Thanks LILEVO for the instruction manual, it helped a lot today.
I just modified my 90 Colt hach last weekend, from 89 N/A 4G61 ECU and wiring, to 91 4G63T coil, power transistor, TPS, MAF, DSMlink ECU, and the extra solenoids and knock sensor. I have the turbo throttle body on there but I still have the N/A engine in it. I just wanted to work out the bugs electronically before I drop my $10,000,000 freshly built 4G63T in there. So the ISC was shot, for anyone wondering the 89-90 ISC WILL fit on a 91 engine.
Anyways to the point, my tach stopped working as well and I worried it was some kind of disasterous compatiblity problem. My fears were put to rest after I read this link: http://www.lilevo.com/turbo/tach/body_tach.html
. I put my 89 coil pack with the tacho interface back on the car, retained the 91 PT, and put the 4 wire coil plug back on my engine harness.
The shortcut I found that may be helpful to anyone reading this thread, was to run the yellow wire (fourth wire from the coil pack), from the tacho interface, to the noise filter for the original 89-90 tach. It's already mounted on/near the manifold on the 89-90 CSM's. Assuming you still have that part of the wiring from your original engine harness, you only have to run the wire a few inches instead of going through the firewall.
There should be a small rectangular metal box about an inch long by a half inch wide, screwed to the manifold or the bracket underneath it. That's the tach filter. Two wires come up from it and go to a two blade plug like what's on your purge or wastegate solenoids. On the 4G61 and 4G63 engines wiring, it plugs into the harness right behind the fuel pressure regulator.
Simply cut the side of that pigtail which splices into the (Power Transistor to ECU) tach signal wire, and connect that side of the plug to the tacho interface wire from the 89-90 coil pack. The other end of that pigtail that comes OUT of the filter should already go directly to your Tach and DOES NOT not pass the ECU nor does it collect $200. Thanks again for the pointers and the pictures. Now that that pain in the rectum is finally over with, I can move on to the next stage of my build.