Grand Cherokee 2WD V8-5.7L VIN 2 (2005)
Wiper Relay: Description and Operation
Rear
The rear wiper relay (2) conforms to ISO specifications for current capacities, terminal functions and patterns. This relay is contained within a small,
rectangular, housing and is connected to all of the required inputs and outputs through hard-wired connections to the base plate of the junction block (JB)
(1).
The JB is located in the passenger compartment, underneath the driver side of the instrument panel. The rear wiper relay cannot be adjusted or repaired
and, if found faulty or damaged, the entire junction block base plate must be replaced.
The rear wiper relay is an electromechanical switch that uses a low current input from the front control module (FCM) to control a high current output to
the rear wiper motor. The movable common feed contact point is held against the fixed normally closed contact point by spring pressure. When the relay
coil is energized, an electromagnetic field is produced by the coil windings. This electromagnetic field draws the movable relay contact point away from
the fixed normally closed contact point, and holds it against the fixed normally open contact point. When the relay coil is de-energized, spring pressure
returns the movable contact point back against the fixed normally closed contact point. A resistor is connected in parallel with the relay coil in the relay,
and helps to dissipate voltage spikes and electromagnetic interference that can be generated as the electromagnetic field of the relay coil collapses.
The rear wiper relay terminals are connected to the vehicle electrical system through hard-wired connections to the junction block (JB). The inputs and
outputs of the rear wiper relay include:
-
Common Feed Terminal - The common feed terminal (30) receives battery (B+) current through a fuse in the JB at all times.
-
Coil Ground Terminal - The coil ground terminal (85) is connected to a control output of the FCM through a rear wiper control circuit. The FCM
controls rear wiper motor operation by controlling a ground path through this circuit.
-
Coil Battery Terminal - The coil battery terminal (86) receives fused battery current at all times.
-
Normally Open Terminal - The normally open terminal (87) is connected to the rear wiper motor control circuity whenever the relay is
energized.
-
Normally Closed Terminal - The normally closed terminal (87A) is not connected to any circuit in this application, but provides a battery current
output only when the rear wiper relay coil is de-energized.
The rear wiper relay can be diagnosed using conventional diagnostic tools and methods. However, conventional diagnostic methods may not prove
conclusive in the diagnosis of the steering control module (SCM), FCM or the electronic message inputs to or outputs from the SCM and the FCM that
control the operation of the rear wiper relay. The most reliable, efficient, and accurate means to diagnose the rear wiper relay, SCM, FCM or the
electronic message inputs and outputs related to the rear wiper relay operation requires the use of a diagnostic scan tool.
