Low Voltage Readings

[FlatBlack77]

im going to try and make this as understandable as i can so bear with me.

ive had my 77 c10(350/350) since 2006 and its been on and off the road the whole time. this problem started years ago but wasnt as frequent as it is now untill this year when i started working on it again and getting it road ready.

the problem is about 75% of the time the voltmeter is reading around 12volts when running. its not just the gauge either ive checked that. i can see it in the headlights and even the sterio display that its just not getting the full 14 and some that it should.

i had this exact problem with my 79k10 when i got it. i replaced the alternator on the 79 and added/replaced all the grounds that i knew to
battery to rad support
battery to frame
battery to alternator bracket
pass side rear motor to firewall
driver side front motor to frame
dash to kick panel by the ebrake bracket

that fixed the 79 right up it runs and charges 100% with no fluctuation in voltage whatsoever


so I tried the same things on the 77. i got this alternator(exact same one i put on the 79)
http://shop.advanceautoparts.com/p/driveworks-alternator-remanufactured-61-amps-7127m-3/5030250-P?navigationPath=L1*14920|L2*14999

added/replaced all the same ground wires, brand new battery terminals, brand new battery, brand new starter, checked every connection, every wire, and everything else i could see and the problem still persists. sometimes ill start it up and the voltage is up where it should be at idle and while driving. other times its down around 12. the truck never has trouble running or starting. every connection looks good no wires are burned up or loose and i dont have anything electrical added other than the sterio, tachometer, and voltmeter. ive tried unhooking the tach and turning the radio off but it makes no difference.

i am not much good with electrical stuff and this is beyond me

sorry for the sloppy post thanks in advance everyone

[bd]

That was a very thorough writeup.

Did you replace both battery cables with 2 gauge copper wire?  Make sure the alternator belt is properly adjusted, then using a volt-ohmmeter on the 20 volts DC scale determine whether and where there is voltage loss...

• check the charging voltage at the battery and at the back of the alternator, preferably both while the dash voltmeter indicates 14.2 volts and while the gauge & lights are acting up.  Find out what the charging system is doing and post the voltage readings.
  
  
• while the dash voltmeter and lights are acting up, perform voltage drop tests (the maximum allowable voltage loss across any given wire at full load is between 0.3 and 0.5 volt).  Check voltage...
  
  
  • across each section of the alternator charge lead all the way back to the battery
    
    
  • from the alternator case to the battery negative post
    
    
  • from bare metal on the cab to the battery negative post
    
    
  • between the firewall junction block and the back of the voltmeter
    
    
• check the voltage across the dash voltmeter
  

Post the results.

[FlatBlack77]

thanks for the reply

i didnt use copper wire im pretty sure it was just regular wire. i didnt replace the positive cable(it looks fine at both ends) but i am going to replace it and see if that helps along with doing all the power checks you mentioned

the alternator belt is nice and tight i checked that first

I forgot to mention that the sterio in the truck is nothing more than a head unit and CD changer. no amps or subs.

and the problem can happen at idle or while driving it comes and goes as it wants. revving the engine makes no difference.

[bd]

First you need to verify that voltage is not being lost across poor connections or faulty wire.  The alternator may have a regulator causing problems or internal connection issues.

Not sure what you mean by "regular wire."  Most battery cables are stranded copper.  Some are aluminum.  Some are copper clad aluminum.  Copper wire is the best choice.  For greatest service life and performance with a near stock engine, 2-gauge copper cable is optimum.

The problem with battery cables is their propensity to wick electrolyte (sulfuric acid) along the strands of the cable and corrode beneath the insulation where it can't be observed.  Although cables can be effectively evaluated using voltage drop tests, many people simply replace them periodically for peace of mind.

[FlatBlack77]

i mean regular wire as in it doesnt look gold like copper it looks like steel or i guess aluminum lol

the alternator could be the problem but i kinda doubt it because i had the same problem with the last 2 alternators(used ones) and this third one brand new well remanned from advance auto.

thanks again you are being most helpful

[FlatBlack77]

ive been working 64 hours a week and havent had much time to work on the truck till this weekend.

i picked up a volt-ohmeter and did some checking.

i couldnt get the truck to stay at 14.2 volts which is nothing new. when testing the voltage at different spots i pretty much had the same readings as the dash gauge was reading(8-14 here and there)

battery was 13.9 volts at idle. so i used one of those battery testers that tests the batttery and charging system. it tested good. 14ish volts and the charging system is working. now i already knew that cause ive never had a dead battery and everything always works just not like it should 100% at 14.2v

i got some 2 gauge copper wire and a brass terminal to replace the main cable from battery to starter.

while i was down there i found a bad section of wire the "main hot wire"(im guessing thats what you would call it) the one that comes off the starter on the same terminal as the battery cable and goes up in the wire harness. it looked like it was pieced together about 3 times with electricl tape and it had a nasty corroded break in the rubber coating 3  inches from the starter

im not sure where exaclty that wire goes once in the harness(goes into the plastic wire sleeve that runs along the back of the motor) and i iddnt have the time to fallow it and replace the whole cable so i cut it and used a  crimp connector and double shrink tubed it aswell as put a good copper connector on the end for the terminal on the starter.

so i start the truck and it reads 12volts. i rev it up a little bit and it kicks up to 14.2 and stays there all the time other than when i come to a stop and the truck drops to idle(voltage doesnt always drop to 12 and never goes under 12 like it used to)

i drove alot today and it was the same all day. start the truck its at 12 rev it up it jumps to 14 and stays while driving.

replacing that entire wire is on my todo list but id like to know where it goes in the harness. im guessing it goes to the bolted terminal on the back of the alternator?

sorry for the cobbled together post  and the lack of a list of drop test readings

[bd]

...i got some 2 gauge copper wire and a brass terminal to replace the main cable from battery to starter.

while i was down there i found a bad section of wire the "main hot wire" (im guessing thats what you would call it) the one that comes off the starter on the same terminal as the battery cable and goes up in the wire harness. it looked like it was pieced together about 3 times with electrical tape and it had a nasty corroded break in the rubber coating 3  inches from the starter

im not sure where exaclty that wire goes once in the harness (goes into the plastic wire sleeve that runs along the back of the motor) and i didnt have the time to fallow it...so i cut it and used a crimp connector and double shrink tubed it as well as put a good copper connector on the end for the terminal on the starter.

...replacing that entire wire is on my todo list but id like to know where it goes in the harness....

The 1977 Wiring Manual is your friend.  You should become familiar with it.  That 'main hot wire' is the 10-gauge red wire (Circuit #2J) that connects to the junction block located above and behind the engine on the firewall.  It's the main feed to the electrical system.  It also carries charge current to the battery, since the alternator also connects to the firewall junction block through its own fusible link.  It sounds like you discovered the problem; or at least most of it.  That's the good news.

Now, the bad news.  That cobbled connection to the starter is supposed to run through a 6-inch length of 14-gauge fusible link.  The fusible link is the first and primary line of defense to protect the truck from electrical fire! 

So, now, your urgent focus is to repeat the repair you just made, replacing the last 6 inches of the 10-gauge red wire with 14-gauge fusible link.  I recommend that you crimp, solder and shrink seal the new link to both the 10-gauge wire and the 3/8" lug, rather than just crimp it together.  Soldering improves the electrical connection over the long run for improved service performance.