Alternator not charging.

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trailblazer50

Member
Joined
Aug 24, 2009
Posts
13
Location
Brownsville, Tn
I recently purchased a 1988 Triple E Regency Class A motor coach.  I knew when I bought it she had a charging problem, but being a shade tree mechanic,  I wasnt concerned.  Well now Im about to pull whats left of the hair on my head out!!  I replaced the chassis battery and both house batteries and the altenator.  Still no charge.  The previous owner told me that it was some how conected to the cruise control,  (which is not working either) but I have yet to figure that one out.  He said that previously a fuse would blow causing the cruise control to stop working and the charging system to also stop working.  I checked EVERY single fuse and none are blown.  The heavy gauge "Batt" wire from the alt. goes to some blue finned box on the firewall.  I dont know what it does though.  This is my first RV so I am treading on new ground for me.....any input would greatly be appreciated!

PS....the new alt. was tested before I installed it also.
 
The blue finned thing is your battery isolator.  It is there to allow charging of all batteries, but not allow the house batteries to drain the chassis battery when charging is not present.  Just two big, high current, diodes in there.

 
If one of the diodes in your isolator are blown, that would stop your charging current from reaching the battery.

Try moving the starting battery lead off of it's post and connecting it directly to the alternator (put it on the alternator post on the isolator).  If you get charging voltage to the battery you've found the problem.
 
trailblazer50 said:
The heavy gauge "Batt" wire from the alt. goes to some blue finned box on the firewall.  I dont know what it does though.  This is my first RV so I am treading on new ground for me.....any input would greatly be appreciated!

Are there three posts on that blue fined thingie?  Measure the DC voltages on all three posts in respect to the chassis ground.  The center is from your alternator. The other two go to one battery each. More than likely, you will find your problem right there. There should only be a small voltage drop (less than a volt) between the center post and either of the other two when your engine is running.

You can buy those diode isolators in the battery section of almost every auto parts store. IIRC, they cost about $25.00 or so.

If that blue fined thingie is a diode isolator, if it's defective, it can prevent charging to either or both batteries. Usually just one battery but it certainly is possible for both diodes (in that isolator) to blow out.

-Don- Reno, NV​
 
"The previous owner told me that it was some how conected to the cruise control,  (which is not working either) but I have yet to figure that one out.  He said that previously a fuse would blow causing the cruise control to stop working and the charging system to also stop working."

Perhaps when the battery gets too discharged, the cruise control doesn't get enough voltage to work normally.

Other than that, I don't buy that the two are related, so for now, forget about the cruise control. 

-Don- Reno, NV ​
                                                       
 
Lou Schneider said:
If one of the diodes in your isolator are blown, that would stop your charging current from reaching the battery.

Try moving the starting battery lead off of it's post and connecting it directly to the alternator (put it on the alternator post on the isolator).  If you get charging voltage to the battery you've found the problem.

Not sure what you mean.....I did disconnect the Main battery wire of the altenator from the "blue box" and ran it directly to the +side of the battery...no help...
 
Lou (onaquest) said:
The blue finned thing is your battery isolator.  It is there to allow charging of all batteries, but not allow the house batteries to drain the chassis battery when charging is not present.  Just two big, high current, diodes in there.

How do I access these Diodes to change or test them?  It appears to be a sealed unit.  How expensive is it to just replace this Isolator and just be done  with it all?????  (can u tell Im getting frustrated?????..lol...)
 
Mark R. said:
Throw that blue junk out, connect the power wire from alt, to batt, it will work.

Already tried that....no help.....I ran a jumper from the altenator batt wire directly to the + side of the battery and still no charge....and when I disconnected the battery from the main + batt cable the engine just shut down......
 
DonTom said:
Are there three posts on that blue fined thingie?  Measure the DC voltages on all three posts in respect to the chassis ground.  The center is from your alternator. The other two go to one battery each. More than likely, you will find your problem right there. There should only be a small voltage drop (less than a volt) between the center post and either of the other two when your engine is running.

You can buy those diode isolators in the battery section of almost every auto parts store. IIRC, they cost about $25.00 or so.

If that blue fined thingie is a diode isolator, if it's defective, it can prevent charging to either or both batteries. Usually just one battery but it certainly is possible for both diodes (in that isolator) to blow out.

-Don- Reno, NV​

I have not tried  that yet...Will do so tomorrow.....
 
What type of engine does that thing have? What make? I am trying to figure out what can be involved other than the alternator.

Does it use a serpentine belt on the alternator?  Or a separate belt for the alternator?

If it has a separate belt, make sure it is VERY tight at least until the trouble is found. I have seen alternator belts do weird things with no systems at all other than no charge. I had a Chevy van where the battery voltage would drop as the RPM's were raised. It was tricky, but it was just the belt causing the problem with the help of some tranny fluid  that leaked onto  the belt. If the bad belt was put on extra tight (as tight as I could get it) the problem cleared for 30 miles or so. The slightest amount of slack prevented the charge. A new belt as well as fixing a hole in the tanny dipstick that ran right above the belt is what fixed it like new.

The best way to test the diode isolator is with a DC voltmeter, with the engine running, as I mentioned before. But if you already bypassed the isolator  and it still would not charge, that's not your problem.

How many wires go to the alternator? If GM, is there a brown wire?

-Don-​
 
If you already worked on the positive side with no success, then try now the negative side. That comment you made about engine shutting down when you remove the cable made me remember a similar issue I had with a false ground. make sure all of the grounds are tight.
 
