Voltage low at low idle how low is too low at 800RPM ??

[MarsMonster]

I noticed that my amp meter reads on the negative side when the engine is idleing at 800RMP with the head lights on AND the heater turned on. I measered the current at the battery and it was only about 12.7 volts. The battery itself puts out 12.5 volts, with the engine off.

As a test I disconnected the ground from the battery when the engine was running, the engine ran for a while ( about 30 seconds ) then stopped.

So my question is this, based on my limited test do you believe that I have a charging issue?

At fast idle (2,000 RMP) the voltage at the battery is 13.6 volts even with the lights and heater on.

I believe that the battery may be about 5 years old or older.

Thanks again,
Mars

[wideblock]

on my crew cab, i experience the same thing you do. mine idles at 500rpm in gear. so when i come to a stop with anything electric on the amp gauge shows a drain, the more i have on, the worse it gets. volt gauge reads right at 12 at idle regaurdless of whats on. but, upon any raise in idle above 1000rpm, the amp gauge goes to the charge side and levels back out rather quickly and the volt gauge reads a shade over 14v. if your only reading 13.6 worth of charge, id say your battery is getting weak. 5yrs is about max life on one, and cold weather kills them fast when they are old. but, your test unhooking the battery does proove that at idle your not charging enough. common for these trucks, mine does the same thing (at idle). try running it at about 1000rpm with the cable off. it should stay running. if it doesnt, then id say yes to you having a charging problem. you need the extra rpm to get the antique reg to kick on and the alt to deliver a charge.

[MarsMonster]

Thanks Trey,

Traffic is so bad here in Seattle that if I were to drive my truck during rush hour I would be in gear at idle for an hour. I think that might result in a call to triple A.

I have already replaced my alternator and voltage regulator. The regulator that I have does not look stock as it does not have the set screw on it that allow you to turn up the voltage on it. I kind of wish I had the original model.

Dave

[RussRoth]

I have an aftermarket volt meter and it goes waaay down at idle. Part of it is my small pulley broke when it was pulled off to change to a larger capacity alternator. Couldn't find a replacement that small so used a larger one. It has been that way for 7 yrs this summer on the same battery so I guess it isn't working too bad. That battery was unused and new when I put it in but had sat on a shelf for about 3 years so it is about 10 years old now. Probably due for a new one before I take it hunting again next fall.

[Jeffc]

what size is your alt pully??......
rebuilt alt's often come with too large of one,
at 800 rpm your out put should be 13.5
to 14 volts.....500rpm should be lower, somewere
around 12.5 to 13.....

Above with everything off but the engine.

[Russ]

Mars, I don't think you have a problem. The measurements you gave are pretty much in the normal range. It's normal for the amp hand to go a little to the discharge side when the lights and blower are running and the truck is idling. I'd also recommend against pulling the battery cable with the engine running. You can cause a higher than normal voltage that might damage something.

[wideblock]

hey jeff, splain dis fo me.....

500rpm, nothing on, idleing in gear. volt and amp gauge steady, amp just below zero on the discharge side, volts at 12.5.

600rpm, in park, same thing.

800rpm, in gear or in park, both gauges bounce like crazy. amp on the + side from zero to about 1/4 gauge, volts from 12-14.5.

1000rpm, amp smooth, on the + side and slowly settles to just above zero, volts smooth and steady at 14.5.

1000rpm to 5000rpm, same thing


so why the heck does it go crazy at 800 rpm?? iis it because of the manual reg?? well, i have the elect replacement for the manual reg. maybe the voltage output from the alt isnt steady at the lower rpm?? is it because im still useing the 1 wire reg on a dual field alt?? one field grounded to the battery.

[Jeffc]

hey jeff, splain dis fo me.....


so why the heck does it go crazy at 800 rpm?? iis it because of the manual reg?? well, i have the elect replacement for the manual reg. maybe the voltage output from the alt isnt steady at the lower rpm?? is it because im still useing the 1 wire reg on a dual field alt?? one field grounded to the battery.

@500 alt is not truning fast enough to produce enough
volts/amps,
possable problems> weak dio in alt, or too large of a pully
on alt (smaller pully will turn the alt faster
at lower engine rpm),power drain. One part of this could also
be the gap set of the regulator points, too wide-
does not contact enough, too close- contacts too long.

