Usage:
The 6V7925 Fuel Rate Indicator measures flow rates between 3 gallons per hour and 80 gallons per hour. Horsepower calculation and rpm measurement circuitry are not included in the fuel rate indicator. For rpm measurements, use the 6V3131 Multitach Group.
Features
ADAPTABILITY: The indicator can measure fuel flow rates on all Caterpillar 1100, 3200, 3300 and 3400 series engines, and 6 1/4 bore "V" engines.
HOUSING: A rugged, break-resistant plastic case protects the instrument from damage.
GPH READOUT: Gallons per hour (gph) are directly displayed on a liquid crystal four digit display in numbers .5 inches high. The display is visible in direct sunlight and low light conditions.
INPUT: Connection is provided for the 5P2157 Flowmeter Cable from the 5P2148 Flowmeter.
POWER: Power for the indicator is supplied by an internal 9 Volt battery.
AUTOMATIC SHUTOFF: A battery saver circuit automatically turns off the indicator when there is no input signal (less than approximately 3 gph).
LOW BATTERY LIGHT: A red lamp glows on the front of the indicator when it is time to change the battery. If this lamp is on, the indicated flow may not be correct.
PORTABILITY: The fuel rate indicator is a lightweight, hand-held instrument; an improvement over the more cumbersome 5P2150 Engine Horsepower Meter.
CALIBRATION: A 6V3030 Multimeter (or equivalent) and a 1.5 Volt flashlight battery are used for easy bench calibration of the indicator.
RELIABILITY: The 6V7925 Fuel Rate Indicator with the new 8T5118 Circuit Board has been designed and tested to withstand severe environmental factors such as high humidity, extreme temperatures, etc.
Specifications
POWER: An internal 9 Volt battery (alkaline), type NEDA 1604A, is used to provide power for the fuel rate indicator.
BATTERY LIFE: Approximately 20 hours of continuous operation.
MEASUREMENT RANGE: 3.0 to 80 gallons per hour.
ACCURACY: 6V7925 Indicator - ± .1 gph typical (± .2 gph max) in specified measurement and operating temperature range.
6V7925 Indicator and 5P2148 Flowmeter - ± .5% of reading plus .1 gph (Example: At 20 gph, .5% = .1gph; plus .1 gph = ± .2 gph accuracy).
READOUT UPDATE RATE: Approximately once per second.
INPUT: Directly compatible with 5P2148 Flowmeter.
CALIBRATION FACTOR: ... 1.250 Volts dc = 40.0 gph (31.25 mV = 1 gph).
OPERATING TEMPERATURE RANGE: ... 32°F to 140°F(0°C to 60°C)
STORAGE TEMPERATURE RANGE: ... +14°F to +140°F(-10°C to +90°C) with battery removed.
SIZE: ... 7.5 in.(190 mm) x ... 4.4 in.(112 mm) x ... 1.6 in.(40 mm).
WEIGHT: ... 1 lb(.45 kg)
Components Of The 6V7925 Fuel Rate Indicator
The 6V7925 Fuel Rate Indicator includes the following service parts:
- 1. 6V3063 Plastic Case (includes front and rear halves)
- 2. 6V3114 Liquid Crystal Display
- 3. 6V7756 Connector (dual Zebra strip, mounting frame and clear bezel for LCD) (not shown)
- 4. 6V4179 A/D Converter (not shown)
- 5. Front decal - SEES5603
- 6. Rear decal - SEES5645 (not shown), gallons/hour to liters/hour conversions
- 2. 6V3114 Liquid Crystal Display
Items To Be Used For Fuel Measurement
1. 5P2148 Flowmeter (discontinued)
2. 5P2157 Flowmeter Cable
3. 6V7925 Fuel Rate Indicator
Connections And Controls
1. Input connector for 5P2157 Flowmeter Cable.
2. Power on switch - push and hold for approximately two seconds, then release to read the flow rate in gallons per hour (gph).
3. Digital readout - displays fuel rate to 0.1 gph.
4. Low battery light - A red LED lights when battery voltage is low enough to cause changes in the accuracy of measurements. A new 9 Volt battery should be installed at this time.
5. Operating Instructions - procedure for operating the fuel rate indicator is shown on the front panel of the instrument.
Operation Instructions
This Special Instruction applies only to fuel rate indicator (1) and replaces the 5P2150 Horsepower Meter (2) readout which is now cancelled. The 5P2160 Engine Horsepower Meter Arrangement is also cancelled. The 6V7925 Fuel Rate Indicator (1) will work with the 5P2149 Attachment Group (3). For application and use of attachment group (3) and flowmeter (4), refer to Special Instruction Form SEHS7050.
