Using remote voltage sensing to increase voltage

[Blazer406]

I have been reading about how to wire up the older 10SI and 12SI GM alternators for our old camp tractor. There is a ton of info on the net and I ran across that most of the newer than 30 years old alternators (GM ones at least) have a "voltage sensing" lead. Basically you hook this up to a common central power distribution point, and that voltage there is what the alternator regulates it's voltage to...... like it takes the voltage drop you normally see out of the equation. Example..... car running.... you can measure 14-14.2 volts at the back of the alternator.... but on my datalogger.... hooked up to the factory ECU, it only sees 13.2 volts or something...... basically if you hook the voltage sensing wire up near the ECU, it would make the alternator keep upping it's output until the voltage at the ECU (or wherever the remote voltage sensing wire was hooked up) got to the 14 - 14.2 volts.

So riddle me this, why can't we take this wire loose from the harness where it is in our car and run it to somewhere like the fuel pump area that is always low voltage in our cars.... probably due to the long wire lengths?

If we don't want or need this higher voltage all the time, why can't we hook this to a hobbs switch that at 3 psi boost or something changes the location of the remote voltage sense by using a relay with the 87 and 87A poles on it?

Anyway... it has my wheels turning about 14volts at my FP under boost and that being 15 volts probably at my ignition..... similar to a volt booster I guess..... just DIY.

 

[Pronto]

Interesting. It would sense the voltage drop at the fuelpump and bump it back to 14. Have you found the sensing wire?

 

[Blazer406]

I'm still hunting..... maybe I shouldn't have posted up just yet....... maybe the CS144's don't have it?

 

[Blazer406]

After some more reading..... it only looks like we use the "batt" which is the stud on the back of the alternator....... and the "L" or "I" pin..... which is the lamp or indicator light. The "S" is unused.

I am wondering if we can get a wiring connector off a car in the JY that does use the "S" wire and wire it up?

FYI... "S" = sense

Still looking....

 

[Blazer406]

Copy and paste off the Monte SS boad about the sense wire:

The sense circuit (when used) is intended to be a remote sense of the voltage at the battery, not a local sense of the alternator output. I haven't looked into the details of the sense circuit, but I presume it is a high impedance input, meaning the voltage drop from the sense line is almost nothing. That way the voltage at the battery is what the sense circuit sees at the sense input to alternator. If sense was measured at the alternator battery output, the voltage drop (IR loss for those who understand electrical stuff) across the battery cable would not be taken into account. By having a remote sense, the length and gage of cable between alternator and battery (cable resistance), and the amount of current being drawn through that cable are irrelevant. The alternator supplies whatever power is required to keep the battery within the specified voltage range, as sensed remotely.

I also see mention of possibly adding the "S" wire to the connector..... so this might be an option......

 

[TurboBob]

there is no sense wire in our cars. that pin is not used.

But, this is how all the "volt boosters" work. feed a reduced feedback voltage into the sense pin, and the alternator voltage rises.

There are some tricks to doing it correctly but it works well.


If you connected the sense wire to the fuel pump near the back of the car, the alternator voltage would certainly be higher, and rise at WOT. But you might wind up with 17V or more feeding your stereo...

When I was experimenting with my "volt blaster" I could easily get 20V or more out of the alternator.

Bob

 

[Blazer406]

I found replacement alternator connectors in Oreilly's Auto Parts for around 7 bucks.... It had a wire on the S terminal... I bet you could make this work.... The trick would be to relay this in only when under boost..... There is a bunch of ways to approach this....

 

[TurboBob]

I would do it like this:

connect a resistor (470 ohm would be where I start) from the S terminal to the charging post.

Then connect another resistor (470 ohm again)to the S terminal, go thru a hobbs switch, and then to the place you want to "sense".

This arrangement does a couple things:

1, the alternator is always running in "sense" mode, and doesn't have to change modes when you go into boost (changing modes can cause a voltage surge)

2, the voltage rise of the alternator can be controlled by the resistors. With equal resistances, the alternator will compensate for half of the voltage droop. So less chance for bad stuff to happen.

Bob

 

[Blazer406]

Cool Bob.....

That is why you are the man!

Neat idea...