Alternator or Regulator fail ?

[percy]

Have done a search, but couldn't find exactly what i needed

1988 911 Cabriolet -- chain of events
Slightly slow crank on starting, but spirited 15 minute freeway run
Exit freeway, first traffic lights, rough idle, tacho bounces around 3000 rpm at idle.
Next traffic light, idle all over place, followed by stall. Unable to get engine to crank.
Rolled car onto footpath -- insufficient charge for hazard lights ! (can just hear relay ticking)
Call nice AAA bloke -- Battery shows 8 volts !
He suggests alternator failure, as no warning light appeared on dash panel at any time.
Take car home, stick on trickle charger overnight, starts fine next day. However, this time, intermittent blinky warning light at low rpms
Questions
How to work out whether alternator or regulator has failed ?
Should replacement alternator have integral regulator?
Or can I get a Bosch reman alternator, and pair it with a Marchal regulator (Pelican Parts)
Thanks in advance

[T77911S]

pull the alt and have it rebuilt.

[Discseven]

Originally Posted by percy

Questions
How to work out whether alternator or regulator has failed ?
Should replacement alternator have integral regulator?
Or can I get a Bosch reman alternator, and pair it with a Marchal regulator (Pelican Parts)
Thanks in advance

Percy... I had same odd tach readings when my alt was dying. And same ability to charge battery and start car but to no lasting avail. Way to diagnosis alternator vs voltage regulator = volt reg basically stops alt from OVERCHARGING system. So, if system is not charging sufficiently to over charge... it's alternator problem. If system is overcharging... it's voltage regulator not doing its' job.

Lack of charging could possibly be ground strap connecting engine/trans to chassis coming loose. It's a long shot but worth checking before anything else. Not 100% sure where strap is on your '88. Typically it's on right side of trans connected to bulkhead of car next to where shifter linkage enters/exits car.

Replaced my alt with this BOSCH rebuilt alt for $300 (after handing over my core.) Voltage reg is internal to this--VALEO. Will suggest return of old core and buy of new part is more economical to do locally as compared to shipping these parts due to weight. When alt was out of fan housing, had fan housing lathed so alt would fit into/out of housing with perfect slip fit.

ORANGE TAG WARNING NOTICE: when installing alt, battery should be disconnected and battery should be fully charged or alt may be damaged.

[T77911S]

diodes can be bad in the alt.
the ONLY real way to tell is to have the alt tested.
you can drive the car to some place like advance, they test the charging system for free. make sure the battery is fully charged when you go.

send it off and have it rebuilt.

[DRACO A5OG]

Make sure you take your pulley half, washer and nut with you, the shop will most likely not have one to test your alternator.

+1 have it tested and/or remove regulator to see if the brushes are worn down.

[wwest]

If the alternator charging light only comes on with the engine off and always goes off with the engine running, or slightly above idle for early 911's, then you have 1 of 2 problems.

1. The battery is being overcharged... Not bloody likely with your symptoms!

2. The battery cannot accept a full charge or has an internal short....

[scarceller]

Charge the battery with a good charger so it has >12.0vdc then start car and measure voltage at the battery it should be 14.0 to 14.7vdc with engine running. If not you need to have the alternator tested or replaced.

Also check the ground strap at the front of the transmission mount, this strap grounds the drive train to the car's body and if it fails it compromises the alternators ground to the battery. The strap is near where the trans shift rod enters the center tunnel right under the rear seat area on the underside of the car.

[wwest]

Originally Posted by Discseven

Percy... I had same odd tach readings when my alt was dying. And same ability to charge battery and start car but to no lasting avail. Way to diagnosis alternator vs voltage regulator = volt reg basically stops alt from OVERCHARGING system. So, if system is not charging sufficiently to over charge... it's alternator problem. If system is overcharging... it's voltage regulator not doing its' job.

Lack of charging could possibly be ground strap connecting engine/trans to chassis coming loose. It's a long shot but worth checking before anything else. Not 100% sure where strap is on your '88. Typically it's on right side of trans connected to bulkhead of car next to where shifter linkage enters/exits car.

Replaced my alt with this BOSCH rebuilt alt for $300 (after handing over my core.) Voltage reg is internal to this--VALEO. Will suggest return of old core and buy of new part is more economical to do locally as compared to shipping these parts due to weight. When alt was out of fan housing, had fan housing lathed so alt would fit into/out of housing with perfect slip fit.

ORANGE TAG WARNING NOTICE: when installing alt, battery should be disconnected and

"battery should be fully charged or alt may be damaged."

Another "old wives tale" that should have been put out to pasture many years ago.

Alternators have a built-in self limiting current flow, an 80A alternator will self-limit to ~80A, but certainly no more than 100A. Once the stator(s) magnetic field created by the generated, output, current flow rises high enough to overcome the rotor magnetic field the limit is reached.

The alternator cannot produce this level of current except with engine RPM elevated well above idle in which case the cooling airflow is sufficient to cool the diodes and VR.

The diodes and VR are designed, specified, to provide and sustain the maximum rated current flow provided sufficient cooling airflow is available.

Over my MANY years of driving I have jump started many cars with a dead battery, many of my own. The Alternator CANNOT be damaged simply recharging a dead battery.

On the other hand the alternator/VR combination WILL/CAN damage a battery, boil the electrolyte away, if it is used to recharge the dead battery time after time, repetitively.

Your VR has no method of sensing the battery charge level, SOC, it will "signal" the alternator to put out the maximum current flow possible given the engine RPM limiting factor, OR a voltage maximum of ~14 volts.

https://books.google.com/books?id=kXcNH5XwNHIC&pg=PA193&lpg=PA193&dq=current+self-limiting+alternator&source=bl&ots=h4XScpjqC4&sig=qa3P7pSUfqXOopfyqTWMXt_EboY&hl=en&sa=X&ei=H-CqVLKsA8SuogTWiIH4Cw&ved=0CDAQ6AEwAzgK#v=onepage&q=current%20self-limiting%20alternator&f=false