Dash / fuel pump problems

Hi , I have a 88 mk1 gti convertable & the battery is over charging at 13.7v when the mk1 is running, I tought this was normal but the auto electrician said that the battery might be nackered & to put a different one in. I have an old battery but it's 630amps from a mk1 diesel I had before. The battery in the gti is a 540amp. The problem I have is when I start the car it's fine but if I turn on the lights or wipers etc the fuel pump get quieter and vice a versa if I turn them off, eventually stalling , is the wrong battery in it? Will the 630amp one be ok ? What's the verdict on overcharging ! I never heard of it . Plus when I reconnect the battery the hazards go for 30 secs by them self ! Is that normal ? If I'm driving, turn on the lights the rev counter stops working , turn off the lights and it's fine , maybe my battery is goosed , any one shed any light. Engine is a dx if that helps in any way. Thanks jukes :roll:

Sounds like earthing probs, check the earths before you replace anything.

It shohuld be ~12 V when engine is off and 13-15 Volts running, 13.7 if anything is a little low.

Battery could be knackered but certainly doesn't sound like the problems you are describing. Agree it sounds like an earthing issue

This turned out to be faulty alternator and a old alarm system that was going gaga on it's onw accord.
All sorted .
Thanks for the replies