Skip to main content ↑ ↓
Topic: Alternator output (Read 707 times) previous topic - next topic

Alternator output

A few months ago when I was on my way to pick her up from the shop the mechanic called me and let me know that the alternator had failed. He said he could have one in by the time I picked it up. Haven't given it much thought since then but the voltage guage has been reading low ever since so I finally decided to check voltage while running.
So what's the best spot besides the alternator (little tight there and with my luck I'll get my voltmeter wires tangled in the pulley) to check..I did check at the coach batteries at idle and it's only reading 12.9v so I'm assuming there's an issue.
I tried to get a picture of the part# on the alternator. Looks like 24SI or M010C21C?? I'm wondering if he just grabbed anything he could get his hands on but even so it should be putting out more than 12.9 right?
Dylan and Aimee
2001 U320 4013
Unit# 5896
450 HP/IFS
 

Re: Alternator output

Reply #1 –
There are so many variables leading up to that 12.9V voltage reading.  If you want to start at the beginning, check the resting battery voltage, just to make sure the batteries are fully charged.  3 hours rest is good, 24 hours is the gold standard.

If you want to check starting at the alternator, get some alligator clips for your test leads. then have someone run the engine up to highway speed and read your measurement.

So:  Batteries fully charged and engine running at normal driving RPM.

Art
1992 Foretravel Grand Villa
U225 SBID Build No. 4134
1986 Rockwood Driftwood
1968 S.I.A.T.A. Spring
1962 Studebaker Lark
1986 Honda VF700C
1983 Honda VF750C
Charlie, the Dog was broken out of jail 24 Oct 2023
N1RPN
AA1OH (H)e who must be obeyed.

Re: Alternator output

Reply #2 –
Quote from: Old Toolmaker –
There are so many variables leading up to that 12.9V voltage reading.  If you want to start at the beginning, check the resting battery voltage, just to make sure the batteries are fully charged.  3 hours rest is good, 24 hours is the gold standard.

If you want to check starting at the alternator, get some alligator clips for your test leads. then have someone run the engine up to highway speed and read your measurement.

So:  Batteries fully charged and engine running at normal driving RPM.

Art
Will do, batteries are definitely fully charged...I'll check at alternator once I get a 2nd person.
Dylan and Aimee
2001 U320 4013
Unit# 5896
450 HP/IFS
 

Re: Alternator output

Reply #3 –
Check alternator output at center tap on isolator. No fan belts to tangle with...

Re: Alternator output

Reply #4 –
Quote from: Barry & Cindy –
Check alternator output at center tap on isolator. No fan belts to tangle with...

Voltmeter black/ground on any clean piece of metal.  Red lead as follows:

Yes, the center lug of the diode-based battery isolator is from the alternator B+ and will read the same as at the B+ terminal.

BUT, you also need to check the outer lugs as one goes to each battery bank.

Do the readings with the engine at 1,100 RPM or so.
Brett Wolfe
EX: 1993 U240
Moderator, ForeForum 2001-
Moderator Diesel RV Club 2002-
Moderator, FMCA Forum 2009-2020
Chairman FMCA Technical Advisory Committee 2011-2020

Re: Alternator output

Reply #5 –
Turn the headlights on while the engine is running and the voltage AT the batteries (house and start) should read 13+ volts if it's charging. The headlights are enough draw to excercise the alternator regulator.  Anything less and THEN it's time to troubleshoot.
1994 U280, Build 4490
Deming, NM.
 

Re: Alternator output

Reply #6 –
Quote from: dlkj07 –
...voltage gauge has been reading low....
I did check at the coach batteries at idle and it's only reading 12.9v...
Looks like 24SI...
...it should be putting out more than 12.9 right?
Yes, it should put out more than 12.9 volts, even at idle.

The Delco 24SI is a excellent alternator, but it must be installed correctly.  If the mechanic who installed yours was not familiar with the Foretravel battery isolator charging system, he may have done it incorrectly.

The Delco 24SI alternator does not require a "hot with ignition" EXCITE wire.  There is no place to connect one on the Delco 24SI.

The only small wire that should be connected to a Delco 24SI is the "always hot" SENSE wire.  It goes on the small "S" post.

With the Delco 24SI alternator, B+ voltage on the output stud is necessary for initial turn-on (excitation).  See the link below for Peter's info on connecting the Delco 28SI (or 24SI) alternator and how to make it work with a battery isolator.

DelcoRemy 28SI alternator

Sentence below is taken from 24SI instruction sheet found in Peter's post linked above:

"IMPORTANT! Please note that if the unit is not connected to the system correctly with the proper battery isolator, it may appear that the alternator is not charging or it is regulating at a low voltage creating a low state-of-charge condition."

I mounted a 28SI on our coach.  It installs exactly the same as the 24SI.  The thread linked below describes that project, and may be of interest.

Delco Remy 28SI Alternator Installation

1993 U280 SE 40' WTBI, Build: 4359
C8.3 300hp, 6-Speed, Exhaust Brake
960 watts on the roof (6 x 160)
Sorento (or BOLT) on a Kar Kaddy SS
"Nature abhors a vacuum"