Bought a 90 D150 for our son and fighting a no spark issue. Thinking at this point it is the ECM. Problem is everywhere the replacement is out of stock or discontinued. OE number is 4379989. Any ideas on where to find or an alternative that would work? It's 5.2, 4spd automatic with overdrive.

 

While ECU's do fail from time to time, I'd look for other reasons you aren't getting spark. Your truck is in that little gap between carburetors and Magnum engines that I've never worked on. I wonder if a failed sensor just isn't telling the computer to make a spark. I don't know if the fuel injected LA engines use a crank sensor or not. It could just be a bad wire connection too. Possibly a problem with the ignition switch system.

 

It's a fairly simple design, Distributor pickup tells the pcm when to fire the coil. Any codes? If you are going to get another pcm you can search that # on ebay/the net and probably find one.

 

Codes are a good first step. Pre-1996 Mopars use the "key dance". Turn the ignition key on, off, on, off, and on. Leave it on the third times and do NOT bump the starter. This tells the computer to go into self test mode. The check engine light will start to flash. On flash followed by two flashes is code 12. You will usually get that because it says the battery was disconnected in the last 50 or 100 key starts. Five flashes followed by 5 flashes is code 55 and means the end of the test. Any other codes could lead to your issue.

 

I had a similar no-start issue on my 90 D150 (also 5.2L and 4spd auto).

It did yield a code that pointed to the hall-effect sensor. Put in a new one and it started fine again.