911 SC Alternator Battery Issues
02-25-2005, 12:58 AM
#1
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911 SC Alternator Battery Issues
About a month ago I started having issues with my alternator, oxygen sensor and parking/hand-brake lights coming on and getting brighter as revs increased while driving. A quick check with my multi-meter showed the voltage at the battery getting up to 17.9 volts (eek). Now, after replacing the (external) volatage regulator, the voltage never goes above 14.8 volts, but the alternator light is never quite going out. Either the high voltage fried my expensive optima battery, or the alternator is on its way out as well. Anyone any ideas which one I should pick 
?
?
02-25-2005, 01:27 AM
#2
A glowing alt light may result from a bad alt diode or a voltage drop in the alt
feed wire to the battery. On a 911SC, the most common connection problem
is either at the alt (no solder connection & just crimped connector) or at the
starter. If the light gets brighter with an increasing load, it's a connection problem.
If the light glows dimly, most likely it a bad connection from the alt.
Actually, the 14.8 value is a little too high of a voltage. It should be about
< 14.3. With the voltage being 14.8, some "monster" batteries could damage
the alt.
The later Porsches (964/993) with Bosch alts had the more ideal voltage setting
of 13.8 to 14.2 which placed less stress on the alt.
feed wire to the battery. On a 911SC, the most common connection problem
is either at the alt (no solder connection & just crimped connector) or at the
starter. If the light gets brighter with an increasing load, it's a connection problem.
If the light glows dimly, most likely it a bad connection from the alt.
Actually, the 14.8 value is a little too high of a voltage. It should be about
< 14.3. With the voltage being 14.8, some "monster" batteries could damage
the alt.
The later Porsches (964/993) with Bosch alts had the more ideal voltage setting
of 13.8 to 14.2 which placed less stress on the alt.
02-25-2005, 07:35 AM
#3
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If you decide to replace your alternator, you may already know that Porsche went to a new alternator design in, I believe, 1982, which has the replaceable regulator on the alternator. If you don't have your existing one rebuilt, you may have difficulty finding one without the internal regulator. Porsche published a bulletin which steps through what wires to cut and which to use to install the newer style. If you happen to have the Porsche parts book that Porsche sold for a few years, the bulletin in there, otherwise any dealer should be able to provide it to you.
02-25-2005, 11:24 AM
#4
Bill is correct, which was one of Porsche's mistakes, i.e. to use the Paris-Rhone
alt. in '82 & '83 (the worst years for Porsche alts.). That alt was the most unreliable
alt of any Porsche used. The one used in the SC ('78 - '81 a Marchal) was one of the
best alt.
One should always try to have the original extrenal regulator alt rebuilt vs using the
later internal regulator Paris-Rhone alt.
alt. in '82 & '83 (the worst years for Porsche alts.). That alt was the most unreliable
alt of any Porsche used. The one used in the SC ('78 - '81 a Marchal) was one of the
best alt.
One should always try to have the original extrenal regulator alt rebuilt vs using the
later internal regulator Paris-Rhone alt.
03-01-2005, 03:44 AM
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Thanks for the replies. I do have the tech note for how to switch to and internal voltage regulator should I decide to update to a newer alt... Does anyone know a source for rebuild kits for alternators?
Andrew
Andrew
09-12-2016, 02:33 PM
#6
Former Vendor
Sorry to revive an old thread but my '83 just started doing the same thing. The alternator light gets brighter as the RPMs increases and it actually shuts down the Alpine amplifier the higher the RPMs get. Is it still the preferred method to rebuild the existing alternator or has a decent replacement come on the market over the course of the past 10 years? One other strange thing that happens is my clock stops working when the car is on but the minute I turn of the engine it starts ticking again. I am hoping it is related to the alternator or the regulator but if its not, does anyone have any experience with the same strange behavior? Is something wrong outside of the regulator/alternator arena?
10-09-2016, 12:37 AM
#7
Rennlist Member
For the community...my pre-start voltage is good...just over 12V. My voltage after start goes just above 14 then settles down to mid to low 13s. A high load (lights on, heater fans or a/c fans running) and voltage drops below 13. I think this is all normal for the last 10 years of ownership. Only oddity is that I have noticed in the past few weeks, every few days I drive my car, my original clock on my 88 is indicating 5-15 minutes fast. Any thoughts?
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10-09-2016, 11:30 AM
#9
Former Vendor
Just a data point for others- my non-working clock when the engine was running was a sign of the pending failure of the voltage regulator. Upon replacing both the alternator and regulator all is well again. Battery light is gone and the clock is functioning normally.
10-09-2016, 03:26 PM
#10
go to an elec. shop and get things tested - the guy will have a large cart with a load tester on it for the battery, and can also test the VR and alt. easily since your VR is external
if he uses a handheld gizmo and insists it is a "load tester" then find another shop
replace the fan belt - if the old one is in good shape and not glazed, cracked, etc. then keep it in the trunk as a spare -- repeat every few years
if he uses a handheld gizmo and insists it is a "load tester" then find another shop
replace the fan belt - if the old one is in good shape and not glazed, cracked, etc. then keep it in the trunk as a spare -- repeat every few years
