24SI Alternator check

Topic: 24SI Alternator check (Read 2712 times) previous topic - next topic

Re: 24SI Alternator check

Reply #25 – March 18, 2025, 11:09:28 am

Quote from: dlkj07 – March 18, 2025, 10:51:28 am

Chuck,
My chassis and house batteries are AGM, though I'm going to throw a wrench into all this when I upgrade to house lithium's in the future.

In that case, then I would change my recommendation and suggest you seriously consider upgrading to the 200A Delco 28Si (model 8600307)

At the same time, you could totally remove your diode based isolator and install some type of "smart" DC-to-DC charging device.

But before you buy anything, you will want to read through the Forum posts by members who have switched to lithium house batteries to get their thoughts on how to charge the lithium bank while protecting your alternator. Charging mixed battery types can get tricky, but others here have made this move, so no need to reinvent the wheel.

See sample DC-to-DC charging info post below from a popular ForeForum Influencer:

Update Isolator/Auxiliary Start Panel

Re: 24SI Alternator check

Reply #26 – March 18, 2025, 11:16:29 am

Quote from: dlkj07 – March 18, 2025, 10:45:15 am

I'm using a reading from my Bluefire app, which I believe comes from the ECM correct? All those other readings are working properly on my dash.

Correct, so real numbers.

It's only the dash gauges that read incorrectly/wonky with the EL/headlight issue, and the usual workaround has been to use VMSpc, ScanGauge-D, or Bluefire. We use Bluefire.

Re: 24SI Alternator check

Reply #27 – March 18, 2025, 02:05:25 pm

Quote from: Chuck & Jeannie – March 18, 2025, 11:09:28 am

In that case, then I would change my recommendation and suggest you seriously consider upgrading to the 200A Delco 28Si (model 8600307)

At the same time, you could totally remove your diode based isolator and install some type of "smart" DC-to-DC charging device.

But before you buy anything, you will want to read through the Forum posts by members who have switched to lithium house batteries to get their thoughts on how to charge the lithium bank while protecting your alternator. Charging mixed battery types can get tricky, but others here have made this move, so no need to reinvent the wheel.

See sample DC-to-DC charging info post below from a popular ForeForum Influencer:

Update Isolator/Auxiliary Start Panel

I was leaning that way too because I've already done quite a bit of reading on the DC DC charger and I believe in my current situation I could add it with my AGM's and then it would already be there when I add lithiums. But I'll do a little more digging just to make sure... at this point I'll go with your recommendation and get the 28SI.

Re: 24SI Alternator check

Reply #28 – March 18, 2025, 03:19:33 pm

Quote from: dlkj07 – March 18, 2025, 02:05:25 pm

...at this point I'll go with your recommendation and get the 28SI.

So remember, the 28Si must see voltage on the B+ terminal to start charging. The easiest way to accomplish this is to move the alternator charging cable from the center post on your isolator over to the chassis battery post. This will eliminate the need for a SENSE wire, because the alternator will directly sense the voltage on the chassis battery through the charging cable.

EDIT: Ignore my statement (above) about the SENSE wire. Even if alternator is connected directly to chassis battery, you should also run a SENSE wire from the chassis battery POS(+) post to the alternator "S" terminal.

With the chassis battery being directly charged by the alternator, you can still use the BOOST switch to combine the coach batteries with the chassis battery. OR, just keep your coach batteries charged using your solar panels when dry camping, or the inverter/charger when plugged in or running generator.

Re: 24SI Alternator check

Reply #29 – March 18, 2025, 06:35:17 pm

Quote from: Chuck & Jeannie – March 18, 2025, 03:19:33 pm

This will eliminate the need for a SENSE wire, because the alternator will directly sense the voltage on the chassis battery through the charging cable.

Yes, the Delco 28SI will function without a SENSE wire, but as recommended by the Delco brochure, a SENSE wire connected to the chassis battery terminal will allow the alternator to compensate for any voltage drop in the cabling for optimum performance.

The selected media item is not currently available.

Re: 24SI Alternator check

Reply #30 – March 18, 2025, 10:20:51 pm

Peter,

I understand the purpose of the sense wire with the 28Si. In fact, as you may remember, when I installed a new 28Si on our coach, connected directly to our start battery, I DID elect to run a sense wire. So I'm not disagreeing with your comment in any way.

