Faulty alternator or battery???

eadmr04

Member
I don't use my 68' P6b that regularly and have it attached to a trickle charger. We went for a 70 mile drive on Friday and I went to move the car after being parked for about 20 minutes and the battery was completely dead? We had a breakdown company jump start us on the Sunday and drove straight home. I got back in the garage and tried to restart it straight away and dead again? I put it on the trickle charger overnight and this evening it has started again but once again I've just tried to start it for a second time and it's dead again? I've put a multimeter across the battery terminals and it's showing about 13.5 to 13.8 v from the battery without the engine running.

Any simple steps or checks I can do to try to see if it's the alternator or the battery that's starting to fail? I believe the alternator to be a 11AC early one as doesn't seem to have the more modern modular type connector on the back?

Thanks
 
Are you sure the problem is not the starter or the connections? The voltage you measured across the battery seems very healthy, assuming it wasn't measured with the trickle charger still connected. With the engine running you should see 14 to 14.4 volts across the battery if the alternator is ok.
 
When the garage jumped started, did they connect to both battery terminal or did they connect the jump lead ,negative, to the vehicle body . If they didn't connect to both battery terminals it would point to a bad connection battery to the body, as it would have been bypassed. I was convinced I had a duff starter motor on my old LR and so did the AA when attending. Turned out to be a bad connection. It may be worth checking all the main battery cable connections.
 
Are you sure the problem is not the starter or the connections? The voltage you measured across the battery seems very healthy, assuming it wasn't measured with the trickle charger still connected. With the engine running you should see 14 to 14.4 volts across the battery if the alternator is ok.
Will do a few more investigations, I did think it was strange to see 14v to 14.5v without the engine running or charger connected? I wonder if the battery is showing that voltage but just hasn't got enough current?
 
When the garage jumped started, did they connect to both battery terminal or did they connect the jump lead ,negative, to the vehicle body . If they didn't connect to both battery terminals it would point to a bad connection battery to the body, as it would have been bypassed. I was convinced I had a duff starter motor on my old LR and so did the AA when attending. Turned out to be a bad connection. It may be worth checking all the main battery cable connections.
I'm sure the went straight on to the battery terminals?
 
we can get a garage to do a 'drop test' on battery .these are not perfect but can give an idea if battery is on way out. ideally one ought to check we do not have a slow leakage to earth ( boot light? etc) a garage can check for power drain or if we can use a multimeter? we can check for self ( U tube ?)
if battery is OK then we likely have a diode failure in alternator . just one failed will allow battery to drain down over say 24 hours. though can vary according to battery capacity and condition. DO check for loose connections and corrosion points. poor battery connections often causing high resistance an thus low /poor recharge rates and cranking power reduction. volts tend to show Ok as we check across battery .again a decent garage can check alternator for you. sadly all can easily cost a few quid so check what you can for self first. I took easy way out and simply bought a brand new high capacity battery and fitted new alternator ( turned out alternator was faulty ) good luck
 
When you check that you do not have a current draw, without the motor running, make sure that all doors are closed properly. I have heard of someone searching for a current draw, when the door was open and the interior light running.
I would also check you battery terminals, and earth straps etc. I believe that 3500's can utilise the kickdown cable as an earth strap, although you will have slow cracking, and eventually you will have no kickdown at all, once your cable melts through.
 
I've been doing a few checks over the last couple of days. All the connections seem ok with no obvious looseness or corrosion. The battery always seems to have about 13.5v to 14v on multimeter but only seems to start the car if it's been left on trickle charge over night?

I saw a suggested test on youtube to check the alternator which involves starting the car and then disconnecting the battery. If the alternator is ok you should be able to put all the lights on etc. to draw current and the car should cope fine as the alternator should provided power for these as well as enough current to charge the battery. I started the car and then took the positive terminal off of the battery. I tested the voltage coming to the positive lead and it read approximately 14.3v so thought everything ok. I then turned on the side lights and they seemed a bit flickery and as soon as I switched on the main beem the motor died. I repeated this several times including just flashing high beam on and it cut the motor every time. Would this suggest that the alternator is not providing enough power and is the problem? Is the above theory a sound one to be working on?

Thanks.
 
If you have multimeter put across battery terminals should show approx. 14v with engine running ie charging. Now put all lights etc on see volts drop now rev engine slightly to see if alternator stays above 13v. This will give an indication as to alternator performance.
Don't forget if you have 11AC alternator there is a 4TR regulator also this is located in side of passenger glove box on mine.
 
I saw a suggested test on youtube to check the alternator which involves starting the car and then disconnecting the battery.

You shouldn't do that. What you should do is disconnect the positive terminal with the engine running while keeping the system complete by having an ammeter between the cable and the battery post, without any disconnection. Then you can check the charge rate with the ammeter. Try to start the car with just the ammeter in the line and you'll blow the ammeter. Run with the battery disconnected and you'll blow the alternator or one of the ancilliaries in the case of an 11AC.
 
Just got it started again after a day of trickle charge and then put all the lights on including main beam and was getting 14v at the battery at 15oo rpm. Would this suggest that the alternator would appear to be working ok. I'll see if I can get another battery tomorrow and see what happens.
 
Update.

Although the battery was always showing about 14v on the multimeter, I have just got a replacement battery and think it's sorted. Starts every time and even if I leave alll the lights on without it running for 5 mins still starts straight away. So looks like the battery was the problem.

Thanks for all the help and suggestions.
 
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