Voltage drop at idle

AFQC

What was that noise?
Mar 30, 2008
57
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6
Vancouver WA
Over the last few weeks I have been driving more and more in the dark and have noticed that when I have a bunch of things running like headlights and heater fan on at idle (waiting for stoplights) I am showing about 11.5v on the dash gauge.

Symptoms:
Headlights and foglights on: almost no drop
Heater fan and rear defroster: 1v drop
Turn signals: transient 1v drop at first flash then maybe .25v
When the engine is cold (and idling at 1000 rpm) drops are minimal.
As soon as I get back on the gas things go back up to 12.5-13.5v on the gauge.

So far I have done the following:
Got rid of the bullshit Rockford Fosgate battery terminals and installed marine type terminals and a new alternator today.
New Exide battery less than 2 months ago.
New 4 ga wire from + batt split to 2 8ga primary wires that go into fusebox.
New fusible link.
New 8ga ground wire from right rear of head to firewall.
New 4ga – batt to left fender.
Cleaned up ground point C.

The engine runs fine and there are no other effects but with everything I have put into this I should have a steady 12v at all times.

Voltage readings at the battery terminals after new batt terminals and alternator installation. Idle @ 650 rpm.
Engine off: 12.4v
Running no accessories: 13.5v
Running all lights/accessories: 9.4v WTF!?

Tomorrow morning I will do a new 4ga Batt – to block and I’m thinking of running 8ga ground wires from the left fender batt point to the following:
Alternator bracket
Firewall point C
Junction box ground points in drivers’ kickpanel at D and E

Do you guys think this will do the trick or should I be working more on the + end of things? I’m about out of ideas.

Thanks!
 

dumbo

Supramania Contributor
Jul 16, 2008
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Albera, Too Far North
almost sounds like you got a short on one of your accesories, i would check voltage at your bat running, turning on one accesory at a time, heater, headlights, rear defrost...ect maybe try and isolate the bad circuit

seems funny how your volts raise once you get some rpm up, check to make sure your alt belt is tight, does your battery die quickly??, whats your volts if you leave key on eng off?
 

hvyman

Dang Dude! No Way Man.
Staff member
Apr 17, 2007
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Fullerton,CA
check it with the engine running one lead on the 10mm nut on the alt and the other on the neg of battery for alt output. when you check it on the battery posts you will have a very slight decrease due to resistance in the wires. you should have somewhere between 13.5 to 14 at idle and 14- 14.2 with the rpms up. the more rpms the faster the alt pully is spinning and more volts you are producing. the reason you dont go over 14.2 or so is the regulator.

if you are losing power some where find which component it is and trace the circuit to the problem.
 

AFQC

What was that noise?
Mar 30, 2008
57
0
6
Vancouver WA
dumbo;1163545 said:
check to make sure your alt belt is tight, does your battery die quickly??, whats your volts if you leave key on eng off?

When I got the new battery a few months ago I thought that it might have been part of the problem and changed it out first so I would have a known quantity to deal with. New battery led to same performance. It doesn’t have the usual symptoms of parasitic draw. When it comes to starting it is rock solid, cranks and starts quick every time even if it sits for a week. Dash gauge reads about 11.5v with the key on and the engine off and then hops to about 13.5v when started cold and 13.0v when warmed up. Also belt tension is to spec.

I’ll head out to the garage in a bit and test the voltage from alt B/batt- and measure accessories one by one and post up the results.

Thanks again
 

Nick M

Black Rifles Matter
Sep 9, 2005
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AFQC;1163384 said:
Voltage readings at the battery terminals after new batt terminals and alternator installation. Idle @ 650 rpm.
Engine off: 12.4v
Running no accessories: 13.5v
Running all lights/accessories: 9.4v WTF!?



Thanks!

Is that on the guage, or open circuit voltage with a multimeter?
 

Nick M

Black Rifles Matter
Sep 9, 2005
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U.S.
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Do you know how to do voltage drop testing? Put the leads on the same wire. If you haven't had issues with needing a jump start, then you have to have a lot of drop through that wiring. Strange, since you already replaced it.
 

AFQC

What was that noise?
Mar 30, 2008
57
0
6
Vancouver WA
Ok, I took a bunch of measurements and now I’m even more confused.
All measurements are from Alternator point B to Battery negative post.
Engine warm, idle at 700-750 in neutral.

Headlight+fog+turn signal+brake on 12.45v
Above + hi heater+rear defrost 11.64v
second test 11.66v
third test 11.63v
dash gauge reading ~10.0v

Heater
High 14.3v
Low 14.2v
Headlights 14.2v
Fog (mod) 14.2v
Rear Defrost 12.9v
Brake lights 14.1v

Accessories off taken at intervals throughout the process
14.19v
14.21v
14.13v
14.13v

Engine off between batt +/- 12.6v

With the exception of the rear defroster, none of the accessories by themselves seem to be the culprit. But together the drop is enough to slow down the heater fan and make the lights dim somewhat. Maybe I am being too picky but having lights that brighten and fans speed up when I hit the gas from a stop has fail all over it.:3d_frown:
 

Pantaloon007

Permanent Ban
Apr 2, 2008
76
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Vegas
Just because a alt. doesnt put out voltage doesnt mean its not going bad- you could have one that is not putting out enough current(amps) under load. You might consider switching one out with a friend if thats possible. Maybe just check your belt tension- I know I have to have mine very tight to keep it from slipping.

If youve replaced the wiring and checked/ceaned grounds, the next in line is a alt or faulty voltage regulator, which I believe is in the alt.
 

dumbo

Supramania Contributor
Jul 16, 2008
1,911
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Albera, Too Far North
12.6v is a fully charged battery.

sorry i cant provide anymore input, but if you've changed everything i'm not to sure, if you dont have a carbon pile, i would go to crappy tire or something and have them test your alternator for yah.
 

AFQC

What was that noise?
Mar 30, 2008
57
0
6
Vancouver WA
Pantaloon007;1163744 said:
the next in line is a alt or faulty voltage regulator, which I believe is in the alt.

I replaced the alternator with a new one yesterday. Belt is good and tight.
Gonna go ahead and run the extra grounds listed in my first post and see what that does.
 

hvyman

Dang Dude! No Way Man.
Staff member
Apr 17, 2007
12,568
1
0
Fullerton,CA
do the same tests with all the load with the engine at like 2000 -2500 rpm. if you get 14 volts when doing that there is nothing wrong. the alt usually will put out 13.5v on idle and 14-14.2 with more engine rpm
 

AFQC

What was that noise?
Mar 30, 2008
57
0
6
Vancouver WA
Lots of work today!

p1163986_1.jpg


Put in a new batt to block ground made of #2 welding cable, getting to that bolt on the block was a bitch! While I was underneath I replaced the lower left side block to chassis ground with 8ga wire.

Added a line from the ground post to the front hoist hook (head)

Ran an 8ga line from -batt through the firewall to the fusebox/relay ground points in the drivers kickpanel.

It appears that the mods made a difference for the better but I ran out of time. I’ll do the idle and 2000rpm tests tomorrow at work.