was operating the alt. at "full field" or max output for too long and damaging the regulator. He insisted a $700.00 GM RV Alt would cure the problem. (NOT, A new M/H solved the problem).

I also found the dash voltmeter was not accurate, it would show a discharge when the actual volts at the batt was 13.5 vdc. I installed a separate voltmeter.
Good Luck & Welcome Home.

OR>>>I must have moved the batt. (bottom) wire to the center tap or the house batts would never have charged

I moved input from center connector to lower connector.

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Can somebody here refresh my memory and tell me what that fourth post is for that's on some diode isolators?

Don, if all four posts are the same size, the disconnect is for three batteries or some special marine applications.
If the forth post is smaller, it is an alternator feedback voltage signal.  I don't profess to know how, or which, alternators make use of this.

Lou (onaquest) said:

Don, if all four posts are the same size, the disconnect is for three batteries or some special marine applications.
If the forth post is smaller, it is an alternator feedback voltage signal.  I don't profess to know how, or which, alternators make use of this.

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Oh, I don't think I have ever even seen the three diode isolators, so it's the other type I was thinking about with the smaller post. I know I have seen those, I just could not remember what it was for. Thanks.

The older Dodge chassis used an external voltage regulator, which of course could use the external feedback terminal to compensate for the isolator's diode drop.  The design is probably a holdover from then.

Lou Schneider said:

The older Dodge chassis used an external voltage regulator, which of course could use the external feedback terminal to compensate for the isolator's diode drop.   The design is probably a holdover from then.

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Is that 4th post is an output to go to the regulator to raise the regulator's voltage when the battery voltage would have been otherwise lower? What does it come from inside  the diode isolator?

Must be some voltage divider or tap within the unit.  That's probably why they sometimes cost more.

Well, I dont know what I did, but after playing with the wiring going to the isolator, now the alternator is charging!!  Thats the good news, of course the bad news is I have no idea why it started working again..... ???  Im still going to replace the isolator as a precautionary measure....

Oh yeah.......The cruise control is working again also!!......go figure!

Sounds like a loose wire someplace.  I would track that down before I replaced the isolator.  There are no "iffy" parts in the isolator, they either work or they don't.

trailblazer50 said:

Well, I dont know what I did, but after playing with the wiring going to the isolator, now the alternator is charging!!  Thats the good news, of course the bad news is I have no idea why it started working again..... ???  Im still going to replace the isolator as a precautionary measure....

Oh yeah.......The cruise control is working again also!!......go figure!

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Didn't you say this was an intermittent problem to begin with? Does the cruise control now work? If so, that's an important hint and proves it's NOT the isolator.

Ever check the brown wire voltage?

DonTom said:

Didn't you say this was an intermittent problem to begin with? Does the cruise control now work? If so, that's an important hint and proves it's NOT the isolator.

Ever check the brown wire voltage?

-Don-  Burns, OR​

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Well the previous owner said this happened before so I guess that could be classified as "intermittent".....Yes,  the cruise control now works.....still gonna change out the isolator...as far as the brown wire goes....kind of a moot point now......everything is working....just gonna have to wait until it breaks again.....In the mean time,  I will check all the wireing for a bad connection and see what I can find...Surely dont want this happening on the road!

trailblazer50 said:

Well the previous owner said this happened before so I guess that could be classified as "intermittent".....Yes,  the cruise control now works.....still gonna change out the isolator...as far as the brown wire goes....kind of a moot point now......everything is working....just gonna have to wait until it breaks again.....In the mean time,  I will check all the wireing for a bad connection and see what I can find...Surely dont want this happening on the road

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Don't waste your money on a new isolator.  Your alternator must be getting its "brown wire" voltage from your cruise control circuit. so when it fails, your alternator goes dead too.

I split off Partymer's question into its own topic.  No need to hijack trailblazers alternator and isolator discussion. 

The new topic is here and titled Lost Power!
http://www.rvforum.net/SMF_forum/index.php/topic,25730.0.html