I have an 89 3.0L Auto tranny that runs like a Swiss watch for 50 miles today, 30 miles next time, but dies. After it dies, it will turn over but there is no spark. Sounded like a coil opening up to me because it will start after it cools down completely. I've replaced the coil twice, the plug and coil wires, distributor, cap and rotor, and engine relay. I'm starting to think it is the ECU. Any help would be appreciated. Another symptom might be that the tach is very irradic when it is about to die.
Thank you.

I should have also pointed out that when running right there is 13 volts to the coil. When it is dead there is only 6 or 7 volts at the coil.

You check the the voltage going to the power unit? If you have a ohm meter check the resistance in the primary circuit if you find a spot with high resistance then focus on that area.

I have to claim a little ignorance. I don't know what the power unit is. On the circuit drawings it looks like the 13 volts for the coils comes from the ECU. Is this not right?

Coil voltage goes from the battery to the key switch through about 14 connections, including the connections for fuses.

Some voltage issues have been resolved by replacing the pigtail connector on the rear half of the switch.

A couple of weeks ago, another issue which was eerily similar to yours was resolved by repairing corrosion damage to connector B-67. B-67 is on the passenger side inner fender between the headlight and battery - I "think" it's under the air filter housing. It's a square connector with about 6 wires. Take a good look at that connector.

Frank

I have 89 Raider and it would run for a while and then after turning off, it would turn over but not crank. I changed the small realy on the bottom of the right kick panel under the plastic cover which I think controls TBI and no longer have starting issues. I think as it heated up it caused the relay to stop working. You can get this relay from Napa. Might help.......

Thanks for the replies. I'm all over it. These are things I have not checked yet.

I think he means the power transistor, or some other names for it.. It is right there near the coil. A little black box
1x2x.25 inches I think

Take a bottle of Dasani and pour it on the coil bracket the next time it won't crank. Has to be Dasani...

This will check for a thermal fault.

My bet is the caps in the ecu just starting to leak. We've seen this a LOT of times... Check for fishy smell in the ecu, under the glovebox.

Thanks to the advice on here and the pay it forward thread, I've solved the problem. 93Montero Sr sent me a Power Unit and solved my starting problem. When it would die, there was voltage at the coil, but, the Power Unit provides the ground path for the coil voltage and it was apparently opening up when it heated up. Now for some wheels and paint and the old 89 should be like new.
Thanks for everything.