Alternator issues

[cruiseliner64]

Hi all,the alternator on our Cruiseliner is not charging.It is a L/N 2500 JB unit.We have replaced drive belts,checked/cleaned all connections and grounds still no charge.We took the alternator to an auto electrician and he tested it on his bench test rig and it worked perfect....What are we missing??? He was surprised there is no charging light on the dash,his question was what excites the aternator on start up? We confirmed with Nathen in the UK that his Cruiseliner does not have a charging light either.We feel its some thing simple or something not common on Europian vehicles.Any ideas???

Paul

[mrsmackpaul]

Regulator is crook

So do you have power (12 volts or light globe test lamp) the alternator 

 

Paul

[cruiseliner64]

Yes.tested the alternator with a lamp across both terminals and then a multi meter...no output when in the truck but gives 14.3V on the test rig......

Paul

[Geoff Weeks]

If the light doesn't light across the two big terminals with the engine stopped, then you have a wiring problem. There should be 12 volts across the two big terminals at all times.

I can see the - goes right to the block, trace the +

[mechohaulic]

these conditions a test light will give faulty info . a meter  is the only way. from this point of view. 14.3 volts off the alternator yet the terminals look like crap. did anyone physically remove the terminals OFF the alternator and with velcro/ sandpaper CLEAN  terminals AND post ????? ALSO  the ground cable going to block needs to be remove and excellently cleaned from rust/ paint. did anyone take a HD jumper cable from ground terminal on alt to separate location for ground ??? anyone check  chassis grounds ?? 

[Geoff Weeks]

Test light will work fine, for seeing if he has power to the output terminals! There should be power there all the time, even with the engine stopped or the key off.

[mowerman]

7 minutes ago, mechohaulic said:

these conditions a test light will give faulty info . a meter  is the only way. from this point of view. 14.3 volts off the alternator yet the terminals look like crap. did anyone physically remove the terminals OFF the alternator and with velcro/ sandpaper CLEAN  terminals AND post ????? ALSO  the ground cable going to block needs to be remove and excellently cleaned from rust/ paint. did anyone take a HD jumper cable from ground terminal on alt to separate location for ground ??? anyone check  chassis grounds ?? 

lol i was thinking the same thing,,,,terminals all look rusty...lol.bob

[mowerman]

it doesnt take much to loose connection,,,good luck,,,,if the unit is putting out ,,its probably just a wire ,,,bob

[mrsmackpaul]

5 hours ago, Geoff Weeks said:

Test light will work fine, for seeing if he has power to the output terminals! There should be power there all the time, even with the engine stopped or the key off.

100% correct, if someone isn't competent with basic electrical knowledge a test lamp is far better than a multimeter 

Understanding how to read the instrument and interpret the results correctly comes from propper training and thorough understanding of what people are doing 

So a basic test lamp is adequate at this stage 

 

Keep it simple and let's not over complicate things 

KEEP IN MIND YOU MIGHT HAVE TO SCRATCH AROUND A BIT ON THE STUDS AND CONNECTIONS TO GET A GOOD READING

so maybe get some emery cloth and clean the nuts, lugs and studs up spotlessly clean before you begin testing, I doubt this is your issue but it gives you a good spot to start testing 

and yes big dark letters because this is really important to give propper test results 

Grab a test lamp, see if you have power at the alternator or not

First thing first, alligator clamp on the negative ( - ) battery terminal and the pointy end on the positive battery terminal ( + ) , the lamp should light up, if it doesn't you might have a dud globe in the tester

That is your most important test, always test the tester on a known lice supply 

Then grab your test lamp, alligator clamp on stud I circled in black, with the key turned on put the probe end on the stud circled in red, does the globe in test lamp light up ? If yes skip a few steps here

If no, then shift the alligator clamp to a good clean ground or frame or better still to the battery terminal marked - 

Try again on the stud circled in red, if you have power at the terminal the fault is between the negative ( - ) terminal and the alternator 

If no power put the alligator clamp on the positive battery terminal ( + )  test between the stud circled in black 

You should have power here if not

Try testing further back were the ground strap from the alternator bolts to the frame 

Still no good test back to were the battery lead bolts to the frame and so on

If yes from above, follow the same principle as the the negative, slowly test back through the circuit, tracing out wires etc until you get the lamp to light up 

When it lights up you have gone passed the fault

I think it will be something simple here to find if you know what your doing and a slow frustrating job if you don't 

But stay patient and we will help as best as we can 

But if to much bull shit advice, like I reckon, or old mate says these always do

And so on I'll butt out as it becomes messy over the internet 

 

Paul

[cruiseliner64]

Thanks for the replys...I should have pointed out the pictures were taken before work started,more to show what type of alternator is on the truck. So far we have ...

