I can confirm that the TR7 requires the special starter relay...
Years ago I bought a dead 1977 TR7 and it had no power to the ignition system. I found that the beureka wireb (resistive wire) which acted as a ballast wire to power the coil was fried. The moment I patched it and turned the ignition to ON (not START), the car started turning over and the eureka wire started smoking!
Turns out that the previous owner had replaced the starter relay with a normal Bosch relay and now the coil positive was electrically connected to the starter solenoid!
So I recall that the TR7 starter relay also functioned to bypass the eureka wire (ballast resistor) only when the key was in the START position (hotter spark during starting the car). Using a regular relay forced power to flow from the positive side of the coil to the starter solenoid, which activated the starter as soon as the ignition was ON and fried the eureka wire within probably a minute! Since the eureka wire runs through the dash wiring harness, you would have to pull the dash to repair it and there is a high probability that other wires will be damaged as well.
So the moral of the story is DO NOT replace the starter relay with a standard one, at least on a TR7!
Incidentally, I have owned a couple TR7bs (besides this b77) with failing starter relays. As Dave says, there is a big inductive load that tends to arc across the contacts. One car I carefully opened the relay and cleaned the contacts (they were rather burnt looking) and the other I just kept a screwdriver in the glove box and would hit it the relay with the handle any time it acted up...

Greg
Calgary
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