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Harry,
I can't promise this information is accurate because the drawing for the electronic ignition I have is in the Haynes manual for '76 - up 240 and I don't have a schematic for the 1800 at all. I do have a schematic for the 123GT. I think the 123GT and the P1800 should be the same for the tachometer. BUT I DON'T KNOW THAT FOR A FACT.
The '76 tach has only three wires so it is obviously a bit different from the 123GT tach. The 240 tach uses one wire connected to the (-) side of the coil. The tach circuitry isolates the tach from the ignition controller, senses the ignition pulse, shapes the pulse, and amplifies it to drive the needle.
Your tach (if it is as per the schematic I have for the 123GT) is a d'Arsonval movement. It uses the two white wires to complete the circuit between the distributor and the coil, eliminating the black wire that usually connects from the (-) side of the coil to the distributor.
I think you can use your tach with the electronic ignition if you remove the white wire running from the (-) side of the coil that runs to the ignition controller, connect one white wire of your tachometer to the (-) side of the coil and the other white wire from your tach to the white wire connected to the ignition controller. Watch the tachometer carefully when you turn the key on to make sure it doesn't deflect backwards. If it does, reverse the connections before you start the car.
It is possible that the tach movement can load the ignition circuit. I don't know how sensitive the electronic ignition control might be to interference on the coil drive side. It shouldn't have any effect at all, I think. The tach might cause misfires or it might prevent the controller from firing the ignition at all. If the car doesn't want to start, or runs rough, remove the tach and return the ignition controller white wire to it's home.
It is also possible that the tach will peg full scale or will read significantly higher or lower than actual RPM. It isn't likely though.
If you have problems, we can research methods of isolating the tach from the coil drive signal.
I hope that helps.
--
Mr. Shannon DeWolfe -- (I've taken to using Mr. because my name tends to mislead folks on the WWW. I am a 51 year old fat man ;-) -- KD5QBL
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