Normal alternator field response time?

How quickly should the alternator respond to the voltage regulator? Seconds? Milliseconds?

Depending on battery charge, I measure between 13.8 - 14.25 VDC on my Bosch 80A alt (older Saab 900). Seems normal

Turning on any load will drop the V by 0.1 to 0.4 V (and stay there). If the alt was originally putting out 14.00 V DC, prior to load, why doesn't the Voltage Reg direct the alt to 'bring it back up' to 14.00 V, if that presumably was what the batt needed for it's SOC? Is this normal and/or is it a safeguard against overvoltage when turning the load off (seems like the systems should be able to easily handle that)?

Possibly related: whenever I turn on any load, I notice the fuel pump pitch/whine moderately decreases. I've jumpered the fuel pump's leg ground, and the fuel pump's component ground wire directly to the batt neg terminal. Also jumpered the fuel pump to receive direct hot feed from batt positive terminal. Have also replaced, cleaned, secured batt cables, chassis ground, and jumpered various other leg grounds to the batt neg terminal - all with no change to the changing fuel pump pitch.

Other dash lights will slightly dim with an additional electrical load. Engine decreases ~50 - 75 RPM with additional load

Alt belts proper tension and not glazed. Alt amperage output tests good. Connections clean and secure. Nominal AC ripple current

Thanks for any input (posting from public library, so may take a few days to answer any follow-up questions)

Lance

Reply to
Lance Morgan
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Did you speed up the rotation, by opening the throttle? Alternators cannot deliver anything near their rated output when the engine is idling. With the engine uo to 2000-3000 rpm, the alternator will be running at a speed sufficient to keep up with heavier electrical demand.

Reply to
the fly

I've jumpered the fuel pump's leg

It's early and the coffee hasn't kicked in yet. Seems to me your fuel pump will stay on 24/7 creating that unseen battery drain. Fuel pump should only come on when ignition key is turned on. Are you measuring alt output at the batt terminals? If so, the 14v reading, IMHO, is high, indicating a batt that is undercharged. Possibly from overnight draining by the fuel pump? Hope this gives you some food for thought. Al

Reply to
snelln2

46 A @ 850 RPM 69 A @ 1900 RPM 75 A @ 2800 RPM
Reply to
Lance Morgan

No - fuel pump doesn't stay on all the time. V-drop between alternator and batt is neglible. Fuel is just an one example, of several. May be normal behavior

Central questions are how quickly VR adjusts alt field, and why doesn't voltage restabilize at initial reading, after load(s) have been applied

Reply to
Lance Morgan

Milliseconds. Certainly not "seconds."

When currents are flowing (say from the alternator to the load you just turned on) then voltage isn't constant throughout the system. Even in a perfectly normal system, the load will "pull down" the voltage on systems wired near it more than other systems (such as the voltage regulator) that might tap off the main power bus closer to the alternator or battery.

It sounds like you might have a particularly bad connection that is causing a big enough voltage drop to make the fuel pump pitch change. Remember that the bad connection can be on the *ground return* side of the load just as easily as on the feed side.

Reply to
Steve

It can be seconds on some alternators, that are designed to slow down the rate of response to voltage drops (especially at idle speed) to prevent sudden loading of the engine and corresponding idle speed drop.

Reply to
Robert Hancock

Thanks for the response. I've jumpered the fuel pump directly to the pos batt terminal, and have also grounded it directly to the neg batt terminal, using 12 AWG wire: the fuel pump pitch change still occcurs, as does the slight dimming of dash lights, headlights, etc. Occurs both at idle and when slightly revved (V output will generally increase by nominal 0.03 V)

Voltage drop at the fuel pump is neglible (hundreths of a volt), but the voltage drop slightly increases when another electrical load is induced

Reply to
Lance Morgan

Thank you. Just so I'm clear - if the initial V @ idle is 14.00 V, w/just nominal loads pulling (ignition system, fuel pump, etc, at a given SOC and temp), and another additional load is added, it is normal for the V to drop a bit - but then should it recover to the original 14.00 V after a second or two (or whatever the response time is?

My alt V drops a bit, and stays dropped when an additional load is induced. And my idle RPM is also slightly lowered. New batt cables, clean and secure grounds, 2d voltage regulator (brushes getting to minimum spec w/orig VR: same performance with both VRs), etc

Reply to
Lance Morgan

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