What's your volt meter reading when cruising around town? Options

[raynekat]

Just curious what the typical volt meter reading is:

1) when you're sitting at the stoplight

2) cruising down the boulevard at 3-4000 rpm

Trying to see if I'm seeing typical or low readings
Thanks

I just recently noted a low charging issue that I believe came down to an alternator/voltage regulator mismatch.
I was using a Bosch VR with my Motorola alternator.
Now I've switched to a Marchal VR and the charging voltages look better.
About 13.75 volts while cruising.

[raynekat]

My 13.75 volts is coming from a voltmeter across the battery terminals.

With the Bosch VR's, I was only getting about 13.2 volts at 3000 rpm.
Driving at night with the lights on and wipers going, you could see the lights visibly dimming at a stoplight as the revs came down.

I also tried a modern Motorola VR that doesn't plug into the socket (had to do some creative wiring to get it to work) and it showed the same as the Bosch VR's.

Still looking around for a "vintage" Motorola VR to try, but perhaps the Marchal is doing a good enough job?
I'll get the car out at night again and check it out.

I've got one of those volt meters/USB things that plug into your cigarette lighter socket for reading voltage while I'm driving.
I'm assuming they are fairly accurate.
Anyone else tried one of those?

[Superhawk996]

My 13.75 volts is coming from a voltmeter across the battery terminals.

With the Bosch VR's, I was only getting about 13.2 volts at 3000 rpm.
Driving at night with the lights on and wipers going, you could see the lights visibly dimming at a stoplight as the revs came down.

Not trying to be an Dope here but welcome to 1969-1976. 13.75 volts is more than enough voltage. Even 13.2 is sufficient to charge.

What you have is a power defecit when you see lights dimming. Said differently you have a current defecit.

Minor dimming is to be expected . . . again welcome to 1970's tech.

If you feel the dimming is excessive, you have to address the current defecit. The 1st suspect would be the battery. When there is a current defecit the battery acts a temporary current source. It should be providing sufficient electrons to keep things powered if the alternator output droops temporarily. Weak battery can start the vehicle and then sill have problems sourcing extra current under continious running load (like lights, wipers, etc.)

All alternators have a current output curve. The 914 alternator doesn't put out much current at idle and/or low revs. I don't know what the specific 914 curve is but the point is they all have a curve. During this low output condition, the battery is expcted to either be just barely charging or with a heavy load (lights, wipers, brake lamps, etc.) it could be net negative for short periods of time. The battery fills the void until the engine rev's come back up, and more current is sourced from the alternator.

If you already have a fresh battery, you could put a current clamp on the battery terminal and monitor it. Current flowing INTO the battery means the battery is charging. Current out of the battery is discharging. Excessive battery discharge with a 13.2 - 13.75 volts across the terminals is a sign that you have a bad alternator this isn't able to keep up with the current demand. Batteries go bad far more often than alternators. A bad battery can act like an excessive load on the alternator due to internal cell shorting within the batttery and can destroy and alternator in short order.

As a theoretical conversation, if your battery were a dead short, the alternator would output maximum current yet the battery voltage across the terminals would read ZERO. The continued max output of the alternator would eventually overheat the windings, brushes, diodes, until the alternator fails some internal component.

The last resort is to replace your alternator but remember, you're replacing it with 1970's tech.

Whatever you do, don't expect a 914 to behave like a modern vehicle. You will have some minor light dimming on a perfectly operating 914 whereas that is not accepted anymore on modern vehicles. Modern vehicle have very elaborate load managment systems. So elaborate that they can for example turn loads off & on without you even knowing in order to keep the voltage and current flowing within the system at an optimal level to protect the battery, and to minimize the fluctuations you see.

For example, your heated seats draw a lot of current. If there is a current defecit, the seats can be turned off yet the heat doesn't go away immediately. They can then be turned back on for a bit to keep them warm, and then off again a few minutes later. All this is done while the LED on the instrument panel remains lit. You have no idea the seats are effectively being pulse width modulated to control the sytem voltage and current. Don't expect this type of system management in a 914.

I've got one of those volt meters/USB things that plug into your cigarette lighter socket for reading voltage while I'm driving.
I'm assuming they are fairly accurate.
Anyone else tried one of those?

Yes, it will be more accurate than the analog VDO gauge thanks to the mircale of modern electronics. The circuit to monitor voltage accurately will fit on a surface mount chip that only costs pennies and is no bigger than a gnat's ass.