Replaced belts

Cleaned all connections

Done voltage tests at batterys..12.4v

Voltage test at alternator also 12.4v

Test lamp also lights up across the altenator terminals as decribed by Paul

No change in voltage at alternator with engine running.

Alternator put out 14.6v on test bench...but stays at 12.4 when in truck.

The auto electrician we went to has a great reputation in our area but is a young guy.He did admit to me he has never seen an alternator with only 2 terminals.All the alternators he is use to have 3 terminals.The 3rd is connected to the charging light on the dash.This shows the driver if the vehicle is charging but also excites the alternator on start up.With only 2 terminals and no light on the dash he was wondering how the alternator is excited on start up.

With all our batterys,wireing,connections and grounds checking out as in good condition we are scratching our heads wondering why the alternator will charge on the bench but not in the truck.....

Thanks for reading guys.

Paul

[Geoff Weeks]

Ok, that is good news, You do have power to the alternator, so a wiring fault between it and the batteries is unlikely.

In North America these are referred to as "one wire" type alternators, A bit of misnomer on the L/N because the case isn't the "ground" for the alternator. They "turn on" by residual magnetizem in the rotor producing a small amount of current in the stator, which runs through a separate set of diodes (diode trio) and is fed  back into the field (rotor) regulator to turn it on. The regulator also is connected to the main output studs internally so when it turns on can supply battery to the field.

 The little wire between the two big wires is the feed from the diode trio to the regulator, to tell it to turn on.

If your electrical guy doesn't know this, he may have supplied external power to that wire and the regulator turned on and worked. If that is the case the diode trio is the problem. It is the white piece that is connected to the three small studs and has the wire coming out of it to the little ring terminal up by the regulator. That trio is replaceable

I am assuming (never a great thing) that he knows how to test the output current and main rectifiers when he did get to to charge on the bench. So assuming that is true it looks like you need a diode trio.

Edited yesterday at 11:43 AM by Geoff Weeks
clarify how the reg turns on

[Geoff Weeks]

Diode trio and connection marked in black

https://www.ebay.com/itm/302551471197

 

Edited yesterday at 12:31 PM by Geoff Weeks

[cruiseliner64]

Thanks very much Geoff,that explains clearly what we were wondering.I wasnt present when he tested it so I dont know exactly how/what he done.I wonder if that part is readly availible over here.I'll get my son to get a google search going...Thanks again.

Paul

[Geoff Weeks]

This ad lists a bunch of interchange part numbers which may help in your search:

https://kuwait.whizzcart.com/product/19384513/new-diode-trio-compatible-with-1a-leece-neville-79021-33-500-l79027-l79027hd-505202-l79027hd-1-33-500-1-l79027-1-171-16000/

 

I looked up 79021 and 79027 on my search.

[cruiseliner64]

Thanks Geoff this does appear to be the problem.I spoke to the auto electrican and he said that he did feed a current to the diode trio block and the alternator then charged.He is a young guy who has built a good business for himself but admitted to me he had never seen this type of alternator before having worked on more up to date vehicles.I let him read your posts and asked me to thank you for furthering his knowledge.I was impressed by this because he may never see another alternator like this but he still wanted to know how it worked.A rareity these days among younger folk.....

Paul

Edited 15 hours ago by cruiseliner64

[Geoff Weeks]

I'm glad to help/

[mechohaulic]

On 6/4/2025 at 8:54 PM, Geoff Weeks said:

Test light will work fine, for seeing if he has power to the output terminals! There should be power there all the time, even with the engine stopped or the key off.

not down playing the usefulness of a test light. when a person attempting to find voltage drop uses a test light how does one know if the lighted test light has 14 0r  12 or even 11 1/2 volts lighting the tester. to see if there is any electrical voltage at all  ;  test light quick and easy. I used test light many times.

[Geoff Weeks]

29 minutes ago, mechohaulic said:

not down playing the usefulness of a test light. when a person attempting to find voltage drop uses a test light how does one know if the lighted test light has 14 0r  12 or even 11 1/2 volts lighting the tester. to see if there is any electrical voltage at all  ;  test light quick and easy. I used test light many times.

What I was looking for, was if there was battery voltage at the output stud. A test light is simple and easy to use for that.

I wasn't concerned about regulator settings, as these type are fixed. either it charges to the set voltage or it doesn't.

Another quick check you can do with the engine running is hold a small piece of steel close to the rear bearing, (A steel key ring will do) and if it is attracted to the alternator, you know the rotor field is energized.

Once you know it is charging but have other problems a voltmeter is handy to have.  A test light is quick and visual to see from a distance and almost impossible to mis-read. It will not tell the voltage, although if you "know your light" you can make an educated guess, but for work/doesn't work determinations it is a useful and rugged tool to have in your tool box.

I wouldn't carry my Simpson meter with me in the truck, but I can diagnose most simple electrical faults with a testlight.