Disco 2 TD5 flashing dashboard lights

Apologies if a similar problem has been posted but I couldn't find this issue elsewhere.
On startup, a number of dashboard lights come on for a second then go off. This happens every few seconds. The rev counter also jumps at the same time!
I changed the battery earlier but this has not sorted it.
If any one has any ideas I would be very grateful.
Thanks in advance.
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rev counter only works when lights come on, its as if ignition is turning off but engines still running, wouldnt have gone for battery alt maybe, what voltage is showing on battery positive terminal when this happens and is it only at idle

jamesmartin said:

rev counter only works when lights come on, its as if ignition is turning off but engines still running, wouldnt have gone for battery alt maybe, what voltage is showing on battery positive terminal when this happens and is it only at idle

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I will check voltage tomorrow thanks.
Yes, changed battery because I have a another TD5 but no joy.