We've had two '73 Tiis for about 20 years. Neither car has working cold start timer boxes. Both have the manual cold start injector bypass button under the dashboard.
Both cars run well and always start. They have been without a timer box for so long I stopped thinking about it.
I originally considered about modernizing the timer circuit. Build it out of a micro-controller, a voltage regulator and a relay, jam it all in the box and call it good. However, a few years back I got the itch to build a box. A reproduction, not a "make do". I want to have a reason to learn more about low voltage electronics, its a fun hobby for me. The electronics (passives) are not particularly complicated. However, some are poorly labeled, or not labeled at all. Some of the parts are no longer made. Some parts would need to be fabricated (rubs hands together gleefully).
The first challenge is the circuit board itself. Not simple to reproduce. The second are the passives on the boards. They are very old now and unlikely to have the same electrical characteristics as they did when new. Missing labels doesn't help. The third challenge is the spade connectors. They are intricate and likely unique. I spent hours looking online for them and have decided on a couple of paths. One is a daughter board. The other is to make them myself. Bend them out of proper gauge sheet aluminum. (Most likely the way I'll go with it.)
The fourth is that the system is not as simple as it looks. There are three primary components - the cold start timer box, the cold start switch, and the injection valve. How do they work together?
More on this project in an upcoming post, as well as the status and views into the test harness.. Should be fun!