Cessna 170, Service Manual

Cessna 170 Service Manual 
Electrical System 
 

7

generator.  Therefore, it is important that both generator and regulator be of the same capacity to obtain best 
performance without damage.   
 

TROUBLESHOOTING THE GENERATOR/REGULATOR SYSTEM  
 
NO CHARGE INDICATED ON AMMETER

.  If the charging circuit does not indicate a charge or if the 

ammeter indicates the generator is not producing current at normal cruising RPM, the trouble can be 
isolated without special tools or instrumentation.  First determine that no obvious difficulty exists with the 
system wiring, i.e., that the generator ARMature terminal (large one) and the generator FieLD terminal ends 
are secure with no broken wires.   Also check at the voltage regulator  BATtery, ARMature, and FieLD 
terminals that the wiring is secure, terminals are tight and that no wires are broken.  Confirm that the voltage 
regulator base is grounded to the firewall and that the rubber shock mounts do not insulate it from achieving 
such a ground (ground strap is secure.)  Confirm the generator fuse or circuit breaker is good and/or reset.  
If none of these items are defective, then using a small jumper wire, MOMENTARILY connect the voltage 
regulator ARMature terminal to it’s BATtery terminal while the Master switch is ON, but the engine NOT 
RUNNING.  This will insure that the generator and voltage regulator is polarized.  Start the engine and run it 
at approx. 2,000 RPM and observe if the generator is now charging.   
 If the generator still does not charge, then shut down the engine and with a small jumper wire connect the      
generator FieLD terminal (small one) directly to ground and then start the engine and run it at 2,000 RPM.   
If the generator still does not produce a current then the generator is defective and should be removed and 
repaired, overhauled, or exchanged with a serviceable one.   
If the generator DID produce a charge then the fault lies with the voltage regulator or the aircraft wiring 
system.  Next, remove the small test jumper wire from the generator FieLD terminal, and use it to connect 
the Voltage REGULATOR CASE to ground as in Appdx. EL-Fig.EL-09, TEST 1 (Confirm Regulator Ground.)  
Again run the engine at approx. 2,000 RPM and observe if the generator is now charging.  Is so, then the 
problem is a faulty regulator ground.  Repair the regulator ground connection between the reg. case and 
aircraft ground.  If there was STILL NO CHARGE, then connect the small test jumper from the regulator 
FieLD terminal to ground as in Appdx. EL, Fig. EL-09, TEST 2 (Isolate Regulator.)  Start the engine and run 
it at 2,000 RPM and if the generator now produces a charge then the fault lies with the voltage regulator and 
it must be repaired, overhauled, or replaced.   However, it there is STILL NO CHARGE then connect a small 
test jumper wire between the ARMature and BATtery terminals of the voltage regulator as illustrated in 
Appdx. EL, Fig EL-09, TEST 3 (Cut Out Relay.)  This shorts out the Reverse Current Relay and the Current 
Regulating Relay and connects the regulator directly to the battery.  Run the engine again at approx. 2,000 
RPM and if a charge now occurs the voltage regulator is still at fault and must be replaced, overhauled, or 
repaired.   
If the generator STILL did not produce a charge, then the fault lies with the aircraft wiring and/or master 
switch.  Use a jumper wire to connect the generator FieLD terminal directly to the voltage regulator FieLD 
terminal.  Again, start the engine and run it at 2,000 RPM, and if the generator now produces a charge then 
the fault lies in the generator-to-master switch-to-Voltage regulator FieLD circuit.   This may be due to a 
faulty Master switch. 
Check the wiring at the cockpit master switch terminals for continuity when the switch is actuated.  
Remember that there are four terminals at the master switch.  Two are used to connect the battery solenoid 
(relay) to ground, and the other two (the ones we’re interested in here) connect the generator FieLD terminal 
to the voltage regulator FieLD terminal.  Confirm this circuit makes continuity across those two terminals 
when the switch is activated.   If found defective, repair or replace that switch and its associated wiring. 
 

GENERATOR TESTS, REPAIRS AND ADJUSTMENTS.  

There is very little to go wrong with the Delco-

Remy generators if they are polarized and have not suffered mechanical failure such as damage from a 
leaking oil seal, or failed brushes or bearings resulting in contact between the armature and the field pole 
shoes, or burned windings from failed or incorrect regulator.  Inspecting the generator in-situ is a simple 
process and may be done with the engine cowling removed.  Remove the blast-tube/brush shield-cover to 
examine the brushes.  When new the brushes are approximately three-quarters inch long.  When the 
brushes are more than half worn, they should be replaced.  It is best to replace them on the workbench.  
Remove the generator in accordance with the previous instructions.  Examine for evidence of oil 
contamination.  Using a screwdriver, pry the brush holders away from the commutator and prop them in that 
position with a wooden stick or tongue depressors.  Be careful not to damage the brushes if they are not to 
be replaced.  Examine for rough or worn bearings by spinning the armature by hand and listening and/or 
feeling for roughness, grinding, or points of increased friction.  Remove the brush end-plate by removing the 
long generator thru-bolts.  Replace the brushes at this time if desired.  Be certain that brush leads do not 
ground against the case or springs.  This is also the time to remove the armature and inspect, and/or 
replace the bearings and oil seal.