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 got back to the car today and retook measurements after ensuring it was completely warmed up.

850-900 RPM
D+ = 0.80
D- = 0
DF = 0.61
Batt = 12.2

1000 RPM
D+ = 12.3
D- = 0
DF = 6.5
Batt = 12.4

1400 RPM
D+ = 13.7
D- = 0
DF = 5.0
Batt = 13.3

2000 RPM
D+ = 13.3
D- = 0
DF = 3.3
Batt = 13.2

If I go to 1400 then back down to 850, The numbers come back down.

So no alternator output at low levels ... does that point to a rebuild?

Many thanks.

[Spoke]

I got back to the car today and retook measurements after ensuring it was completely warmed up.

850-900 RPM
D+ = 0.80
D- = 0
DF = 0.61
Batt = 12.2

1000 RPM
D+ = 12.3
D- = 0
DF = 6.5
Batt = 12.4

1400 RPM
D+ = 13.7
D- = 0
DF = 5.0
Batt = 13.3

2000 RPM
D+ = 13.3
D- = 0
DF = 3.3
Batt = 13.2

If I go to 1400 then back down to 850, The numbers come back down.

So no alternator output at low levels ... does that point to a rebuild?

Many thanks.

@Chaznaster
When you go from 1400 to 850, does D+ go back to 0.6V? D+ at 0.6V means the alternator is not running.

One other test to do is the D+ to DF short with VR pulled. Check D+ and VBATT at 850, 1400, 2k. Do them quick since D+ shorted to DF is maximum output. I would want to know if the alternator can work at 850.

[Chaznaster]

I got back to the car today and retook measurements after ensuring it was completely warmed up.

850-900 RPM
D+ = 0.80
D- = 0
DF = 0.61
Batt = 12.2

1000 RPM
D+ = 12.3
D- = 0
DF = 6.5
Batt = 12.4

1400 RPM
D+ = 13.7
D- = 0
DF = 5.0
Batt = 13.3

2000 RPM
D+ = 13.3
D- = 0
DF = 3.3
Batt = 13.2

If I go to 1400 then back down to 850, The numbers come back down.

So no alternator output at low levels ... does that point to a rebuild?

Many thanks.

@Chaznaster
When you go from 1400 to 850, does D+ go back to 0.6V? D+ at 0.6V means the alternator is not running.

One other test to do is the D+ to DF short with VR pulled. Check D+ and VBATT at 850, 1400, 2k. Do them quick since D+ shorted to DF is maximum output. I would want to know if the alternator can work at 850.

When I go from 1400 to 850 D+ does go back to 0.6V. And back up upon returning to 1400.

I'll try the next test and get back as soon as I have some more numbers.

As always - thanks.

EDIT: This might not make any sense, but could the belt be slipping at low engine speeds. I would think that would happen when fast ...

[Spoke]

When I go from 1400 to 850 D+ does go back to 0.6V. And back up upon returning to 1400.

EDIT: This might not make any sense, but could the belt be slipping at low engine speeds. I would think that would happen when fast ...

That is weird that going from 1400 to 850 causes the alternator to turn off. Usually once the alternator is bootstrapped it keeps on running.

BTW, when at 850 and V+ = 0.6V, is the GEN light on?


About the belt, it should be tight. Is it?