ECM problem

I have put an 87 5.0 into my 67. The car was running fine one minute then the next time i went to start it I have no spark and the fuel pump sounded week. Once i started probing around with the voltmeter i found that the 12v signal from the ignition switch would only measure 1.8v with the ecm plugged in, but if i unplug the ecm the voltage goes back upto 12+. Any ideas.
thanks

the 12 v is being measured as it comes from the ignition switch and then again from the backside of the EEC relay or the TIF power in. When the ecm is connected the voltage drops to around 1.8 but when unhooked it returns to 12v.