Delco Remy 28SI Alternator Installation

However, looking at the Delco factory brochure that you posted (above), it does raise a question in my mind. In the drawing labeled "With Remote Sense" the red sense wire appears to be longer than the B+ charging cable. Assume (for arguments sake) that this was the case in "real life". That is, the sense wire is actually longer than the B+ charging cable. The purpose of the sense wire is to compensate for voltage drop in the charging cable. Why would we not see the same, or greater, voltage drop in the sense wire?

Going back to my coach as an example, when I installed the 28Si I decided to upgrade the charging cable and the sense wire. The OEM charging cable was 2 AWG and the sense wire was 18 AWG. I elected to upgrade to 2/0 welding cable for the charge cable, and to 10 AWG for the sense wire. My new charging cable (unlike the diagram in your post) runs in one unbroken section from the alternator straight to my AGM8D starting battery. My new sense wire follows the exact same (unbroken) path. In fact, they are tied together along the whole length with zip ties.

So, on my coach at least, the charging cable and the sense wire are exactly the same length. In addition, the charging cable is much larger diameter than the sense wire. Thick wire has less electrical resistance than thin wire. Why is the battery voltage seen by the "S" terminal on my 28Si "better" (more accurate) than the voltage seen on the B+ terminal?

Like I said, I'm not disagreeing with your comment...just curious about the reasoning behind the whole "sense wire" concept.

Re: 24SI Alternator check

Reply #31 – March 18, 2025, 10:37:29 pm

Chuck,

Good question. The reason why the SENSE wire can be much smaller (and possibly even longer) and work fine is that it is sensing voltage with very little current (probably just a few micro-amps). So there is virtually no voltage drop on the sense wire.

The main alternator cable is carrying a very large current - possibly up to 200 amps - so the cable needs to be sized to handle this current. Your use of 2/0 cable was a good choice. The 10 gauge SENSE wire is fine, but it could have been 18 gauge with no issue.

Edit:
The reason the alternator B+ voltage is not accurate as the SENSE voltage is the alternator will be maintaining the voltage only at that point - to say 14 volts. But let's say the current demand is 200 amps with a resulting voltage drop of say .5 volts at the battery, now the battery is only getting 13.5 volts. A sense wire to the battery will tell the alternator now to maintain THAT point at 14 volts, raising the B+ voltage to 14.5 volts to cover the cable voltage drop under high load. Once the current goes back down the alternator will adjust to keep the battery at 14 volts at all times.

Re: 24SI Alternator check

Reply #32 – March 18, 2025, 10:40:39 pm

Peter,

I figured you would supply a logical technical answer to my question. That is why I value your comments so highly.

Cheers!

Re: 24SI Alternator check

Reply #33 – March 18, 2025, 10:53:11 pm

Quote from: prfleming – March 18, 2025, 06:35:17 pm

Yes, the Delco 28SI will function without a SENSE wire, but as recommended by the Delco brochure, a SENSE wire connected to the chassis battery terminal will allow the alternator to compensate for any voltage drop in the cabling for optimum performance.

So the wire that was hot with the key on and connected to the "I" lug on my current configuration wouldn't work as the SENSE wire once I install the 28SI correct? Or does it matter that it's only hot when key is on. As previously mentioned I do not know where those 2 small wires that were connected go once they leave the alternator, just that one is hot with key on ("I" lug) and the other (connected to the "R" lug) read 6 volts when the engine is running. I realize that these wires were probably connected incorrectly and maybe not needed at all with the 24SI without a SENSE lug.

Re: 24SI Alternator check

Reply #34 – March 18, 2025, 10:59:05 pm

Also, I drove the coach today and dropped it off to get some work done and the voltage on my Bluefire was reading 13.4 consistently. That is with the B+ from the alternator connected directly to the chassis battery lug on the isolator. Normally it would fluctuate to a lower voltage then back up as I drove so something is better.

Re: 24SI Alternator check

Reply #35 – March 18, 2025, 11:13:20 pm

Quote from: dlkj07 – March 18, 2025, 10:53:11 pm

So the wire that was hot with the key on and connected to the "I" lug on my current configuration wouldn't work as the SENSE wire once I install the 28SI correct?

Yes - that is correct. "Hot with key on" won't work as sense wire.

