Skip to main content ↑ ↓
Topic: 24SI Alternator check (Read 9633 times) previous topic - next topic

24SI Alternator check

I've searched and read plenty on this subject but unfortunately i may have confused myself.

Someone else installed a Delco Remy 24si so not sure if they transferred the wires correctly...pretty sure it's had low voltage off and on since. Getting 13.6 volts at B+ and center lug of my isolator while running at idle and 1100 RPM. Sometimes going down the road the voltage drops to 11.9 or 12. I have 2 other wires installed on the I and the R. The "S" lug does not exist on my alternator. (See Pic) Tried to follow the 2 wires on the I and R but they disappear in the wiring harness so not sure where they go.

I have solar so it's masked the problem some what because I can use the boost button to charge my chassis batteries when I dry camp.

Not sure but I think something is wrong.

Should I buy the other isolator that has 4 lugs that I read about? Should I just replace isolator with a combiner?

Re: 24SI Alternator check

Reply #1 –
Quote from: dlkj07 –
Not sure but I think something is wrong.
Dylan,

Let's try to figure out what you've got going there.  Looking at photo of back of alternator, that does NOT look like a normal 24Si model.  AFAIK, a standard 24Si should have the large B+ lug, then 3 smaller terminals labeled (from left to right) "S", "I" and "R".  See the diagram below.  As you have noted, your alternator is missing the small "S" terminal.  I don't know what 24Si version that is, but I DO know it is probably not connected correctly.

On a normal 24Si alternator the "S" terminal is the SENSE terminal.  The "I" and "R" posts should NOT have any wires connected.  Connecting a "12V always hot" wire to either of these posts can damage the alternator.

A properly wired 24Si alternator requires only two cables: the large B+ charging cable, and a smaller gauge "SENSE" wire.  The large B+ cable goes to the alternator post on the isolator, or to a Voltage Sensing Relay (VSR).  The small "SENSE" wire goes from the alternator "S" terminal to the POSITIVE post on the chassis (START) battery, or to the START battery post on the isolator.

If a diode type isolator is used with the 24Si it must include a special "IGNITION" or "ENERGIZE" terminal.  This terminal is connected to a "HOT with ignition key ON" 12V source.  When this terminal is activated by turning the ignition key ON, voltage is supplied to the ALTERNATOR post on the isolator.  This voltage is required on the B+ charging cable in order to "turn on" the 24Si.  Two examples of isolators with this feature are linked below.

Amazon.com: Victron Energy Argofet Battery Isolators 200-2AC (2 Batteries...

Amazon.com: Cole Hersee 48162 Battery Isolator : Automotive

As an Amazon Associate Foretravel Owners' Forum earns from qualifying purchases.

Re: 24SI Alternator check

Reply #2 –
You could try removing, and then testing, the two small wires that are now attached to the "I" and "R" terminals.  With any luck at all, you will find one of them is HOT all the time.  That could be the original SENSE wire.  The other might be HOT only with ignition key in ON position.  That could be the original EXCITE wire.  Report back on what you find.

Re: 24SI Alternator check

Reply #3 –
I think I found an example of the 24Si model that looks like the one on your coach.
It is, indeed, a genuine 24Si that does NOT come with a "S" terminal.  It is "Self excited" and it DOES require voltage on the B+ terminal to turn on (start charging).

Genuine Delco Remy 24 SI, 12V, 160A Alternator with Pulley - Seaboard Marine

Because this alternator does not have the SENSE terminal, and DOES require voltage on the B+ terminal, it is not suitable for use with a standard diode isolator such as the one found on your coach.  Even if the alternator puts out proper charging voltage (14.0 to 14.6 volts) on a test machine, it will never work properly as it is presently wired on your coach.

I see two possible ways to correct your situation:

1.  Remove your isolator.  Remove the two small wires, and connect the 24Si B+ terminal directly to the chassis (start) battery bank.  With this "One Wire" hookup it should work properly.  You would then need a way to direct alternator charging current from the chassis batteries to the coach batteries.  This could be accomplished with either a manual combiner switch (or the BOOST switch), or with an automatic combiner relay (VSR).

2.  Replace the 24Si with a proper DUVAC alternator that includes both the SENSE terminal and the EXCITE terminal.  When connected correctly (including the SENSE and EXCITE wires) this should work fine using the isolator already in place on your coach.

