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Hey Doc, still working on the Datsun 720 Truckl problem, just doing a bit more research on the ignition module. By the way I had already priced these out and found that the module is like 246.00 cheapest but an entire distributor, including module is only 136.00, although my professional mechanic friend reminded me of the old mechanic wisdom that new parts are often bad and not to get too discouraged if the first one doesnt work. Anyway, in my research I came across this post which has a very neat little piece of info at the end. Perhaps I'll try this little trick before purchasing the new unit. Enjoy.
'81 Datsun 200 SX which would sometimes start severely missing and running very poorly, almost undriveable. It never would do it when I could check it out, always out on the freeways, usually at highway speeds. It would clear up as quickly and it occurred, usually after about 30 seconds of running extremely rough.
Timing was ok, fuel pressure ok, new fuel injectors, new cap, rotor, wires and plugs. Both coils ok, new condensers, O2 sensor ok, no vacuum leaks, no leakage air, no temperature problems, no smoke, nothing apparently to trigger the problem. It would just be running perfectly down the road and suddenly out of the blue, start running ragged for a short time and then straighten out and run perfectly again. Sometimes for several months at a time.
Suspected computer, but could not be sure. Finally it did it once in the driveway. I immediately clamped a timing light on the secondary coil wires and found that the intake side plugs weren't firing. Traced it down to the ignition control module in the distributor which has 2 outputs, one output for each coil, and the output for the intake side coil was intermittent. Since the intake plugs are more important that the exhaust side plugs, when they would quit firing at high speed, the engine could hardly run. However the engine could idle perfectly on the exhaust plugs.
Since both plugs in each cylinder fire at the same time, I simply switched coil wires to allow the good side of the ignition module to control the intake plugs. It ran perfectly again with no intermittent problem as before. I went and ordered a new ignition module and kept the old one as a spare. The new module was $170.00. Copied from Bob Hewitts automotive repair page.