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Discussion Starter #1
I’m helping a friend with an electrical problem on his “84 Chey van / motor home. At first looks like it’s not charging, but all the alternator tests prove that there is not an open circuit and when I isolate the system, it will get up to 13.8VDC. But here is where it starts to get weird, when we increase the RPM (from idle to 1200-1500 RPM) the voltage decreases and just starts draining the battery. As I stated, it’s a motor home, w/ a dual battery system (has the isolator) and when every thing is disconnected from battery #2, the out put of the isolator is almost 14V (13.8). But when we put any load / drain on the system (lights, wipers, heater, etc.) it just starts to drain the battery. Other important facts: 2 new batteries, new alternator (72 amp), and the engine was recently rebuilt, and reinstalled by a reputable shop, and they claim that they didn’t do any thing to the vehicle.
Long story short, why is it acting backwards?
Mark (A.K.A. Z28 Mark)
 

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First thing I would do is to remove the BAT wire from the alt and add an inline 100-0-100 ammeter. 60-0-60 amp would work to the alt BAT terminal.

Now you can see(read) the output of alt in amps as well as read volts with DVM.

I would suspect some thing wrong with the isolator. I personally never like them. I subbed a Ford solenoid in place to charge the 2nd battery. Hook the control wire to IGN so it gets charged when engine running.

Back to the problem, if volts go down, do the amps go up? If it does, then alt is doing its job. It sensing low voltage and tries to make up for it, but, too much load. Does alt get really hot to the touch? Signs of it working.

You might isolate the camper 12 volt wiring from the chassis wiring to help troubleshoot. RV manufacturers don't use the same wiring as chassis builders. Start to pull fuses, checking meters after each one.

My money is on the isolator.....let me know what you find.



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Everett "OBJECTS IN THE MIRROR APPEAR QUICKLY UPON RAPID DECELERATION"
 

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Another trick is to check it on AC volts. If there is AC present, its discharging as fast as it charging and you have a bad diode

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69 convertible
LT1 4L60E
 
Z

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Discussion Starter #4
The isolator was my first thought, but I don’t think so now. I really think that the engine shop hooked up1 of the 2 wires labeled “terminal 1 & 2” incorrectly not allowing the alternator to get the needed voltage to “excite” it to allow it to charge. Between term. 1 and 2 there is about 1volt of difference and this is incorrect and this also just feeds the alternator so the alternator is just “turned on” all the time, draining the battery.
I will try the AC setting that Red Camaro talked about and see what readings I get.
Any more help is always Welcome! Please keep it coming.
Mark (A.K.A. Z28 Mark)
 

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I had a similar problem on my 77 camaro. The problem actually turned out to be a glazed fan belt that was slipping, but not squealing when the rmps increase . A simple solution but I change the alternator twice before I noticed the condition of the belt ...
 

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Okay, if that's the case of swapped wires, the alt light goes to term #1 and ign goes to term #2(?).

I understand the alt has two prongs in-line with each other. This signifies an internal reg alt. The plastic plug is keyed, however, the Q/D's (contacts) inside can be swapped.

I'd try Red's idea, then sdt's idea. But, 72 amp alt for a RV? Really light! S/B 90-120 amp, take a look at the load involved, two batteries, especially if the aux bat has been used for a day, lights, water pump, & frig.

Anyway, keep us posted.......Everett
 
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Discussion Starter #7
I should have been a little clearer on my last post. When I say terminal 1 and 2 swapped, I mean at the other ends of the wire, not at the alternator. Like down at the starter or the spot where terminal 2 hooks up to (I think it’s the switch [key]). You never know what that engine shop could have done.
Well my friend is out duck hunting this weekend, and I will not get a chance to work on the M/H until Monday. But I will keep everybody posted.
Again, please keep the ideas coming, I can use all the help I can get!
Mark (A.K.A. Z28 Mark)
 

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A 70 amp alt. is not enough fot a two bat. system.Do not use a Ford solenoid in place of the isolator.Ford solenoids are olny on momenterly,and they can not be on all the time.The will burn up!There is a solenoid that is made for M/H blue streak makes them. I just can remember the part #.If i were you I would install a 120 amp.alt with a smaller pulley for low RPMS. Mr
 

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check the connections at the starter, clean them and check for broken terminal ends. The charging system goes thru the starting system, get wiring diagram to help tracing wires. feel your pain
 
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Discussion Starter #10
Well no crossed wires found, but we did check the voltage w/ the meter on AC…. Some 25 volts were present. Red Camaro, looks like you may be right. Also it turns out that one of his neighbors works for the local Honda dealer and says that he will bring home some Snap-On tool that diagnose the complete starter/ alternator / battery system. As of now I just think the alternator has 1 bad diode (shorted) (this is the 2nd alternator this way, from Kragens). I think this explains a lot of why it acts “backwards”.
Mark (A.K.A. Z28 Mark)
 

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Yes, if AC starts to appear with meter, you can bet one or more diodes broke.

I would get another alt from Kragen and have them bench test it under full load for 2 minutes. Start with charged batteries in the RV.

I'd still look into getting a larger alt, 70 amp for an RV with two batteries isn't enough.

Everett
 
Z

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Well the Snap-On “Verri-Mat” tool says that the alternator IS bad. So he plans to pull the thing and take it back to Kragens some time this week. Everett, even the other neighbor (with the Snap-On tool) now also says that a 72 amp alternator is a little low also. But my friend claims that he tried a 90 amp and it would not bolt up to the brackets, so he went back and picked up the 72 amp model that fit. All I want is this problem to be solved.
Mark (A.K.A. Z28 Mark)
 

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Yes Mark, I can understand.

Good work, man! (Patting you on the back)

Everett
 
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