@600 still not enough rpm

@800 voltage reg trying to keep pace, switching off
and on the alt trying to keep pace with needed load
and/or recharge 'used' battery shortage every
time the alt is told to turn off.
Large part of this is how the point type reg works,
how much of a magnetic field is made or not depending
on how much the alt is putting out vs useage.
This is the main problem if used with a newer EI (dist/box mopar, pertronix, ect)
systems, which needs a smoother supply of voltage
to preform as it should.
(the above could also be part of the problem
along with grounding issues and/or battery plate
short in just one cell).
In this case the voltage is there in peaks, but the
amp's made are not high enough to recharge the battery,
so the reg is fighting back and forth, loss, gain, loss, gain.......because there is not enough alt
output to run the system and fill the battery
(amp output problem, volts are there in peaks).


1000 above-
high output- battery recharging, grounding issues, ect.... also falty reg (points staying closed too long), power drain (short).
Once the battery is fully charged, gauge should
return to close to center, just to the + side.

If your voltage useage is higher than the alt
output its pulled from the battery, once pulled
will not be replaced till alt output is higher
than useage.

With the later EI reg/alt (2 field wire system)
the alt becomes a 'constant output system'.
In other words the alt never shuts off as long
as it has enough rpm to make power. Voltage
and amp output is smoother, has less peaks
and valleys. The reg tells the alt how much
to produce, never shuts the alt off like in
the old points type reg.

A bouncing needle to the + side means the
battery is being fulled or there is high use
of power or both.

[wideblock]

i agree with what you said on the points type reg. but thats not what i have. mine is the electronic reg. still the single field type, but its a sealed electronic box. all my grounds are good, battery is almost new, same as the alt. and, its a 65amp alt. so output isnt an issue. the pully is the smallest dual groove they offer, only about the size of a silver dollar. but the 600-800 rpm jumpy just baffles me. its smooth as glass any other time and the needle settles to just above zero on the + side within seconds of leaving the stop light. everything works great, i have never been left with a dead battery, never had a problem recovering a charge, and i can jump start a dead battery in seconds. LOLOL but the bouncy needle still bugs me.

[Jeffc]

The sealed type reg works almost the same way
that the points type does, has to because of the
way that it balances the system between alt and
battery. It is differant inside than the
2 field wire system reg, does do a better
job than the points type, is 'solid state', gives better balance
and works faster switching,
but it still will turn the alt off and on.
It is better than the points type, but it
is a compromise between the points type
and later system...

I run both systems, needle movement pulse
is not uncommon on the single field wire 'sealed'
blue box, though it would be even worse with the
old points box.....
Want it to stop? install the other 2 field late
reg., that should put a end to it.....

[MountainMoparRobin]

This is unusual, have to keep all this info in mind, I have a 120amp Alt with the electronic Reg, and idle in gear at 500 rpm still 13-15 volts, I don't care what ya turn on, can't drag down the 120amp Mopar Altenator, those little 65 amps don't seem to be doin something right, thats the same size on the 83 D150 and it is fully loaded with AC, cruise, elect windows, etc. etc. and it holds up in traffic good, same reg as on the 120amp

[Hobcobble]

This is unusual, have to keep all this info in mind, I have a 120amp Alt with the electronic Reg, and idle in gear at 500 rpm still 13-15 volts, I don't care what ya turn on, can't drag down the 120amp Mopar Altenator, those little 65 amps don't seem to be doin something right, thats the same size on the 83 D150 and it is fully loaded with AC, cruise, elect windows, etc. etc. and it holds up in traffic good, same reg as on the 120amp

120amp!?.... are you worried about baking your wiring
harness? That seems like a lot of output.
John

[Jeffc]

As long as the alt is not putting out the full 120amps your not going to cook wires, however, once you
hit around 70 amp output things will start cooking
if there has not been a wiring up grade by-passing the
alt gauge, heavy wire from alt to battery, ect.

This will also depend on the year, after 69 trucks
often had higher amp units (camper special, adventure)
and there wires should handle the load pretty well.
Most of us that have the stock 35 to 45 amp units
would have a problem if we hit over 65 amps for any time.

Just depends on how much load is put into the system.

[MountainMoparRobin]

The 68-78 cars all had 120amp (with electric windows,etc. etc.) I've had the altenator on since 1990 as Jeff mentioned with the regulator working properly it is no different than any of the others, it will only put out what the regulator allows, works great with the solid state regulators. and when the electricity goes out I just plug the house into my truck in the 70's the 120amp was highly sought after, they used them on everything. eventually I'll have alot more electronics in my truck