Dual batteries

[Blazin]

Well just finished installing a second battery in my 86 K30. Ran new ground wires for the primary battery too. Also installed my 140 amp alternator. Batteries are both almost new, so they took a load test fine. Did a crank test, normal. Did a charge test, said it was charging good. My problem is that with everything on, heater, head lights, strobes, reverse lights, the dash gage reads about 10 or 11 volts at an idle. When I raise or angle the plow it drops to 7 or 8 volts. It takes a few seconds to recover. Rev the motor to about 1500 RPMs and it reads 14 plus volts. With nothing on at and idle it reads 14 plus volts. I have clean all connections, grounds, replaced the alternator plug, run a ground directly from the brand new plow solenoid. Any ideas??

[808SS]

Im no pro but from what I went threw.
Are both batterys the same? What kind of batterys you using?
Im thinking the 140 Alt is to small, I was running a 200 Alt w/two yellow tops
What size cables you using for ground and power?

[hilton850]

Alternators are usually rated at peak output...which is spinning a lot faster than idle.  I think a 140 alternator should be fine...I only have 60 amps out of the one on my truck.  Most alternators charge to 13.5-14.5 volts unless the electrical load is drawing more power than the alt can make.  It sounds to me like you have everything hooked up right and working, but the alternator just can't keep up with the electrical load at idle with everything on.  Sounds odd to me because I've never had that problem with my small alt. but I have less load since my plow is hydraulic.  The 2000 gmc we have at work doesn't like it when we run the plow and heater and lights all at once...the volt gage is all over the place in that thing (the heater slows down and the lights dim every time you lift the plow) so I guess it sounds kinda normal to me (but that doesn't make it right).  Sounds to me like you can do a couple things...get a bigger alternator or use a smaller pulley on the one you have to spin it faster and get it charging better at lower engine RPMs.  Good luck!

[VileZambonie]

Where did you get the alternator and what kind of alternator is it? Did you verify the accuracy of the gauge? Is the field circuit in series with the gauge?

[Blazin]

The load test, crank test, charge test all say normal. But the gage reflects the light dim, heater slow down etc. Been having a problem with the plow not going left when gage is low / when truck is hot. Hold it left, and would not work until all the lights and heater where shut off. It uses two coils to go left where as only one to raise, and one to go right. More power draw. Plowed around the yard last night, that did not seem to happen. Had the alt rebuilt by a reputable shop, have used them before, know others that have too.

[choptop]

What rpm are you running at idle? The reason I ask, is alternators are similar to the generators, and if a certain rpm on the alternator isnt met, then the regualtor cannot rectify the voltage to an adequate amount. If the rpm is correct, which I can imagine it is, then the internal regualtor isnt  reguating the voltage correctly.
  Had a co worker that had alot of Hella lights on his truck, and if they were on at idle, it would pull the voltage down. Hey found a smaller diameter pulley, and put it on. This made the alternator spin faster at idle than normally which was fine because even at hi way speeds, the alternator was no where near max rpm. Just a thought.

[Jay2D2]

Is it a new or old alternator... because contrary to what a lot of ppl believe an alternators output will decrease over time.

Also verify that it is the correct pully on it.

I would bring the alternator to a reputable rebuilder and have him bench test it.  After that test the system with an an inductive type ammeter(type that clips over the wire) and see what you have with the engine idling and no electrical load then do the same test with everything on, compare your readings

[Blazin]

Had the alt rebuilt by a reputable shop, have used them before, know others that have too.

Idle is between around 900 RPMs Set it, and the timming with a vacume gage and, a timming light

[choptop]

RPM is good. I think the regulator needs changing. I know you said you have used them before, but its probably a part issue, not an ability issue. Tell them what it is doing, they may be able to put n a different regualtor that will work. Most industrial alternators are regulated to work well at less than 900 rpms because that is where they are run. I have have many new electronic parts fail right out of the box. This doesnt sound like a failure, just on the low side of pass/fail. If it runs good at 1500rpm, and they dont know what to do, look into getting a smaller pulley. This will speed up the alternator, and get it in the proper charge zone.

[VileZambonie]

Take the alternator back to them. Is it a CS or SI alternator?

[dunedigger]

Did you put in a battery isolator?  If you don't the alternator will try and charge both at the same time, which is a lot of work for it.  The other problem is batteries in parralell will only take the charge of the weaker battery without an isolator.  So your batts will always be as dischared as they can be.  With the iso, the alt will jump back and fourth charging to keep them individually topped off.

[Blazin]

It is better with the second battery. Doesn't seem to be as low on the truck gage now that it is hooked up. I plan on an isolator but have not yet. I called him he said bring it back. Want to take the truck not just the alt. only problem is he is 40 plus minutes away in my Camry, or Camaro. Which means over an hour in a my dump truck ! Plow on, and rough state road the whole way.

[VileZambonie]

You don't need an isolator unless you are deep cycling the batteries.

[808SS]

I didnt use a isolator, but using the same type of battery will help too

[Blazin]

Its an SI alternator.