Volt gauge in dash

[weq92f]

I've got this volt meter gauge in the dash that is not reading what I expect. It's consistently reading around 12 volts when I observe nearly 13 volts both at the battery and at the single wire alternator.

This occurs with the engine running and all major electrics operating ( including dual electric fans, relay operated headlights and blower fan on high ).

I would rather the gauge read true. Any ideas?

Thanks,

-klb

[VetteVet]

This is a fallacy of one wire alternators and is why I never recommend them over the standard three wire units.
In order to supply the required voltage for all the electrical needs of the truck's circuits, the alternator's internal regulator needs to sense the downstream voltage drops in the systems circuits. With the one-wire alternator the internal regulator senses the voltage at the charge wire connection. It is important to have the charge wire connection at the main harness junction. Most installers make this connection directly to the battery's positive post.

In your case you should make the charging connection at the terminal junction where you have the biggest current draws, I.E. electric fans and cab supply junction. Have you read the voltages with a multi-meter? The biggest problem could simply be that the alternator doesn't have enough output to supply the 14.2 volts that you need at 1500 rpms.

[weq92f]

The big electrics draws under the hood: dual rad fans, ignition, fuel pump, EFI, headlamp relays, etc... are fed directly from the alternator charge wire.

Whether the AC blower fan and the main harness to the cab is fed from there is questionable.


Upon cold start measuring with a multmeter I get 14.1 at the alt and 13 at the battery with the dash gauge reading just over 13.

Once coolant reaches 195 and both fans get full power, nothing else extra running I get 13.35 at the battery and 13.70 at the alt with the inside gauge reading 13.

Next cycle of the fans I added lights on high beam and the numbers were:
13.30 battery
13.60 alt
12.8 gauge

Next cycle on of the fans I add full power on the cab AC blower fan:
13.05 battery
13.35 alt
12.5 gauge

Just flicking on the lights with no fans or blower motor
13.70 battery
13.80 alt
12.9 gauge

All this was done at idle of about 750-800 rpm. Bring RPMs up to 1500 or so did not change the position of the gauge needle when under load. However I did notice the engine RPMs drop slightly when the fans kick on and the gauge did seem to recover immediately after with a bump up after the initial "hit" on voltage so I think the alt is sensing the voltage draw.

Interesting to me was that I was able to produce more of a gauge drop with the cab AC fans than the much more robust and demanding radiator fans did when they kicked on. This is likely due to the wiring of the gauge being much closer to the draw given that the gauge doesn't really seem to be reflecting what is actually happening under the hood anyways.


-klb

[VetteVet]

All your readings suggest that the alternator output amperage is not sufficient for the current loads that you have. While they are keeping up with the current draws, you could use a higher current alternator. What amps alternator do you have installed. For your loads a 100 amp should get the job done.

[weq92f]

My alt is a CS 130 style unit. Best I can tell is it's about an 80 or 90 amp.

I found the main ground from the battery to frame was not tight! Ran the tests again today but it was too cold out for the fans to kick on during the time frame I had. The results were better...

Readings in volts:

alt/battery/gauge
14.20/14.05/13.3 cold start
14.12/13.98/12.9 added full high AC fan motor
14.00/13.85/12.6 added High beams

my guess is if the fans kicked on the readings would be
13.50/13.10/12

Still don't like the gauge reading so far off from actual though.


-klb