Coil and/or Tach Electrical Problem

[DJThom]

Hi Folks,

I've got a bit of a problem and need some advice. I have a '76 MGB that I rewired for a 3 ohm coil. I'm feeding the coil 12V from the ignition relay and have done away with the resistive wiring. Its a Lucas Sport Coil DLB 105. Essentially I've wired it like the older cars that first had alternators.

When I start the car, within 30-60 seconds, the green accessory circuit blows its fuse. With the engine off and key turned forward to run position, I can have everything on that circuit turned on simultaneously with no problem.

I narrowed the problem to my tach. The green circuit also feeds the tach, and if I disconnect it, the car runs fine, and no fuse blows. I just don't have a tach.

The wires going to the tach (other than lights) are the green power and the white/black coming back from the -ve post of the coil. The engine has to be running AND the tach has to be hooked up for the fuse to blow.

Any help would be greatly appreciated.

Thanks
Darren

 

[Mickey Richaud]

That's a head-scratcher, alright!

From what you've said, it seems like it has to do with the tach. Is its ground wire secure?

 

[DJThom]

Mickey, the ground wire is secure or seems to be since the tach works perfect up to the point where the fuse blows. Also, the ground is a "traveler" there carrying on to other gauges and switches which also work.

I'm thinking its a tach problem, although I have never had a problem with it before.

I've emailed Peter at Nisonger Instruments to see if he can shed some light

 

[Mickey Richaud]

I've emailed Peter at Nisonger Instruments to see if he can shed some light

Good call.

Maybe someone else with more electrical savvy will pop in here.

Be sure to keep this one going, so we can learn!

 

[Roger]

Do you still have an original, unmodified tach and points and condenser, or have you gone electronic?

In either case, the sensor wire in the Smiths/Lucas common British wiring is attached to the + post on the coil in a negative-earth car.

 

[Mickey Richaud]

Now, this is confusing! According to the diagrams I've checked, the tach wire is connected to the negative post along with the distributor wire (both white w/ black stripe) from '72-on. There are two diagrams for the '76; one for cat. converter equipped and another for non-cat. The one for cat. models shows a ballast resistor and a distributor resistor.

Does this have any bearing here? Or am I throwing in the proverbial red herring?

LINK to diagram.

 

[DJThom]

I heard back from Nisonger saying i probably have one or more transisters blown in the tach which is causing it to ground out. I had the tach off to replace the glass and change to chrome bezel, but I was super gentle and careful.

Regarding the coil wiring, the way I have it now is identical to the first alternator cars which had 3 ohm tachs.

 

[Roger]

Mickey, I looked at those diagrams and confess to being more familiar with the earlier wiring, which is as I wrote.

Though as an amusing aside, the 62-64 diagram appears to show no wiring to the tach apart from the dashboard light.

 

[Mickey Richaud]

Hey, Roger -

Confused is the state I live in!

It hasn't helped that I've recently had to "engineer" the wiring for the V6 conversion. THAT was a task!