Other Forum members may suggest other ideas, but that's the best I could come up with.  Good Luck!

Re: 24SI Alternator check

Reply #4 –
Quote from: Chuck & Jeannie –
1.  Remove your isolator.  Connect the 24Si alternator you have directly to the chassis (start) battery bank, and remove the two small wires.  With this "One Wire" hookup it should work properly.  You would then need a way to direct charging current from the chassis batteries to the coach batteries.  This could be accomplished with either a manual combiner switch (or the BOOST switch), or with an automatic combiner relay (VSR).

Easy  to test:  At the diode-based battery isolator, remove the wire that comes from the B+ terminal of the alternator (should be the middle lug of the isolator).

ADD it to the lug going to the chassis battery (you are now direct wired as mentioned by Chuck).

Let us know what it reads with engine at around 1,100 RPM.

Re: 24SI Alternator check

Reply #5 –
If you try Brett's test (above) I would advise also removing the small wires from the "I" and "R" terminals.  No matter where those wires terminate, they shouldn't be connected to those terminals on the alternator.

Re: 24SI Alternator check

Reply #6 –
Quote from: Chuck & Jeannie –
If you try Brett's test (above) I would advise also removing the small wires from the "I" and "R" terminals.  No matter where those wires terminate, they shouldn't be connected to those terminals on the alternator.

Agreed.

Re: 24SI Alternator check

Reply #7 –
What about just removing the two wires that we're talking about on the back of the alternator but leaving the isolator alone, would that tell us anything?

Re: 24SI Alternator check

Reply #8 –
I'm checking this right now and I also forgot to mention that the little wire that is connected to the "I" has 12 volts when the key is on. Neither of them have any voltage when the key is off

Re: 24SI Alternator check

Reply #9 –
Both small wires disconnected, wire from B+ of alternator connected to chassis battery lug of isolater

13.6 volts at idle
13.5 volts at 1100 rpm with lights and dash fan on high

Should I be getting more voltage or does it depend on how low the voltage is on my chassis batteries. They are at 12.6 resting right now.

Alternator bad??

Re: 24SI Alternator check

Reply #10 –
Where are you measuring the voltage? At the alternator or battery? Typical the alternator should be trying to create 13.8 to 14.2 volts depending on regulator set point. One of those terminals should be for remote sense. For a diode setup this is connected to the chassis battery or a terminal on the isolator and is used to increase the alternator voltage to compensate for the voltage drop across the diodes. This is ~1.1volts so with his working the alternator should be trying to produce ~ 15 volts. As you are getting 13.6 volts that indicates the alternator is at least trying. You need to know the current it is delivering to know if it is just maxed out or if it is weak.  A bad diode or winding in the alternator can reduce output. A clamp on ammeter will allow you to get that current measurement to have a better diagnosis.

Re: 24SI Alternator check

Reply #11 –
Quote from: dlkj07 –
...wire from B+ of alternator connected to chassis battery lug of isolator
13.6 volts at idle
13.5 volts at 1100 rpm with lights and dash fan on high
13.6 is somewhat low, but not conclusive, so don't condemn the alternator yet.

Where are you taking the voltage reading?  Ideally you should check the reading at the alternator B+ post, then at the isolator chassis battery lug, and finally at the chassis battery itself.  This is to eliminate any chance of a bad cable or connections.

Try leaving everything connected as you have it.  Turn the engine off.  Turn ON headlights and switch to high beams.  You could also turn on fog lights and docking lights for more load.  Wait 5-10 minutes, then turn all the lights OFF.  Check (record) the voltage at the three points listed above.

Next (with all the lights still OFF), start the engine, and set to about 1100 rpm.  Check (record) the voltage at the three points.

Finally, with the engine still running at 1100 rpm, turn all the lights (including high beams) back on.  Check (record) voltage at the three points.

Let us know the results.

Re: 24SI Alternator check

Reply #13 –
What are you using as a volt meter?

 I was given a Milwaukee volt, continuity,  and ohm meter that was totally automatic. It would automatically change to whatever I was looking for. I then learned it was about  1/2  or more volts off when checking DC and way more on AC. When I learned i couldn't trust it, ithrew it in the trash.

Re: 24SI Alternator check

Reply #14 –
Quote from: Chuck & Jeannie –
13.6 is somewhat low, but not conclusive, so don't condemn the alternator yet.

