Battery Voltage Low - most likely culprit?, Does not start well when warm Options

[Chaznaster]

Hi All.

I am having a trouble starting my carbureted 1.8 when the engine is up to temperature (heads at 310 degrees for over 30min). The engine is definitely turns over and the starter engages.

The car starts immediately when cool/ cold. It is also fine when I let it cool after being hot.

My battery seems to have settled out at about 12.2V. Per the Google, that equates to 60% useful charge. That is definitely a problem.

My theory on the non-hot start is that, as copper wire resistance increases with increasing temperature (also something learned on Google), it is just not getting enough current to the coil when hot. Cold wires = lower resistance = better current flow, and so, even with the marginal battery, it gets enough juice to start.

Now on to why the battery is running low. At warmed-up idle speed (800-900 RPM), the multimeter only reads 12.3. That is most certainly the cause of the low voltage.

I'd very much like your input as to whether this is likely caused by a voltage regulator problem or an alternator problem (or both).

EDIT: It will always start with a push start ...

Many thanks!

[Chaznaster]

I am back with some data ...

Battery after full charge and 3hr sitting : 12.5v

Battery with VR removed and DF / D+ jumped : 17v (at 1200 RPM. Did not get this at 850 but expect it would be low)

Alternator to ground
At 1200 RPM
D+ 13.6v
D0 0v
DF 5v

At 850 RPM - note alternator dashboard light often flickers here
D+ 0.7v
D0 0v
DF 0.5v

I recently cleaned the ground between the transmission and body, as well a the multi chassis ground below the relay board.

Do alternators or VRs start by failing at the low end of voltage range?

Thoughts??

[Spoke]

I am back with some data ...

Battery after full charge and 3hr sitting : 12.5v

Battery with VR removed and DF / D+ jumped : 17v (at 1200 RPM. Did not get this at 850 but expect it would be low)

OK that number looks good for the alternator at maximum output. Was this the battery voltage?

Alternator to ground
At 1200 RPM
D+ 13.6v
D0 0v
DF 5v

At 850 RPM - note alternator dashboard light often flickers here
D+ 0.7v
D0 0v
DF 0.5v

If you measure these voltages again, please measure the battery voltage as well.

At 1200 the numbers look good. D0 being zero is expected. Also D+ and DF looks good.

At 850, did you measure these voltages after doing the 1200 test? Or a different run? With D+ at 0.7V, the alternator is not functioning.

If you go from 850 to 1200 then to 850, does DF go from 0.7 to 13.6 then back to 0.7?