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
 
Now Im scratching my head. I would first put a test light, not meter, between batt. + on rear of alternator and alternator case. A test light will give you a true "load" reading, not ghost reading like some meters. You obviously have a ground at the engine block or the distributor would not produce voltage. Next I would check the 12 volts at the top plug of the alternator. If thats Ok the last check would be the regulator. On the back of the alternator opposite side of the batt. plug and diode trio (you can look in from the rear and see the heat sinks) you will see a half circle  hole in the case.(looks like a "D") Inside the hole you will see a small tab, this is the case of the regulator. If you touch the tab with a small screwdriver and ground it to the case and it starts to charge the regulator is bad. When the alternator starts charging you will notice a slight drop in engine rpm and slight "humming" in alternator.
If it works on the bench and you duplicate the input voltage in the M/H it has to work.

 
trailblazer50 said:
Sorry, I failed to mention that its a 454 chevy, with a 3 wire alt.  There is power going to the alt. to excite the altenator.

Is there an alternator charging lamp involved or a voltmeter? If the charge indication lamp burns out, you won't charge. The lamp goes to  the brown wire I mentioned, in series with your ignition keyed 12 volts.  And even if there's a voltmeter, that brown wire turns the alternator "on" and needs voltage from somewhere. I would expect the brown wire voltage not being there and this will explain why the alternator works in the shop but not in your vehicle.

If you have a charge lamp indicator, the GM design is much better than any volt or current meter. It's a great tool for troubleshooting. In fact, in my old RV I added the lamp and had both, the voltmeter as well as the lamp.

If you have the charge indicator lamp, mention what that lamp indicates under all conditions, such as  ignition on with engine not running and also running. And even when key is out.

If there's a voltmeter, but no voltage on your brown wire, and you want to see it charge, just go from batt A+, through a small 12 volt indicator lamp to the brown wire to turn the alternator on. And then when you see that's the problem, we can troubleshoot your brown wire and fix it correctly.

Use a small 12 volt indicator lamp. The rating is not critical for this test. Put it in series with your 12 Volt battery positive to your brown wire to see if it then charges.

If you have the voltmeter, you might decide to leave the lamp in permanently mounted, like I did. If you experiment with different lamps, you can find one that will work perfectly which will be completely out when the engine is running,  and on when the engine is not running or if the alternator belt breaks.  You will get an instant failure indication, unlike a voltmeter. 

For now, you  want to check if you have voltage on that small brown wire, which will explain fully why it works in the shop but not in your RV. Ignition must be "on" for this test, but no need to start the engine.

-Don- Reno, NV​
 
DonTom said:
Is there an alternator charging lamp involved or a voltmeter? If the charge indication lamp burns out, you won't charge. The lamp goes to  the brown wire I mentioned, in series with your ignition keyed 12 volts.  And even if there's a voltmeter, that brown wire turns the alternator "on" and needs voltage from somewhere. I would expect the brown wire voltage not being there and this will explain why the alternator works in the shop but not in your vehicle.

If you have a charge lamp indicator, the GM design is much better than any volt or current meter. It's a great tool for troubleshooting. In fact, in my old RV I added the lamp and had both, the voltmeter as well as the lamp.

If you have the charge indicator lamp, mention what that lamp indicates under all conditions, such as  ignition on with engine not running and also running. And even when key is out.

If there's a voltmeter, but no voltage on your brown wire, and you want to see it charge, just go from batt A+, through a small 12 volt indicator lamp to the brown wire to turn the alternator on. And then when you see that's the problem, we can troubleshoot your brown wire and fix it correctly.

Use a small 12 volt indicator lamp. The rating is not critical for this test. Put it in series with your 12 Volt battery positive to your brown wire to see if it then charges.

If you have the voltmeter, you might decide to leave the lamp in permanently mounted, like I did. If you experiment with different lamps, you can find one that will work perfectly which will be completely out when the engine is running,  and on when the engine is not running or if the alternator belt breaks.   You will get an instant failure indication, unlike a voltmeter.  

For now, you  want to check if you have voltage on that small brown wire, which will explain fully why it works in the shop but not in your RV. Ignition must be "on" for this test, but no need to start the engine.

-Don- Reno, NV​

7 Ohms resistance is what you are looking in this light for If I recall. You should be able to find an Indicator light that is the right one pretty easy.

Nice explination by the way Don

wayne
 
zukIzzy said:
7 Ohms resistance is what you are looking in this light for If I recall. You should be able to find an Indicator light that is the right one pretty easy.

Thanks, I could not remember what the lamp number I used in my old RV. I didn't know seven ohms was what I was looking for when I added mine. I did it by experimentation and found the perfect lamp which worked just like a stock GM charge indicator lamp.

GM seems to have the only decent design for a charge indication lamp. My 1997 Chrysler Sebring is the opposite. That idiot light will only show when the battery is so low that the car won't start. It works by voltage only. In fact, there's no indication for hours if the alternator belt breaks. But the lamp in the GM tells you all you need to know, as well as being a great troubleshooting tool. And it's conclusive, as even if the lamp burns out (very unlikely, in fact, I have never seen it happen, because it's not  lit often)  you know you're not charging, because the lamp won't light when the ignition is on, engine not running, showing a problem. 

With GM, I would rather have the lamp than the voltmeter. With any other make, I would take the voltmeter over the idiot light. I won't call the GM alternator lamp an "idiot" light.

-Don- Reno, NV​

 

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