Quote:
Originally Posted by Dynomite
OK....thanks for that explanation....I will clean up my posts so as not to confuse folks 
I was thinking maybe the capacitor C4 was also failing........here I go again thinking that a possibly failing C4 Capacitor actually changed the resistance in the circuit and may continue to progress in failure without replacement. But.....that does not explain the addition of basically a new 220K Ohm Resistor as the only fix required in ghlkal's case  |
Well, who knows zackly what failed in his case, except to say these particular printed resistors value tends to increase as it deteriorates, resulting in rpm values too high.
Electrolytic "caps" do fail too. But, when they fail, (typically) I find the DC resistance (respective of polarity) across them falls or even shorts out as opposed to resistance increasing - the exact opposite failure mode.
An unusually large error of 300% could be the result of the printed resistor perhaps opening or at least having become extremely high. In which case a 220 Ω across C4 might work quite well. Too many variables to say "this or that" is an absolute fix, I guess is the point. But, placing an arbitrary value resistor across C4 without eliminating the printed value (between pin 4 and 10) is iffy at best, and a temporary (Bandaid).
Example...
Given:
Rt (Resistance total) = 220k Ω = correct calibration
R1 = unknown (degraded) resistance value
R2 = the parallel value required to bring the tach into calibration = 300k Ω
Traditional formula to determine total resistance of parallel resistors:
Rt = 1/(1/R2 + 1/R1)
Solving for R1:
R1 = 1/(1/Rt - 1/R2)
inserting known values for Rt and R2:
R1 = 1/(1/220k - 1/300k)
R1 = 825 Ω
825 Ω?? Well,
who knows what the actual value of the printed circuit degrades to for any given case without measuring it. Even then, the calibration value tends to fall within a range of several thousand ohms (welcome to analog!!). So, considering all the blanks, it is anybody's guess what parallel value will actually result in achieving reasonable calibration. Furthermore,
the printed resistor just continues to degrade.
SO!
Nuts to all that nonsense, sez me. I say just clip out the failing resistor, and good ridence. Then insert a (temporary) variable one and achieve calibration. Then measure the resulting variable resistance value and replace it with the proper fixed resistor of the same (or close) value (across C4) and call it good!
And, of course, the tach lags actual rpm changes; worst in the low gears especially. So, the tach is useful for troubleshooting, etc, when rpm is relatively constant. But for drag racing, for example, and especially in the lower gears where rpm values are changing more rapidly, a digitally triggered shift light is ever so much better than a tach anyway (to the point I don't even pay attention to the tach when rowing the gears while drag racing).
Paul.