Cessna 170, Service Manual

Cessna 170 Service Manual 
Electrical System 
 

8

A SIMPLE GENERATOR TEST

 involves recognition that a generator is merely the mirror image of an 

electric motor.  A generator may be easily tested on the workbench by securely clamping it in a vice or 
placing it upon the floor between two aircraft chocks to prevent its rolling, and placing one’s foot firmly upon 
it.  Using your hand, spin the armature to confirm that no interference exists between the armature windings 
and the field windings/shoes.  
 Using a heavy gauge wire or jumper lead connect the generator FIELD terminal (small terminal) to the 
generator case.  Using a 12-volt storage battery (car or aircraft battery will do) and starting jumper cables, 
connect one end of the red/positive cable to the POSitive terminal of the battery and the other end of the 
red/positive jumper cable to the generator ARMature (large terminal).  Next connect the black/negative 
jumper cable to the generator case and…while firmly holding the generator from violently jumping away from 
it’s location…connect the other end of the black/negative jumper cable to the NEGative terminal of the 
battery.  (Alternatively, a 10 Amp or larger battery charger may be used for this test.)   
  A healthy generator will instantly and vigorously spin its armature shaft as it becomes motorized.  Bearings 
may be listened to for roughness or chattering. (Limit this test to only short bursts of 5 seconds or less as 
armature RPM (speed) can be exceeded and damage can subsequently occur.)   See Appdx EL, Fig 07 for 
diagram of this test. 
 

TEST THE GENERATOR FOR SHORTS

.  A simple test for a shorted generator is to remove the two leads 

from the generator FieLD and ARMature terminals.  Raise the grounded brush and insert a piece of 
cardboard beneath it to insulate it from the commutator.  Using a Volt-Ohm meter or battery powered test 
lamp, test for a short between the ARMature terminal and the generator case.  If current flows with this 
setup, the generator is internally shorted either in its field coils, its armature or its brush holders.  Send the 
generator for repair, overhaul, or exchange.  See Appdx EL, Fig. 08 for a diagram of this test. 

 
VOLTAGE REGULATOR REPAIR AND ADJUSTMENT

 Most owners feel ill equipped to undertake voltage 

regulator repair and/or adjustment.  However certain minor adjustments may be made with unsophisticated 
tools.   
 

HIGH RPM GENERATOR ON-LINE SPEED

.  Ordinarily the generator should come online at 1200 engine 

RPM or slightly above.  If this does not occur, then adjustment may be made to cause a reduction of the 
Cut-Out relay spring tension.  To adjust the ON-Line speed the battery must be less than fully charged.  A 
voltage meter reading of approximately 11.5 volts is ideal.  Use a short test jumper lead as in Appdx. EL, 
Fig. EL-09, TEST 3 (Cut Out Relay) and from engine idle, slowly increase engine speed until the first 
indication of a charge occurs.  If this speed is appreciably below the previously observed ON LINE speed, 
then adjustment of the Cut Out Relay spring tension to a lower tension value may be warranted.  .  Most 
regulators will have either a tension-type coil-spring attached to bend-able steel hooks (use needle nose 
pliers), or a screw may adjust the spring tension with a screwdriver.  In either case a reduction of spring 
tension will lower the required engine RPM (generator voltage actually) to bring the generator on line. 
Carefully note that when the Cut-out Relay brings the generator online that indicated voltage does not 
decrease but should increase with any additional engine RPM.  After the battery has charged above 12.6 
volts, the engine should be slowly brought back to low idle and it should be determined that the Cut-out relay 
opened before 800 RPM and/or before battery voltage drops below 12.6 volts.    

Note:

  On regulators that 

have dual springs (two springs on each relay) then only one spring should be adjusted to increase or 
decrease total tension. 
 

ADJUSTMENT OF VOLTAGE

:  Examine the voltage regulator with the cover removed.  Determine if the 

relays are operated by direct spring tension through “hangers” or hooks, or if they are adjusted by screws.  
(Caution:  Some models utilize screws with left-hand threads.)  Voltage is adjusted by adjusting the spring 
tension of the Voltage Regulator Relay (connected to the FieLD terminal).  Operating voltage is Increased by 
increasing spring tension and decreased by decreasing spring tension.   It is recommended that a DIGITAL 
voltmeter be used if adjusting voltage for accuracy.  Maximum voltage should be 13.8 volts. 
 

ADJUSTMENT OF CURRENT

 (Amperage): To check this setting the voltage regulator relay must be 

disabled.  To do this connect a jumper across the voltage regulator contacts.   Turn on landing/taxi lights and 
non-avionic accessories to prevent high voltage during this test.  With the engine running at 1800-2000 RPM 
note the current reading on the ammeter.  If the current reading is not within specifications, adjust the spring 
tensions of the Current Relay (attached to the ARMature terminal) in the same manner as that outlined for 
the voltage regulator relay.  Increase of spring tension will INCREASE current output.  NOTE:  It is important 
that operating accessories demand the maximum possible current in order to determine the output of the 
generator;/regulator for this method to be valid.  Consult your electrical loads to confirm this.   

 
ISOLATING “Too Low or Too High CHARGE” TROUBLE

.  There are three common symptoms of high or 

low charging difficulties.