Chrysler has more "firsts" under their belt than any other manufacturer and the Engine Computer is one of those. They were first with the alternator, (they copyrighted the term), in 1960, first with a trouble-free electronic voltage regulator in 1970, first with anti-lock brakes in 1969, first with electronic ignition instead of the miserable breaker points in 1972, first with the electronically controlled carburetor that got a very peppy 54 mpg in the Horizon Miser in the early '80s, first with a lockup torque converter for better fuel mileage in 1977, first with a computer-controlled transmission in 1989, first with air bags in 1987. I could go on but you get the idea. They were also the first in 1976 with a computer to calculate spark timing based on numerous variables. Before that we could only adjust timing based on engine speed and load. That first computer was indeed the "Lean Burn" system. It was called that because it could make the engine run well on a 17:1 air / fuel mixture instead of the normal 15.7:1. Running too lean normally results in hesitation and sputtering, but running a little too rich wasn't noticed so all cars ran too rich most of the time. Carburetors can only be adjusted to provide the proper mixture at idle and highway speed. Anything in between had to be too rich enough to insure it never fell below the perfect 15.7:1.
That computer is exactly the same as was used in the front-wheel-drive cars except it was mounted on the side of the inner fender. In both cases intake air is run through it to cool the high-current coil switching transistor. Long before 1988 Chrysler had their computers perfected so they hardly ever had a problem. I worked for a very nice family-owned Dodge dealership from 1989 through 1999. I never replaced a single Engine Computer until they came out with the redesigned Caravan disaster in 1996.
While it's possible your computer is defective, that would be very surprising. It's the last thing I would suspect. Many years ago I had, (still have) a 1978 model with that system and I noticed it wasn't creating any timing advance. That severely hurt the fuel mileage. At that time I didn't understand how to troubleshoot the system, but now I know that was caused by a sensor that I didn't understand. Instead, I dropped in a distributor from an older engine that had the vacuum and mechanical advance built in. In another misguided attempt at a repair gone wrong, I wired in the five-pin ignition module that you can still buy for 25 bucks. That engine ran great for another 90,000 miles until there was more rust than body!
I think it's normal to find voltage on both coil terminals when the engine isn't cranking. If necessary, I can double-check on another one of my cars tomorrow. The computer pulls the negative terminal to ground like you think, and holds it there until it wants a spark to occur, then it opens the circuit to abruptly stop current flow through the primary. If it would hold that circuit at ground with the engine not running, that might overheat the coil and the switching transistor in the computer. Remember that air flow to cool it when the engine is running?
270 ohms for the pickup coil should be okay too. One thing you might consider is checking the air gap for that pickup between it and the reluctor wheel. It must be.018" and must be checked with a brass, (non-magnetic) feeler gauge. One time I couldn't get the rotor off to replace it so I cracked it with a hammer and screwdriver. When it spread open, it pushed the coil a little and I had a no-start. As I recall, the gap opened up to only about.022" but that was enough to cause a problem.
Just for fun you also might try laying the air cleaner housing upside-down with some tension on the wiring harness to see if you get spark. If you do, suspect loose solder connections on the computer's electrical pins. I haven't really run into that but there is a lot of vibration from the engine so I suppose it's possible.
If none of these things work I'll search for a service manual to figure out where to go next. Please don't scrap the car. They were way too nice and they're a lot more reliable than the newer stuff. If you think no spark is a problem now, wait until you have that on a 2005 model!
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Wednesday, April 20th, 2011 AT 8:32 AM