Tennant 7400, Service Manual

Page: 293 / 322

ENGINE--GAS/LP
6-29
7400 MM428 (12--98)
“FAILURE MODE OF DIS” IGNITION HARNESS
The Ford DIS Universal Spark Control was
checked for “poor connection”.
The results of open leads were:
Term 11 or 12 Ignition Coil Signal Leads --
The governor received 1/2 of its intended
signal and caused the actuator to hold the
carb arm wide open. At the same time, the
engine missed due to incorrect firing of
plugs, but the speed still climbed to about
3,000--4,000 rpm. When either of these
leads were opened at the start speed, the
engine missed slightly and rose to double
the start speed or about 3,000 rpm.
Term 9 Ground -- Any failure here killed the
engine instantly or caused the engine to
slow down if it were cycled.
Term 6 & 7 Octane Selector -- Opening
these terminals retards the spark in varying
degrees. Under light to moderate load
conditions, no adverse effect will take place.
However, run-on or pre-ignition may take
place.
Term 4 & 10 Coolant Sensor -- Contrary to
previous thoughts, opening the lead doesn’t
noticeably affect the engine. The timing only
changes a few degrees.
Term 2 & 3 Crank Position Sensor --
Opening either of these leads kills the
engine quickly and it will not start.
Term 8 Power Lead -- Opening this lead kills
the engine and it will not start.
In summary, no alarming effect happens except if
the coil signal leads are interrupted. These are the
end leads on the module and if the harness has
too much tension on it, the signal will become
intermittent and cause the engine to overspeed.
The only obvious damage will probably occur to
the propelling pump, as the fan is rated for
operation to 10,000 rpm. If the acc. are on when
the lead becomes intermittent, the engine will not
develop enough power to overspeed excessively.