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Alternator Voltages on TR6


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A question for any of the electrical experts out there .......

My TR6 alternator is simply marked as 14v 45A  A115 type  (it has a Lucas Electrik sticker with 24 026 on it).

The concern is I'm only seeing 13v across the battery terminals at tickover, and only 13.8v when revved harder. If I turn the headlamps on the voltage drops back towards 13v again.

I think this is 1v less than it should be, doesn't sound much difference I know but the battery isn't that old and I'm having to separately charge it. The alternator is under 2yrs old btw.

Has anyone taken an alternator voltage reading I could use for comparison please?

Cheers

Bob

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It does sound too low to me, the battery will never get fully charged if it only has 13.8 V on it.

Mayby your alternator has a blown diode.

Bob.

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22 minutes ago, Lebro said:

It does sound too low to me, the battery will never get fully charged if it only has 13.8 V on it.

Mayby your alternator has a blown diode.

Bob.

Thanks Bob, is there any way I could  test /measure for that ?

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my porsche runs 13.8v when revved. it has no driving negligible affect. i haven't put the trickle charger in it for 2 years.

 

I don't run any ancillaries and rarely is it out in the dark or bad weather. electrical drain is at a minimum.

 

After shorting my brand new alternator 2-3 years ago i had it refurbished. cheaper than a new one and keeps these old school skilled guys in business.

 

if you're not sure whip it off and give it to someone who refurbs auto electrical components

 

 

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Well those voltages are actually within the specification for the Lucas A115 which is 14.2V +/-0.5V measured at 6000rpm (alternator rpm, not engine) so it could be as low as 13.7V and still meet the spec and it's perfectly possible to charge the battery fully with that voltage - it just takes a bit longer.  

If your battery reads 13V at idle then it is still getting a small charge. The open circuit voltage for a fully-charged lead acid battery is around 12.6 to 12.8V depending on temperature so there must be current going into it to raise the voltage to 13. 

Even so, the voltage drop when you turn the lights on doesn't sound right - if the diodes are OK maybe you have a poor connection on the alternator wiring which is dropping voltage ?

It's worth noting that Lucas alternators are designed for standard lead acid batteries and cannot provide sufficient voltage to fully charge a more modern calcium type, which has a slightly higher voltage requirement. If you have one of those it may be why you need an external charger. 

Edited by RobH
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Thanks Rob,  I'll go over the wiring connections as a next step, it is a newish wiring loom (3yrs) but worth a check.

When you say "...if the diodes are OK"  is there a check for diode performance I could do ?

Alternator has done approx 3k miles but as usual now out of warranty !

thanks

Bob

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There really isn't any easy check you can do Bob - the only real way is to open up the alternator and do continuity measurements on each diode.  I guess from what you have told us, that your car doesn't have an ammeter ?  That would have been useful for diagnostic purposes in this case. 

 

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it does sound on the low side.  My TR3a has a Nippon Denso 40a alternator and that shows up to 14.4v when the engine is running.  (I have a dual USB socket hidden under the dash which incorporates an LED voltmeter).

Rgds Ian

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On 8/29/2020 at 1:24 AM, ed_h said:

I might try to check the voltage right on the alternator tab. If it is higher than at the battery, look for marginal connections.

Ed

Good tip Ed thanks,   at 1000rpm its 14v directly on the alternator terminal.

Still low unfortunately.

Bob

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On 8/29/2020 at 3:34 PM, Ian Vincent said:

it does sound on the low side.  My TR3a has a Nippon Denso 40a alternator and that shows up to 14.4v when the engine is running.  (I have a dual USB socket hidden under the dash which incorporates an LED voltmeter).

Rgds Ian

Hi Ian,

14.4v is what I would like - but thats never going to happen with my present alternator, its marked as 14v and thats all its supplying.

Bob

 

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On 8/29/2020 at 10:04 AM, RogerH said:

Hi Bob,

check all the connections - positive and earth.  Make sure all contacts are clean. 