1. Connect 5P2148 Flowmeter to the engine (see Special Instruction Form SEHS7050).
2. Connect 5P2147 Flowmeter Cable to the flowmeter and fuel rate indicator (replaces 5P2150 Engine Horsepower Meter).
3. Press POWER ON switch. Display will read ± 00.0 or ± 00.1. Release POWER ON switch. The display will go blank (automatic shutoff).
4. Start Engine.
5. Press POWER ON switch. Displayed number is fuel rate in gallons per hour (gph) to the nearest 0.1 gallon. If gph is less than 3.0, display will go blank within twenty seconds.
6. To reactivate display, press POWER ON switch. With gph input greater than 3.0, display will remain on and update once per second.
7. Replace battery when LOW BAT light turns on.
System Troubleshooting
NOTE: In following procedure, the indicator must be connected to the 5P2148 Flowmeter with the 5P2157 Cable.
Application And Basic Interpretation Of Test Results
The "horsepower calculation" feature of the 5P2150 Engine Horsepower Meter is no longer required for engine diagnostics because it was too often misinterpreted. Instead, a simple measurement of fuel consumption is adequate to check fuel system performance. If fuel consumption of an engine is within the specifications shown in the FUEL SETTING INFORMATION, the fuel system is performing correctly and no additional time should be spent checking fuel delivery.
Engine horsepower output and overall performance can be determined easily and quickly with the following diagnostic tools.
There are two basic requirements for generating horsepower - fuel and air. If the proper quantity of fuel is being injected at the correct time, the engine is probably delivering output within the rating tolerance.
- 1. Fuel Consumption: If the specified amount of fuel is being injected into the engine, the fuel delivery specification is being met. This suggests that the basic fuel system is within limits:
- A. fuel pump/lines
- B. transfer pump
- C. fuel filter
- D. fuel pressure (primary)
- B. transfer pump
Additional time spent troubleshooting these components is probably not justified.
- 2. Fuel system timing: Fuel can not be burned efficiently if it is not injected into the cylinder at the correct time. Because engines only develop horsepower when they are running, timing must be measured when they are running. The "fuel flow timing procedure" is not adequate for this requirement since it is a static (engine dead) procedure. Timing must be measured throughout the speed range (this also checks the timing advance operation).
- 3. Intake manifold pressure: Manifold pressure is an indication of the overall health of the engine. Boost is affected by any one or all of the following:
- A. fuel consumption
- B. compression
- 1. valve condition
- 2. ring condition
- C. turbocharger performance
- D. intake restriction (air filters)
- E. exhaust restriction (muffler)
- F. timing
- B. compression
When troubleshooting a "low horsepower" complaint, whether it be an earthmoving vehicle, truck, boat, etc., if a) fuel consumption, b) timing, c) speeds, and d) manifold pressure are within specification, do not spend additional time looking for "low horsepower". If it is still believed that performance of the vehicle is below that expected, look for horsepower losses in the transmission/marine gear, differential, clutches, parasitic loads (fan, air compressor, air conditioner, etc.), tires, low fuel density or high temperatures, but not the engine.
- A. fuel pump/lines
Low Battery Detector
The electronic circuitry in the fuel rate indicator constantly compares the battery voltage to a fixed reference dc voltage. When the battery voltage drops to or below the reference voltage, the LOW BAT lamp turns on. Below this point, the accuracy of the indicator may not be acceptable. The 9 Volt battery should be replaced at this time.
Changing The Battery
1. Remove six screws (1) from the rear cover of the fuel rate indicator. Pull cover (2) off the unit.
2. Disconnect battery (3) from connector (4). Remove battery from battery holder (5).
3. Install a new 9 Volt battery in battery holder (5), and insert top of battery into connector (4).
4. Install rear cover and the six screws.
Calibration Of The Fuel Rate Indicator
Calibration of the 6V7925 Fuel Rate Indicator, and all diagnostic tools, should be checked (calibration procedure may not be required) every six months; this includes the 5P2148 Flowmeter (see Special Instruction Form SEHS6948).
There are two methods for calibrating the fuel rate indicator. The first method requires a 6V3030 Multimeter and a variable dc voltage supply. The second method requires a 1 1/2 Volt flashlight battery (size AA, C, or D) and the 6V3030 Multimeter (or equivalent).
(A) Method 1
1. Remove six screws (1) from rear cover (2). Pull cover (2) off the indicator.
2. Locate R1 potentiometer (3), DISPLAY CAL, on the printed circuit board.
3. Set the dc variable power supply for 1.250 Volts output.
4. Apply 1.250 Volts to FLOWMETER connector (4); positive to pin B and negative to pin A.
5. Push POWER ON switch (5). The display should be on and reading approximately 40 gph.
6. Use the multimeter to make sure the 9 Volt battery is at least 8.0 Volts. If not, replace battery.
7. Use multimeter (6) to measure the output supply voltage. Adjust the variable voltage supply to exactly 1.250 Volts dc.
