Alternator confusion...

[hector]

Hi everyone.

So, the domestic alternator on my Barrus Shire (a CAV) is causing me confusion...

The boat is new to us, so I don't know the history, but with the engine at revving a bit the voltage charging the domestic batteries was around 14.5 and all fine, but at idle it slowly dropped and at around 14.3 - 14.2 the alarm went off... the voltage would carry on dropping below 14.0... rev the engine and it would pick up the alarm would go silent.

(NB. the LCD odometer was also not working... see below)

 

This dropping voltage was diagnosed by a couple of people as a faulty regulator, so I replaced the regulator and the brushes while at it.

Having done so, I started the engine, gave it a good rev and then when I let it settled back to idle, the alarm stayed quiet! Great. Also: The odometer was now giving a readout! A bonus.

Two hours later, I restarted the engine and it had gone back to it's old ways - almost... The voltage seems to stay above 14 even at idle, but the alarm kicks in as it drops to 14.3 again, and the odometer has gone blank again.

I wonder what anyone would try next?

Is the alternator a red herring, even? I thought that 14.3 - 14.2 volts was fine and shouldn't be triggering the alarm so maybe something else is going on? Any idea why it would work once after a new reg/brushes, then go wrong again? Any idea what the connection with the odometer is?

 

Probably just time to phone an electrician, but if this makes sense to anyone I'd love any other advice. There doesn't seem to be any other loose connections.

Thanks!

Hector.

[Tony Brooks]

I would suspect a failed field diode (I am all but certain the CAV alternators call them  aux. Diodes). With a diode out you only have two phases so the output may well drop on idle. That would also explain why the rev counter has stopped working if it's the rev counter pulse phase that has failed.

 

Edited to add - Got that wrong, its probably a main diode that has failed.

Edited March 25, 2021 by Tony Brooks

[Sir Nibble]

Any chance of a picture of the alternator? 

[hector]

Yes @Sir Nibble, and thanks! - there should be pics below of the alternator while it was out, and the regulator / brush set I replaced (not much writing on either, *just* enough to find that particular part online...)

 

@Tony Brooks - thanks for that, if it's something like that it's definitely above my grade and I'll send it off. To clarify, though - RPMs read fine, it was the LCD display for running hours (I assume... it read 1522) which suddenly came on after I replaced the reg, and then stopped showing anything when the alarm problem returned.

[Flyboy]

15 hours ago, Tony Brooks said:

I would suspect a failed field diode (I am all but certain the CAV alternators call them  aux. Diodes). With a diode out you only have two phases so the output may well drop on idle. That would also explain why the rev counter has stopped working if it's the rev counter pulse phase that has failed.

 

Edited to add - Got that wrong, its probably a main diode that has failed.

The OP said the LCD odometer had stopped working, I assume he meant the hours meter. This is a very common problem with these displays due to the ribbon cable connection to the glass LCD substrate ( worse during damp conditions ). Can usually be fixed by application of heat from an electric flat iron after stripping it out of the rev counter. Totally separate fault to the dodgy alternator.

[Tony Brooks]

1 hour ago, hector said:

Yes @Sir Nibble, and thanks! - there should be pics below of the alternator while it was out, and the regulator / brush set I replaced (not much writing on either, *just* enough to find that particular part online...)

 

@Tony Brooks - thanks for that, if it's something like that it's definitely above my grade and I'll send it off. To clarify, though - RPMs read fine, it was the LCD display for running hours (I assume... it read 1522) which suddenly came on after I replaced the reg, and then stopped showing anything when the alarm problem returned.

 

 

 

 

In that case I think that I have been spouting ollox. Assuming a typical combined taco & hour counter then if the taco is working the alternator is probably fine. However both the feed to the tack/hour counter are from the ignition switch so if the switch went open circuit or highly resistive the alternator would try to supply the instruments, including the tacho/hour counter from the field diodes and that can put the warning lamp and buzzer on. How that fits with the hour counter failing but the tacho working I have no idea though. If it is something like this I suspect a loose connection that   vibrates more on idle. Look at the connections on the ignition switch and also the fuse if one is used in the ignition switch main feed (unusual) or feed from switch to instruments/warning lamps.

 

I await SirN's input with interest.

 

4 minutes ago, Flyboy said:

The OP said the LCD odometer had stopped working, I assume he meant the hours meter. This is a very common problem with these displays due to the ribbon cable connection to the glass LCD substrate ( worse during damp conditions ). Can usually be fixed by application of heat from an electric flat iron after stripping it out of the rev counter. Totally separate fault to the dodgy alternator.

 

I agree it might well be coincidence.

[Sir Nibble]

A lot of information in those pictures. That's an AC5 R. Let me tell you what goes wrong with them. Until the slip ring end bearing starts rotating in its housing and wears the bracket, nothing goes wrong, they are long lived and reliable machines.

