drops under 12 v. with New Alternator
 

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!!
:confused: :confused: :confused:

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

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:confused:

"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?

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.

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.

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

- Wil

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!!

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.

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?

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)

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.

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

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. :)

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

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.

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

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!!

"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?

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.

Seriously, check the voltage drop between the starter and battery.

Also check the alternator charge-back wire from the alt to the starter.

Your alternator may be putting out 14 volts, but the battery never sees it because either of these wires may be building up resistance when they get warm.

thanks I will check that voltages at the starter. Wire corrosion isn't normally a heat related issue. I have a volt meter in the car and I watch it when Im driving, its around 14 volts them BAM! to drops to 12.XX(or whatever the battery voltage is). anywhere from 5-10 min. after starting the car.

+1 for Tyson's thinking. Heat does in fact play a huge role in resistance and wires, as they get hot resistance builds. A bad wire may be fine when cold, but fail when warm. At this time, disconnect the alternator and battery. Get out a good ohm meter and start checking all the wires that effect your charging circuit.

911Pcars:

I guess I would word it differently,..my bad...(a EE talking...) Pop wouldn't be the right word....... The essence was to CHECK the diodes with a meter (with a diode function),..and, if suspecting thermally induced failures of THESE components within the bridge, one could create that environment with a heat gun (ever so carefully) and measure (i.e.,: look) once (again) carefully heated.....

Although I don't think this to be the problem....hard to say with the original terminal removals while running (hello??)...

Inanycase, hope that clears it up........(bad choice of words on my part, as "pop" has negative connotations relating to a P-N barrier..)

My best,

Doyle,
My comment was more about your assuming this owner to disassemble and diagnose the alternator. Sounds like he may not have the necessary electrical background (nothing personal Dralph12) . For example, in order to check the diodes, they must first be de-soldered so they can be individually inspected with a meter.

If it's faulty, and snce this is a rebuilt unit, the best course of action is to return it for a good one. If it's the regulator (maybe), do likewise.

As Tyson suggested, measuring voltage drop is a good test, but not many people know how to measure it. It isn't the same as measuring continuity with with an ohmmeter. The former is an active test under load, the latter is a static test for a continuous path but not necessarily a path able to support the load. A thin wire is continuous and an ohmmeter will indicate near zero resistance. However, if connected between battery and starter, then the circuit activated, it will surely fry, but if it somehow remains intact, will create excessive current resistance in the circuit due to it's size.

A voltage drop test measures voltage loss between two points. For example, to measure voltage drop between alternator and battery, use two long test leads, one at battery term. on solenoid (the alternator output wire is here) , the other at the + term. on battery. Engine ON, then observe the voltage drop (voltage loss via connections, broken conductor, etc.). Should be <.1 V or so. Test and observe under different engine speeds and temperatures.

For example, if source voltage = 12, and voltage drop test shows 12 volts, it's an open circuit; a reading of 6 volts indicates half of the available voltage is lost/not being used.

Hope this helps,
Sherwood

On my '85 I was driving 70MPH when one battery terminal let loose due to corrosion. Engine 'immediately' died and there was no damage. So I'm surprised you can run with battery disconnected.

I can't explain it but fortunately rush hour traffic was not too bad so I could get off the road.

New regulator should be part of the rebuild since it contains brushes. I would verify this with the rebuild shop after discussing problem. (Oh maybe you did not use a shop for rebuild. In this case the reg almost certainly different if not new.) Does seem like a bad reg though.

Look for manual or Alldata (database) at library for schematic.

Glencoff says ..." On my '85 I was driving 70MPH when one battery terminal let loose due to corrosion. Engine 'immediately' died and there was no damage. So I'm surprised you can run with battery disconnected...."

If it runs then a "substitute" return path was found to complete the DC circuit. I've heard ..that sometimes the electrons will " find " the shift linkage as a suitable "fat" return wire !!! to complete the circuit.......

- Wil

Understood, 911 P-cars. I'm an electronics engineer so I'v not much trouble with assessing the elements of this (relatively) primative charge circuit. Agreed, you are correct in overshooting a bit with this Pelicanite. And I would rescind the reccomendation, based on your points. All agreed on the wiring checks...

I do wish the gent well....and thanks for your observations, comments and question..

My best,

All the alternator shops I've been to have motorized alternator test benches. They put the alternator under load at a predetermined speed. If the alternator output is at the rated amperage/wattage, the diodes are good. You really don't have to disassemble a bridge to test the individual diodes.
You mention early on the alternator light eventually goes out after the engine starts up. How much time? My '85 goes out immediately after the engine fires up.
+1 on what Tyson commented on. Check the posts on the starter where the other alternator leads attach. It would be interesting to see what amperage the alternator is putting out when it drops in voltage at the battery. If possible, start pulling fuses out with the volt meter connected to the battery. If the voltage returns to normal after one of the fuses is pulled, that's your faulty circuit.

