Bizarre tacho issues - Toyota 3F LandCruiser

[dave#3]

I installed a MJLJ in my 1985 Toyota LandCruiser (FJ73 with a 3F petrol engine running duel fuel) a couple of years back. I'm yet to put in on the dyno for a proper tune (when I do I'll post the map), but it's been pretty reliable and runs well. The only issue I have is I've never got the tacho to work correctly.

If the tacho pickup is connected to a single coil I get a reasonably solid signal, albeit reading low, so I believe the tacho works OK. If I connect it to the tach adapter board output (with the tach adapter connected to all 3 coils) I get no signal to the tacho, but, and here's the bizarre part - if I add my finger to the circuit (i.e. I hold both the output from the tach adapter and the tacho pickup against one finger but not touching each other) I get a perfect tacho signal, showing correct revs.

I assumed my finger was adding resistance (which I measured at about 3-4 M ohm), but I've tried adding everything from 0.5M ohm to about 25M ohm resistors to the circuit to no avail. I can't really imagine what else my finger is adding to or removing from the circuit in an electrical sense, except perhaps smoothing pulse peaks, and I don't have a CRO to test that, so I thought I'd see if anybody has come across something similar before or has any ideas of things I could try before I try to get one?

FWIW, the 3F engine is an inline 6cyl, 4.0l petrol engine, carbied, originally running a distributor and points, with a single coil, wasted spark configuration. The firing order is 1-5-3-6-2-4 (which in and of itself caused issues with the EDIS system which had a different firing order). To overcome the firing order issue I've had to map the coil firing order to the original firing order. I'm not sure if this could have any impact, but it's the only thing I had to do at all differently.

Any and all thoughts would be greatly appreciated!!

[stieg]

Hey Dave,

Tough situation. Lets start with some clairfication...

hold both the output from the tach adapter and the tacho pickup against one finger but not touching each other

In this description do you have the multi-coil tach adapter circuit https://www.autosportlabs.com/product/tach-adapter/ wired in or not? Put differently, is the circuit bypassed completely?

If I had to guess, your finger may be acting as a sort of cleanup circuit, removing short spikes that are causing your tach to not read correctly (would read high, but your tach may be just showing nothing because it reads something it can't display). Would be wonderful to see the signal on an oscilloscope to verify this theory.