Interpreting Voltmeter readings?

[98GTW]

I need a little help here.
Short story: the starter had been spinning more slowly lately and finally wouldn't crank at all last Friday (also have had some funny turn-signal modalities, too). Cleaned the battery terminals Saturday morning and jump-started and I've been able to start since. Finally installed the voltmeter I bought a year or so back. Now I need to know what it's telling me.

Readings
IGN on, not running: ~12
cranking: ~11
low idle: a bit over 12
road speeds: just under 14

After searching, and reading about not expecting the alternator to recharge a dead battery, I bought a small (Sears 1.5A) charger and put it on overnight last night. It indicated full charge this morning and the meter readings are pretty near the same today.
Am I right in thinking the system is functioning normally?

Thanks,

[Porsche_monkey]

Your low idle voltage seems too low to me. You should have 13 volts minimum forn the alternator, I think.

[Rot 911]

What is the voltage of your battery with everything turned off? should be around 12.3-12.6. Your low idle voltage should be close to 13 volts. Your voltage at higher rpm's looks good.

[Porsche_monkey]

You're voltage are a bit vague. Do you have a reasonably good meter you could use to measure the voltages to one decimal place?

[WERK I]

Also would recommend that you put the alternator under more of a load. Turn on headlights, radio, fan(high). Monitor the voltage at varying engine speeds under these conditions as you did before. This will give you a clearer indication that the charging system(alternator/regulator/battery) is functioning properly. Look forward to hearing your results.

[Gunter]

Voltage gauge at the dash is not accurate enough, use a good Voltmeter.
How old is your battery? If it is more than 4 years, it's suspect.
You can have it tested under load by a shop without cost.
Do you have an external Voltage Regulator on your '77? It would be located next to the CDI module left side of engine compartment.

[98GTW]

Originally posted by WERK-I
Also would recommend that you put the alternator under more of a load. Turn on headlights, radio, fan(high). Monitor the voltage at varying engine speeds under these conditions as you did before. This will give you a clearer indication that the charging system(alternator/regulator/battery) is functioning properly. Look forward to hearing your results.

Dave,
The 14-V reading at road speed has so far been unaffected by load (lights, etc.). The voltage at idle is low when the idle is down around 750-800RPM, which it settles to under some circumstances. Warmed up idle is closer to 950, and the voltage there is closer to 12.5.
I'm really most interested in two things:
1) preventing another no-start situation (caused, I think by the battery not getting well charged because the connections were flaky.)
2) not overcharging the battery or overworking the alternator.
Am I correct in thinking that if the connections were to go high-resistance again it would be indicated by a large drop in indicated voltage when cranking?

[Porsche_monkey]

Maybe you need to buy the pepboys voltage monitor that plugs in the lighter. They're cheap and should work fine for ongoing diagnosis.

[WERK I]

Dave,
The best avenue when testing the entire circuit(charging, battery) would be going to an AutoZone or PepBoys and the like where they have a tool that diagnoses the state of the battery and the charging circuit. They generally do it for free since it may wind up in a sale of some sort or at the least good customer relations.
One the second item; testing the grounds with a digital ohm meter gives a good indication of the chassis to battery ground and engine to chassis wiring. They all should indicate less than 1 ohm resistance.
When an engine is cranking at startup, high current draw from the battery will give the indication of low voltage. It is not a good test of the wiring, since the amperage draw is so dependent upon other factors....compression of the engine which will effect the amount of work the starter has to do, oil on reciprocating parts which reduces the torque required to turn the engine over, and finally the state of the starter itself(age/wear).

[911pcars]

Notwithstanding the comments from Dave (Werk 1), if the battery is able to sustain more than 9 volts during cranking, it should be fine (yours drops to only 11V). As mentioned by others, the charging specs seem a little on the low side. I would expect +14 volts at speed with an electrical load on the system.

Sherwood

[98GTW]

I appreciate all of your comments. I'll keep an eye on things and see where they go. If I begin to have trouble, at least I've got more information now.

Thanks, all.

[ZOA NOM]

Sounds like a loose belt to me. If the charging voltage is good at higher rpms, it should work fine at lower rpm. Check the simple stuff first.

[copper]

Is your idle too low maybe?

[WERK I]

Dave,
How old is your alternator? When I replaced the alternator on the '85 911, it had approximately 70K on the odometer. Upon inspection of the internals, I found that the brushes had worn to the point of being ineffective and the commutator the brushes ride on were very worn down. This produced low current output at low engine speeds, put sufficient current at road speeds.

I still recommend the car be taken to a place where the charging system can be exercised and diagnosed.

[98GTW]

Dave,
I have no reason to believe the alternator is anything but original. Odometer shows ~111K.

I'll get it all looked at and go from there. Maybe a rebuild or replace project in my future.