Alt. gauge issues?

[69supercj]

I've noticed on occassion that when I start my '66 the Alt. gauge will temporarily jump to the high side but then immediately come back to straight up. Is this normal? Why would it be intermittent? I haven't noticed any charging or lighting issues. Just wondering.

[ICEMAN6166]

its not unusual, the most drain comes from the starter,then when the alt kicks in the amps quickly charge the battery and the gauge returns to normal.
if your battery is somewhat discharged from things like forgetting to turn off headlights or radio the gauge should show charging until the battery is once again at a full charge.
cleaning my truck last week i inadvertently turned on the cb radio and it was on all night. charging started out above 30A then over the course of 170 miles by the time i got home it was only a hair above 0 which is its usual location.
normally when i start it the ammeter hits 30 then back down almost immediately after its running.

if the gauge does not quickly return to normal after starting and there are no other known issues such as battery drain then its time to look into load tests on the starter and battery as a weak starter or battery will draw more from the alt upon starting.

[slickstysix]

Don't mean to hijack, but since it came up...Mine does the same, but it has always read in the negative once it passes 0. I've seen no electrical issues while running, but always disconnect the battery when it's shut off. Actually, my oil pressure reads low also. Could I have an issue with the gage cluster?

[ICEMAN6166]

ammeter is hard wired, oil pressure runs thru instrument voltage regulator.

possibility if you know oil pressure is good that the sending unit needs replacing.

negative readings on an ammeter suggest the alt is not keeping up fully with electrical use.
sitting at idle with all electric items on its likely to see slight negative but should be positive or even at increased rpm

[LM14]

"Normal" may be different for each gauge. I have had them that stayed just below -0- and others that were just above -0-. If all has to do with what is normal for your gauge. I've seen a lot of gauges that normal was slightly to the left or right of -0-. If it looks like it returns to it's normal position I wouldn't worry about it.

SPark

[Toyz]

If concerned about the readings, get one of the cheap voltmeters which plug into the cigarette lighter socket.
Paul

[slickstysix]

Ice - I replaced the oil pressure sending unit when i rebuilt the motor last year. a mechanical gage read well over 50lbs, but my dash gage reads only about 20.
The alt always reads negative after the first minute or so running, no matter how fast the motor is revving, and it doesn't always go to the same spot....weird.

[ICEMAN6166]

nearly 50 year old gauges, not necessarily weird
i run a tee with both oil light and mechanical gauge and have a stewart warner ammeter which functions better imo than the factory ammeter i had in my 65.

also what Steve mentioned about normal being different on different gauges is true too.

[Toyz]

Does your fuel gauge seem accurate? The dash voltage regulator will have no effect on the ammeter, but if both fuel and oil gauges seem inaccurate, it may well be the gauge voltage regulator. As Ice said, "fifty year old..........". Often the replacement sending units also can be incorrect for the original gauge.
Paul

[ICEMAN6166]

Often the replacement sending units also can be incorrect for the original gauge.

had that issue with my temp gauge
rebuilder put the wrong threaded plug in the head so a different sending unit had to be used.
was 35* below correct.
i had to modify the correct unit to fit, a real pita but now its reading properly.

[slickstysix]

My fuel gauge does seem to be accurate, although i haven't tested it on full empty! My temp gauge reads ~180F when warmed up nicely, which is what the thermostat is. So I guess it's mainly the ammeter and oil pressure.....I wonder if I should just get a new resistor and try it. Doesn't sound like big $$.

[slickstysix]

Thanx guys for the hints.
Sorry 69supercj to hijack your thread!

[Toyz]

The "resistor", actually a pulsing regulator, will have no effect on the ammeter, and would affect fuel and temperature gauges if it were the cause of your in-accurate oil pressure readings.
Paul