Do I have a bad alternator?? help!

Imcoz · Imcoz's Garage

Thought it was my battery (5 yrs old), but my 88 911 now has a brand new one. Car still has the same issue. Car runs fine with a charged battery, but eventually (After and hour or two) bucks and dies (like running out of gas). Tested the voltage at the battery and it had close to 14v. Could the VR be going and showing good voltage at idle but crap out due to heat or rpms? Battery is definitely discharging. Also not getting a light at the dash.

I might add that this car sees mostly DEs. Not so much a daily driver. My spidy sence tells me my 3-4 year old Alt is probably the issue, but I'm thrown off by the reading at the battery. Could there be another common culprit?

TIA-need to get on the track tomorow!

Gogar · 08:17 PM

When the car "dies", have you read the voltage then?

Check the voltage with the car off, and idling, and at RPM. 2-3k is enough.

"car off" voltage should be around 12.4, car idling around 14v, and NEVER more than about 15 at the very high end.

Seems to me like if you have good "car off", "idle", and "at speed' voltages, you should look somewhere else.

Imcoz · Imcoz's Garage

When battery is charged (and this car sits on a trickle charger) at "off" it's fine. At the time of it dies, battery is mostly depleted (very dim lights). Haven't been able to check at 2k, but will once battery is recharged. I've read quite a few similar posts and it seems to be the VR. I just don't get why there wouldn't be a lit red light at the dash.

Gogar · 10-01-2009, 10:37 PM

Does the light light up when you turn the key to "ON"? Maybe it's burnt out!

Just covering all the bases.

mca · 10-02-2009, 03:15 AM

Did you check your fan belt?

Vin-barrett · Vin-barrett's Garage

Your dumping voltage somewhere
Pull the alternator out and see if one of the terminal nuts rattled loose, also ground strap underneath between body and trans, and of course the battery.

DonMo · DonMo's Garage

I hate to say it but if you take the car by Autozone, Pep Boys, Sears automotive, they have a machine that will tell you if it is the alternator, VR or battery. They have the ability to put a load on the system and see if the system adjusts. That assumes your DE has tags and is street legal.

burgermeister · burgermeister's Garage

+1 checking the alt light with the ignition in "on", engine "off". Should light up. If it doesn't, either the bulb is burned out, which will keep the alternator from charging, or the brushes are toast (bulb gets its ground through the brushes & alternator - try ignition-on & ground the bulb manually to see if it lights up).

mca · 10-02-2009, 01:17 PM

Also, when the VR goes you will see some other electrical failures.

Usually the tach starts going crazy first - bouncy. Then other electrical items such as lights will flicker / dim.

This is what happened to me when mine failed. I only replaced the VR b/c I had the alternator checked out and it was fine.

The reason I suggested checking the fan belt is b/c I had alternator issues following my rebuild (I thought so anyhow). I used the same amount of inner / outer shims before and after the rebuild. But for some reason the belt was too loose following the engine install. Could not tell by feel and belt wasn't squeaking. It is an ez and cheap thing to check.

chamilun · 10-02-2009, 01:33 PM

plug your voltmeter in to say the radio battery source and drive around watching the voltage. If its fluctuating, youve got a bad VR

Imcoz · Imcoz's Garage

Thanks guys! Took the alternator out and turns out the VR was hanging loose on the back of it, the two little bolts backed out. The guys at Action Auto in Vineland got me one overnight and I was back on the track.

Canada Kev · 10-03-2009, 07:39 PM

Checking the VR is pretty easy. I was getting dash lights that would get rather bright and then go back to normal randomly. I checked the voltage at idle and at 2-3K rpm, but was getting the correct reading of about 14.5 volts which would suggest that all was well. With the varying dash brightness, I wasn't convinced. I got a cigarette lighter plug and clipped it to my meter and watched it as I drove around. It was mostly around 14 v, but quite often it went up to as much as 16.5 volts. Yikes!

I parked it and ordered a new regulator. It installed quite easily, but getting the fan out was a bit of a PITA. I would suggest to anyone doing this to take a rag or two and place over the fan pulley. I scratched up my fan surround a bit getting it out. The padding from the rags should prevent this.

Imcoz · Imcoz's Garage

One other tid bit for those refering to this post at a later date with possibly the same issue. I confirmed that the idiot light on the dash was working, but it never came on, even when the battery was fully charged and no current coming from the Alt/VR.

burgermeister · burgermeister's Garage

So, the best insurance is probably to install a voltmeter; if it's reading 12.5 instead of 14, something is amiss ... it's how I found my worn-out brushes issue (brushes are part of the VR on my car - are they on others as well?)

mattn · 11-11-2009, 06:04 AM

If you guys are still tracking this post, have a related question. I would like to check the voltage at the battery and the current generated by the alternator. I have only a small handheld multimeter. Do I need something more robust? Thin wires, lots of current..... I'm obviously not an electrician.

Formerly Steve Wilkinson · 11-11-2009, 07:37 AM

The thickness of the multimeter's wires is not relevant. It will check battery and charging voltage just fine, just as it will check 120 volts inside your house, the latter assuming you have the correct scale and "ac" selected."

Walt Fricke · 11-11-2009, 02:36 PM

Burgermeister - the earlier external VRs (example - a '77 2.7) obviously did not include the brushes as part of the VR package. I think all of the internal regulator alternaters include the brushes, but maybe not - my sample is one SC and one 3.2 alternator.

As to measuring voltage, Formerly Steve is right about wire thickness, as measuring voltage in the way we are discussing involves miniscule amounts of current.

The same cannot be said for measuring current. If you are expecting your alternator to put out, say, 50 amps with a lot of accessories on (or maybe a battery that is run down), you need a current meter and wiring that will handle that. Your multimeter is good for maybe 10A if you plug one of the wires into the high current jack (if it has one). All I have used the ammeter function of a multimeter for is to check current draw with the ignition turned off (I could see the clock relay pulling little spikes of current), as those usually aren't high.

I think one might figure out a way to use a multimeter to calculate high current flows. Measure the resistance accurately of the thick cable connecting the alternator to the starter (or the starter to the battery - even thicker), consider that the alternator shunt, hook up the multimeter as a voltmeter between both ends, do the calculations, etc.

My local parts store had an LCD voltmeter for ~$20 you could plug into the cig lighter and leave there to read while driving. I bought a couple for my various cars. Better mousetrap than the LED one I bought from JEGS and have used in the 911 for several years.

Walt Fricke

Laneco · 11-11-2009, 03:11 PM

Quote:

Originally Posted by mattn

If you guys are still tracking this post, have a related question. I would like to check the voltage at the battery and the current generated by the alternator. I have only a small handheld multimeter. Do I need something more robust? Thin wires, lots of current..... I'm obviously not an electrician.

Pop your clock out, grab the positive off the back and the ground, tape or clip each to your respective voltmeter leads.

Drive away and check your voltage as you go. I'd like to take credit for this idea but I'm not that smart - another Pelican told me how to do it and I did it on my car to diagnose a failed regulator.

angela