jeff jackson

Bill...you must be referring to the wire from the battery to the starter...Not the charging wire from the alternator to the battery... I am thinking this wire should be around AWG 8 after the upgrade...

Lorenfb

"Increasing the excitation current at low RPM's may help with the voltage, but is also likely to cause rotor heating that may lead to premature failure." - dr bob -

1. The diode inserted into the ground circuit of an alt's. regulator just increases
the alt's. output voltage and ONLY when an adequate RPM is reached.
2. The diode will NOT really increase the field current. Although the field voltage
increases with the regulator voltage increase, there's another voltage drop now
in series with the field winding because of the diode reducing the field voltage.
3. As stated in #2, the field current will remain the same and not cause a premature
alt failure. Even if the field current increased, it would be about a 7% increase or
a about 14% in field power/heat.

A. This approach has questionable merit for idle charging issues because an alt's output
is a function of RPM & not just field current.

B. This approach is problematic for a premature alt failure because of the now much
high alt. output current. Remember, I = V / R. This is why the Paris-Rhone of the
'82-'83 928 alts fail, i.e. regulator set at 14.75 versus a lower Bosch alt regulator,
besides the poor overall design. Also remember that as the battery voltage exceeds
its design value its current increases significantly potentially causes an alt failure,
e.g. Paris-Rhone, as do bascially all load current.

dr bob

Responding to Loren--

It looks to me like the diode is in the sense line to the regulator. That would cause the regulator to read an output voltage about .7 volts lower than actual, right? (silicon junction voltage drop is about 0.67 IIRC, assuming a silicon diode)

Lorenfb

The Bosch regulator has only three leads:

1. the sense line - measures the input voltage
2. the ground line - a reference for the sense line & the return line for the
field switch line
3. the field switch line - the line which switches the field on/off

The diode shown in the image on this thread appears to go to the ground screw.
This causes the alt. voltage to be higher by the forward diode voltage .65 - .90
depending on its current. It doesn't matter whether the diode is placed in the
input sense line or the ground line, the regulator "forces" the alt. to acheive
a higher output voltage before it switches "off' the field current.

T_MaX

From 86-95 Porsche used several differant regulators in the 928. I wonder what the specs are for these?



I like the idea of upgrading the battery cable to a multi stand -0- gauge, however if you are running 100+watt lights and an aftermarket amp or two. You may want to look into doing something about upping the alt output a little as well.

Again, JMHO

checkmate1996

Another 'work around' for you guys who put stereos and other electronics in your car...is I put a 3 Farard capacitor in my car and then ran all my accessory electronics stereo,etc from that. It captures the stored energy from the battery so the alternator and battery aren't working as hard. Completely solved the dimming lights problem at night...

Bill Ball

Thanks for answering this, Loren.

SharkSkin

Some good points above -- I have found in the past that some alternators are simply not capable of providing much current at idle. While I have not dug into the 928 system as yet I had a related experience with my Ramcharger about 17 years ago.

There is a local rebuilder, Pacific Auto Electric, that does rebuilds for most of the high-end shops nearby(at least, the ones that consider Mex rebuilds unacceptable). In the process of trying to get the charging system sorted out I had to go back a couple of times, since the first regulator they sold me had a Mfr. defect and even after that I wasn't getting enough charge at idle to handle my aux lights, defroster fan, etc.

So I talked to their top guy at length, and what he told me is essentially the same sort of thing that Bob and Loren were talking about above, that many alternators will simply not charge well below a certain RPM, which varies by model. He illustrated the point by providing 16V to the field circuit from a separate power source, which only changed my charge current @ idle by about 5 amps or so. I already had the smallest possible pulley on the alternator. He recommended against a larger crank pulley because he felt that the next size up would overrev the alternator at 5K RPM. Fortunately for me, he knew how to combine stator, rotor, and diode assemblies from three different Mopar alternators to create a unit that would charge 80 amps at idle, and provide up to 130 amps at full tilt. Shortly afterward, I was stuck in 3 hours of bumper-to-bumper traffic trying to return home after the quake of '89 and with my lights and heater running constantly I had no problem whatsoever with charge @ idle.

Moral of the story -- some alternators are simply not capable of providing significant current at idle, no matter what what tricks you play with the field current/voltage, and if you have such an alternator there is no fix short of replacing it with one that will give the desired output.

I had a recent scare with the shark, charge light coming on while stuck in traffic with defroster, lights, wipers, and aux cooling fan on. This has me seriously considering the Delco conversion mentioned in another recent thread: https://rennlist.com/forums/928-forum/245494-delco-alternator-conversion.html

borland

This PDF file I believe describes the same Bosch alternator that the later 928's have, however the housing is for a Audi. It gives some specs, also states that the alternator should put out 20-30 amps at idle, and 90-110 amps at 2000 rpm. Still a good reference.

http://www.easternbeaver.com/Home/Main/Faqs/Alternator.pdf#search='slip%20rings%20bosch%20alternator'

borland

FlyingDog

Hmm... I think my father has a spool of 0 gauge welding wire in the garage for his boat wiring.

Alan

For those of you with high load issues - while the issue is probably most of concern at idle - if the alternator voltage is increased across the spectrum (and actually probably moreso @ high rpm) your loading will actually go up under normal operating conditions so it a bit self defeating... Running systems at say 0.7v higher than nominal (say 14.5v) will increase electronic wearout mechanisms somewhat (still within spec in all likelihood - but these cars are already old right...). Things like headlamp bulb lifetimes suffer significantly at higher voltages.

Given your Alternator is struggling with a high load and thats why you'd do this mod - increasing its O/P voltage just makes that load higher under normal conditions and will likely stress the alternator more.

Seems what you really need is an alternator which will supply a higher current @ nominal voltage at both low & high rpms. This would be a higher current rated (and a good low rpm source) alternator - no?

Alan

jeff jackson

Alan...
Thats probably true (about the higher current delivery being desireable over the .7V increased voltage)...but my 86.5 alternator already has a 115 amp alternator...is the GTS unit of higher capacity, or some other known compatible upgrade unit available ?? My car is only upgraded by a 300W 4 channel Alpine stereo amp, a 200W Infinity Basslink sub, and a Valentine 1 (which I know is nothing in terms of load), and I doubt the other amps are a severe strain...but with the headlights...the foglights...the HVAC fan...(let alone the rear AC)...the voltmeter struggles (especially at idle)...to keep its nose above 12. ...and the pod instrument illumination, as well as the HVAC controller lighting, are not all they should be. Any options I am unaware of that could "improve" the situation ?? Thanks for your help.

SharkSkin

That's exactly what I was getting at... and as relates to Jeff's follow-up question, that 115 amp alternator isn't putting out 115A at idle, guaranteed. Even if it's in perfect condition.

Bill Ball

Dave said: "I already had the smallest possible pulley on the alternator."

Now you've got me thinking that maybe my problem is related to the crank pulley that replaced the stock pulley in my SC setup. I'm using the stock alternator pulley, but the crank pulley may be smaller. Need to check that, and if so, look for a smaller alternator pulley.