Will not start after ECM replacement

The Engine Computer is the last thing to suspect. It causes very little trouble on this model. The only thing you can do to damage a computer is to speed up a running engine while the battery cable is disconnected. The charging system can reach over 30 volts, so obviously, don't do that.

Why did you replace the computer? What was the problem you were trying to solve? Have you read the diagnostic fault codes?

You're a couple of steps ahead of most people with the diagnosis. You found both spark and fuel missing. Those are two separate systems in the computer, so you have to look at what they have in common. That is the crankshaft position sensor, the camshaft position sensor inside the distributor, and the automatic shutdown, (ASD) and fuel pump relays. The two relays are turned on at the same time by one circuit.

If you haven't read the fault codes yet, Chrysler made doing that yourself much easier than any other manufacturer. Cycle the ignition switch from "off" to "run" three times within five seconds without cranking the engine, leave it in "run", then count the flashes of the Check Engine light. Check this page for the procedure and the code definitions:

https://www.2carpros.com/articles/retrieve-trouble-codes-for-chrysler-dodge-plymouth-odb1-1995-and-earlier-car-mini-van-and-light-trucks

or I can interpret them for you. Be aware any fault codes in the old computer are erased once it was unplugged, so that valuable information was lost. Fault codes for these two sensors often do not set just from cranking the engine. They commonly need more time to set, as in when a stalled engine is coasting to a stop from highway speed. For that reason, we can't rule anything out just because there is no fault code related to either of these sensors.

When you do get a fault code for a sensor, it never says to replace it or it is bad. Fault codes only indicate the circuit or system that needs further diagnosis, or the unacceptable operating condition. First we have to rule out wiring and connector terminal problems, and things like that.

When you do not have a code set related to a crank/no-start, professionals use a scanner to view live data and see what the computer is seeing and responding to. If you're lucky enough to have a scanner, display the cam and crank sensors, and observe the state they're listed in. Mine lists them as "Present" or "No". Both will be listed as "No", or signal missing, when the ignition switch is on, but they should both switch to "Present" or some similar indication when the engine is rotating, meaning you're cranking the engine. If one doesn't switch to "Present", that is the circuit that needs to be diagnosed.

When you don't have a scanner, you can do this a different way. A test light works best because digital voltmeters don't respond fast enough. Measure the voltage on the dark green / black wire on the ignition coil, or by back-probing into that wire on any injector, or on either of the smaller wires bolted to the back of the alternator. You will see 12 volts on that wire for one second when you turn on the ignition switch, then it will go back to 0 volts. What is important is if that voltage comes back when you crank the engine. If it does not, the signal pulses are missing from one of those sensors.

Did you measure the voltage on the dark green / black wire?

Many auto parts stores rent or borrow tools. You might ask if they have a scanner you can borrow. I have a Chrysler DRB3 for all of my vehicles. With an extra plug-in card, they will do emissions-related stuff on any brand of car sold in the U.S. Starting with '96 models. For that reason a lot of independent shops bought them. They are obsolete now so you might find a local shop that wants to sell theirs, or will borrow it to you. Word of warning though, it will require a plug-in card that allows it to work on '93 and older models.

You might also look on eBay for the previous Chrysler DRB2. Those are going now for around $100.00. These require a plug-in cartridge. The original ones were year and model specific, but if you can find one with a yellow-label "1994 Supercartridge", I'm pretty sure that will cover all models and years through 1995 models.

Also look for a Monitor 4000. It's almost identical to the DRB2, it was made by the same manufacturer, and uses a cartridge too, but it's a different one from the DRB2. These cartridges cover all GM, Ford, and Chrysler models up to the year listed on the label.

I suspect you overlooked a short pause and the first three was really a one and a two. Code 12 just means power was lost to the Engine Computer, meaning you turned the ignition switch off. Code 55 is "end of message", meaning it's done sending codes. All cars will give code 55. Some models are programmed to not read out code 12 when the ignition switch being turned off is the cause. Where it becomes useful is when you have an intermittent stalling problem while you're driving. Typically these hiccups last only a few seconds, then the engine resumes running normally. That usually will not show up on a recording with a scanner because those 12-volt lines aren't monitored or displayed on the scanner. Instead, if there's an intermittent connection on a 12-volt feed wire, that would set code 12. The clue is you've already recorded, then erased all the fault codes with the engine running, then set out on a test-drive. If the hiccup occurs, code 12 shows up, and you never turned the ignition switch off, that code tells you it's that 12-volt feed circuit you need to look at as the cause of this problem.

Basically what you have is what you'll normally have when there is no defect being detected. Codes 12 and 55 can be ignored. When you don't have a scanner and you have a crank/no-start, it's the voltage on the dark green/orange wire you need to look for during cranking. We have to know what's there to know where to go next.