The '85 did not use a tach amp. Pre-85 (or possibly pre-84) vehicles used a VR sensor mounted to read a pin on the harmonic damper and the signal is routed through the tach amp and then to the tach module in the cluster. The 85 on the other hand takes its signal off the flywheel teeth and routes it through the EGR controller and then to the tach module in the cluster. Some have reported that the signal can be routed directly to the tach module, bypassing the EGR controller completely. In my swap, I kept the EGR controller mostly because I thought I would need it to pass the BAR inspection. I don't know if there is a difference in the tach module itself between the 85 and the earlier but I suspect there might be - and your experience might point to this reality.
Here's the diagram
Hope that helps.