On Fri, 2 May 2008 23:13:37 +0000 (UTC), Saab C900 Viggenist
<c900@[EMAIL PROTECTED]
> wrote:
>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.
It's possible that the ignition is being upset, but it's more likely
that the alternator really is slowing the engine down mechanically. At
a guess, the rad fan might pull 50A or so as it starts. That's getting
on for 1 BHP, which is significant for an idling engine. In the 80's
cars often had "idle-up" systems. A solenoid valve in the carb would
increase engine power slightly if any big electrical loads (or aircon)
were turned on. These days, the idle speed has closed-loop control.
>
>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.
[snip]
That's not right. The battery can't provide any power at-all when it's
charging. During normal running, the alternator provides *all* of the
power, with extra to charge the battery. If a peak load happens that
the alternator can't manage, the voltage drops to the point where the
battery stops charging and starts supplying the deficit. The voltage
drop when this happens is quite noticeable. Normally these peaks are
very short - maybe 0.5 seconds or so for the rad fan starting. If they
happen for long periods (say, all the time the rad fan is running at
idle) then the danger of flattening the battery starts to increase.
Cheers,
Colin.
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