Re: 24SI Alternator check

Reply #36 – March 18, 2025, 11:19:31 pm

To add to Chuck's comment,

No, the "hot" with ignition won't work as the SENSE wire. The SENSE wire needs to be directly connected to the chassis battery. I would recommend to run a new SENSE wire if you upgrade to an alternator with SENSE capability.

The "hot" with ignition could still be useful if you decide to put in a 28SI and also change your isolator to one with an excite terminal. If you remove the isolator and connect the 28SI alternator directly to the chassis battery you won't need the excite wire. As mentioned previously, connecting an excite wire to a Delco alternator can cause permanent damage. The "I" terminal is for an Indicator lamp connection.

It's hard to know where the other wire is connected with a reading of only 6V, I would leave that one disconnected and just tape it.

Re: 24SI Alternator check

Reply #37 – March 18, 2025, 11:40:39 pm

Alternator brands is a vigorously debated topic on this forum, I wanted to throw a suggestion into the mix.

Here is a source (DelStar - made in Canada) that would be worth considering for anyone looking at upgrading. (I have no financial connection with this company).

DelStar offers alternators very similar to Delco models including the 28SI, here is a 200 amp model that is brushless which will provide improved performance and reliability over a brush design.

https://www.delstar-hd.com/pdf/160/D100-16100.pdf

Re: 24SI Alternator check

Reply #38 – March 19, 2025, 08:21:17 am

Quote from: prfleming – March 18, 2025, 11:19:31 pm

No, the "hot" with ignition won't work as the SENSE wire. The SENSE wire needs to be directly connected to the chassis battery. I would recommend to run a new SENSE wire if you upgrade to an alternator with SENSE capability.

Agreed. And a little more "reason why":
You want correct voltage to the chassis battery. A sense wire connected to the chassis battery or the chassis battery side of the diode-based battery isolator gives the alternator accurate data.

But, an ignition-hot wire is does not accurately represent chassis battery voltage. Depending on where it is connected to an ignition- hot source has gone through fuses, relays, connections and lengths of wire.

And, depending on where the ignition-hot source comes from, changing in loads on that circuit (head lights, dash HVAC fan, etc) will send falsely low voltage signals to the alternator which would lead to over-charging voltage to the chassis battery.

Re: 24SI Alternator check

Reply #39 – March 20, 2025, 02:38:52 am

So would this eliminate both the islolator and the boost switch?

ML Solenoid - 12V DC - Blue Sea Systems

Re: 24SI Alternator check

Reply #40 – March 20, 2025, 02:51:39 am

This is my go-to for battery combination. Automated and manual override for combination or isolation.
Mechanical interlock for service or if you really need to force it one way or the other.
ML-ACR Automatic Charging Relay with Manual Control - 12V DC 500A - Blue Sea...
In automatic both lithium and AGM will keep the batteries combined even after charging stops due to the high resting voltage. Lithium House and AGM will actually complement each other with this set up especially for float-storage. This relay is rock solid reliable.

Re: 24SI Alternator check

Reply #41 – March 20, 2025, 08:36:48 am

Quote from: dlkj07 – March 20, 2025, 02:38:52 am

So would this eliminate both the isolator and the boost switch?

Yes. The ML 7701 can eliminate both isolator and boost SOLENOID. This is also true of the ML-ACR mentioned (above) by Dennis. They are both the same basic solenoid. The 7701 is pure manual control. The ML-ACR adds some very nice "automatic" features.

To use the full functionality of the remote control switch that comes with either of these solenoids, you MAY have to run a new multi conductor cable from your dash area to the solenoid. Your original boost switch used a single conductor wire to connect to the solenoid. The Blue Sea solenoid switches require 3 (or possibly more) conductors. I detailed the installation of my ML 7701 switch in the post linked below:

'93 U280 Isolator Panel Upgrade

Re: 24SI Alternator check

Reply #42 – March 20, 2025, 10:30:27 am

Also, I didn't realize that when you search up 28SI alternator there would be so many...I know usually you get what you pay for but is it a horrible mistake to get one of the knock off's or am I asking for trouble? It's a 3-400 dollar difference depending on where you look between the Delco Remy and other brands.

Re: 24SI Alternator check

Reply #43 – March 20, 2025, 11:08:54 am

Be careful with the imitators. I had a 28SI knockoff and then replaced it with a real one. In comparing the wire windings looking through the air slots the wire size was a noticably smaller gauge on the knockoff. Less copper, more resistance and I think would run hotter.

