My battery went flat, so I charged it and started the vehicle and noticed that it was only charging at 10 volts, I fitted a new alternator, and the battery now charges at 13.8 volts. however if I switch the radio and the lights on together the vehicle stops. I thought maybe a dud cell in the battery, so exchanged the battery and still the same problem. any ideas ? the voltage regulator is external from the alternator and its a 2.8 1996 diesel Hilux with a rear pump type alternator

13.85 volts is barely sufficient and would indicate an alternator fault if the reading was taken "off-load"..

An alternator is not designed to charge the battery and is incapable of doing so because the terminal voltage of the battery quickly rises to fool the voltage regulator into believing even a discharged battery is fully charged and will quickly reduce the charge rate to a trickle.

The alternator is therefore designed to keep a fully charged battery fully charged by supplying all the current for the vehicle equipment and the output rating is chosen for this purpose.

The voltage regulator should limit the off-load voltage to about 14.5 volts but that is only a small part of the alternator test. A professional would apply a load across the battery equal to the rated output while monitoring the voltage and current. Most healthy alternators exceed specifications.

For a sensible in-service test when an adjustable load isn't available, with a fully-charged battery it is sufficient to start the engine, leaving it to idle and switch on all the vehicle equipment. Fit a voltmeter to the battery and wait while the battery voltage is pulled down by the load - to around 12.5 volts. Raising the engine speed to 2500/3000 rpm should cause the voltmeter to suddenly rise.
13.85 would be just about acceptable "on-load" but in excess of 14.0 volts would be ideal.

Is there any dash lights on? Check the alternator fuses!

You definitely have a problem with the charging system. As it is, the engine and other electricals are running your battery down. You need to put a voltmeter on the battery with the engine running to verify no charging. When the alternator is working and charging, the voltage will be about 14 volts, or somewhere between 13.5 volts and 14.5 volts. It may not even crank the engine over fast enough to start if your battery voltage is now under 12 volts. Then you would need a boost, but if alternator is not working, what charge you now have in your battery may be quickly depleted and the engine will shut down.
Test the alternator as I stated, or take it off and have it tested at a parts store.

Sounds like a bad alternator. Drained your battery and when you turned on the lights it took all the power left in your battery. When you drain a battery down that far, they are real hard to jump, and coupled with the alternator, the car is not producing enough current to start and stay running.

I would take the alternator off and take it to oyur local auto parts store to be tested. If bad, replace it and charge the battery fully before starting. Then check voltage at battery with motor running. Should be 13.2 -14 Volts DC. If not, you may have a bad battery cable or blown fusible link. (especially that if the alternator tests good).....Mike

if your battery died while driving and the car cut off while your driving,, your problem is from your alternator!!!,, on a normal working car, you can start, remove ther battery terminals and the car would still run cause the alternator will be powering the car. In your case the alternator isnt working and the car was working on battery power therefore when the battery went dead the car cut off.. Your alternator need \s to be charging between 13.5 and 14.2 volts.. as little as 10 volts will keep the vehicle running.. I'd suggest you have the alternator replaced.

You may have bought a bad battery, it happens. A battery's strength is measured in amps, you should take the battery to a parts store and have it checked to see what the amperage output is. If it is a good battery, then you need to check the terminals and see if they are worn out. Also, have someone jumpstart the car and then test the voltage output of the alternator. It should put out at least eleven volts. If not, the alternator or it's regulator may have gone out. You would then need to take the alternator to a parts shop and have it tested. Finally, sometimes the ground wires or even the lead wire from the alternator can get corroded or short out. If this happens, your vehicle will not function. Hope I could help!

Your alternator is going bad. It should only put out 13-14 volts and therefore is overcharging your battery. This causes your battery to get a dead cell. If you replace the alternator and battery, make sure you check the cables for any shorts.

charge it up .crank engine while its idling get a volt meter hook it to battery .volts should be 13.5 to 14.5 if its 12 to 10 or lower.alternator is not charging.if alternator is charging . turn engine off battery should stay 12.5 volts if it drops after engine is off to 11.0 or 10 volts battery weak replace it.

Here are a couple of ideas for you to try. First check in the power ditribution box under the hood, there is a fuse for the charging system, either use a circuit tester or just pull each fuse one at a time. Usually it is a 10 amp fuse as it is on my 94 cherekee. if you find any bad fuses replace them with same type and amperage. you should also check the fuse box on the right side of the dash, door has to be open for acess to the cover on the end of the dashboard, check all of those fuses also. If you live near an AUTOZONE auto parts store, take your jeep there and they can test your alternator on the vehicle , or you can remove it from vehicle and take it inside, they can bench test it. If you have a volt meter you can test it yourself, start vehicle, set meter to volts at least 20 or higher , place black probe on negative post on battery, red on positive, look at readout, a good alternator will put out 13.5 to14.0 volts, which is how much it takes to run vehicle and accesories and charge battery. If alternator doesn't put out 13.5 to 14.0 volts it is defective. I hope these ideas help.

Start vehicle up and take vehicle to an AUTO-ZONE they will check the entire charging system for FREE. This could BAD battery or a BAD alternator or even maybe both. So let not guess or if you have a volt meter at home with vehicle running voltage at battery polls should be at least 14 volts and without vehicle is running battery should be at least 12volts how ever with voltmeter you can't LOAD-TEST battery or alternator, But if alternator is less than 14volts it may be charging too low and if batter is less than 12 after it was fully charged or after car had been running for at least 15 minutes> BATTERY IS BAD> Good Luck and contact if you need more help. Thanks for using FIX YA