Peugeot alternator/charging problem.

[Squiffy]

My neighbour has a Peugeot based motorhome 2004 that has a charging problem, he asked if I could look at it, it started apparently not to charge the cab battery a couple of months back so he would take the cab battery of and recharge it off the vehicle. The ignition light comes on when turning the key and goes out as it should with the engine running suggesting that the battery is being charged, but when checking the charge rate with a multi meter it shows only the battery voltage 12.5 volts or less depending on the state of charge when refitted.
I went over and checked the alternator when running and it shows only battery battery voltage at bat ve+ but oddly only 1.5volts at the exciter wire, in the past I would immediately have condemned the alternator, but he has had it replaced with a brand new one last week but it has not changed from the original problem. I personally am at a loss as to knowing the reason for it. All earth's are good and the strange thing is he says the engine and head lights still keep running when he disconnected the cab battery, which suggests that the alternator is actually supplying a charge. Any ideas would be greatly appreciated because its driving me nuts apart from him. Phil

The hab battery is disconnected so not supplying any voltage to the system.

 

[Squiffy]

Just for the interest of those that would like to be informed. The problem turned out to be the earth strap from chassis to the gearbox. The gearbox end was heavily corroded, once removed and cleaned up all started working as it should. The strange thing was that the starter motor which shares the B+ and engine earth with the alternator showed no sign what so ever of being compromised as it operated as you would expect, so the only thing that I can think is that the heavy ampage drawn through starting the engine overcame the high resistance of the earth strap but was too high to deliver the exciter voltage to the alternator, lesson learned for future reference, something that in 40 years in the motor trade that I had never come across. Phil

I.e. I've had the situation whereby the starter was sluggish because of a bad earth but once started the alternator/ dynamo, still charged the battery but never the other way round.