OK
My dead speedo, tach, and check engine light malfunction has now happened three times with a low battery. So, I put in a new Ford OEM unit (cheaper than an Optima, even tho' it is one!) So far, problem solved. However, I wonder if there are any folk on the forum or contactable who know the intricacies of the GT electrical system? Whilst I've been thinking it is a low feed voltage issue,
Based on my personal experience, this doesn't explain the failures among those who use battery tenders (unless they have less-than optimal charging voltage). So now I'm wondering if there are capacitors somewhere in the gauges or related wiring? This might explain my belief that its all somehow linked to frequency of use; if a capacitor is used to maintain an adequate feed to the gauges, and it bleeds down over a period of time, could this explain the root of the problem, i.e. The gauge somehow needs a 'reference' voltage and when that goes away and you start up, it doesn't spring to life. I am not an electrical engineer but have wired several vehicles and repaired the diodes on many many old Ferrari gauges, so am not a complete ignaramous....Anyway, bring on the experts!