Grizzly wrote:I was asked this but want to make sure i have it right, if you put a Multimeter from the battery to the Engine block (get a good a low Ohm reading) then crank the engine with the gauge still attached and the reading shoots up until the Starter is turned off at which point it drops back to more or less the original Ohm reading does that point to a bad ground? I thought it does, but having said that i went and checked mine knowing i have a ground from the battery through the chassis to the starter and it does go up when you crank the engine which i wasn't expecting?
Oh dear...
Assuming you mean from the Earthed terminal of the battery to the block:
That could damage your ohm meter if the earth continuity was really bad.. The increased reading isn't the ohms going up it is the volt drop of the earth cable feeding back into the ohm meter & giving a false reading.
You fed some volts into it when it was switched into a mode using its internal battery and that expected to be connected only to unpowered equipment!
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Wildly changing anything is not going to get you (or your friend) anywhere, you need to be systematic and analytic.
1. Ignoring the tach, is the engine starting and running OK? if so don't change anything else on the main ignition circuit. Coil, contact breaker, ballat resistor (if any).
2. The problem will be something to do with the tacho itself. In particular the signal feed from the CB end of the coil to the trigger terminal of the tacho. It is likely that on the car it came from there was either a feed-out from its electronic ignition box (if any) or some sort of filter circuit between the coil and the trigger.
3. The simplest filter as mentioned above would be to reduce the magnitude of the signal by inserting a series resistor. This assumes that there is an existing resistance inside the tacho to provide the 'bottom end' of a resistive voltage divider.
4. Other possible filters might include a series capacitor so that only the edges of the pulses from the coil are received by the tacho.
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Most voltage triggered tachos would work by detecting each trigger pulse and then ignore all but the leading edge of that pulse using that edge to generate one nice short fixed length internal pulse per trigger. The rest of the circuit then uses the short pulses to feed into a capacitor that is deliberately 'leaky' by having a resistor accross it. The effect is that the voltage across that capacitor averages the pulses and so the voltage is proportional to the rate of the trigger pulses. This voltage is then fed to the dial meter to produce a deflection, which is now proportional to the rate of trigger pulses & hence RPM.
The symptoms you describe would occure if the internal circuit was not triggered by the EDGE of the signal pulse but by its magnitude. When you switch on the ignition the engine is stopped or spins slowly on the starter motor. so the pulses from the contact breaker are looooooong. So it would make looong pulses in the internal circuitry and the voltage across the 'leaky' capacitor would rise to maximum and then take a long time (seconds) to leak away. Meanwhile the pointer would be hard over in the max RPM indication.
You can only check for this by inspecting the internal circuit of the tacho and/or by testing/calibrating it using a controled signal source, such as a signal generator and it hels if you can see what is happening by using an oscilloscope.
In developing their conversion kit Spyida designed a neat way of using a computer as a signal generator by using the audio output (speaker/headphone) socket and making the computer play specific .WAV files which simulate clean trigger signals.
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I would be inclined to take the Tacho out of the car and check its characteristics on the bench.