88 Grand caravan computer problems

I usually spend most of my time in the Pontiac forum but my fathers car has a problem now so i thought i would ask here.
Almost a year ago the car stoped charging the battery and we found the problem was with the voltage regulator on the computer. We swaped in an old computer we salvaged off my uncles van when he junked it and every thing ran fine untill three months ago when suddenly it refused to start. We found it had a week or no spark we changed the coil which didnt help. So on a hunch we swaped back in the old computor that wouldnt charge and the car started on first try. So knowing it was in the computor we bought a rebuilt one and put that in so that the car would charge. It's had no problems for almost three months now untill the other night when same thing happend. He stoped somewere quickly and when he went to restart it it just cranked and cranked. with a few back fires which i assume is from gas getting in the hot exhaust pipes. So again we put the old computer in and it works fine again. My question is how does the computor control the spark and why did we kill two of them is there something wrong with the car or is it two bad computors? The computers give no fault codes at all. If the coil shorts out will it fry the computer?

The computer controls the spark by monitoring all functions of the car. It sends signals to the various components when the timing is correct, the crank is in the proper position and etc. The computer problems you indicate may be the result of a problem with one of the other components that when you change the computer it resets the component and the car starts. I would check the other components and then replace the computer with another one to see what happens. Unfortunately this is a hit an miss situation.

your theory seems plausable but why when now if we try puting the computer that didn't work before back in it still doesn't work. It seems to me that something is physically damaged and not just in need of a reset. Any other theorys out there?

From a distance I'd say you've probably got a bad crankshaft or camshaft position sensor. Also, those computers are very specific to each car (ie, part#'s REALLY matter when putting in new (or used) ones.

ASsuming the battery is charged and the engine is turning over when you are trying to start, I'd guess the crank or cam sensor....

Hope this helps!

You probably have the 3.0 Liter engine. It does not have crankshaft or camshaft sensors in it. The ignition system is simple.

You have a hall effect distributor that signals the computer when a cyclinder is to fire. You have a coil that has a hot wire and a ground wire going to the computer. When a cyclinder is to fire, the computer grounds out the coil wire creating spark.

Since most of your problems seem to be corrected by replacing computers, I'd say the problem may be in the connector to the computer that provides the grounds. Bad ground wire.

Also, I remember there was a recall about bad wiring harnesses in the 1988 model.

Also, my 1988 Caravan was great until it rained. I used to dread the rain. The alternator charging system would stop and I never did resolve that problem.

have you tested the output voltage with a volt meter ? If the battery is showing 16 volts when it is running , that would fry the computer. I think the voltage regulator is built into the altenator , not the computer.