Alternator not charging at idle

[iamtherealJayy]

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]

Check the alternator voltage output at the alternator, should be about 13.5 volts

 

[iamtherealJayy]

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]

Sounds like a faulty diode in the alternator. Also check for a loose belt and poor grounds.

 

[iamtherealJayy]

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]

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]

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]

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]

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]

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Code Z53, voltage gauge.

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[Trucksareforwork]

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]

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]

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]

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]

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.