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

[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.

[rhodyguy]

Starts fine when cold? When it’s hot it still spins up, just won’t catch? Have you considered hot fuel? When you are starting when hot, are you touching the gas pedal? My car (Webers) stumbles at startup when hot. 3 pedal pumps then feathering the pedal. You have to get those throttle plates open. Things calm down when the carbs are full of fresh tank fuel. Actually it’s only a few seconds. Are you using phenolic spacers/gaskets on the intake bases? I thought the voltage numbers were more about what the ‘brain’ needed to function.

[emerygt350]

You want 13.5 at idle. After I switched to a modern VR I get 13.7 at idle and 13.7 at whatever rpm you like. At the battery. When you have to turn your wipers on and your heater blower and your cool air blower and your headlights you don't want to be wondering about your battery.

[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]

Starts fine when cold? When it’s hot it still spins up, just won’t catch? Have you considered hot fuel? When you are starting when hot, are you touching the gas pedal? My car (Webers) stumbles at startup when hot. 3 pedal pumps then feathering the pedal. You have to get those throttle plates open. Things calm down when the carbs are full of fresh tank fuel. Actually it’s only a few seconds. Are you using phenolic spacers/gaskets on the intake bases? I thought the voltage numbers were more about what the ‘brain’ needed to function.

It starts great cold and most of the time hot. I have a single, central Weber 32/36. It is not over the heads so should not get excessively hot. I am running the spacers, as well, just in case. I usually give it a single pump on the throttle to reset to fast idle. When it does start it comes right to life.

[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 ...

[iankarr]

Just dealt with this on my green car. Replaced the VR and battery and running 13.5-13.7 all the time now.

[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?

[Chaznaster]

Just dealt with this on my green car. Replaced the VR and battery and running 13.5-13.7 all the time now.

That would be better than an alternator rebuild ...

And congrats on the Bumblebee project!

[Chaznaster]

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?

At 850 the gen light is flickering. It goes out as soon as I rev the engine.

I'll check the loose belt and do other tests this weekend. I recently had the engine out and pulled the alternator / replaced and tensioned the belt but maybe I did not get it right ...

[Chaznaster]

So ... I feel a bit foolish about this ... the cause of the wonky alternator voltages was a loose fan belt. Must have been slipping at lower engine speeds.

Thanks to all for all the help, as always. Special thanks to @Spoke for walking me through the diagnostics.

Now we will see whether we still have a hard start when warm.

[emerygt350]

So ... I feel a bit foolish about this ... the cause of the wonky alternator voltages was a loose fan belt. Must have been slipping at lower engine speeds.

Thanks to all for all the help, as always. Special thanks to @Spoke for walking me through the diagnostics.

Now we will see whether we still have a hard start when warm.

Don't feel bad, happened to me after I rebuilt my engine. I freaked out for a couple days and then realized I was hearing some strange sounds...