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Car Forum / Nissan / Nissan Cars / December 2005Strange voltage readings from alternator
The battery on my 92 primera 1600 failed yesterday and as it was about ten years old i thought no more and a replacement was fitted.Today I checked the battery voltage and running at tickover and no loads on I was getting just under 14 volts, I decided to check the voltage at the alternator and on the main terminal I was getting the same voltage but on the warning light terminal I was getting near on 17 volts, I checked the batt warning light and it goes out as it should. I rechecked when the engine was switched off and there was no votage on the w/l terminal.The alternator is a replacment one from Halfords and about 2 years old and it appears to be a Lucas A127 type. Now I thought the w/l output should be the same as the main output or it would light the w/l but acording to Haynes manual it shows a diode on the w/l circuit in the instrument panel so I gather thats why it's not alight when the engines running. Main question is would it be either the regulator or the diodes that is causing this and if left would it do any damage or fail. Regards Trevor Smith > The battery on my 92 primera 1600 failed yesterday and as it was about > ten years old i thought no more and a replacement was fitted.Today I [quoted text clipped - 14 lines] > Regards > Trevor Smith I'm not sure about the warning light, but a good fully charged battery has a voltage of 13.8 Volts. So that part of it is ok at least. A tired battery with high internal resistance can reach 15V under charge, Try charging the old battery if you still have it, and read the voltage while under charge. That should comfirm the problem, unless you have a good reason for suspecting the altenator. No I had no reason to suspect the alternator as the battery was old and I only discovered the voltage anomaly out of curiosity, I have no idea if the high reading on the w/l terminal has just happened or has always been like it. > No I had no reason to suspect the alternator as the battery was old and > I only discovered the voltage anomaly out of curiosity, I have no idea > if the high reading on the w/l terminal has just happened or has always > been like it. I sure wouldn't get all upset about that and I'm not sure if anyone would know if this is normal or not, that isn't where you test an alternator! Also digital meters can show all sorts of odd voltages where there is almost no current. Test this yourself, attach the negative test lead on the battery negative, hold the positive lead in one hand and then touch the battery positive with the other hand. How much current do you think is there but it still reads 12+V..  SignatureSteve http://www.atlantaracing.com Normal readings for a DVM. 10Mohm input resistance combined with a body resistance of perhaps 10-100kohm mean that your body is almost as good as a length of wire as far as the meter is concerned. > Normal readings for a DVM. 10Mohm input resistance combined with a body > resistance of perhaps 10-100kohm mean that your body is almost as good as > a length of wire as far as the meter is concerned. Exactly why people get LOST using a DVM working on their cars. You can find some crazy voltage readings on a car when you probe without any meter load. SignatureSteve http://www.atlantaracing.com The warning light most likely gets its voltage from the alternator windings, while you were reading after it had gone through the regulator, which on most modern alternators are built in. As for the diode, I would have to see the schematic to be able to tell you its purpose. > The battery on my 92 primera 1600 failed yesterday and as it was about > ten years old i thought no more and a replacement was fitted.Today I [quoted text clipped - 14 lines] > Regards > Trevor Smith Hereby ends the tale. Got home from work then had to nip out again quess what no batt warning light, checked voltage 11.25 and no charge. Serves me right for thinking it was a ten year old battery that had failed and it turns out to be a two year old Lucas alternator. Happy Christmas. Trevor Smith > Hereby ends the tale. Got home from work then had to nip out again > quess what no batt warning light, checked voltage 11.25 and no charge. > Serves me right for thinking it was a ten year old battery that had > failed and it turns out to be a two year old Lucas alternator. Happy > Christmas. > Trevor Smith It's good to see you figured out the problem but don't be so hard on yourself for the miss-diagnosis. I mean, even after only 5 years it would be normal to start suspecting a failing battery. The fact that your battery has lasted 10 years is quite impressive. I hope the lessoned learned here is to only buy a Nissan ALT:) CD >> Hereby ends the tale. Got home from work then had to nip out again >> quess what no batt warning light, checked voltage 11.25 and no charge. [quoted text clipped - 9 lines] > >I hope the lessoned learned here is to only buy a Nissan ALT:) No experience yet with new Nissan Alt's, mines done 140K miles and is 15 years old. The slip ring tracks are better finished on modern alternators and brushes last longer. When I goes I hope I'll just need put new brushes in. Did this on a 104K mile / 10 year old Toyota alt and I found some that would fit for £1 the pair at the local auto electrical place. I paid for recon 20 years ago before I learnt that it was usually just a pair of carbon brushes that needed replacing. 10 year old batteries are normal on Nissan's in my experience. All 3 were blue top translucent white case Hitachi on 910 Bluebirds. 1st came on the coupe when it was bought new. 2nd was on a 2 year old estate and was clearly the original battery. 3rd was to replace the estates battery when it was 10 years old. On selling the estate the coupe's 10 year old battery was put on it and I kept the 1 year old battery for the coupe. Coupe was T boned 5 years later and battery fitted to a Celica on which it served for a further 5 years. Exide only last 7 years. Crap cheap batteries only last 3.01 years. I did have one that was replaced in warranty but that was down to it not being filled correctly - a rush job that was botched. You get what you pay for. The Hitachi cost £40 while a cheap battery would have cost £26 (it didn't fit right so I got the Hitachi). >The battery on my 92 primera 1600 failed yesterday and as it was about >ten years old i thought no more and a replacement was fitted.Today I [quoted text clipped - 14 lines] >Regards >Trevor Smith Diode is to stop a reverse circuit though the bulb check relay coil. It would light the dash lamps when the engine was running down having been switched off or when the alternator is failing to generate enough field coil current and is drawing current though the bulb. The L terminal will also go the ABS and dim dip unit if fitted. |
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