Also check eBay. I bought my Delco 300 amp alternator on eBay and it was in perfect shape and ran like a champ.

Re: 24SI Alternator check

Reply #44 – March 20, 2025, 01:49:36 pm

Quote from: dlkj07 – March 20, 2025, 10:30:27 am

Also, I didn't realize that when you search up 28SI alternator there would be so many...I know usually you get what you pay for but is it a horrible mistake to get one of the knock off's or am I asking for trouble? It's a 3-400 dollar difference depending on where you look between the Delco Remy and other brands.

I have installed some knock offs. If you get past a few thousand miles you will be good for some time. Early failures tend to be the regulators going full on. For the long term bearings. For older vehicles, mostly VW Vanagons I have had much better luck with the knock off staters and alternators than the name brand rebuilds.
For the windings wire size and number of turns is affected by capacity. Higher amps bring larger wires, but less turns so output at low speed is often reduced. Everything is a compromise.

Re: 24SI Alternator check

Reply #45 – May 26, 2025, 12:51:44 pm

So I did end up purchasing a 28SI Part# 8600212

It's been awhile since this post so want summarize...

Since the 28SI I bought has the "S" terminal, I'm going to run a wire from chassis batteries to "S" on back of alternator.

What I'm a little confused about is can I use my 3 lug isolator currently installed or do I need to install one of the isolators with the ignition lug like Chuck mentioned above in reply #1 above for it to work correctly?

I am not ready to install a DC-DC at this time so just trying to get this back together without throwing more parts at it for now.

Re: 24SI Alternator check

Reply #46 – May 26, 2025, 03:49:29 pm

Dylan,

The 28Si must "see" 12 volt on the alternator B+ (output) terminal for it to turn on. This can be accomplished in one of two ways.

1. You can connect the alternator output cable directly to one of the battery banks - normally it would be the chassis batteries. Or, as Brett mentioned in one of the earlier posts, you could just move the alternator cable on the isolator from the center post over to the chassis battery post. This will provide the required 12 volt on the output cable to turn the alternator on. This setup basically bypasses the stock isolator. yYou could use the BOOST switch to combine/charge both battery banks, OR you could replace the isolator with either a dumb or smart combiner solenoid as mentioned in earlier posts.

2. You can replace your diode isolator with a model like the ones I mentioned in Reply #1.

Either way works. Do whatever works best considering your future plans for electric system upgrade/modification.

Re: 24SI Alternator check

Reply #47 – May 26, 2025, 04:07:42 pm

Ok, so what you're saying is I can't connect the 28SI B+ to the middle lug of my current 3 lug isolator as it was previously connected and be on my way.

It's either hooked directly to the chassis batteries (or lug on current isolator that go to chassis batteries) or I need to purchase the 4 lug isolator in reply #1 with the ignition hot lug. Then hook up the wire that has 12V when ignition is on to the 4th lug on new isolator.

Just curious, why does the 28SI have to use the ignition hot wire to work correctly?

Re: 24SI Alternator check

Reply #48 – May 26, 2025, 04:14:03 pm

Quote from: dlkj07 – May 26, 2025, 04:07:42 pm

1. Ok, so what you're saying is I can't connect the 28SI B+ to the middle lug of my current 3 lug isolator as it was previously connected and be on my way.

2. It's either hooked directly to the chassis batteries (or lug on current isolator that go to chassis batteries) or I need to purchase the 4 lug isolator in reply #1 with the ignition hot lug. Then hook up the wire that has 12V when ignition is on to the 4th lug on new isolator.

3. Just curious, why does the 28SI have to use the ignition hot wire to work correctly?

1. Correct
2. Correct
3. I don't know. That's a question for a EE major. I was a English major...to my great sorrow...shoulda studied computer science (this was in the time before Bill Gates became an unemployed college dropout).

Re: 24SI Alternator check

Reply #49 – May 26, 2025, 04:35:14 pm

Quote from: Chuck & Jeannie – May 26, 2025, 04:14:03 pm

1. Correct
2. Correct
3. I don't know. That's a question for a EE major. I was a English major...to my great sorrow...shoulda studied computer science (this was in the time before Bill Gates became an unemployed college dropout).

Ha, well I don't have any major...except the major of hard knocks but that doesn't count I'm sure. 😋

Affiliate Notice