Charging problems

Hello all, more help needed, again !!! If I run my 90 2.5 tdi during the day I have no charging problems, ie she will start off the battery o.k. but at night when running with the lights on full beam and a pair of driving lights for a few hours the thing will not start without a boost. The battery is in good condition and the alternater is pushing out 13.5v. how do I boost the ampage/find out what the alternator is pushing out.
Thanks

Either your alt is not happy or you have voltage drop caused by dirty connections/corroded wire. Output from the alternator measured at the battery terminals should be 14.4v or very close

Is the alternator reading 13.5V with a load or no-load? Either way that is too low, as already stated it should be reading up to 14.4V so either you have poor connections or a faulty alternator.

ok...

to be sure get a good meter (digital)measure the voltage across batt,it should be around 14.4v,13.9v is ok on load(ie lights on full,heater fan on etc..)

if you getting lower then measure the voltage drop across the earth cable from battery to engine.(one test probe on battery,other on engine block).now if its ok you should only read 0.1v MAX,if more then you will find the earth cable needs removing,clean up connections,block and bolts and refit..measure again..looking for .1v max..

do the same if you can get at it for live cable from alternator to battery..

if all ok then your alt is fecked..OR your batt is fecked as it is never fully charging..

WARNING..be carefull whilt doing tests please,rotating alt pulley,belt etc!

have you drop tested your battery?no i dont mean dropping it,but putting a proper battery tester across it?(one that draws a high currect while monitoring voltage)

Thanks I will try in the daylight in the morning. A mate said I need a bigger alternator, as the one I have on is only a small one (35 or 45 amps) and I should upgrade to a 70 or 100 amps, as I said it only fails to start after a stint on the moors with heater on full blast headlights on full and a search light, the rest of the time all is well.
Thanks again.

I think the smallest was a 45A but it is easy to work out the load you are putting on it to see if you are overloading it but it sounds like you are.
Don't forget to upgrade the alternator cable when you fit a higher output one otherwise you will overheat the existing cable.