1. could be stuck open thermostat (normal op. temp should be 80 C)
2. there's a sensor above the harmonic balancer; on my 83 it was bad, i don't think it's that expensive...i believe the wire runs from above the harmonic balancer to the big round looking thing with a screw on cover on near the driver's side fender (that was on a SD; yours might be different)
3. do you mean it lights intermittently when the engine is cold (i.e. when trying to pre-glow?) if so it's probably bad (mine was)...the new and "improved" glowplug relay is only about $100...i would check the the glowplugs first thought (ohm meter) before replacing the relay...there's a fuse in that relay too...kinda big if i remember ...

hope this helps,

p.s. i would get a factory manual, they usually have them on ebay