8. The gph readout should now be exactly 40.0 gph. If not, adjust by turning R1 potentiometer (3) clockwise to increase the readout or counterclockwise to decrease the readout.
9. Disconnect the variable power supply and multimeter from the printed circuit board.
10. Install the rear cover and six screws.
(B) Method 2
1. Remove six screws (1) from the rear cover. Pull cover (2) off the indicator.
2. Locate R1 potentiometer (3), DISPLAY CAL, on the printed circuit board.
3. Connect the plus terminal of a 1 1/2 Volt battery to pin B of FLOWMETER connector (4). Connect the negative terminal of the battery to pin A of connector (4). If necessary, use a rubber band to hold the wires in place.
4. Push POWER ON switch. The display should be on continuously.
5. Use the multimeter to make sure the 9 Volt battery is at least 8.0 Volts. If not, replace battery.
6. Use the multimeter to measure the voltage across pins A and B of the FLOWMETER connector. This is the voltage supplied by the 1 1/2 Volt battery when the fuel rate indicator is on. Call this voltage Vin.
7. Vin represents a gph rate determined by the following equation:
EXAMPLE: If the battery measures 1.581 Volts, then the calculated gph equals:
The display should be calibrated for 50.59 gph.
8. The gph readout should now be exactly equal to the calculated gph. If not, adjust R22 potentiometer clockwise to increase the readout or counterclockwise to decrease the readout.
9. Disconnect the 1 1/2 Volt battery and multimeter from the FLOWMETER connector.
10. Install the rear cover and six screws.
Checking The 5P2157 Flowmeter Cable
1. Use the 8S4627 Circuit Tester or the 6V3030 Multimeter to check for continuity in each wire. Each wire should measure .000 K Ohms on the 2K Ohm scale of the multimeter.
2. Check for short circuits between the conductors. The measurement should be OL (infinity Ohms) on the 2K Ohm scale of the multimeter.
Repair Of The 6V7925 Fuel Rate Indicator
(A) Removal Of The Rear Cover
Remove six screws (1) from rear cover (2) of the fuel rate indicator. Pull straight out to remove the rear cover.
(B) Removal Of Circuit Board
1. Put the fuel rate indicator face down on a soft surface to prevent damage to the face. Use a screwdriver to remove six screws (1) from outer edge of printed circuit board. Do not remove two screws (2).
2. Hold connector panel (3) with one hand, and battery holder (4) with the other hand. Carefully lift printed circuit board (5) out of the front half of the case.
(C) Removal Of Display Assembly
1. Remove the rear cover and circuit board as shown on page 11.
2. While holding display assembly (1) around the edges, use a small screwdriver to remove two screws (2). Remove the display assembly from the printed circuit board. Be careful not to touch the display except at the edges.
3. If flexible conductors [Zebra strips] (3) are to be removed, use tweezers to carefully remove them from the slots in the display bracket. Do not touch the flexible conductors; skin contact can leave oil on the conductors.
(D) Cleaning The Display Assembly
1. The LCD can be cleaned by gently wiping it with a soft cloth. IMPORTANT: Rubbing the LCD too hard can remove the contacts deposited on the edges of the glass.
2. The flexible conductors (Zebra strips) can be cleaned by dusting them with a camel's hair brush. If grease or fingerprints are on the conductors, the conductors may be washed with clean alcohol, and then air dried. Do not wipe the flexible conductors with any material that could leave lint or other deposits on the flexible conductors.
3. Contacts (1) on the circuit board can be cleaned with a pencil eraser.
(E) Installation Of Display Assembly
1. Put display bracket (1) in position on the printed circuit board. Use tweezers to carefully position flexible conductors (2) in slots (A) of bracket (1).
2. Being careful to touch display (3) at the edges only, hold the display to the light and observe the numbers and decimals "1.8.8.8". Put display (3) in position on bracket (1) so the decimals are at locations (B).
3. Install display cover (4) and two screws (5). Make sure the screws are tightened evenly so the display and flexible conductors make good contact. Install the printed circuit board and rear cover.
NOTE: If the display does not operate correctly (segments missing) after assembly of the fuel rate indicator, press the display cover while the indicator is on. If this corrects the problem, either cover (4) is not fastened securely or the terminals are oxidized (still dirty). If the correct digits do not appear, make sure the display has not been installed upside down.
Block Diagram
Parts Layout
Schematic Diagram
Component Function
INPUT - J5
IC1 is used to invert the polarity output of IC7 so that the display reads a positive number when a positive voltage exists at TP1. IC1 is also used to invert the backplane squarewave in order to drive the decimal point. The decimal point is on whenever the unit is on.
LOW PASS FILTER - IC2
The signal is next taken through the low pass filter IC2, which has a cutoff frequency of 0.4 Hz. This stage has a gain of -.75 (inverts the signal and has a gain of 3/4).