"If possible, start pulling fuses out with the volt meter connected to the battery. If the voltage returns to normal after one of the fuses is pulled, that's your faulty circuit."

More accurately, ammeter and current. With a voltmeter, voltage remains fairly constant for most circuits.

Sherwood

Load testing will verify bridge integrity, run long enough...agreed: no need for (personally) testing the diodes...the convenience of taking it to the shop, eh?

...Ammeter measurements only...just as 911pcars stated..

Keep on it....esp. the wiring integrity..

Best of luck

Stupid possibility. Is the belt new and adjusted correctly? Old belts can slip silently.

Diverdan

Found the problem!
Took car to local shop, they check system and said it still was the Alternator.
I removed the Alternator and had it tested at a electrical shop.
Bad Solder joints on this Alternator diodes were found.
My advice!
Don't order Autolite rebuilt alternators from Kragen Auto Parts.
I received Two bad Alternators in a row.
I have installed a new 95 amp Pelican Alternator
Problem Solved!!

I assume you're speaking of your ORIGINAL alt? (Since the 2 replacements were "bad", we would assume those have long been returned) ....

Goes back to looking at the bridge (and measuring),..assuming that one has the knowledge and eye to do so...these solder joints can be "bad" in a few ways,..thermally is definately high on the list for an original alt insofar as solder joints go...lots of times this timed heat process will provide VISUAL clues as to solder connection integrity (cracking, etc.)

This weekend, just for the hell of it, I put my bridge on a curve tracer (on my o-scope) just to get gnat's ass on PN junction integrities in this primitive electrical circuit in my beloved baby.....I've longed measured Mr. silicon's forward/reverse bias voltages (yes, using the diode function of the meter...) but this never tells you all about the junctions at the level I'm looking at......even applied light heat and observed the tracer results....I had one diode that had an ever so slightly "less-than" it's brethren'...but still quite functional...interesting data points, for me. I reflowed ALL the points long ago, decreasing the chances of solder problems (ie.,:NOT components)....

Anywaze,...glad to know you finally resolved the source problem......feels good, eh?

Best to all

I'm glad the problem above was solved. I had a minor question that was in a way related to this post, so I thought I'd ask it here...

Does anyone know what an in-dash voltmeter should read with the a/c turned on fan position 2 or 3 and headlights turned on? At idle, my voltmeter (just installed one too) reads above 14V. At night, running with the stereo, headlights and a/c fan in position 1, I think it reads about 13V. With the a/c fan in position 2 or 3, it is reading only 12V or so.

I just thought 12V seemed quite low so I was wondering if this is normal or not. I don't really think it is an alternator issue since when I turn off the a/c it goes right back to 14V at idle...

On my 86 I installed a Oil Cooler fan, fan on the rear A/C Condenser, and a Voltmeter.
At night with the lights, A/C Fans and the Oil cooler fan running the volt meter reads 13.5 volts with the car above 20k rpm's
but at Ideal it reads 12.5
With everything off during the day the volt meter reads 13.8 to 14.2 at ideal.
I don't know what it should read, but I can say that after normal city driving at night with everything on the battery still reads 12.5 the next morning,
GOOD at NIGHT !!! Ready to Run in the morning !!! thats how I like my Cars and Woman !!
AND thats all I need !!!

One more thing You may want to check the battery voltage with the car off
it should read 12.5 of not the battery may be running low.
remember if you recharge ithe battery remove the Neg. cable to isolate the battery during charging.
My Original problem was that after recharging the battery I would go out at night with ever thing on "Lights and fans" the next day I would check the battery and if would be below the full charge "say 12.35 volts" another day or so of driving and the battery would read 12.2.
So the alternator wasn't recharging the battery. Make sure to check all grounds on the car before diving into the alternator as a problem.

Do you know what the reverse standoff voltage is for those (8) diodes?

For the run the car without a battery crowd what is the open circuit peak AC voltage?

dralph12,

You have made reference to 'at/@ Ideal' more than a dozen times in this thread, and I would venture to say there isn't anyone out here reading your thread that knows what you are talking about!!!

Just what is this condition you call '@ Ideal???'

"I reflowed ALL the points long ago, decreasing the chances of solder problems (ie.,:NOT components)...."

Not easy (requires a hot plate or a torch) and not a good idea
as the diodes can be damaged with too much heat. The diodes
are usually damaged by using those "monster" batteries
and/or revving the engine when the battery is very low to
charge it quickly, i.e. exceeds the 90 amp rating.

"Do you know what the reverse standoff voltage is for those (8) diodes?"

There're more than 8 diodes (14).
The PIV of the diodes are usually 400 volts.

"For the run the car without a battery crowd what is the open circuit peak AC voltage?"

The peak output of the alternator can reach 30 volts (bad regulator) which
usually destroys the DME or a CDI in the older cars.