GMC 1976 ZEO 6083 Maintenance Manual page 417

Regardless of instrument or method used, the
coil must be tested at normal operating temperature
because internal defects often fail to show up on a
cold test .
DISTRIBUTOR CONDENSER TEST
When a condenser is suspected of being faulty it
should be tested with a reliable condenser tester to
determine whether it is actually the cause of ignition
trouble. The condenser should be tested for (a) high
series resistance (b) insufficient or excessive capacity
(c) low insulation resistance .
A special condenser tester is required to make
these tests. When using a condenser tester the in-
structions of the manufacturer must be carefully fol-
lowed.
NOTE : The condenser must be at normal operat-
ing temperature when it is being tested .
A. High Series Resistance
High series resistance in the condenser causes the
condenser to be slow in taking the charge and, conse-
quently, a higher than normal voltage is developed
across the contact points when they first start to
open . The higher voltage causes more disturbance at
the contact points, which in turn causes more rapid
wear and more tendency toward oxidized surfaces .
The condition can become severe enough to cause
complete failure of the ignition system . It would first
show up during starting and low speed operation.
High series resistance may be caused by internal
resistance in condenser or by resistance in the con-
nections . Any defect caused by internal resistance
should show up at low mileage since this does not
change very much with time or use. The damaging
changes are in the connections, in which looseness,
corrosion, or broken strands may develop.
New condensers may have a series resistance as
low as .05 ohm. Some condenser testers are set to
reject condensers which have a resistance of .3 ohm;
however, test show that the resistance can go to .5
ohm before ignition performance is affected .
B. Insufficient or Excessive Capacity
The condenser specified for use in the ignition
system has a capacity of .18 to .23 microfarads.
If a condenser is used which does not have the
specified capacity of .18 to .23 microfarads, excessive
pitting of one contact point and a corresponding
buildup of metal on the other contact point will re-
sult . A condenser having insufficient capacity will
cause build-up of metal on the breaker arm (positive)
ENGINE ELECTRICAL
6Y- 3 7
point. A condenser having excessive capacity will
cause build-up of metal on the contact support (nega-
tive) point.
In exceptional cases, pitting and metal buildup
on contact points may be experienced even when
condenser capacity is within the specified limits . In
such cases the life of contact points will be improved
by installing a condenser of high-limit capacity if
metal build-up is on breaker arm point, or a con-
denser of low-limit capacity if metal build-up is on
contact support point. There is usually sufficient
variation in the capacities of stock condensers to
permit selection of a high or low limit condenser by
testing the available stock.
C. Low Insulation Resistance
A weak or leaking condenser is usually one that
has absorbed water so that the insulation resistance
of the winding is lowered to the extent that the con
denser will not hold a charge satisfactorily . A con-
denser with low insulation resistance will drain
sufficient energy from the ignition system to lower
the secondary voltage seriously . The condenser
specified for use in the ignition system is sealed to
prevent absorption of water, and no other type
should be used .
A leaky condenser usually does not affect engine
performance except when hot. It is unlikely that a
condenser with low insulation resistance would
cause missing at low or medium speeds under condi-
tions where the condenser does not get hot. A con-
denser that has low enough resistance to affect
engine performance when cold would probably be
indicated as broken down on most condenser testers.
Condenser testers equipped to check condensers
for low insulation resistance usually give a reading
megohms, a megohm being one million ohms . The
scale is marked to indicate whether the condenser is
good or bad.
When testing a condenser for low insulation re-
sistance the lead should always be disconnected from
the distributor . Since the distributor terminals and
the connected circuit have much lower insulation
resistance than the condenser, failure to disconnect
the condenser lead will give a reading much too low.
IGNITION SYSTEM RESISTANCE TEST
Check for proper functioning of the resistance in
the primary ignition circuit by turning on the igni-
tion . With the engine not running, a voltmeter con
nected from the battery side of the coil to ground
should read approximately 5 to 5 .5 volts. If the read-