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Discussion Starter · #1 ·
Having been adjusting my new windscreen wiper arm positions with the ignition on did the dumb thing of leaving the ignition on overnight on my '72 Ghia.

Went into the garage in the morning to be greeted by foul electrical smell, coil collapsed its contents over the distributor, some local wires cooked and free of insulation and completely flat battery!

Anyway, replaced points, condenser, coil, local wiring, battery, dizzy cap, leads etc and everything runs fine except my aftermarket VDO tacho misreads. The reading floats up way ahead of actual engine revs. Not exactly eratic, more a case of over reads and seemingly load sensitive!

Thought the melt down must have goosed the tacho so bought another and this is just the same. So, checked feed to coil- 12v ok, checked feed to tacho from switched side of ignition- good, checked tacho earth -good.
Am now wondering if I have the right coil and or whether some gremlin to do with ballast resistors has been involved.

Any body know anything about these things? - would appreciate it
 

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The tach input circuitry is very sensitive to the signal sent from
the points/coil. Just an extra little wiggle in the signal can make
the tach read off.

I would be suspicious of the coil and the condensor as possible cause.
Did you get a generic coil or the correct Bosch unit?
The stock Bosch coil doesn't use an external ballast; it's built-in.
 

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Discussion Starter · #3 ·
Hi SJ,
The replacement is a Bosch but not sure if the part number is suited so will check when I get home. By the way, I have 12.4v at the +ve terminal on the coil and about 7v at the negative terminal. Does that sound about right? I have tried a second condenser and it made no effect so will concentrate upon the coil for the mo' ........
Thanks for you help, cheers
 

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about 7v at the negative terminal
No, that doesn't sound right. The neg terminal should swing
from 12V down to near zero volts, depending on whether the
points are open or closed.

Measure again with the tach disconnected. The tach shouldn't affect
the voltage reading.

If the 7V reading is with points closed, then there is a problem with
the points or else the advance plate is not getting proper earth.
If this is a vacuum advance distributor, look for the tiny braided
earth lead attached to the advance plate; it could have burned out.
 

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Discussion Starter · #5 ·
The distributor is centrifugal but I will check the voltage with points points closed etc.
Cheers Jim
 
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