drops under 12 v. with New Alternator

[dralph12]

I have a 1986 911 Coupe failed to regulate with Second New Rebuilt Alternator.

Here the issue:

Battery voltage at startup is 13.8 volts with Car @ Ideal. Battery voltage drops while the car is running when the car warms up the voltage drops from 13.8 to 12.xx (whatever the true battery voltage is at the time).
I have a voltmeter connected so I can watch it while I drive the car. I can park the car after I see the voltage drop and remove the Neg. side battery cable and between the car ground and the positive side of the battery I read below 12 volts.
So I would hate to think the Second New Rebuilt Alternator is bad. . I took the car to the shop and they found the ground strip on
the transmission loose but it still didn't fix the problem.

Here is the story:
Second New Rebuilt Alternator same issue it will now keep the new battery charged.
I removed battery and rechecked at the shop yesterday (batt. is good).
Also put a DC amp meter in series with battery, there is no current flow when car is off.
Dash Battery indicator lamp illuminates at startup then goes out.

Car starts strong and after removing Neg. cable from battery car continues to run with alternator voltage below 12 volts.
Even when I turn on head lights,aftermarket oil cooler fan (Spal 8" 7amp fan), and AC with aftermarket fan (Spal 8" 7amp fan)
on the rear condenser.


I have no wiring diagrams, and no idea where to start looking. I have removed the alternator 4 times
to review the connections and make sure there tight. I have removed the indicator lamp and check the connections on the gauge.

Any Ideas?
HELP!!

[Por_sha911]

Wild guesses:
Does the rebuilt alt include a new regulator?
What takes power once the car is warmed up? Maybe it has a short?

[Wil Ferch]

Dan Ralph:
You say ( in two places).." I can park the car after I see the voltage drop and remove the Neg. side battery cable and between the car ground and the positive side of the battery I read below 12 volts.

,.....what am I missing? Why remove the ground cable when either checking voltage or checking the running condition electrics of the car? You can ( make that "should") measure voltage directly across the two battery terminals ( either car "running" or "off").

??????

- Wil

[dralph12]

"Battery voltage at startup is 13.8 volts with Car @ Ideal. Battery voltage drops while the car is running when the car warms up the voltage drops from 13.8 to 12.xx (whatever the true battery voltage is at the time).
I have a voltmeter connected so I can watch it while I drive the car. I can park the car after I see the voltage drop and remove the Neg. side battery cable and between the car ground and the positive side of the battery I read below 12 volts.
So I would hate to think the Second New Rebuilt Alternator is bad. . I took the car to the shop and they found the ground strip on
the transmission loose but it still didn't fix the problem. "

One way to see iv the alternator is working correctly is to remove the battery once the car is running?

[Wil Ferch]

Don't disconnect a battery with a running car (alternator possibly still charging.....)

- Wil

EDIT-> your electrical system needs a ground path to complete a proper DC circuit. Don't ever remove a ground strap from a battery and run the car or its alternator. If it runs it means somehow a ground path was "found" by the system....I've heard sometimes the shift rod linkage "becomes" a ground path !! You can measure battery voltage across both battery terminals...either in a non-running standby condition or when the car is running. A fully charge battery is 12.6 volts. It drops fast after that:
12.6= full charge
12.4 is 75% charge
12.2 = 50% charge
12.0 = 25% charge
below 12 = "dead" for all intents and purposes.

With car at 2000-3000 rpm, measured voltage across the battery terminals ( everything still connected !!!) should be about 13.8 to 14.4 volts or so. Anything above 15V would indicate a possible bad voltage regulator, anything less than 13.8 would likely indicate something wrong with the alternator , although it still could be a voltage regulator ( just less chance) and possibly grounds. Also...go nuts and check ALL ground conditions, especially the main cable on the battery and the ground strap on the trans-to-body.

[foamy911]

If you disconnect the battery with the engine running, the regulator senses 0 volts and sends all the alternator output possible to try and charge the battery. But if it was disconnected where would that voltage go ? So disconnecting the battery while running will most likely damage your alternator/regulator.