Before going any further, is there any other way of charging the domestic battery? Solar panels? Split output from engine alternator? What are you measuring voltage with and where?

[hector]

Thanks @Sir Nibble , that's very helpful (and reassuring if it's supposed to be a reliable alternator...).

 

No solar panels and as far as I can tell the starter alternator and travel power generator are completely separate. The only other factor is the shoreline and the Sterling power management thing.

There is a voltage display which has been installed under the engine panel which can be flicked up for domestic and down for starter... but I have also checked these readings by putting a multimeter across the battery terminals with the engine and alternators running, and the numbers match for both systems. So I can say for sure that the voltage across the leisure battery hovers around 14.2 - 14.5 and at 14.3 or below the alarm is sounding.

 

[Sir Nibble]

53 minutes ago, hector said:

Thanks @Sir Nibble , that's very helpful (and reassuring if it's supposed to be a reliable alternator...).

 

No solar panels and as far as I can tell the starter alternator and travel power generator are completely separate. The only other factor is the shoreline and the Sterling power management thing.

There is a voltage display which has been installed under the engine panel which can be flicked up for domestic and down for starter... but I have also checked these readings by putting a multimeter across the battery terminals with the engine and alternators running, and the numbers match for both systems. So I can say for sure that the voltage across the leisure battery hovers around 14.2 - 14.5 and at 14.3 or below the alarm is sounding.

 

OK. Those voltage readings tell us the alternator is charging. What I would like you to do is to take two voltage readings with the alternator running. One across the two main terminals on the back of the alternator, and one from the small warning light terminal to main -ve. The reason for this is to establish that there is no difference in output from the main +ve diodes and the "auxiliary" diodes. A voltage imbalance here can make funny things happen. I don't expect a problem but it's as well to eliminate it with one simple check. What I do suspect is what Tony suggested above, a faliure in the instrument supply. Try this. with the fault symptoms present, alarm sounding. Disconnect the warning light wire from the alternator, does the alarm stop? If not see backup theory below.* If so, connect warning light wire to -ve, does alarm restart? if so we have a weird alternator problem to investigate, if not the problem is most likely what Tony suggested.

 

* Backup theory. Odometer is connected to oil switch, so is alarm, oil pressure issue.

 

[hector]

Thanks so much for your help with this @Sir Nibble - I went through these yesterday evening with an unhelpful mixed bag of results!

 

I did the voltage reading comparison three times with different results... First time the reading from the alarm wire seemed to be jumping around quickly between 10v and 14v - possibly a loose connection as once I cleaned the contacts I got a reading that matched the main terminals, but later in the day it was reading consistently a volt lower (14.3 at mains, 13.5 between alarm wire and common ground). I'll come back to this a few times this week, but not sure what behaviour to expect from a bad diode. Would it jump around, or just be steady but different?

Disconnecting the alarm wire stops the alarm, and the voltage readout on the control panel drops to around 13v. I didn't have a spare length of wire to get the alarm connection to reach the main -ve (it's on a banana plug in an awkward place) but I'll be able to check that.

The whole system seemed to behave itself at times, then consistently give lower voltages and alarm when idling. At one point the V across the alternator started dropping off below 14v on idle. Can't find any obvious loose connections in the control panel, but the alarm going off does definitely correlate with a dropping voltage, it's not sounding randomly: i.e. sometimes I get 14.3 or more with engine idling and no alarm so I don't think it's a mechanical bug connected to engine vibration.

Anyway - for now, I have a workaround I hadn't noticed, which is that the on-board genny powers the same charging circuit as the shore line, so as long as it's on as the 240v power source whenever the engine is running, the domestic batteries stay up at 14.7 - 14.8 and all is well... 

Hope some of this makes sense, but I'm not sure if it's useful intel ?

 

H

[Tony Brooks]

Rather than try to take two readings connect the voltmeter between the main positive output and the warning lamp connections on the alternator without disconnecting any cables. When ever the engine is running I would hope the meter read zero volts but I would accept up to 0.5 volts as maximum. If it's higher than that  then the chances are it's a diode problem in the alternator (probably a field/aux diode).

 

A bad solder joint at one of the diodes could make the problem intermittent and the reading jump about.

 

Hopefully SirN will soon give his input as well.

 

Edited to add: Unless it has been attended to in the past I would not expect a CAV alternator to suffer a bad solder joint but you can never be 100% sure. It would explain the random voltage variations and sounder.

Edited March 28, 2021 by Tony Brooks

[Sir Nibble]

On 28/03/2021 at 18:43, hector said:

Thanks so much for your help with this @Sir Nibble - I went through these yesterday evening with an unhelpful mixed bag of results!

