RWD - Exciter wire voltages

Volvo RWD 900 Forum

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Exciter wire voltages 900 1992


I don’t usually answer any questions on the 900’s because I have never owned one or worked on one. Since this is an alternator and a alternator is a alternator! Maybe I got a chance?

The statement about checking the voltage on the exciter wire, it seems that you are asking, What should the voltage be on the exciter wire if is disconnected from the alternator?
It will be very low!
I’ll explain as best I can.

If it’s disconnected the alternator, the alternator does not get a tickle of current to get thing up and going!
So, it will put out no voltage rise or have actual current to raise the voltage above the resting voltage of the battery. It needs about one-volt above the system voltage for current to flow towards and into the battery.
As the car is running, the battery is getting drained down to the 12.65V surface charge or lower if allowed to keep going.

The exciter wire gets its power through the “red battery light” under normal operation.
This voltage is considerably lower because the bulb acts as a current/ load dropping resistance.
Only a few volts and a low amount of current flows to the regulator. From there it goes into the rotor to create a magnetic field. The magnetic lines of force cuts through the many turns of windings and out pops electrons!
This only starts the output and from here it gets greater.

The regulator then starts watching the systems B+ voltage on its output windings and related diodes.
It is comparing or watching for a few tenths of a volt rise and ups the ante with more current.
There are two set limits of “roughly” 13.2 to 14.2-.6 volts. Variances are allowed for ambient temperatures but the idea is to be holding the pushing power of 1.0 to 1.5V and the getting the current flow to hold it there!
No matter what sudden but “normal” withdrawals happen.
Jump starting is not normal. If the two systems can get closer equalized, say, with passage of some time hooked up, before drawing on the circuits, it is safely done to both of the systems to suppress surges on diodes.

When voltage is lower, it self feeds some of its own current, being produced, back in towards the rotor.
It then becomes a “self regulated” charging system.
Some models of newer alternators are slang called “one wire” because it’s all internally controlled.
I haven’t seen or used one but in my younger days the early hot car roadsters used them for their simplicity. A solid housing mount makes the ground wire nonexistent, right there!

Anyhow, The output now opposes the voltage coming from the dash to ground that’s through the regulator,
The idiot light will now glow dim and then goes completely out!
You have seen it hundreds of times!
It does not take much of an output to do this, because the voltage is so low it’s easily overcome!
The light bulb arrangement was explained before.
This is why it is a false indicator that the alternator is all OK or hunky-dori!

Remember tenths of a volt count big time here on current output.
There has to be good connections to get the best readings, in tenths of a volt, to that regulator.
He is the middle man! Just like in food prices, he truly affects things!

A dash mounted voltmeter is the best indicator, to watch that middle man work on the system!
A flickering of headlights helps wake you up to look at the voltmeter but a “scanning habit” is best as you don’t see, them headlights, in the daylight very well!
A real good airplane pilot develops this scanning habit in order to survive!
He is always looking for a possible place to land on his route too! Because #### happens everywhere!

If your belts get too loose the voltmeter wiggles or sits parked tilted and consequently lower than usual.
The red idiot does nothing until it’s real bad! Maybe in like, the alternator is about to fall off, bad!
Remember that, it’s a rubber bushing mounted unit and that is not better than sliced bread!

The whole ticket is to be an “overseer” of the middleman’s whole system by using the voltmeter!
If you don’t have an in-dash meter, then when checking oil, flop a handheld digital voltmeter on there!
Look for a steady hold on the voltages held between this two readings above. A battery in a good charged condition, the tenth numbers are the best heads up to worn brushes or corrosion.

Hope this helps you understand how it works and how maintenance pays out dividends with no tow bills.
I like that you are pondering and asking questions, for better insight!

Insight is what grows in between the foresight and hindsight of a human!



New Exciter wire voltages [900][1992]
posted by  someone claiming to be Tim  on Wed Jun 5 13:49 CST 2019 >

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