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.

 

No crank sensor on this one. We've been trying to trace wires but haven't found anything yet. The transmission witing harness was pretty chewed up and we fixed that but not sure how that would contribute to the issues here.

 

The codes we initially pulled were 12, 46, and 37. Haven't tried to pull codes again since we have replaced some parts but might be worth another go around with it.

 

New coils, wires, distributor, auto shutdown relay. Checked both sides of the fuesable link for the sbec and auto shut down relay with a test light, the bulb lit up on both places. Power is going to the positive and negative sides of the coil. The transmission wiring harness was pretty ratty. And looked like a mouse or rat chewed completely through 1 wire. I spliced that wire back together with a 14 gauge wire and heat shrink butt connector. Put heat shrink over the exposed wires and then wrapped everything up with electrical tape. The codes are 12, 46, 37. Going to go through some more test procedures from the FSM this morning.

 

Code 37 and 46 will let you know where to start. I don't have a manual for anything that old handy or I'd tell you what they are. One thing I notice you haven't mentioned is the ballast resistor. I think they were still being used in '90. They were a common failure on the Mopar electronic ignition systems. I always used to carry a spare wrapped in a towel in my tool box. It's a simle, cheap part that when it fails will stop you dead.