 

I did the voltage reading comparison three times with different results... First time the reading from the alarm wire seemed to be jumping around quickly between 10v and 14v - possibly a loose connection as once I cleaned the contacts I got a reading that matched the main terminals, but later in the day it was reading consistently a volt lower (14.3 at mains, 13.5 between alarm wire and common ground). I'll come back to this a few times this week, but not sure what behaviour to expect from a bad diode. Would it jump around, or just be steady but different?

It would be steady but different.

Disconnecting the alarm wire stops the alarm, and the voltage readout on the control panel drops to around 13v. I didn't have a spare length of wire to get the alarm connection to reach the main -ve (it's on a banana plug in an awkward place) but I'll be able to check that.

This on the other hand, does look like an alternator problem.

The whole system seemed to behave itself at times, then consistently give lower voltages and alarm when idling. At one point the V across the alternator started dropping off below 14v on idle. Can't find any obvious loose connections in the control panel, but the alarm going off does definitely correlate with a dropping voltage, it's not sounding randomly: i.e. sometimes I get 14.3 or more with engine idling and no alarm so I don't think it's a mechanical bug connected to engine vibration.
That makes sense.
Anyway - for now, I have a workaround I hadn't noticed, which is that the on-board genny powers the same charging circuit as the shore line, so as long as it's on as the 240v power source whenever the engine is running, the domestic batteries stay up at 14.7 - 14.8 and all is well... 

Hope some of this makes sense, but I'm not sure if it's useful intel ?

 

H

Normally I would say it's almost vanishingly unlikely, but I think Tony's right, solder connection. Unlike modern alternators, the rectifier in there is not a throw away component, it's built. There are three heatsinks, one for each phase. Each heatsink has three diodes, one each for main pos and neg and the third is the "Auxiliary" diode. The diodes on the three heatsinks are connected with soldered wire links. the top of the diode has a stem with a forked top. The wire should be in this fork and soldered. I have seen lots of broken joints on these. Do you fancy looking it yourself? So long as you have a decent soldering iron the most difficult bit is to get the pulley off.

[Tony Brooks]

If you do decide to DIY I would suggest that you get a pin nosed pliers and put an elastic band around the handles so they clamp shut. The clip them to the stem close to the actual diode. That will go some way to protecting the actual diode from excess heat travelling down the stem. Such excess heat can be caused by an undersized iron, the iron too cold, or keeping the iron in contact for too long.

 

Probably more vital on the Lucas ACR series but it won't do any harm and may help save a diode.

[Sir Nibble]

Well look what I found. This shows the wire links soldered between diodes.

[hector]

@Tony Brooks and @Sir Nibble - apologies, I've been away from the computer (and the boat)... I can do some basic soldering and certainly have more experienced friends if I find anything that makes me lose my nerve. Definitely up for giving this a go, I'll just have to wait until I have time to remove the alt again. But that photo is very helpful!

I'll report back! Many thanks again both.

Hector.

 

 

[Tracy D'arth]

On 31/03/2021 at 14:17, Sir Nibble said:

Well look what I found. This shows the wire links soldered between diodes.

Far right top diode soldering looks a bit dodgy.

Not a nice assembly with just blob soldered connections carrying all the current. Suppose this is what passes as acceptable these days. My old Admiralty QC guy would have flung it out the door.

[Sir Nibble]

14 hours ago, Tracy D'arth said:

Far right top diode soldering looks a bit dodgy.

Not a nice assembly with just blob soldered connections carrying all the current. Suppose this is what passes as acceptable these days. My old Admiralty QC guy would have flung it out the door.

It's a 1960s design widely used by the MOD and fitted to Rolls Royce cars. Most early alternators from CAV, Simms, Lucas and Bosch had similar wire connections to the diodes. The way they do it these days is a vast improvement with much better support and a greater wetted area.

[Tracy D'arth]

13 minutes ago, Sir Nibble said:

It's a 1960s design widely used by the MOD and fitted to Rolls Royce cars. Most early alternators from CAV, Simms, Lucas and Bosch had similar wire connections to the diodes. The way they do it these days is a vast improvement with much better support and a greater wetted area.

I remember the Lucas ACR rectifier diode stacks.  With the diodes on plates bolted together and rigid connections they were very robust. The connection spades tabs on the plate sides were prone to rusting however.

[Tony Brooks]

I am sure when I was training MOD apprentices the bared end cable went through the eye and back on itself a little but that may have been the result of in use "modification."

[Sir Nibble]

1 hour ago, Tracy D'arth said:

I remember the Lucas ACR rectifier diode stacks.  With the diodes on plates bolted together and rigid connections they were very robust. The connection spades tabs on the plate sides were prone to rusting however.

Robust indeed but the 10/11 AC machines that came before them still had this wire on the diode performance. This is where a nice flexible braid and woven sleeve comes in.