1993 900S turbo convertible (USA)
Simple question, but complicated lead-in, so that you may understand
where I have been. Let me open by saying that I am electrical engineer.
I have had 3 Saab 900's, all of the same vintage. Only the one above is
left. I love these vehicles, but they all have shared one problem - they
eat batteries. On each of them, the day comes, repeatedly, and sooner
rather than later, that when I go to start it, the battery is flat for
no apparent reason. I've done all the standard stuff - checked the
grounds, checked the connections throughout, maintained the batteries
properly, looked for intermittent parasitic drains (stuck switch, etc).
I replaced the voltage regulator with a new Bosch unit. I checked the
alternator terminal voltages in accordance with Bentley. No apparent
problems, and no fixes. As a desperation measure, I ran separate wires
(#10) directly from the alternator output terminals to the battery
terminals, and that helped. But not enough.
Recently, the thing stranded my wife. Again. And I got the ultimatum.
Fix it or sell it. So I got more seriously interested in what's going
on. Instead of spot checks, I attached a permanent voltmeter to the cig
lighter circuit, and drove it for several days. And lo and behold, the
problem quickly became obvious. The alternator is inadequate to the task!
With the air conditioning off, and no big drains (engine cold so
radiator fans off, power windows/seats quiet), the voltage stays well
up, in the range of 13.8-14.0 volts. But as soon as a big drain shows
up, the voltage drops to the 12.5 region, and that is not enough to
maintain a battery.
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.
So. Now I know what is happening. The alternator cannot put out
sufficient current to support 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.
Bill
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