[Wil Ferch]

Damage may already be done with multiple atempts running car with battery disconnected..... that's my bet, sorry to say.

- Wil

[dralph12]

Ok the alternator is grounded at the engine. the car and engine are GROUND when the Car is running. The Battery is only used to start the alternator and the engine starter. Once the car is running the current and voltage is maintained by the alternator. removing the battery negative cable at this point shouldn't damage the car, and it should show if the alternator is maintaining the correct voltage/current need to run the car. BUT I have replaced the alternator twice and the voltage still drops after the car is warmed up. it starts at 13.8 but drops all at once when the car is out on the road ( about 10 min) or less.


I have a 1986 911 Coupe failed to regulate with Second New Rebuilt Alternator.

Here the issue:

Battery voltage at startup is 13.8 volts with Car @ Ideal. Battery voltage drops while the car is running when the car warms up the voltage drops from 13.8 to 12.xx (whatever the true battery voltage is at the time).
I have a voltmeter connected so I can watch it while I drive the car. I can park the car after I see the voltage drop and remove the Neg. side battery cable and between the car ground and the positive side of the battery I read below 12 volts.
So I would hate to think the Second New Rebuilt Alternator is bad. . I took the car to the shop and they found the ground strip on
the transmission loose but it still didn't fix the problem.

Here is the story:
Second New Rebuilt Alternator same issue it will now keep the new battery charged.
I removed battery and rechecked at the shop yesterday (batt. is good).
Also put a DC amp meter in series with battery, there is no current flow when car is off.
Dash Battery indicator lamp illuminates at startup then goes out.

Car starts strong and after removing Neg. cable from battery car continues to run with alternator voltage below 12 volts.
Even when I turn on head lights,aftermarket oil cooler fan (Spal 8" 7amp fan), and AC with aftermarket fan (Spal 8" 7amp fan)
on the rear condenser.


I have no wiring diagrams, and no idea where to start looking. I have removed the alternator 4 times
to review the connections and make sure there tight. I have removed the indicator lamp and check the connections on the gauge.

Any Ideas?
HELP!!

[foamy911]

Originally posted by dralph12

Any Ideas?
HELP!!

We are trying to tell you that running the car with the battery disconnected will damage your alternator as the voltage has NO WHERE TO GO. My next piece of advice is for you to sell your car and buy a Honda, or listen to the Pelicanites who are trying to help you.

[dralph12]

Wait
Im not running the car with the battery disconnected!
I just did it as a test to see if the alternator would KEEP the car running PARKED!
Please I'm trying to ask if anyone has seen this issue:
Battery voltage at startup is 13.8 volts with Car @ Ideal. Battery voltage drops while the car is running when the car warms up the voltage drops from 13.8 to 12.xx (whatever the true battery voltage is at the time).
I have a voltmeter connected so I can watch it while I drive the car.
So I would hate to think the Second New Rebuilt Alternator is bad. . I took the car to the shop and they found the ground strip on
the transmission loose but it still didn't fix the problem.

ANY IDEAS?

[Wil Ferch]

Parked or not parked .. answer this:

Did you run the engine with the negative cable disconnected from the battery?.

If "yes", you may have already done-in the alternator. It doesn't take long or it dosn't need a "loaded" engine...as if you're driving.

PARKED may do it too !

- Wil

PS- perhaps RoninLB or other electrical gurus can chime in here.

PPS - Bentley Manual for Carrera, page 900-2 , Electrical System-General... fourth bullet paragraph under "CAUTION"- - ->

(quote) ...."do not disconnect the battery with engine running" ...(unquote)

[dralph12]

Ok !! Ok
I don't think I did any damage!
I just drove the car and my volt meter across the Battery reads 13.8 volts, but what happens is that when the car warms up the voltage drop off to the battery voltage 12.4 If I let the car sit a while and re-start it its back to the 13.8 volts. The time it takes to drop off to the 12.4 volts depends on how cold the car is when I start it. Today here in SoCal its about 80 Deg. so I can get a few miles before the car warms up, but once the car is warm it doesn't matter when I start it the voltage stays @ 12.4 or that ever the battery voltage is at the time. after awhile I need to recharge the battery.

