Alternator output voltage, low?

[HarryD]

My charging light came on suddenly tonight after having a new alternator for a week.
The belt was fine but the lamp stayed brightly lit even with all accessories and lights out.
Disconnecting the large cable that feeds back to the battery from terminal B+, I measured the output from the alternator as 3.5 volts.
Is this good/bad? With lead connected the voltage from the battery showed as 12.5 but was dropping. Battery is 4 months old.

95 Discovery 300TDi Auto

Harry

[TEMPL4R]

Should show 14.4v when running. Depends which make and model alternator is fitted. Check the other connections and fuses, then take it back.

[HarryD]

Thanks TEMPL4R.
Is that 14.4 off the back with the battery off or is that with the battery connected.

H

[TEMPL4R]

Battery connected.

Some are battery sensed, so would play up if you disconnected them.

I was always told not to disconnect and run an alternator as they could blow the regulator or diode pack ( From my RAC days at a course in Birmingham) Have never tried it, so don't know if it does.


JonV8... is it true?


Chris

[HarryD]

Right....nipping outside to run the beast on the drive.
Will give you a reading across the battery soon.

[CDNrover]

Are you sure you have the belt routed correctly? I changed my alternator out in my Disco a couple months ago (put a 120A one in) and was sure I had the belt routed correctly but upon a second look something didn't look right and realized I missed a pully. Might be worth a second look-see.

As for output, mine is a tiny bit higher than 14.4 but not sure if that's due to the higher output alternator.

[HarryD]

Thanks CDNrover.
Yup, belt fitted by landrover garage (I could have done it but they did it as they were swapping out the last alternator). All was well for a couple of days and then the light came on.

OK, just been out on the drive and tested voltage across battery terminals with engine at idle and it reads as 12.43v.....then from rear of alternator it also reads 12.43v.

Harry

[TEMPL4R]

Usually 14.4 -14.8 is OK. a couple of volts higher and they dry the battery out.

[TEMPL4R]

That is battery voltage, it's not charging.

We fitted an exchange alternator on a Rover 200 a couple of weeks ago, it was faulty. Never worked.

Chris

[HarryD]

Praise be to the lord on high for on the seventh day, he somehow found the time to invent warranties.
Thanks for verifying that for me TEMPL4R, as it was charging and isnt any more, I will assume that it's the alty thats at fault as opposed to anything else. The wiring is all fine and I have continuity otherwise.
Should have enough juice in the battery to allow me to get it to the garage for a swap out.

[TEMPL4R]

If all the wires are there and it isn't a fuse, then it's failed.

They will check and confirm, then change it.

Chris

[HarryD]

Yup...all wires are there. My first thought was that either the belt had gone or a cable had come off a terminal. Was slightly disappointed that it wasnt either of them. Ah well....a little drive on Monday. Downside is that I will have to borrow the wife's petrol freely tomorrow. It's nice enough....I think.

[HarryD]

Am I going mad?????.....Got home today (used a different motor) and decided to check the voltages again. With the car cold, voltage across the battery was about 12.5v. Ran it fo a few minutes at tickover, nothing really exciting happened. Turned it up to 2000 and 3000 rpm progressively and it went up to 15.3 volts on average. Then gradually dropped to 12.1 or on occassions 12.0 with everything on....fan, lights, heated rear screen etc. Could this be a battery problem?....if it is...how would that account for the red lamp?. Getting really confused now and dont want to look a proper nanna when I take the alternator back (still attached to hitherto trusty Disco)

[TEMPL4R]

Sounds more like a brush sticking in its carrier.

When everything is on, there should still be 14-15 volts, the alternator regulator adjusts to suit the load.

I'd still take it back, explain what is happening,and get them to check it.

[HarryD]

Thanks yet again Chris. I figured pretty much the same that even with everything on, the alternator needed to be putting out more than 12v.
Ah well, I will give them a call and explain. Only last week I remember him telling me that he hadnt (to his recollection) had a new one returned so perhaps there could be a problem with the batch?? I am still a bit wary about the reading across the battery being 15v plus though when everything else is off. Should I be?....

Harry

[HarryD]

And the reason that I dont bin the car and just get on with life may be seen from the image "Looks tidy" seen at http://www.harrydevine.com/Wigtown2005/Landrover/

It's an Auto ES and although now 147k road miles, the engine has only done 27k of that. So....some spending is deserved. I only wish it would end. This was a £4k purchase from a workmate and I have spent £1500 on it in the last 3 months. As far as another alternator is concerned, well that's hopefully a warranty issue.

