AT 261 - Power Balancing and Compression Testing - Chapter 3
Name: _________________________________________ Group: ______________
Date: __________________________________________
Power balancing will pinpoint a low or weak (missing) cylinder. This is done by comparing the RPM drop from one
cylinder to against the other cylinders. Several operating parameters must be met to get accurate results. In order to
pinpoint a weak or low cylinder you must either remove spark, or fuel from the cylinder.
This narrows your search down and saves valuable time! It also verifies that the vehicle is not running at its full
power. Items like the oxygen sensor and EGR valve must be disconnected. These items are computer controlled,
and will skew your readings. The computer can compensate within time as short as 20 milliseconds of detecting the
misfire (when you short out the cylinders). Disconnecting these items prevents the computer for correcting for the
problems; in fact some customers never realize that their engine is not running properly!
STATION 1 Power Balance (#1) Tools you will need:
1999 Ford Escort (White)
Infrared Pyrometer
12 Volt Test Lamp
DVOM with RPM pickup (Tachometer)
Noid Light
Spark Tester
Firing order and plug/coil pack schematic
WARNING! High Voltage! Be caution while performing this lab. Ignition system voltages are
approximately 25,000~50,000 volts DC. NEVER run an open (disconnected, ungrounded) ignition
system! This will lead to damage to the vehicles ignition system!
Warm up the engine to operating temperature (Typ 170~190 Deg F). Verify temperature by pointing infrared
pyrometer at the water outlet (Thermostat housing). Run the vehicle at fast idle (2000 RPM).
The (primary) Oxygen Sensor(s) must be disconnected. (This is already done for you)
Connect the tachometer to a spark plug wire.
Unplug each injector one at a time, noting the DROP in RPM. Write down that number that the RPM drops to
while it is shorted out.
Cyl 1 Cyl 2 Cyl 3 Cyl 4
RPM
The paperclip method of shorting out cylinders works as well (see demo) the best. Some (lame) techs short out
each plug wire by just stabbing the test light into the spark plug wire. This is a bad idea since this makes a hole in the
plug wire that can lead to corrosion or a short to ground form the ignition system. WARNING! Do NOT short a
cylinder out for more than 10 seconds at a time! Doing so causes raw fuel to be "dumped" into the exhaust.
Q- The cylinder the least drop or no drop in RPM is the weak cylinder.
The weak cylinder is ______________________
Check for spark with the spark tester on the weak cylinder, is their spark? ___________________
Check for injector power with the noid light. Is their power? ________________
Instructor’s Signature _________________________________________________________________
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