Is my alternator getting weak
 

Put one those digital readout voltmeters in the car. When running I get 13.8 to 14.1 volts with 3 fans running. Shut the fans off and turn on the parking lights I get 13.4 and with the headlights on I get 12.8 and then with the fog lamps on also I get 12.5 volts. All this is happening at 3K rpm. Is this pretty normal or am I looking at an alternator thats not putting out properly under load? If you didn't notice the avitar then its an "86" 911 carrera and the belt is new and properly adj. I know lights make a big draw on the elec system.

Janz, I think the voltages you are seeing are pretty normal and can reflect the condition of the battery to a certain extent as well. What you didn't mention is the history of the alternator or mileage on your car. The alternator on my '87 911 with ~120K miles failed last summer on a trip to Glacier National Park. It had displayed no outward signs of failure beforehand. -John

CAR HAS 146k ON IT AND THE BATTERY IS LESS THAN A YEAR OLD. Not sure about its previous maintainence records though. Just wondering if that voltage drop during that type of electrical draw is normal. I would think its a bit low. I did have the alt out to remove and clean fan and housing and the alt looked to be origonal and I was thinking time for a new one, thats what started me looking at the voltages under load.

Doesn't anyone check their actual voltage while running. This is what I bought and seems to work very nice, just plug into cig lighter.
http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&item=270333726819&sspagenam e=STRK%3AMEWAX%3AIT&viewitem=

Just get it rebuilt at a local auto electric shop. No need to buy a new one. I had mine done for around $100 while I had the motor out. Cheap insurance in my opinion. I also had my starter rebuilt by the same place for about the same price.

I rebuilt mine at a local shop. It seem fine until another batery die rematurely; which get my attention and ask detail questions on this board. It has been running at 14.9v while driving on freway. I thought it was fine until people confirmed with me that is too high.

12.5V is below 100% charge on the battery - you will be losing charge. 100% charge on an automotive battery is 12.85V.

I had a similar issue with my last alternator on a BMW. After I put the new one in, it read 13.9 to 14.1 at idle and stayed pretty much fixed. Just installed it last week.

This was after the car died at the convenience store on a 6 mo. old battery.

Doug

Funny timing on this thread. I'm having trouble with a recently installed alternator in the 993. Taking the car to the shop Thursday for more diagnosis.

The original one died a couple of months ago after 140k+ I swapped in a rebuilt one from a very reputable re-builder and put in a new battery at the same time. Last weekend the battery died after the car had sat only a day and a half. (The car is my wife's DD for a 60 mile a day commute).

Put the battery on the charger and once charged started up the car to check output on alternator...only putting out about 13.2-13.4 volts under no load. Reved it up and turned on lights etc. no change.

I have been checking the state of charge on the bat. every night when the wifey gets home and it is slowly losing about 1 or 2 10ths of a volt per day. I told her no radio or other electrical drains for the commute if possible. Looks like the Alt. can't keep up.

I'm thinking bad rebuild. We'll see what the shop can find out. I would have expected output of 13.8 to 14.2 as others have suggested.

Anyway thats my current story...

Cooper911SC,
and I am getting too high, 14.9-15v while driving :)
isn't 13.x is charging, just slow charge?

I think 13.8 to 14.2 is right around perfect. Did you try taking voltage readings with headlamps and fog lights on? This is where my problem lies, I get a perfect charge voltage until I put the full lighting load on then it drops to 12.5 volts. Anyways just ordered a new Alt from host so when it gets here in 9 days and I install it I'll let everyone know if it made a difference. Mine does look origonal and with 146,000 miles it did look kind of ratty on the inside when I had it out to clean the fan.

janz,
So you meant with headlight/foglight on, the voltage should stay the same?

I recently had my battery & alternator tested using a Midtronics Micro 500 V1.4. The results were similar to your voltage numbers. Diagnosis: "Bad Alternator". 1987 with 180k PO records indicate it had been replaced at 83k.

Static battery test: 13.09V "Good Battery"
Starter test: Cranking voltage 11.78V "Normal"
Charging test: Load off: 14.01V Load on: 12.68V "Low Charging Voltage"
Diode Ripple: "Normal"

Voltage should not stay the same but it should not drop as far as mine does. I would imagine that if you started with 14.5 and turned on all the lighting load it should go down to 13.8 but this still offers enough to keep charging the battery even with the heavy load providing your not just sitting there idling, hence the 3K rpm test.

So you are saying under load, it should be at 14.5 without headlight and such. With headlight, it should be around 13.8 minimum?
Wow, then it's hard to get the good config then. The good and bad is not that far away. What I have might be too high and it is between 14 to 14.5 at idle, high speed freeway is 14.9, with headlight on is 14.5 but if I push it a little over 3k rpm for around 15 mins, it'll come up to 14.8-14.9 again.

Your no-load voltage is fine. The voltage drop with the lights on is too high. That suggests you have high resistance connections somewhere in the lighting circuit. Start at the back of the headlamps (where the sockets are exposed to moisture) and work back to the switch on the turn signal, clean the contacts with electrical contact cleaner and emery cloth and see if you don't get an improvement.

Where is this attached? To measure battery voltage you should put the meter leads on the battery posts.

It plugs into the ciggarette lighter which is a good source rated amperage socket for reading voltages, no need to go directly to the battery. Its nice to able to easily monitor voltages as you drive.

http://forums.pelicanparts.com/uploa...1239983974.jpg

http://www.amazon.com/Equus-3721-Battery-Charging-Monitor/dp/B000EVWDU0/ref=sr_1_6?ie=UTF8&s=automotive&qid=1239983914&sr= 8-6

What is the voltage drop on the wire harness through the fuse panel?

On my 88 current from the battery flows to the ignition switch through two 4 mm^2 wires in parallel (8 mm^2). Current for everything switched by the ignition switch flows through these 2 wires. This path also includes connector T56 and the connector on the back of the ignition switch. To me it seems reasonable that if you turn on all the accessories the full output of the alternator (+ battery discharge) could flow through this path (100 amps????).

To get back to the lighter the current path is through an actual contact in the ignition switch, the connector on the back of the ignition, a 4 mm^2 wire, connector T56 again and the junction of fuse 4,5 and 6 (6 goes to the lighter) 8, 15 and 25 amps respectively.

If you want to estimate 4 mm^2 wire has a resistance of about 1.34 ohms per thousand feet and 8 mm^2 0.67 ohms per thousand feet. I have no clue as to the contact resistance of contacts in a 20 year old car.

To put this in perspective the wire from the alternator to the starter is 10 mm^2 and the starter to the battery is 35 mm^2

My point here is that I wouldn’t use a meter connected in the dash (even though it indicates it is accurate down to 10 milli volts) to diagnose a battery or alternator problem. You may be getting up to a volt drop in the wiring. You can use it to say yes my alternator is charging but not to say: .