78 dodge motorhome wont start.

Older vehicles had Kettering ignition systems, using a coil, points and condensor, etc.

They often had a different circuit to the coil when in RUN position as compared with the START position. If your RUN circuit is not working as it should, you could get this effect. It could be the run wire (which often goes to a ballast resistor) or the actual ignition switch itself.

This might give you a place to start, anyway.

'78 should have the "orange box" ignition but even so the ballast resistor is known to fail. the symptoms described sure sound like a bad ballast resistor, or less likely, bad wiring in the coil circuit (the ballast is bypassed during cranking which is why you're getting spark then)

nate

My 1978 Dodge (318 cubic inch engine) van doesn't have points. cuhulin

But a 78 Dodge motorhome will have Chrysler electronic ignition.

Ballast resistor. Its bypassed when the key is in the "crank" position, so the engine will run until the key is released and current then would have to flow through the ballast resistor. Fairly common Mopar problem, easily solved by carrying a couple of spare ballast resistors in the glove box :-)