Alternator not charging at idle

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.

iamtherealJayy

Full Access Member
Joined
Nov 26, 2021
Posts
1,571
Reaction score
275
Location
Tennessee
First Name
Jacob
Truck Year
1987, 1978, 1976
Truck Model
V20, K10, K10
Engine Size
350, 350, 350
So I’ve had this 1976 Chevrolet K10 for around 8 months now and it has never seen pavement the whole time I’ve owned it. I still need to do some tuning on the quadrajet to get it running ideal, but it’s got an electrical issue. The voltage gauge shows 12 at idle and only goes down from there, wipers super slow etc. if I breathe on the throttle and bring the rpms up the voltage is fine. The truck has a brand new alternator and battery. If I leave the truck idling for a while then shut it off and try to instantly crank it again it cranks really slow for a few rotations and either fires up or starts clicking like a dead battery. The starter is new I think I can’t really remember I’ll have to check.
 

Ontheboulder

Full Access Member
Joined
Feb 27, 2022
Posts
310
Reaction score
365
Location
colville,wa
First Name
greg
Truck Year
1987
Truck Model
gmc sierra classic v2500
Engine Size
350
Check the alternator voltage output at the alternator, should be about 13.5 volts
 

iamtherealJayy

Full Access Member
Joined
Nov 26, 2021
Posts
1,571
Reaction score
275
Location
Tennessee
First Name
Jacob
Truck Year
1987, 1978, 1976
Truck Model
V20, K10, K10
Engine Size
350, 350, 350
On the wire with the eyelet that’s on the stud correct? The two wire plug is a 12v power wire and a ? Yes?
 

Memaloose

Full Access Member
Joined
Dec 2, 2018
Posts
490
Reaction score
1,355
Location
Dolan Springs, AZ
First Name
Tony
Truck Year
1975
Truck Model
K20
Engine Size
350
Sounds like a faulty diode in the alternator. Also check for a loose belt and poor grounds.
 

iamtherealJayy

Full Access Member
Joined
Nov 26, 2021
Posts
1,571
Reaction score
275
Location
Tennessee
First Name
Jacob
Truck Year
1987, 1978, 1976
Truck Model
V20, K10, K10
Engine Size
350, 350, 350
The ground running from alt to battery seems fine but one of the mounting bolts going into the head, the one with the cylinder spacer, is loose maybe? That cylinder spacer piece spins semi freely. The alternator is new but makes a slight ticking tapping sorta noise? Somewhat
 

Ontheboulder

Full Access Member
Joined
Feb 27, 2022
Posts
310
Reaction score
365
Location
colville,wa
First Name
greg
Truck Year
1987
Truck Model
gmc sierra classic v2500
Engine Size
350
On the wire with the eyelet that’s on the stud correct? The two wire plug is a 12v power wire and a ? Yes?
yes. that stud should register 13 to 13.5 volts, if not your alternator is starting to fail
 

iamtherealJayy

Full Access Member
Joined
Nov 26, 2021
Posts
1,571
Reaction score
275
Location
Tennessee
First Name
Jacob
Truck Year
1987, 1978, 1976
Truck Model
V20, K10, K10
Engine Size
350, 350, 350
It shows 13.5 to 14.4, I did increase the timing just a bump because I still had a stall off wot. I assume it bumped the idle up just enough however.
 

75gmck25

Full Access Member
Joined
Oct 13, 2016
Posts
2,017
Reaction score
1,808
Location
Northern Virginia
First Name
Bruce
Truck Year
1975
Truck Model
K25 Camper Special TH350 NP203
Engine Size
5.7
Did your truck have a factory voltage gauge, or was it an ammeter gauge? Or maybe the cluster has been swapped.

The wire on the threaded stud on the alternator is the charge wire. Stock routing is along the valve cover and down to the starter, where it connects to the positive battery cable. There will be a fusible link on the starter end of the cable.

The two wire plug is the sense wire and the activation wire.
- The sense wire sends a voltage measurement back to the alternator and tells it to increase or decrease the charging. It should be connected into the wiring loom to get a voltage reading (including the drop from accessories that are running) from near the firewall, but it’s fairly common to just jump it over to the threaded charge wire terminal.
- the activation wire runs to an ignition-switched circuit to tell the alternator the engine is running - so start charging! If you have trouble with the truck continuing to run after the ignition is switched off, it may need a resistor added to this wire, or to reconnect the dash alternator light.

