Hi. I’ve got a problem that’s baffling everyone. I’ve got a drain on my 64 transit custom. It’s had a new battery which is testing fine and the alternator tests out perfect yet every morning for the last two weeks or so it’s been flat. It’s been in a well respected garage for two days and no drain is showing up at any point on the software. They disconnected the radio over night as a long shot but it’s still flat in a morning. Does anyone have any ideas where to go next with it? 

I recall a car I had where the battery would discharge slowly through the alternator when parked although alternator seemed to charge fine when engine running. Alternator has a rectifier built into it to convert ac to dc . Rectifier is a bunch of diodes. Diodes are like valves that let electricity travel through one way but not the other. A faulty diode can let power back slowly from battery to earth through the alternator. I don’t know how easy it is to get to alternator. Perhaps disconnect that one night.

Thanks. The trouble is it’s at a garage that specialises in alternators and electrics and their adamant there’s nothing wrong with the alternator and everything on it tests out perfect 

There's got to be a drain show up on a multimeter at some point at the garage surely. Did the battery not go flat while in there garage? 

I don’t know what the alternator test entails. It might only test that it charges the battery properly when it’s spinning. I don’t know if they would automatically test it to see if it lets power leak back in the reverse direction when not spinning. 

14 hours ago, iantt said:

There's got to be a drain show up on a multimeter at some point at the garage surely. Did the battery not go flat while in there garage? 

The battery did go flat over night but tested fine during the day. Everyone’s saying the same thing that there’s got to be a drain but nothings showing up. I’m leaning towards just swapping the alternator and seeing what it does 

13 hours ago, isetta said:

I don’t know what the alternator test entails. It might only test that it charges the battery properly when it’s spinning. I don’t know if they would automatically test it to see if it lets power leak back in the reverse direction when not spinning. 

I’m not sure what the alternator test entails but I’m going to ask about that this morning. I’m leaning towards just swapping the alternator and hoping for the best. A few people have spoke about the alternator leaking power it’s just strange for an alternator place to be telling me it’s perfectly fine. You’d imagine if there testing it and saying that they must have some way of testing it for leaking 

58 minutes ago, RichardTransit said:

I’m not sure what the alternator test entails but I’m going to ask about that this morning. I’m leaning towards just swapping the alternator and hoping for the best. A few people have spoke about the alternator leaking power it’s just strange for an alternator place to be telling me it’s perfectly fine. You’d imagine if there testing it and saying that they must have some way of testing it for leaking 

A Clamp Meter on the Battery to Alternator wiring will show if any current is flowing.