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Alternator issue


cutandpolished61
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Hi All,

i'm still having issues with my 12v alternator despite the RCR attending twice.

They have connected lose cables which sorted the problem instantly but on both occasions just for a few running hours.

It's charging at 14.2 v then goes down to 13.1 v and repeat.

Had a look yesterday but can't find any lose connections.

I want to avoid another call out.

Any suggestions from you experts how to troubleshoot the issue?

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1 minute ago, WotEver said:

Where are you measuring this voltage? If at the batteries check also at the back of the alternator. That will help to narrow down if it’s an alternator fault or a wiring fault. 

Agree, that's the best first step. Also a photo or you tell us the make and model of the alternator.

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Post a picture of the back and someone will advise.  Usually you are looking for a connection labelled B+.  Measure between that and the alternator frame or if there is a B- connection as well measure between B+ and B-.

N

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It's a Lucas A127

3 minutes ago, BEngo said:

Post a picture of the back and someone will advise.  Usually you are looking for a connection labelled B+.  Measure between that and the alternator frame or if there is a B- connection as well measure between B+ and B-.

N

Thanks, will do and come back

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13 minutes ago, BEngo said:

Post a picture of the back and someone will advise.  Usually you are looking for a connection labelled B+.  Measure between that and the alternator frame or if there is a B- connection as well measure between B+ and B-.

N

The B+ and if it is there, the B- will have the thickest wires going to them.

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  • 3 weeks later...

Yes, wiring issue or possibly the battery isolator switch. If the alternator fat positive (B+) wire goes via the isolator switch before reaching the battery, I think the battery isolator switch is the most likely culprit.

 

Anyway, you need to trace back from alternator to battery to see where the voltage decreases.

 

As a first step I’d check the voltage between the B+ alternator terminal and the battery +. There should be next to no voltage between them but you might find the missing 1.15v. Then repeat between alternator case or B-, and battery -. That will at least tell you whether the problem lies in the positive wiring or the negative wiring.

Edited by nicknorman
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On 21/09/2020 at 12:49, nicknorman said:

Yes, wiring issue or possibly the battery isolator switch. If the alternator fat positive (B+) wire goes via the isolator switch before reaching the battery, I think the battery isolator switch is the most likely culprit.

 

Anyway, you need to trace back from alternator to battery to see where the voltage decreases.

 

As a first step I’d check the voltage between the B+ alternator terminal and the battery +. There should be next to no voltage between them but you might find the missing 1.15v. Then repeat between alternator case or B-, and battery -. That will at least tell you whether the problem lies in the positive wiring or the negative wiring.

Now i have to question if it is a wiring issue. Had the engine run again and the alternator was charging fine constantly at 14,4v at 1100 rpm. Then after 1 hour i noticed a change in rpm and checked again only to find the voltage going up to 16.6v. Went to tick over and output was still high at 15.2v. Any more advice would be appreciated.

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9 minutes ago, cutandpolished61 said:

Now i have to question if it is a wiring issue. Had the engine run again and the alternator was charging fine constantly at 14,4v at 1100 rpm. Then after 1 hour i noticed a change in rpm and checked again only to find the voltage going up to 16.6v. Went to tick over and output was still high at 15.2v. Any more advice would be appreciated.

I assume these are measured directly at the alternator itself. If so they are typical of a wiring issue, and especially of a problem with the isolator switch.

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2 minutes ago, mrsmelly said:

Are you batteries in good order? I have found over the years that a dodgy battery completely stuffs everything up. Apart from that its a relatively cheap alternater to replace.

no the batteries are not ok. Mistreated them from new a year ago when I didn't charge them well enough due to lack of knowledge.. I think they have lost about 50% of capacity and I will replace them next spring when I get finally the solar fitted. I charge them once a week with a fast charger for 7 hours until the charge rate goes to 14.7v. No float charge, I know I keep molesting them but no alternative available at the moment.

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Just now, cutandpolished61 said:

Yes measured from alternator. That would be an easy fix, many thanks. I'll give it a go.

Simply connect both of the cables on the Isolator switch to one side (it doesn't matter which side) this takes the switch out of the circuit, and doesn't involve any cutting, removing or replacing anything.

If you still have the same problem, then the switch is not at fault.

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30 minutes ago, cutandpolished61 said:

Now i have to question if it is a wiring issue. Had the engine run again and the alternator was charging fine constantly at 14,4v at 1100 rpm. Then after 1 hour i noticed a change in rpm and checked again only to find the voltage going up to 16.6v. Went to tick over and output was still high at 15.2v. Any more advice would be appreciated.

The alternator regulator may struggle with voltage regulation if the output is disconnected. So one cause of high voltage on the alternator, is a disconnection or at least a high resistance connection between the alternator and the batteries. 

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39 minutes ago, cutandpolished61 said:

Now i have to question if it is a wiring issue. Had the engine run again and the alternator was charging fine constantly at 14,4v at 1100 rpm. Then after 1 hour i noticed a change in rpm and checked again only to find the voltage going up to 16.6v. Went to tick over and output was still high at 15.2v. Any more advice would be appreciated.

The change in rpm. Upward or downward?

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5 minutes ago, cutandpolished61 said:

and that could be a faulty isolator switch or something else? I can't see anything between the alternator and batteries apart from the isolator switch and a shunt

rpm goes up when alternator cuts out

Ok. That's important. If there were an alternator fault then revs would dip as the voltage rises. Since they lift as the load comes off, combined with the high alternator voltage pretty much confirms a wiring problem.

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Isolator switches, especially the cheaper ones with the removable red plastic key, are notorious for causing exactly these symptoms. Often switching them on and off multiple times (with the engine NOT running) will cure the problem for a few days. If there are any doubts, change the switch (they are cheap enough) as a complete failure can damage the alternator.

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