Where are you taking the voltage reading?  Ideally you should check the reading at the alternator B+ post, then at the isolator chassis battery lug, and finally at the chassis battery itself.  This is to eliminate any chance of a bad cable or connections.

Try leaving everything connected as you have it.  Turn the engine off.  Turn ON headlights and switch to high beams.  You could also turn on fog lights and docking lights for more load.  Wait 5-10 minutes, then turn all the lights OFF.  Check (record) the voltage at the three points listed above.

ONext (with all the lights still OFF), start the engine, and set to about 1100 rpm.  Check (record) the voltage at the three points.

Finally, with the engine still running at 1100 rpm, turn all the lights (including high beams) back on.  Check (record) voltage at the three points.

Let us know the results.

Ok, here it is...
I also disconnected my amp-l-start just in case it was messing with my readings...

Lights off after draw down, Engine off
Alt  12.5
Isolator chassis lug  12.5
Batteries  12.5

Engine on 1100 rpm, lights off
Alt 13.8
Iso chas lug13.8
Batt 13.7

Engine on 1100 rpm lights on

Alt 13.5
Iso 13.4
Batt 13.4

Re: 24SI Alternator check

Reply #15 –
Quote from: turbojack –
What are you using as a volt meter?

 I was given a Milwaukee volt, continuity,  and ohm meter that was totally automatic. It would automatically change to whatever I was looking for. I then learned it was about  1/2  or more volts off when checking DC and way more on AC. When I learned i couldn't trust it, ithrew it in the trash.

I have a pricy fluke meter, I doubt that's the problem...I'm a lineman so we have the good toys.  ;D

Re: 24SI Alternator check

Reply #16 –
Quote from: Chuck & Jeannie –
Dennis,

We have already determined that the OP's alternator does not have a sense terminal.  See earlier posts in this thread.
If the rig has a diode block for house battery isolation it needs an alternator with remote sense. Sorry for being late to the thread. Just trying to help. Is this a new alternator install with the plan of eliminating the diode block?

Re: 24SI Alternator check

Reply #17 –
Quote from: dlkj07 –
Ok, here it is...
I also disconnected my amp-l-start just in case it was messing with my readings...

Lights off after draw down, Engine off
Alt  12.5
Isolator chassis lug  12.5
Batteries  12.5

Engine on 1100 rpm, lights off
Alt 13.8
Iso chas lug13.8
Batt 13.7

Engine on 1100 rpm lights on

Alt 13.5
Iso 13.4
Batt 13.4
13.8 volts is a common set point for trucks running long hours using standard lead acid batteries. 13.8, 2.3 volts per cell is just below the Gassing voltage for a standard battery and is also desirable for the low maintenance versions. AGM and Lithium need higher. This indicates alternator is functioning. The voltage drop is due to the alternator getting loaded down. Does it increase if you increase engine RPM further? Often takes 1,200 rpm or more to reach full output. Pulley can be changed to increase low speed RPM of alternator. Would be good to know actual current draw on alternator. Regulator can be changed to increase voltage.

Re: 24SI Alternator check

Reply #18 –
Quote from: Dennis H –
If the rig has a diode block for house battery isolation it needs an alternator with remote sense. Sorry for being late to the thread. Just trying to help. Is this a new alternator install with the plan of eliminating the diode block?
This was a new alternator 2 years ago, just now finally trouble shooting intermittent low voltage. If the forum thinks the alternator is still ok, i'm willing to change my isolator to something else if needed....combiner, vsr, etc... though I would like to keep my boost switch operable if possible, but not a deal breaker.

Re: 24SI Alternator check

Reply #19 –
Quote from: Dennis H –
13.8 volts is a common set point for trucks running long hours using standard lead acid batteries. 13.8, 2.3 volts per cell is just below the Gassing voltage for a standard battery and is also desirable for the low maintenance versions. AGM and Lithium need higher. This indicates alternator is functioning. The voltage drop is due to the alternator getting loaded down. Does it increase if you increase engine RPM further? Often takes 1,200 rpm or more to reach full output. Pulley can be changed to increase low speed RPM of alternator. Would be good to know actual current draw on alternator. Regulator can be changed to increase voltage.
The thing is, I often get voltage in the low 12's intermittently when I'm driving at highway speed so not sure that's it...