Voltage Fluctuations

[OK-944]

Alternator readout at the gauge holds at just under 14V from startup until about twenty minutes of driving - then might drop to about 13V for awhile…then maybe go back up to 14V for awhile - or maybe drop further to about 12.5V.

After dropping to 12.5V, the alt needle will then typically jerk around a bit - between 12.5 and 13.5V…and after a few minutes of this, the needle will usually come back up to 14V (where it started), and stay there for the rest of the drive.

All of the above…regardless of whatever accessories (lights, blower, radio, etc.) happens to be in use at the time. I do notice a little bit of a voltage reduction when employing brakes or headlights, but this is something I’d consider normal.

Alternator is original to the car (‘87na), and tested “good” at the local O’Reilly’s.

Keep in mind that I’d cleaned all grounds and contacts during my rebuild/rehab process, and all of these are now “protected” with dielectric grease.

Am thinking either just a wonky gauge contact…or maybe I need to replace a voltage regulator?

[TheSecondChris]

It sounds like a jerky gauge. I saw a post recently from the youtube mechanic Edredas in North Carolina where he wrote "A lot of these cars read a little over 12v on the gauge despite being about 13.9 at the alternator while running. They will also show under 10v when not running despite having 12v at the battery. I think it's mostly due to age, but its rare to see one behaving properly. Thanks!"

As a starting point, I'd recommend getting one of those cheap digital voltage gauges that you can leave plugged into your cigarette lighter and you can drive around for a couple weeks and compare both readings.

I replaced my VR on my '84 last year and yes the brushes were work and the new part helped a little bit. I've cleaned a lot of grounds too so at this point I'm convinced that my alternator output is lower than it should be and a replacement is warranted. When my headlights are illuminated along with the cooling fans I can see the battery light glowing a little bit. I mention this to you because the cost of a quality Bosch voltage regulator is about $45 and in retrospect I wish I'd put this money towards the purchase of a rebuilt alternator.

[944 Ecology]

The voltage will fluctuate, depending on the draw of the various accessories used. As long as it stays about around 13.5 volts.

One thing to check, besides the usual electrical (good clean connections and grounds and cables) stuff, is the tension of the alternator drive belt. When using the factory specs and the gauge, the tension is quite a bit tighter than you might imagine otherwise.

[originalowner]

Make some long clip leads and put them on the battery to inside the cab. Connect your DMM and monitor the voltage at the battery. This will let you know if it is the alternator or the gauge.

Edit: you probably only need one long lead as you can find a closer chassis ground in the cab.

[L Sjoberg]

Have you checked the actual voltage at the battery terminals? My gauge will read at about 14v at startup and then about 13v at operating temp (about 15 min of driving), but the actual voltage at the battery terminals stays pretty constant at just north of 14v.

[Fast Freddy 944]

Could be time for another alternator...

[TheSecondChris]

Originally Posted by 944 Ecology

The voltage will fluctuate, depending on the draw of the various accessories used. As long as it stays about around 13.5 volts.

One thing to check, besides the usual electrical (good clean connections and grounds and cables) stuff, is the tension of the alternator drive belt. When using the factory specs and the gauge, the tension is quite a bit tighter than you might imagine otherwise.

Hey George, Good suggestion to check the belt tension. Does the multi-ribbed drive belt make a squealing sound when undertensioned or is this mostly associated with loose v-belts only?

[944 Ecology]

Originally Posted by TheSecondChris

Hey George, Good suggestion to check the belt tension. Does the multi-ribbed drive belt make a squealing sound when undertensioned or is this mostly associated with loose v-belts only?

Might make a bit of a sound, but the squealing is more prevalent with v belts.

[Lakelab]

I bought one of those digital meters that plugs into the cigarette lighter. It also has 2 usb ports for your phone or GPS. Less than 20 bucks. I leave it plugged in all the time.

[OK-944]

Since I began this post, I, too, bought a digital meter for the lighter socket...which indicates a very steady 13.9 volts while the car is running - sometimes dropping a tenth or two when headlights are on.

The original gauge seems to have become a bit steadier also, but still jumps around sometimes, although when this happens there is no associated dimming of lights, nor anything else indicating any true voltage fluctuation issue.

At any rate...looks like all is well.

[CarGuyNeil]

Originally Posted by OK-944

Since I began this post, I, too, bought a digital meter for the lighter socket...which indicates a very steady 13.9 volts while the car is running - sometimes dropping a tenth or two when headlights are on.

The original gauge seems to have become a bit steadier also, but still jumps around sometimes, although when this happens there is no associated dimming of lights, nor anything else indicating any true voltage fluctuation issue.

At any rate...looks like all is well.

So it sounds like an erratic gauge issue then. Maybe clean the contact points of that gauge if even possible.

Sent from my SM-N960U using Tapatalk

[djnolan]

The ultimate alternator test is to turn on all of the accessories at idle (one at a time) while watching the voltage to make sure it stays around 13V.

Headlights, high beams, fogs, blower, A/C, rear defogger, etc. With everything on at idle speed, and a voltage around 13V your charging systems is good.