I fitted my car with a 80 amp alternator from Simons, about a year ago.
Ever since, the charging lamp and indeed the charging itself will first start after the engine reaches about 1500-2000 rpm.
First question is, since this is a non-standard alternator, what make is it, and is the pulley diameter the same as the original Paris-Rhone A13N10?
Second, the Murena 2.2 and this Paris-Rhone was unusual in having to have an ignition feed to the alternator for it to work. Most alternators only need a live power supply and the warning light connection through which it energizes, so how is this non-standard alternator wired?
Third, the Murena has an 82 ohm resistor wired across the warning light, which no other car I have ever come across in all my years in the motor trade, has ever had. You will notice if you look very carefully, that as you turn the ignition on, there is a slight delay to the warning light coming on, due to this resistor. Why was this resistor fitted? I've always thought it was because the current that the 1.2 watt bulb passes was too small to energize the alternator (since they always used to use a 2.2 watt bulb). However, I'm not positive. If the bulb was blown on a normal car (without resistor) the alternator would not energize and would therefore not charge*, but on the Bagheera and Murena it will as it can pass the energizing current through the resistor (although it doesn't always charge properly but that may have been those particular cars in question had other problems like worn brushes).
So the warning light on the Bagheera and Murena appears to be mainly so that you get a visual indication of under or overcharging. However, with your non-standard alternator, if it is the type that only requires a live feed and the warning light bulb connection, is the resistor actually causing the problem? This would be easier to answer if I knew exactly why the Matra engineers put it there in the first place!
Assuming the warning light comes on immediately you switch the ignition on, then the alternator is getting some energizing current. However, if the alternator has to be revved to 2000 rpm to get the warning light to go out, then it does not appear to be producing enough voltage until it is revved, which may be because it has insufficient current to energize it properly. If the resistor is required but it has blown, it could be the reason. Have you checked it?
Now is this reving to get the alternator to charge a fault? or a known Murena issue?... and in the case of the latter, can the alternator be modified to start charging straight away?
It is not an issue with the standard P-R A13N10 alternator. If anyone else has this non-standard alternator, can they tell us if they have this delayed charging symptom? Did you question Simon about this effect after fitting it originally?
Would the solution be so simple as to change to a different alternator all together?
Well as I said there should be no problem with the original, but then you only have a max. of 55 amp output. Mind you that should be sufficient for the Murena - it has never been a problem with any of my cars. To require an 80 amp output you would need to be using some serious electrical equipment. Do you have anything that makes this necessary?
* Cars with self energizing alternators will work without a warning light bulb, but they have totally different charging systems with separate warning light controls etc. and are not often seen now.
Roy