drops under 12 v. with New Alternator

[Tyson Schmidt]

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.

[dralph12]

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.

[foamy911]

+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.

[dshepp806]

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,

[911pcars]

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

[glenncof]

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.

[Wil Ferch]

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

[dshepp806]

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,

[WERK I]

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.

[911pcars]

"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

[dshepp806]

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

[diverdan]

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

Diverdan

[dralph12]

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!!

[dshepp806]

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

[84porsche9113.2]

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...

[dralph12]

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 !!!

[dralph12]

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.

[rick-l]

Originally Posted by dshepp806

IThis 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)....

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?

[Early_S_Man]

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???'

[Lorenfb]

"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.