Low idle voltage

Disclaimer: Links on this page pointing to Amazon, eBay and other sites may include affiliate code. If you click them and make a purchase, we may earn a small commission.

bigcountry78

Full Access Member
Joined
Apr 16, 2013
Posts
3,600
Reaction score
8,589
Location
Hickory, North Carolina
First Name
James
Truck Year
1978
Truck Model
K10 Custom Deluxe
Engine Size
350
Kast night after work I was waiting on my truck to warm up before I left, with the headlights and radio on, my volt meter was just barely above the red. Is this normal, or is there something that would cause this? Battery and alternator are good, it reads well over 13 volts driving down the road. It just drops when idling.

You must be registered for see images attach
 

yevgenievich

Full Access Member
Joined
Mar 1, 2014
Posts
4,781
Reaction score
3,300
Location
Texas
First Name
Viktor
Truck Year
sad
Truck Model
very sad
Engine Size
less sad
Pulley size on alternator wrong size for motor/belt or alternator failing
 

Charlie

Mopar by Birth. Chevy by Choice.
Joined
Aug 2, 2017
Posts
1,837
Reaction score
909
Location
Euless, Texas
First Name
Don
Truck Year
1974
Truck Model
Cheyenne 10 LWB
Engine Size
350/TH350/AC/4 BBL Quadrajet
If you have a multimeter or access to one, test battery while idling and reving to see if there is much difference. Idling should read approximately 13.5 - 14.5 V. If reading increases at higher RPM's and decreases at idle, I would say alternator bad.
 
Last edited:

bucket

Super Moderator
Staff member
Super Moderator
Joined
Aug 3, 2010
Posts
28,964
Reaction score
23,479
Location
Usually not in Ohio
First Name
Andy
Truck Year
'77, '78, '79, '84, '88
Truck Model
K5 thru K30
Engine Size
350-454
If you have a multimeter or access to one, test battery while idling and reving to see if there is much difference. Idling should read approximately 13.5 - 14.5 V. If reading increases at higher RPM's and decreases at idle, I would say alternator bad.

That and it's a quick way to tell if the dash gauge is accurate at all.
 

yevgenievich

Full Access Member
Joined
Mar 1, 2014
Posts
4,781
Reaction score
3,300
Location
Texas
First Name
Viktor
Truck Year
sad
Truck Model
very sad
Engine Size
less sad
Actually, talking about gauge accuracy. A poor body ground will make the gauge read decently lower than it should
 

yevgenievich

Full Access Member
Joined
Mar 1, 2014
Posts
4,781
Reaction score
3,300
Location
Texas
First Name
Viktor
Truck Year
sad
Truck Model
very sad
Engine Size
less sad
There is a ground to the core support, to the engine block and from engine block to firewall by rear passenger side head. But test with a voltmeter should be done first
 
Last edited:

Snoots

Full Access Member
Joined
Jan 23, 2013
Posts
8,158
Reaction score
15,957
Location
Georgia
First Name
Roger
Truck Year
1973
Truck Model
Jimmy Sierra
Engine Size
350
Which ground should I check?

All of them.

Not trying to be a wise-arse. But it is best to check them all.

Just start at the front and work you way to the back one-by-one. Take 'em off, clean the surfaces, add a touch of copper infused anti-seize and reinstall.

If ANY of the screws or bolts that hold down a ground wire are rusted, replace it. It ain't worth the headaches to try and clean them up. If they're rusty, they're toast.
 
Last edited:

77 K20

Full Access Member
Joined
Nov 9, 2012
Posts
3,075
Reaction score
3,044
Location
Montana
First Name
Mike
Truck Year
1977
Truck Model
K20 5" lift
Engine Size
HT383 fuel injected
I had that same problem on my truck. Idling at a stop light with heater fan on, headlights on, and heaven forbid its raining and I have the wipers on. Volt gauge would be down to where yours was or even lower.
I bought a smaller pulley for the alternator. This helped a bit- then when I added fuel injection I went from the stock 10si alternator to the 12si alternator. It makes a LOT more current at low (idle) RPM.

You must be registered for see images attach


You must be registered for see images attach


You must be registered for see images attach


You must be registered for see images attach




Typical OEM ratios are around 3 to 1, which simply means the crank pulley is three times the diameter of the alternator’s pulley. So in my case the engine would idle at 550 RPM and the alternator is spinning about 1650 RPM. So maybe I was making 16 amps? Slightly smaller puller and the 12si then will put out maybe 30 amps.
 
Last edited:

Matt69olds

Full Access Member
Joined
Jan 8, 2018
Posts
2,338
Reaction score
3,571
Location
Central Indiana
First Name
Matt
Truck Year
81
Truck Model
GMC 1/2 ton
Engine Size
455 Olds
The GM Cs130 alternator is a easy upgrade, with current capabilities up to 105 amps basically at idle. It’s a easy upgrade over the 12Si most of these trucks came equipped with.
 

77 K20

Full Access Member
Joined
Nov 9, 2012
Posts
3,075
Reaction score
3,044
Location
Montana
First Name
Mike
Truck Year
1977
Truck Model
K20 5" lift
Engine Size
HT383 fuel injected
You must be registered for see images attach


I looked at the CS130. It is better than the 12SI at idle. (Looks like about 40 amps) And it wasn't quite as plug and play, but still easy to install.
 

Matt69olds

Full Access Member
Joined
Jan 8, 2018
Posts
2,338
Reaction score
3,571
Location
Central Indiana
First Name
Matt
Truck Year
81
Truck Model
GMC 1/2 ton
Engine Size
455 Olds
I did the CS130 swap on my Olds probably 20 years ago. I can run the heater in the car, headlights, the bottle heater for the nitrous, electric fuel pump, basically anything electrical and the voltage gauge never moves. I did have to swap pulleys (the CS alternator had a multi groove pulley for a serpentine belt) I can load test the alternator to its max output with no belt slip.

The nitrous and bottle heater are intended for those late night grocery trips. Gotta get that ice cream home before it melts!!
 

Latest posts

Forum statistics

Threads
41,856
Posts
903,624
Members
33,370
Latest member
mitchell1128
Top