The earth cable - battery to body often gets iffy. Remove paint under its tab

Also consider an engine to body earth strap.

 

Roger

Roger,

sound advice thanks, I've done this and now seeing 13.9v across the battery terminals at 1000rpm. Its the best I will see from this alternator.

I need someone who rebuilds alternators to select me higher voltage one.

Bob

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14.4 is the recommended maximum charge voltage for standard lead acid batteries, 14.7 i think for calcium.

i’d be happy with 13.9 at 1k revs unless you are finding the battery isnt getting enough charge?

steve

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14 hours ago, Steves_TR6 said:

14.4 is the recommended maximum charge voltage for standard lead acid batteries, 14.7 i think for calcium.

i’d be happy with 13.9 at 1k revs unless you are finding the battery isnt getting enough charge?

steve

I'd agree with your voltages Steve,  the issue is my battery does need charging seperately. After which its fine again but only for a while.

14.4v is ok for continuous charge as mentioned above by Ian on his 3A and I really wanted to find out how many people were seeing over and above 14v across the battery. Have you measured yours by any chance?

thanks

Bob

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Hi Bob

i have not measured the voltage on the battery ‘in anger’ no

am away on a trip , non TR, at the moment but will set something up when i’m back 

steve

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Hi Bob,

I just measured my TR6 with reconditioned. Bosch 55A alternator:

12.8V after 2 weeks not running, contact off.

13.8V after 2 mins running cold @1000 rpm, increasing revs does not increase volts.

13.4V after a 30 mile run, engine warm and at idle, lights off.

I have a new Varta 70Ah battery which cranks the car easily.

Waldi

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On 9/2/2020 at 7:21 AM, John Morrison said:

Hi Bob.

Are sure the battery is 100%, I think your alternator is probobly OK?

and you will be unchuffed if you spend a lot of money fixing something that isn't the issue, or total issue.

John.

Hard to say 100% John but it does hold full charge once its been on the battery charger though - so it seems acceptable.

The voltage from the alternator is most definitely on the low side, but technically a 'pass' (certainly not good).

How long the battery will last while I continue to charge it in between using the car is the question.

Bob

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On 9/3/2020 at 3:35 PM, Steves_TR6 said:

Hi Bob

i have not measured the voltage on the battery ‘in anger’ no

am away on a trip , non TR, at the moment but will set something up when i’m back 

steve

Thank you, another reading would be useful.

Bob

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9 hours ago, Waldi said:

Hi Bob,

I just measured my TR6 with reconditioned. Bosch 55A alternator:

12.8V after 2 weeks not running, contact off.

13.8V after 2 mins running cold @1000 rpm, increasing revs does not increase volts.

13.4V after a 30 mile run, engine warm and at idle, lights off.

I have a new Varta 70Ah battery which cranks the car easily.

Waldi

Thanks Waldi, very useful.

Those figures are very close to mine and my battery is certainly not happy about it. Are your engine running voltage readings taken across the battery or on the alternator terminal?

Also, as you have a new battery your situation is different to mine - did your last battery expire early by any chance?

Thanks

Bob

 

 

 

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I think 13.8v at the battery is sufficient, especially if the battery is well charged and the engine only running at 1000 revs.

Steve

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Hi Bob,

I measured across the battery terminals.

I do not know how long the previous battery lasted, Have never driven it, it came with the car 5 years ago Which I bought as a dismantled project car.

I have little / no doubt about my alternator, it has a sticker indicating it is a factory reconditioned unit (by Bosch). It was “new”.

And I have not charged the battery in between my rides.

Waldi

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  • 2 weeks later...

Thanks Waldi for the confirmation.

I had a new regultor fitted to the alternator and its providing 14.4v now at 1000 rpm.

I'm hoping this will mean less need for the separate battery charger sessions now.

Bob

 

 

Edited by OldBob
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