DISPLAY - IC7/LCD1/IC1
A single chip voltage to digital (A/D) converter IC7 changes the filtered and calibrated signal at TP3 into a digital output on LCD1. This IC requires a very stable voltage reference at pin 36. A drift in this voltage will cause proportional errors in the digital output.
The "polarity" output on pin 20 is not active until a negative signal appears at the input of IC7, pin 31, (TP3). This polarity signal is connected to the decimal in the display LCD1. That is why the decimal is absent with no input signal. If the 5P2148 Flowmeter is connected to the fuel rate indicator, and there is no fuel flow, the indicator will typically read 000 or 001.
VOLTAGE REFERENCE - IC5
IC5 serves as a 2.5 Volt dc voltage reference. It sets the reference voltage of 1) IC3 in the automatic shutoff circuit, 2) comparator IC4 of the low battery detector circuit through a series divider network of R13 and R14, and 3) the digital display IC7.
The automatic shutoff circuit requires a negative reference. A voltage divider of D1 and R12 generate a negative voltage at TP7 between the output of IC5 and the -5 Volt power supply.
AUTOMATIC SHUTOFF - IC3 AND Q1
Anytime the flow rate is less than approximately 3.0 gph, the display of the fuel rate indicator will automatically go blank within twenty seconds. This feature helps to conserve battery life.
A comparator IC3 continually compares the negative output voltage from the low pass filter to a reference volttage of -.051 Volts dc at the junction of R10 and R11. If the signal voltage is less than the reference voltage (more negative), the output of IC3 goes low turning on Q1. S1 no longer needs to be depressed because the circuit is "latched on".
If the signal voltage is greater than the reference voltage (more positive), the output of IC3 is high turning off Q1 and opening the circuit path to the 9 Volt battery. The entire circuit will automatically turn itself off after a time delay determined by C6 and R6. This delay is approximately twenty seconds after the input signal drops to zero.
LOW BATTERY DETECTOR - IC4
IC4 is a comparator used to detect low battery voltage. The fuel rate indicator is designed to have the LOW BAT light, a red LED on the front of the instrument, turn on when the voltage of the 9 V battery goes down to 6.4 Volts.
If the battery voltage is less than 6.36 ± .25 Volts, the output of IC4 is low; D4 is forward biased and will light indicating that it is time to change the battery. In this case, the signal voltage at the junction of R15 and R16 is less than the reference voltage at TP9. If the battery voltage is greater than 6.4 Volts, D4 is reversed biased and will be turned off.
-5 VOLT REGULATOR - IC6
The -5 Volt converter IC6 takes the +9 Volts and converts it to approximately -8 Volts out. R21 and D3 then limit (regulate) this output to -5 Volts. When battery voltage falls below about 7.5 Volts, this -5 Volt output no longer regulates, but "floats" to about -3 Volts during the life of the battery.
INTERFACE - J6
A six pin male connector J6 is located on the circuit board for future expanded use of the indicator. Provisions are made for +9 Volts, ground, -5 Volts, Display Cal., and interface output.
Voltage Test Points
To aid in electronic troubleshooting of the fuel rate indicator, eleven test points are featured on the printed circuit board. To get access to the test points, remove six screws (1) and rear cover (2) from the indicator. To make the following test point measurements, use a variable dc power supply and a 6V3030 Multimeter (or equivalent) set on the 2 Volt dc scale.
1. Adjust the dc power supply to 1.250 Volts. Apply positive to pin B and negative to pin A of the flowmeter connector J5.
2. Push the POWER ON switch for five seconds and release. The display should remain on. If not, short the two leads of the POWER ON switch.
3. Verify that the 9 Volt battery is at least 8.0 Volts. If not replace the battery.
4. Connect the negative voltmeter lead to pin A of J5 for all measurements.
NOTE: The following voltages represent a working fuel rate indicator calibrated to 40.0 gph with 1.250 Vdc input.
Parts List
Warranty And Repair By Manufacturer
If the 6V7925 Fuel Rate Indicator does not operate correctly, the unit can be returned to the manufacturer for repair. Fuel rate indicators less than one year old will be repaired under warranty, except for abuse. Cost of out-of-warranty repairs (or abuse) will be charged to the owner. The 6V7925 Fuel Rate Indicator should be returned to the following address for repair.
General Fabrication Corp1100 S.W. 11th Avenue
Industrial Park
Forest Lake, MN 55025
U.S.A.
Telephone: 612-464-5110
A letter should accompany the fuel rate indicator giving the following information.
- 1. The date of purchase from Caterpillar.
- 2. Description of the problem.
- 3. Person and address the repaired indicator should be returned to.
- 4. Billing address (if different than shipping address).
- 2. Description of the problem.