Electrical problem - tach & headlamps

Not my problem but a friend's

I've been unable to help solve the electrical problem with Emmett’s 1980 MGB. The issue is simple - the tach only works when the headlights are ON, not when the headlights are OFF. All kinds of simple tests haven’t been able to pinpoint the problem. Lots of folks say it’s a bad ground but the solution does not appear to be so.

A couple of other folks have reported the same problem but no solution.

This is a new one - any ideas?

Regards, Graham

Does this happen when the light switch is in parking lights only or when headlights only

That is opposite of the problem I had. On mine the tach would drop out when the lights were turned on.

I'd almost guarantee that the tach is earthing out (read grounded) via the headlamps or teh light switch.

After some instrument work the tach on my 73 stopped when I turned the headlights on. I'd been working on the speedo & had removed the tach to get to it. As soon as the tach dropped I realized in finishing up, I'd missed connecting the little ground wire to one of the "legs" on the tach that secures it to the back of the dash. The wire had dropped down out of sight. I discovered that by turning down the dash light dimmer switch that the tach came back almost to an accurate reading.

I don't know if this bit of trivia helps. Hope so.

Good luck!

Rob

This one is very interesting... My gut tells me that it is ground problems, my head says could be (DPO or DCO) wiring.

What other symptoms are there (if any)?
Are the heater or windshield wipers affected?

IF Not, I would say ground problem, i.e. the tach is probably grounding through the light bulb. Try removing the light bulb from the tach or turn the dash light dimmer rheostat so the dash lights go out and see if the problem persists or if the tach completely fails. IF NEITHER of those makes any difference you should start looking at any wiring mods, especially recent ones. IF EITHER or both of those tests change the symptoms start looking for loose grounds at the Tach.

HTH!

Tony

I suspect DPO connection of radio or cd player. or maybe incorrect installation of a pertronix ignitor Have you had anything installed by an electronix store such as Bestby or circuit city that dont know cht about our LBC's

graham - try to follow the connection. I believe the lamp wires are red. If they are connected to a red/green, they are connected to the light switch it will only work when switched on. The permanent feed is brown. The tach should therefore be connected to a brown. Have you checked a wiring diagram?

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