I bought a scan gauge for my Impreza a while back so I could monitor water temp and alternator performance. I hate it when you can't tell from the stock gauges that things are bad until AFTER they go bad.

 

Scan gauge has warned me about over-heating well before the dash gauge has even moved off it's normal warmed up position.

 

Tonight the gauge has warned me that the alternator is failing without losing one headlight bulb. Returning home from qman's birthday party I saw the alternator voltage climb up into the low 14 volt range. Very unusual since it's normally 13.3 to 13.8V depending on load and rpm. By the time I reached home I saw it hit 14.7V at 700rpm idle with the hi-beams on. Time to replace the alternator and have a new voltage regulator installed on this one. Last time I had the voltage regulator fail, I lost two low beam filaments on the headlights in one day before I realized the alternator was failing.

Depending on how the VR's are set - 14.5 is not outside of what I would consider normal. 14.7 is a bit high. I like to see between 14 and 14.5 when I'm checking directly off the alternator. But if you normally see 13.8 and it's now climbed an entire volt then it's most likely shot. That probably means the reading directly off the alt is somewhere north of 15.

 

GD

I generally look for mid 13 when I check voltage. Anything north of 14 gets me uneasy.

Up to 15 volts is spec for Subaru.

Changed the oil today since it was due and the rains subsided for this weekend. Checked the voltage at the battery terminal with my trusty DVM. Revved engine slightly and reading went from 14.8 to 15.1 VDC. That matches the 14.8 that I saw cruising down the road today going to Pick N Pull in Smokey Point. Time to put in another alternator!

 

Just as an aside, I've lost coolant recently and added almost a quart of pre-mixed last evening when I returned home. Today while checking the alternator I found my leak....a hair-line crack in the top plastic spurting coolant when I revved the engine slightly. Bad timing, but glad I found it today and not tomorrow driving to work.

I generally look for mid 13 when I check voltage. Anything north of 14 gets me uneasy.

Me too.

Depending on how the VR's are set - 14.5 is not outside of what I would consider normal. 14.7 is a bit high. I like to see between 14 and 14.5 when I'm checking directly off the alternator. But if you normally see 13.8 and it's now climbed an entire volt then it's most likely shot. That probably means the reading directly off the alt is somewhere north of 15.

 

GD

15.1 with a calibrated DVM at the battery terminals.

Mid 13, 13.9 max, yoiu have an overcharge condition, which can shorten the life of lightbulbs and electrics. It may not mean anything but check the batter terminals make sure they are clean.

Good job Ed. That's keeping ahead of things.