[Wil Ferch]

Your description isn't entirely clear ( at least not to me)...but from the sounds of it...you're not "charging" at some point in the alternator's function cycle. The alternator isn't charging at times.

I'd take the alternator out and bring it to an electrical shop and have it "load" tested...along with a general "look-see" on the slip rings and commutator brushes. Diodes should be checked too as part of the process. I vote bad alternator that has a temp-dependent fault ( diodes?).

- Wil

[Porsche_monkey]

Turn your fan onto high when you read 12.4 volts. The alternator should be required at that point and you should be at 13volts or more. If not your voltage regulator may be baffed.

And your battery should measure more that 12.0 volts resting (nothing connected). Maybe your battery is near the end of it's useful life? Since this is a replacement alternator I will guess that you had a charging issue, and that may have destroyed your battery also.

And finally, is your battery weak when you start the car? From the info provided it seems like there may be nothing at all wrong, other than all the measuring you are doing, which is driving you crazy.

[dshepp806]

Agree with Wil...check the diodes, F/R biased...then pop 'em with a heat gun . measure again,...looking for thermal induced breakdown..

Best yet, get it to a shop and have em do as Wil noted for you. Hope you're clear on the no-no of battery disconnect WHILE ENGINE IS RUNNING,,...parked, .... slowly rolling,.............. at 150MPH ,...........or even 10 feet of vertical levitation.........ultimately, an expensive bad habit... I'm sure foamy911 would agree that you may even find a Honda shop that could check it for you....?

Once we know the ALT/REG is good then, all can go forward...

Best of luck

[Tyson Schmidt]

I didn't read all the responses, but I'll throw this in the ring:

Check the voltage at the starter junction and compare that to the reading at the battery connection.

You may have some voltage drop through the length of the battery cable that increases with heat.

[911pcars]

Typical charging spec is around 14.2 volts. 13.8 is a little below spec but serviceable. depending on the number of working loads and accessories.

You say there is no parasitic current draw.

The battery voltage falls to 12.4 with engine OFF.

If so, my conclusion is your battery may be kaput. Have it load tested to confirm.

dshepp806,
Did you suggest to this owner to "pop the diodes" and look for thermal induced breakdown?

Sherwood

[dralph12]

Here the issue:

Battery voltage at startup is 13.8 volts with Car @ Ideal. Battery voltage drops while the car is running when the car warms up the voltage drops from 13.8 to 12.xx (whatever the true battery voltage is at the time).
I have a voltmeter connected so I can watch it while I drive the car.

So I would hate to think the Second New Rebuilt Alternator is bad. . I took the car to the shop and they found the ground strip on
the transmission loose but it still didn't fix the problem.

Here is the story:
Second New Rebuilt Alternator same issue it will now keep the new battery charged.
I removed battery and rechecked at the shop yesterday (batt. is good).
Also put a DC amp meter in series with battery, there is no current flow when car is off.
Dash Battery indicator lamp illuminates at startup then goes out.

Car starts strong


I have no wiring diagrams, and no idea where to start looking. I have removed the alternator 4 times
to review the connections and make sure there tight. I have removed the indicator lamp and check the connections on the gauge.

Any Ideas?
HELP!!

[911pcars]

"Here is the story:
Second New Rebuilt Alternator same issue it will now keep the new battery charged.
I removed battery and rechecked at the shop yesterday (batt. is good).
Also put a DC amp meter in series with battery, there is no current flow when car is off.
Dash Battery indicator lamp illuminates at startup then goes out.

Car starts strong "

If all is well, what is the current issue?

[dralph12]

Battery voltage at startup is 13.8 volts with Car @ Ideal. Battery voltage drops while the car is running. The car warms up and the voltage drops from 13.8 to 12.xx (whatever the true battery voltage is at the time). The car is running on the battery voltage and after a few hours of driving the battery voltage drops to below 12 volts and the Tachometer goes wack'o. I then need to remove the battery and recharge it. I have replaced the Alternator twice (they are autolite alternator), and I replaced the battery.

It still seems like the alternator is unable to regulate once the car warms up.