the tach is driven directly off the coil pin 1, with a red-white wire. pin 1 is what fires off the spark, and comes from the power module, which in turn gets signaled by the ICU (which is under the dashboard), based on either the hall sensor (for LH2.2 cars up to 1988) or the crank position sensor (LH2.4, 1989+)

what year is this 740, and is it turbo or normally aspirated ?

 

 

if its running rough AND the tach is bouncing, I suspect there's some glitch still in the ignition.

you might put a timing light on it and see how unstable the timing actually is.

 

 

so my money is on something you replaced is defective. new != guaranteed good.

because the tach is bouncing, its VERY likely an ignition problem.

 

 

the tach is connected to the INPUT side of the coil, suggesting the problem is upstream from it, which could be the crank position sensor, the ICU (ignition control unit), or the Ignition Power Module that you've replaced twice, or the wiring between these things

I'd likely be getting inthere with a test light, and looking at coil pin 1 to confirm its 'jittery' and not a steady series of evenly spaced blinks, then I'd look at the input signal to the power module, if thats also jittery, I'd wonder about that crank position sensor. ICU's are usually rock stable reliable on these cars.

 

 

one side of the coil (pin 15, I think its a blue wire) is connected to the ignition switch.

the other side of the primary coil, pin 1, is selectively grounded via the ignition power module... when it is grounded, current flows through the coil primary winding, when the power module shuts off, the magnetic field collapses and fires the spark via the coil secondary winding.

so when the ignition is on but the engine is not running, you'll see 12V on pin 1. when the engine is running, pin 1 is rapidly pulsing between nearly 0V and 12V, a DC volt meter will probably still see 12V, but an AC Volt setting would see some random AC voltage, and a frequency meter would see twice the RPM/60 in hertz (600RPM is 10 Rev/Second, so you'd see 20 Hz since the coil fires twice per full turn.

 

 

at the power module, pin 5 is the 'input' from the ICU ... this will be a smaller signal than pin 1 on the coil, but should still trigger your test light.... if it too is erratic above idle, the problem is upstream, either the ICU (never heard of one failing) or the CPS (Crank Position Sensor)

 

 

oh, note the wire at pin 5 is a coax, with the shield connected to pin 3... in between them, pin 4, is a blue wire which is switched ignition power.

power module:
pin 1 - ignition output pulses to coil, red-white
pin 2 - ground, black
pin 3 - shield for input, grey
pin 4 - power, blue
pin 5 - input signal, grey

 

 

 

could mean your test light is drawing more current than that control signal can handle?