816730 Mercruiser NUT, ROCKER COVER STUD

The following information given is for repair only of the indicator. Basic troubleshooting information for the complete group is given in Special Instruction Form SEHS7742, "Service Information and Use of 6V3100 Diesel Engine Timing Indicator Group".Operation Of System
The 6V3100 Diesel Engine Timing Indicator Group measures fuel injection timing while the engine is in operation by finding at what degree BTC (before top center) that fuel is injected into number one cylinder. It makes a comparison with TC (top center), and the difference in time between these two points is measured and read out in degrees BTC. The pressure transducer monitors the fuel line pressure and provides an output that is calibrated for a predetermined pressure. When the fuel line pressure reaches this predetermined value of 6895 kPa (1,000 psi), the pressure transducer signals the indicator to start measuring time. The engine continues to rotate to top center, where the TDC transducer (magnetic pickup) outputs a signal corresponding to engine TC. This signal is generated by sensing the location of the static timing pin hole (located in flywheel or timing gear, depending upon the engine model). At this instant, the indicator stops measuring time and gives the number of crankshaft degrees between full injection and top center on the DEG display.Block Diagram - 6V3100
Circuit Operation
R/MIN Readout
6V2192 Injection Transducer - The 6V2192 has a differential output between connector pins J301-4 and J301-2 (see 6V3100 INDICATOR SCHEMATIC insert). At a fuel line pressure of 6895 kPa (1,000 psi), J301-4 is zero Volts with respect to J301-2. Increasing the pressure causes J301-4 to "crossover" from negative to positive with respect to J301-2. When this happens, the output of IC301B, TP13, goes low. Alternate Pressure Transducers - Other types of pressure transducers may be used to sense injection if their outputs equal 2.830 Volts at 6895 kPa (1,000 psi). Because J301-2 is adjusted to 2.830 Volts positive with respect to ground (J301-3), any input on J301-4 that crosses through (from negative to positive) 2.830 Volts with respect to J301-3 will also cause the output of IC301B, TP13, to go low.IC301A is a zero crossing detector. Its output switches (changes state) every time that J301-4 crosses through zero Volts with respect to J301-2. TP13 goes low (-5V) every time the injection pressure in line number one gets to 6985 kPa (1,000 psi) and returns high (+8 V) at the end of each injection pulse. When TP13 goes low, it clocks (triggers) the 23 ms Monostable, IC302A, so its output TP14, goes high. This output serves two purposes.1) Switches (changes state) the output of DEG Flip-Flop, TP20, high.2) Input signal for R/Min Limiter.The R/Min Limiter output, TP16, goes high with TP14 but TP16 amplitude is a precision 5 Volts as determined by REG102. TP16 stays high until 23 ms Monostable IC302A times out. So, TP16 is then a series of constant amplitude, constant duration pulses whose repetition rate is proportional to engine speed. One pulse is generated on TP16 for each fuel pulse.The R/Min Filter, IC304, is an active low pass filter with a gain of 0.20. It filters the pulses from TP16 into a smooth dc. R/Min Calibrate, R313, provides a calibrated output on TP17 that is read by digital voltmeter DVM1/LCD1, as engine speed in R/Min (revolutions per minute). The injection transducer is the input for the speed or R/Min readout, and the reference for the timing or DEG display.DEG Readout
When a fuel pulse causes TP14 to go high (see R/Min Readout), it triggers the DEG Flip-Flop output, TP20, high. TP22 then also goes high, and is a precision 5 Volts as determined by REG102. TP22 stays high until the engine reaches top center.The TDC transducer senses top center of the engine. It outputs a pulse as the static timing pin hole (flywheel or cam gear) passes by. This pulse is first negative going and then positive going. The exact center of this hole is represented by zero Volts when the signal transition goes from negative to positive. A zero crossing detector made up of (two comparators) IC301C detects this zero Volt transition and causes TP18 to go low (-V). This generates a trigger pulse, TP19, that serves two purposes:1) Resets the 0.5 Sec Degree Lockout Monostable, IC204B. This IC is a retriggerable monostable and is reset by every TDC pulse. If IC204B gets a reset pulse with each engine revolution, it will stay reset and never time out. This reset condition is indicated by the NO TDC SIGNAL lamp being off. The output of IC204B, pin 9, inhibits the DEG Flip-Flop from being set by a fuel pulse if there is no associated TDC pulse. (A fuel pulse without a TDC pulse will cause the DEG display to go off scale and read approximately 60 degrees.)2) It also clocks the DEG Flip-Flop output, T20, low. This is now the end of the pulse that represents timing.The pulse train at TP22, output of the DEG Limiter, is of constant amplitude but varying pulse width-varying proportional to timing for a given engine speed. Engine speed and pulse repetition rate are proportional but the duty cycle of this pulse train (ratio of "on time" to "off time") varies proportionally to engine timing and contains the actual timing data.The DEG Filter, IC305, is an active low pass filter with a gain of 2.66. It filters the pulses from TP22 into a smooth dc. DEG Calibrate, R328, provides a calibrated output on TP23 that is read by digital voltmeter DVM2/LCD2 as engine/fuel system timing in degrees before top center. Power Supply
Switch S101 selects polarity of input that is correct for power cable hookup. Diode D101 prevents operation from backward power hookup. Series pass transistor Q101 regulates output to a nominal 12 Vdc. Output will be less than 12 Volts if input is below 14 Vdc.Three terminal regulator, REG101, must be adjusted to 8.00 .01 Volts and supplies regulated power to logic and to the injection transducer. Precision 5 Volt regulator, REG102, provides a reference voltage for