MGB: Tach acting weird

Ok, So i removed my dash to recover it and marked all the wires prior to removing them. So after finishing the recovering job and reinstalling the dash, my tachometer is acting weird. When I turn the car on the tach quickly jumps to 3000 rpm and just sits there. Can anyone help. Also just to make sure i have it weired right...

On the back of the tach there is a metal post connection, and right next to it there is a spade connector. Then on the case there is another spade connector.

I connected the white/black wire to the post
the green wire to the spade next to the post,
and the black to the spade on the case

is this right... any help?

David,
You're connections are correct.

Does the tach eventually come down when the car is started? Are you having battery problems? A few weeks back I brought my 78 out of "hibernation." Needless to say the battery was dead. So dead it wouldn't even take a charge at first (battery charger indicated it was fully charged-so I had to push current through it by using the starter feature and turning on the lights to cause a drain).

Anyhoo, after a few minutes of charging, and seeing the clock to start working and the interior light come on, I decided to see if it would turn over. When I turned the key (just to the on position, not start), the tach shot up to about 3500, or 4000, can't remember exactly but it was high.

The battery eventually took a full charge and since then I haven't had that problem. But I've seen it before and it almost always involved a low battery.

Not sure what year yours is, but the Bentley's for 78+ seems to show the green on the post, white/black on the spade, black to the case, and white/red to the light. Just a suggestion.

Ok so i did some troubleshooting. I checked my connection to the ignition solinoid... its good. Verified 12v on the green connection...good, and verified the ground... all good. However then it starded acting different, instead of pinging to 3000 when the ignition was turned on it then pinged to 7000. So i decided to open it up to check the inner wiring. All the wiring looks good internal. No obvious blown caps, connections etc.

Has anyone had sucess in replacing the internal electronics in these. Anyone have the wiring schematic for the electronics?

I figure ive got a blown tach so now whats the best / most cost effective way of fixing this. Do i send the tach for repair or do i just try to get a new one. Any hints / companies?

Anyone have a spare good tach lying around to sell? Its for a 74

David, what does it do when the car is running?

Still stays pegged at 7000

David,
What is the "Smith" number on the tach? I got a spare from a 76 car, but I suspect its a different model.

When I needed one for a 79, I checked out rework shops, and they want about 100 to 110 for a tach rebuild.

I bought a working used one from Tony.

$70 including shipping.

Ed

RVC 1410/00AF

You might want to have a look here:

[url]www.classictiger.com/techtips/motach.html[/url]

googling "smiths tachometer" brought up some other interesting sites as well.

HTH!

Anyone know,
Will any of the neg earth electric tachs work on other cars? Are they interchangable?

Also,
Tony, any luck on a working tach for a 74?

David,

Here's a quote from the article I cited above. Can't vouch for the accuracy, but it sounds reasonable.

David,
I sent you an email about the tach
Ed

Thanks to all who replied and for all your help. I managed to get my hands on a used working Tach for $50. I got it, cleaned the glass, polished the chrome ring, and hooked it up. Worked like achamp. So in summary, if the tach pegs high when you turn the ignition on and stays pegged when the car is running... you have got a blown tach.

Thanks