The battery negative terminal usually has a large cable running directly over to a bolt on the alternator bracket. Since the bracket is also bolted to the block and head, you have a good ground path.
 

iamtherealJayy

Full Access Member
Joined
Nov 26, 2021
Posts
1,571
Reaction score
275
Location
Tennessee
First Name
Jacob
Truck Year
1987, 1978, 1976
Truck Model
V20, K10, K10
Engine Size
350, 350, 350
I just washed the truck since I got dirty today during the test driving, and when I started it the voltage didn’t increase until I revved it then it started charging. I’ll trace the wires but it looks factory routed. I’m not sure what the factory gauge had is that an rpo option? The sticker is in the glove box I can check it out.
 

iamtherealJayy

Full Access Member
Joined
Nov 26, 2021
Posts
1,571
Reaction score
275
Location
Tennessee
First Name
Jacob
Truck Year
1987, 1978, 1976
Truck Model
V20, K10, K10
Engine Size
350, 350, 350
You must be registered for see images attach

Code Z53, voltage gauge.
You must be registered for see images attach
You must be registered for see images attach
You must be registered for see images attach
 

Trucksareforwork

Supporting Member
Supporting Member
Joined
Feb 22, 2022
Posts
228
Reaction score
449
Location
Spartanburg SC
First Name
Geoff
Truck Year
1985
Truck Model
C10
Engine Size
305
I just washed the truck since I got dirty today during the test driving, and when I started it the voltage didn’t increase until I revved it then it started charging. I’ll trace the wires but it looks factory routed. I’m not sure what the factory gauge had is that an rpo option? The sticker is in the glove box I can check it out.
If it started charging after revving and kept charging from there, I would think that maybe your exciter wire has too much resistance or another problem and isn't throwing enough voltage to the alternator until you rev it. I think you can jump it to any 12V switched source to see if that helps (e.g., electric choke right nearby?).
 

iamtherealJayy

Full Access Member
Joined
Nov 26, 2021
Posts
1,571
Reaction score
275
Location
Tennessee
First Name
Jacob
Truck Year
1987, 1978, 1976
Truck Model
V20, K10, K10
Engine Size
350, 350, 350
Nope, the choke has the rod going into intake, heat rise style? Not positive what kind it is. That was also a warm start without choke so idle was at idle speed not a high idle. Could part of the alternator issue cause the slow crank when warm? Or is that more starter issue? I did verify the starter is newer and has a brace to the block. All connections felt tight.
 

Trucksareforwork

Supporting Member
Supporting Member
Joined
Feb 22, 2022
Posts
228
Reaction score
449
Location
Spartanburg SC
First Name
Geoff
Truck Year
1985
Truck Model
C10
Engine Size
305
Nope, the choke has the rod going into intake, heat rise style? Not positive what kind it is. That was also a warm start without choke so idle was at idle speed not a high idle. Could part of the alternator issue cause the slow crank when warm? Or is that more starter issue? I did verify the starter is newer and has a brace to the block. All connections felt tight.
Slow crank when warm might be:

- timing too advanced
- heat soak on the starter (happens w headers-do you have them and are they heating up the starter?)
- marginal ground or positive connections / bad wires. As wires warm up they gain resistance.
- bad starter…
 

iamtherealJayy

Full Access Member
Joined
Nov 26, 2021
Posts
1,571
Reaction score
275
Location
Tennessee
First Name
Jacob
Truck Year
1987, 1978, 1976
Truck Model
V20, K10, K10
Engine Size
350, 350, 350
No idea where timing is, tab is bent over
Nope has stock manifolds and a shield over the starter
Connections at starter are tight, wire insides maybe?
Starter appears new
 

Trucksareforwork

Supporting Member
Supporting Member
Joined
Feb 22, 2022
Posts
228
Reaction score
449
Location
Spartanburg SC
First Name
Geoff
Truck Year
1985
Truck Model
C10
Engine Size
305
No idea where timing is, tab is bent over
Nope has stock manifolds and a shield over the starter
Connections at starter are tight, wire insides maybe?
Starter appears new
I've read your posts. You should consider finding TDC on Cylinder 1 and marking your timing or applying a timing tape appropriately. It's hard to know what's going on with the engine if you don't know where your timing is set.
 

Latest posts

Forum statistics

Threads
41,849
Posts
903,434
Members
33,362
Latest member
Dhatch84
Top