Voltmeter question

[drohweder]

Finally! I got the dual battery wired up and its working great. Only thing is my voltmeter that was always pegged at 14 before now has a ton of flucuants to it. It will drop clear down to <10 with the lights on. When I tested the batteries they all were fine. >12v at rest on each. Alt is putting our 15.6V and I lose 0.7v through the isolator. So my net voltage at the + cable is 14.5-14.9.

The only change I made in the system was to the alternator (3-wire delco). I took the sensing wire off the rear post (which would have read voltage to the isolator-in post. I relocated it to the + post on my main batter. Now its sensing actual voltage at the batter--taking so the drop across the isolator doesn't cause poor charging.

Could this be the cause of my voltmeter dancing all over the place? Where does a voltmeter normally get it read from (fuse box/battery)???? The third wire off the alt is the "warining light" wire that I think is inactive as its got a fusible link with an in line fuse but no fuse present (shouldn't work right??)?

Anyway, its just a curious thing cuase it all worked fine before. Let me know what you think might be going on. Thanks

 

[drohweder]

Help?

 

[cruiser_guy]

Dan,

According to my FSM the FJ60's take voltage straight off the harness.

 

[drohweder]

I'm going to trace the wire down today. My rig has all aftermarked gauges so who knows where its getting is feed. Time to start eradicating the electrical gremlins from this thing!

 

[hammerhead]

Dan - connect your voltage sensing wire to a main junction where the power is distributed to all systems. Your V-meter is reading system voltage while the Alternator is reading batt voltage thus the dicrepancy...

 

[drohweder]

So where is the main junction box on a 40? Is it where the power is fed into the fuse box?? Thanks.

 

[Pin_Head]

It shouldn't matter where you "sense" the voltage because the wiring resistance is so low that there should be no siginficant voltage drop anywhere in the circuit. Assuming that your volt meter isn't lying to you, the fact that it sees 10 volts tells you that there is a high resistance (bad, corroded) connection somewhere between the voltage gauge and the battery. If the voltage is jumping around the bad connection may be intermitttent, changing witth bumps and vibration.

 

[drohweder]

Problem solved.

So I was testing the voltage on my batteries and I kept on hearing a little snap. At first I just thought it was a leaky header but it was spaced too far apat for that. It turned out to be the 10amp circuit breatker with automatic reset just going on and off. Replaced it with a 25 amp fuse and the voltmeter problem was solved. In many of the dual battery diagrams that I referenced they put a 6-10amp circuit breaker in between the isolator and battery on the charge cable. Anyway, problem solved, Also found the nasty short in my turn signal wires and got the truck %100 street legal.

According to the stock wiring diagram that I have, the voltmeter reads off the alternator. Not sure if that is the case on my truck of not. Thanks for your help. Time to start a new thead about my engine oil.

Dan

 

[toddslater]

I am not so sure you have solved your problem or you have created the potential for one hell of a melt down. Hopefully it will be the solved.

 

[drohweder]

I can't fry a battery unless I am consistently above 16v to the batteries. With the drop through the isolator I am only putting 14.4-14.7v to the batteries. Anyone chime in if you think this is a dangerous set-up. Its basically stock with a dual battery switched with output to the starter. Alt charge wires to the isolator and then to each battery. I took the alt sensor wire and placed it on the main batter so that the alternator tries to keep the batter above 14v instead of reading 14 at the isolator and then my batteries only get 13.25v. Makes sens to me but I am new to this. I plan to rebuild the entire wiring harness over the winter. At that point I am going to add lots of goodies like new fuse boxes and clean up the wiring.

 

[hammerhead]

It shouldn't matter where you "sense" the voltage because the wiring resistance is so low that there should be no siginficant voltage drop anywhere in the circuit.

If you want more info on proper wiring try some seriously fun reading ...

http://www.madelectrical.com/

 

[drohweder]

If you want more info on proper wiring try some seriously fun reading ...

http://www.madelectrical.com/

That site is what I reference for the most part. I have thier entire site printed out and highlighted. I did avoid all the major pitfalls they mentioned. I'm just trying to optimize the 33 year old system. I think I am done with it for now. Its working great. When I tear the harness out this winter I plan to make sure everything is clean, add a hidden ignition kill switch, get all the lights on the dash properly working and get it prepped for a stereo. I also will do the headlight relay job at that time. Great site! Really I've found electrical is some of the cheapest modsyou can do--its just so intimidating for a newbie like myself.