new voltmeter and just noticed an odd occurance

[mwood1985]

what would cause a alternator to run 12.6 volts at idle and jump up to about 13.8 above 1500rpm's? I just pulled the factory cluster and went all aftermarket. the old voltmeter didn't work and now ive noticed the voltmeter reads like above and I verified it with a multimeter at the battery and the back of the voltmeter itself. charges a bit low at 13.8 but still within tolerance of the system as long as you rev it up and I guess excite it

 

[Stomis]

Sounds like typical single wire alt behavior.

 

[mwood1985]

its wired up with the factory 3 wire style at the moment. kinda wondering if its not wired wrong somehow. my boss did it yesterday while I was messin with the bezel. I figured it was single wire the way it acted but it drops to about 11.3 ish when the lights and wipers are on and its idling. normal or what?

 

[ktmoutfront]

All of mine put out 13.8 above idle. Normal.

 

[mwood1985]

we changed bodies from a 86 to a 88 after I crushed the roof in. the engine and all is the same. wired the alternator up identical on both trucks. same alternator just new wiring. the same system in the 86 put out a constant 14.6 no matter what. the new truck with the old trucks engine with zero change other than what cab is there and the charging system is acting totally different? its kinda confusing me

 

[ktmoutfront]

That is more info and is weird. Grounding issues?

 

[diesel4me]

Perhaps there is no voltage getting to the #1 wire on the plug ,it needs a 12V source thats only hot with the key on...its possible for the altenator to still kick in and start charging by itself without any voltage present at that terminal,due to residual magnetism in the stator,but you'll have to rev it up over 1500 rpm or so to get it to start charging..that might be the surge you see...

Once it charges it should keep charging at a steady rate though..there is a possibilty the #2 wire isn't "sensing" battery voltage properly or one or more diodes out of the three in the trio are defective,that will reduce output and make for unsteady output,or the regulator might be going south....

 

[mwood1985]

im gonna have to do some investigating. the post wire is running to the terminal on the firewall and the plug im not sure as of yet without goingto see where my boss ran the plug wires. where SHOULD everything be wired to? what good source of 12 volt switchable does everyone prefer?

 

[diesel4me]

GM wired the #1 altenator wire to the fuse box ,to a keyed hot lead...usually its a tan,brown or white wire coming out of the fuse box on the firewall side of it......any keyed 12V source would work OK though...be aware this wire needs some resistance though!..

Trucks with idiot lamps instead of gauges had the current in that wire running thru the "ALT" lamp in the dash,the bulb provides the nessasary resistance in the circuit to prevent the altenator from putting out full amps all the time..on trucks with an amp gauge the gauge has a resistor or shunt wired into it to do the same thing..

The #2 wire needs to be hooked to a 12V source somewhere between the battery and altenator--many guys just jump the #2 wire to the stud on the back of the altenator--it works,but may not allow it to sense the full range of voltage to allow the regulator to boost or reduce output acorrdingly..good grounds are important too,it wont hurt to add a wire right from the back of the altenator case to the negative battery terminal...doing that solved some weird charging problems and "flickering light" woes on one of my older trucks..

 

[Avery4jc]

many guys just jump the #2 wire to the stud on the back of the altenator--it works,but may not allow it to sense the full range of voltage to allow the regulator to boost or reduce output acorrdingly.

Mine is wired like this on my wheeler and it works fine for me. It runs just above 14 volts almost 100% of the time regardless of rpm.

I know it's not "right" but it works for me.