[Mgs] Excessive Voltage?

[Barney Gaylord]

At 05:13 PM 8/6/2009 -0400, Steve Hughes <Daybell7 at aol.com> wrote:
>....
>I've installed a generic ballast resister in my ignition circuit

Why?  What kind of car, and what model year?  MGB had a ballast 
resistor standard from 1974-1/2 to 1980, earlier cars not.  Ballast 
resistor must be mated with ballast type coil, both or neither 
installed, not one by itself.

Standard coil is about 3.2 ohme resistance in the primary 
winding.  Ballasted coil is about 1.6 ohms plus ballast resistor of 
1.6 ohms in series to make same 3.2 ohms total.

>and voltage is correct while the car is not running: 12+ volts & 6  volts.

Where are you measuring the 6v?

With ballast resistor and ballast type coil installed, and with 
ground circuit through distributor closed (equivalent to points 
closed), you should have 6v between the resistor and coil.  With 
ground circuit through distributor open (equivalent to points open), 
you should have 12v between the resistor and coil.

>When the car is running, I measure 13+ volts on the ignition side of 
>the resister

Good.

>and 11+ on the coil side and at the coil.

Bad.

>Is this  correct?

The voltage reading is not correct, but there may be nothing wrong 
with the car.  While running the voltage between ballast resistor and 
coil is constantly changing from about 14 volts (system voltage) to 
about 7 volts (half system voltage).  When ground curcuit opens 
(solid state ignition equivalent to points opening) the coil primary 
circuit is subject to a momentary high voltage spike (up to 300 
volts) and high frequency "ringing".  This erratic voltage 
fluctuation in the coil primary will confuse a digital volt 
meter.  An analog volt meter should show about 7v at the coil while 
running (maybe, depending on design of the meter).

>Also, does the voltage regulator affect the operating voltage of the 
>car generally, or just the charging system?

The regulator controls voltage output of the alternator (or generator 
on earlier cars).  Alternator output is intimately connected to the 
battery cable, so system voltage goes with alternator output voltage.

>I thought voltage throughout the car should not exceed 
>battery  voltage, i.e. 12.6V.
>  ....

Nope.  Alternator output may be in the 14-14.5v range when running, 
and so goes the rest of the electrical system.  Open circuit voltage 
for a fully charged battery in good contition is 12.6v.  When 
discharging (lights on and engine not running) the voltage will go a 
bit lower (around 11.5-12.2v).  When running and charging the system 
voltage will be higher (13.0-14.5v depending somewhat on where in the 
system you test it).  Checking voltage near the coil when running 
using a digital meter can give false readings.

Barney Gaylord
1958 MGA with an attitude
http://MGAguru.com