Colin Stamp <col.dustbin@[EMAIL PROTECTED]
> writes:
>On Thu, 01 May 2008 18:14:25 -0700, Bill Jeffrey
><wjeffreyAT@[EMAIL PROTECTED]
> wrote:
>>1993 900S turbo convertible (USA)
>>
>>So I checked the alternator specs in Bentley. The "80 amp alternator"
>>puts out 80 amps only when the engine is screaming at 6000 RPM! And I
>>don't drive that way (that's why I still have a 1993 Saab!) At 2000 RPM,
>>it is spec'ed to put out only 54 amps. And below 1900 RPM, the output is
>>essentially zero. And that is pretty pitiful against the requirements of
>>the electric radiator fans and the air conditioner clutch. In these
>>conditions, I can actually see the voltage drop and rise as the
>>directional signals flash. And Lord knows what happens if I turn on the
>>headlights.
In my both my running 900's (81 turbo and 89 900i), the big increase in
load
when the fans turn in is quite noticable, esp. if the car is idling. In
the
89 900i especially, it creates so much load that the engine speed is
noticably affected for a few seconds. I doubt the alternator is doing it
directly though - would be more the reduction in voltage affecting the EZK
ignition box.
I have a voltage gauge (connected not directly on the battery but on the
switched side of the power to the interior devices so it shows the real
voltage) in my 81 turbo and when the fans turn on at idle, the voltage
tem****arily goes down to around 8.5 volts but quickly comes back up to
12.5
or so. I haven't tried watching the voltage level in the 89 car when it's
fans turn on.
>>So. Now I know what is happening. The alternator cannot put out
>>sufficient current to sup****t all the goodies that Saab has installed.
>>And thus the final question (thanks for your patience) is whether there
>>is a way to increase the alternator output at low RPM - OR - is there an
>>aftermarket alternator that will put out a bit more current at low RPM,
>>without making major mechanical mods to the alternator mounting system.
80A isn't really much considering all the potential loads that are
present,
but remember most of this is 'peak' load. All the base-load electrical
power
is actually from the battery, and the alternator just handles peak loads
while also recharging the battery.
Things like the fans, headlights, etc. are not base-load electrical
devices
so take them away (not that you really can, but if you could) and the
alternator wouldn't have any problem at all.
I've noticed that both alternators in my 900's have mounting bushes that
are
a bit worn so they're in need of repair, and the alternators themselves
don't sound the best, so they probably are not giving optimal output. They
do both have new voltage regulator units though.
>Are you sure the RPM figures you've found are for engine RPM and not
>alternator RPM? There's normally some gearing done on the pulleys.
>It's about 2.8:1 on my 2001 9-3. 2000 alternator RPM is well below the
>engine idle speed.
Hmmm, interesting point. The alternator pulley size is about half to 1/3
the
crankshaft pulley size in my c900's (not sure if this is the same in other
Saab models with different belting arragements) so that gives an automatic
'gearing' by virtue of pulley sizes alone of somewhere between 2:1 and
3:1.
The alternator at idle (nominal 850 rpm for a c900) is rotating at
approximately 1500 to 2000 rpm (if my basic math on how pulley size
affects
driven devices is correct!). This shows why things like belt tension, etc.
are im****tant as the bearings must get a thra****ng in alternators.
>It certainly sounds wrong that you can't maintain a charge with the
>engine running though. The 9-3 happily manages 14.4V at idle with
>headlights, foglights, HRW and seat heaters all on. You could get the
>alternator tested off the car if you take it to an auto electrical
>specialist. If it really is bad design, you could try fitting a
>smaller alternator pulley, but be careful that the alternator won't
>get over-speeded at max engine revs. Do you know what the pulley ratio
>is on the 900?
I think a good thing to check would be the voltage regulator in the OP's
car
alternator. If the brushes are worn down, or the v/reg itself is fairly
old,
it might not be able to provide the proper 13.5 to 14.5 nominal DC output
under the full range of normal rotational speeds.
That relates back to my point before about how the alternator is a 'peak'
load device. It's primary function is charging the battery, and the peak
load 'feature' is secondary, though because when charging it's pu****ng
electrons against the battery's internal resistance, much of the
electrical
output from the alternator feeds directly to load devices even though
that's
not the main function of the alternator.
>The other way would be to fit a higher capacity alternator from a
>different model - they do tend to work better all the way down to low
>revs. Check pulley ratios again though. Bigger alternators are often
>intended to run faster.
That's possible. I don't think Saab used serpentine style belts on any
c900's so that could limit the choice as high-amperage alternators all
seem
to be serp-belt types. Standard double-belt pulley alternators are fairly
common on places like Ebay, etc. but I haven't come across many that have
ratings of more than about 80 or 90 A. That might affect what the OP could
achieve with a replacement (non-standard) alternator to replace the normal
one.
Craig.
--
Craig's Saab C900 Page at | Craig's Classic Saab Workshop - Sydney
.au
http://lios.apana.org.au/~c900
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