Potential electrical Gremlin?!

Hi all.

I'm hopefully now in the final stages of the build in terms of getting the car on the road for the first time in years, Still a few bits to be doing but I guess there will always be something!

I have one 'big' issue that I can't figure out, if I turn the headlights on, the rev counter drops to dead on the clocks, has anyone had anything similar or advise on where I need to be looking at/thinking about!?

I've earthed the clocks as they didn't appear to be previously but that hasn't sorted it, the previous owner didn't have MK1 clocks fitted when I brought it and had spliced the loom in an attempt to get Mk3 polo clocks to work, I have removed all the extra that was put in but not sure where I'm now going wrong!

TIA

Jack. 

Does the car have an MFA fitted to the instruments?

You could have a earth cable broken which goes to the back of the engine head, the thinner brown wire fitted to the same stud as the thicker earth lead going from the coil bracket to the head.

When my cable broke my Rev counter would drop when I switched on the headlights..

I'll have answer that with a question! How do I know if it has MFA? - I had to get some second hand clocks as the car didn't come with any. - will it make a difference if it has?

I'll double check the cable to the head as well

Check your Battery to Frame, an Frame to engine/tranny ground for voltage drop.

Then I would relay the headlights to take the load off the lamp switch and give you better current and voltage to your head lamps.

You added a new ground to the clocks pin 2? (brown)

Briano1234 said

Check your Battery to Frame, an Frame to engine/tranny ground for voltage drop.

Then I would relay the headlights to take the load off the lamp switch and give you better current and voltage to your head lamps.

You added a new ground to the clocks pin 2? (brown)

Perfect thanks will have a look.

No the brown wire was loose at the speedo plug so reattached that before it went in

I have found that the ground on the cluster is a little weak, and that by adding a direct to frame ground (behind the clock) to the frame and then use a scotch-lick in line splice to add a newer ground to the clocks on pin2 of the cluster connector (brown) worked wonders for me and improved my bulbs brightness and the gauges seemed more responsive.