2.0 CiH Rev Counter drops when lights on

[Charlie Campbell]

I have a 1979 Opel Rekord with the 2.0 CiH engine. Rev counter works fine until I switch on the lights and the rev counter drops. I've checked connection to the alternator and all seems fine. Any ideas? 

[Jessopia74]

When you say drops, do you mean rpm shown drops or it just stops working?

[Sutty2006]

Do the actual revs drop or just the needle on the clocks? Have you checked the earth cable from the alternator to the block? Check your dash earths too. 

[berni]

Battery test ! 😎

[cam.in.head]

it's a bit annoying when someone comes on and asks for advice and then doesn't answer our questions back !  or some that never say thankyou afterwards !

this particular fault sounds like an unusual one to start with

[Paul Barrett]

Social media generation I’m afraid 

[Charlie Campbell]

Sorry for delay in replying to all. Been too busy to spend time on the Opel the last couple of months but hoping to get back to it this weekend. 

When headlights are switched on the rev counter stops working completely, not just a drop in revs caused by alternator load affecting engine idle. I had checked and cleaned up all connections to the alternator including earth cable. I'm going to remove the instrument cluster and check earth connection there. Fingers crossed.

Thanks to all for offering suggestions.