Harry

[Jon v8]

Sorry I didnt reply before,anyway I wouldnt want to run an alternator disconnected- lots of vehicles used to have stickers warning against that,or disconecting the battery when the engine is running.Mind you I once drove a David Brown 885 about 30 miles with the alternator undone - it still works now! There must be a sound explanation for it,never bothered to find out.Anyway your car is going to boil its battery dry if its getting up to 15v with no load.With all loads on I wouldnt expect to see much more than 12.6v at idle.Above 1200 rpm you should be picking up to a max of 13.8 v approx (all measured across the battery)Because of the layout and in my opinion poor earthing on 300 discos,it is rare to see more than 14v across the battery.It needs a bit more checking,but it sounds like a possible regulator issue.What company recond the unit ? There are more crap ones than good I find.Does the place you bought it from have anyone competent to test it,cos they might not want to believe you.Warranty on "rotating electrics" is a pain to everyone.Much is written about it in a magazine I subscribe to -Vehicle Electrics,Electronics.Suppliers dont even always believe garages that send stuff back.They say that alot of returns have nothing wrong with them.Good luck :rolleyes:

[TEMPL4R]

Knew you could explain better Jon....

We used to put a wire to the small lead on the alternator to set them charging, on cars that had lost feed through the bulb, we weren't allowed to strip dashes at the raodside, on the RAC. Kept charging until you stopped the engine.

As alternators aren't self exciting, unlike a dynamo, they wouldn't do anything until it got a small jolt. We used to get the mechanical regulators on the French cars failing, the battery would boil and smoke. Funnily, the Jap ones didn't fail.

Chris

[HarryD]

........Anyway your car is going to boil its battery dry if its getting up to 15v with no load.With all loads on I wouldnt expect to see much more than 12.6v at idle.Above 1200 rpm you should be picking up to a max of 13.8 v approx (all measured across the battery)Because of the layout and in my opinion poor earthing on 300 discos,it is rare to see more than 14v across the battery.It needs a bit more checking,but it sounds like a possible regulator issue.What company recond the unit ? ...........:rolleyes: Thanks Jon. That is worrying. It's a new unit and not a recon so the plan is to take it back and with a lot of stuff running in order not to boil the battery. It's only 6 miles. I agree with what you say about some garages etc being unhappy potentially but if they are getting duff stuff from suppliers it is in their interest to correct it at this end too.
Will let you all know how I get on. Am I right and assuming that (apart from earthing) that the problem starts at the alternator end and not at the battery end?....could my battery be the problem? (New in October, only been to/fro work and one week in Wigtown)
Harry

[Jon v8]

As long as you are sure the wiring is ok,then I think we can point the finger at the alternator.If you think about it the alternator sets the voltage,(sometimes sensed from the battery ,sometimes from itself - depends on type)the battery can only output its own potential difference,it cant raise it.So your 15v is coming from the alternator and being crammed into the battery.Interesting that its a new unit though,what make is it?

[HarryD]

Thanks Jon, it was a Lucas model.
The problem has now been rectified and you can all now have a laugh....BUT...there is a serious side to this as I believe it could happen again to someone so read on.

A few weeks ago, my original alternator failed. I obtained another from an independant garage who did Landrovers. Essentially it looked very similar. It was a different make but in all other respects it was similar externally.

It worked for a while then failed. I got another, it failed and that took this story up to a couple of days ago. Yesterday, I took the car back to the garage and found out that he had now changed suppliers for his alternators and that they were local. A visit to them soon revealed the problem. The original alternator had an integral cooling fan. The replacements were the type that reqd an external fan fitting and were not supplied with them. Therefore, they eventually overheated. I was unaware of the fan's existence and assumed that as the replacements looked similar to the original that that was it. I suspect that this was an easy mistake to make as if the original had an external fan, this would easily have been seen to be missing when the new one was fitted.
Aaaaarrrrrgggghhhh.

[Jon v8]

So long as its sorted, thats the main thing -We are never too old to learn eh, Each day I try to go home having learnt something - its remembering it all that is my problem !!!!!!

[HarryD]

Thanks Jon. I have a very understanding LR garage too and I have only paid for the one alternator. Dunno if this would be the case with some of the big boys.