MSD Upgrade No Tachnometer

[Simeon]

So with the white wire from the tach adapter you end up with the white wire from the MSD connected to this white wire from the adapter?

 

This is connected via the diode? (which can only work in one direction otherwise the white wire from the MSD wouldn’t be grounded via the distributor and therefore the engine wouldn’t run)

 

The black wire from the 123 is then connected to the white wire from the tach adapter (on the correct side of the diode)? The black wire from the factory harness (which originally goes from tacho to condenser on the distributor and then to the old coil negative) is also connected to this white / black wire conglomerate? This is also connected to the tachometer, along with the other connections at the back of the tacho?

 

The grey wire is a red herring in this case but if it’s really connected as above then I am not sure why it isn’t working. It’s very simple as the tach adapter (a fake coil if you prefer) is energised by the 12V running through the closed ‘points’* in the 123 to ground. The ‘points’ open, cutting the current flow through the fake coil (just like a real coil) which then causes the field to collapse and create the fly back voltage that the tacho sees. 

 

I would say, check your connections, again. Make sure you check the connections at the back of the tacho and, as you obviously have a multimeter, check for continuity between the black tacho wire connection where you have it connected to the white / black wires through to the back of the tacho. It will be something simple at fault. 

 

[Arlo Cooper]

Huughhhhh this is taking a toll on me I can't get my little head around it. 

 

Okay I have triple or quadruply checked everything that you have said.. The white wire from the tach adapter is fed from the black wire from the 123 which is also fed from the white wire from MSD and is also feeding the black original wire which leads to the tacho which has been checked for continuity. Also taco is getting 12v on the green feeder wire and the ground wire is working fine.  The red wire on the adapter is fed from the ignition power as the picture instructs. The only thing I feel like I could try would be feeding the red wire from the Tach Adapter from the red wire from the 123? I don't understand why the tach adapter is spitting out a solid 12V from the white wire. That cannot feed a tach info can it? Why would it be sending a full 12V? 

 

Im sorry I know this is boring and tedious. 

 

But I really do appreciate your help. 

 

[Simeon]

Where are you measuring the 12V?

 

The adapter will give you 12V to ground (measured from the white wire to ground) when the ‘points’ (they are not points) are open. You are measuring the voltage dropped across the open points. If you the close the ‘points’ this will then give you 0V in the same location as there is now no longer a potential difference between the white wire and ground.

 

If you put a test light between this white wire to ground and crank the engine, you should see it flash. When the engine starts the ‘points’ will be opening / closing so quickly that you wouldn’t determine the flash at the light. 

 

One last crazy check for me: can you confirm that the only connections at the coil are the orange and black leads from the MSD? Nothing else?

 

Your engine is running and therefore we can rule out the most complex parts of the system (the 123 and the MSD). This leaves the wiring and connections between the tach adapter and the tach, the adapter as well as the tacho itself. I know you have had this checked but now the tacho is the most complex part of the remaining system.

 

How about taking the MSD out of the equation for a minute and reconfiguring back to the original (Kettering) ignition and seeing if it works then?  

[Arlo Cooper]

Just to clarify. The grey MSD tach wire gives a small 1Vis heading that increases with revs up and up. And the adapter gives a 12V signal that drops to 0Vs. And this 12V signal that drops is what the tach is looking for? I would like to confirm which sort of signal the tach actually runs off.

 

Just used the test lamp method. Can't establish or see any flashing. Will hook it up to a needle meter tomorrow for a fine look but pretty sure the current is solid 12V out of the output (white tach adapter wire). 

 

The only thing attached to the coil is the Orange & Black MSD Coil leads. 

 

Could I try using the negative from the coil as a signal to the adapter instead of the negative from the 123? 

 

What about using the positive feed from the 123 for the tach adapter instead of the positive feed from the ignition. 

 

I could try remove the MSD if I can't come up with anything to try. 

 

 

[Simeon]

5 minutes ago, Arlo Cooper said:

but pretty sure the current is solid 12V out of the output (white tach adapter wire). 

 

That can’t be true because your engine is running and the MSD is triggering. 

[Simeon]

45 minutes ago, Arlo Cooper said:

Could I try using the negative from the coil as a signal to the adapter instead of the negative from the 123? 

 

Don’t connect anything to the coil apart from the orange and black wires from the MSD. The ‘high’ voltage (it’s actually about 400V) output will fry your tacho). 

 

Here is a coil secondary waveform form and the point states in an inductive Kettering ignition that trigger each state based on the coil field charging and collapsing. The tacho is actually looking for a lower voltage version of this that is mirrored on the PRIMARY side of the coil. There is a large (compared to battery 12V) voltage spike on the coil primary when the points open and the field discharges. The wave form looks very similar to the secondary wave shown but the voltage is not thousands of bolts, more like 

 

 

 

Here is a document (from Princeton University no less!) describing what it all means. Your MSD ignition works NOTHING LIKE THIS but this is what the tacho adapter is trying to simulate just to produce the rippling waves for the tacho to read. 

 

https://www.princeton.edu/ssp/tiger_cub/library/ignition_waveforms.pdf

 

Try this for a more in depth description of the whole inductive ignition process. 

 

https://www.motor.com/magazinepdfs/052005_04.pdf

 

Here is a reasonable description of an inductive ignition system and how an MSD CD ignition differs. 

 

https://www.rodauthority.com/tech-stories/the-basic-ins-and-outs-of-an-msd-ignition-system/

 

 

 

 

 

Edited December 12, 2018 by Simeon