Ignition system

MDAVIS72

MEMBER
1978 JEEP CJ5
5.0L
V8
4WD
MANUAL
150,000 MILES

I have, I think, a 78 cj5 with a 304 I just install a fiberglass body and I have everything grounded. It now starts but it only runs with the starter. I have replaced the ignition switch on top the column. The only thing I have tried is jumping the coIL to the battery and it runs like it should. Is this telling me the resistor wire is bad? If not what do I need to check? I'm pretty new to this. Thanks

7 Replies

CARADIODOC

MECHANIC
33,752 POSTS

Yup. I'd have to dig up a wiring diagram to know for sure, but there is going to be a ballast resistor, (Chrysler), or resistor wire, (GM) to limit current flow through the ignition coil when you have breaker points. Because battery voltage is drawn down during cranking, average voltage to the ignition coil would be even lower, so the resistor is bypassed to get the spark voltage back up. Many electronic ignition systems don't use a resistor, but some do. When one is used, it too must be bypassed during cranking. Ford did that with a tap on their starter solenoid. There's at least three different designs that all look the same with the two large copper terminals and the two small ones on top. If you have the wrong solenoid, the starter system might work but you won't have the resistor bypass circuit. Your symptom doesn't agree with this.

GM bypassed their resistors with a "resistor" (R) terminal on the starter solenoid on top of the starter. Early Chryslers did this with a separate extra contact in the ignition switch. By the mid '70s they used an extra contact on their starter relays.

Regardless which system you have, if there's a break or defect in the resistor part of the circuit, the engine can't run due to no spark, but that defect gets bypassed during cranking. By running a hot wire from the battery positive post, you're proving there's a break in the circuit, but that won't tell you where.

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Monday, February 15th, 2016 AT 4:27 PM

MDAVIS72

MEMBER
12 POSTS

I have a resistor wire that is 1.35ohms and runs from the fuse block and where it splices for the coIL and starter solenoid. How do I check it or fix it? I can't just cut it out and install new as easy because it is in the fuse block and is more involved then just cutting and slicing. Would rather be able to check or run a temp to test it

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Monday, February 15th, 2016 AT 4:47 PM

CJ MEDEVAC

MECHANIC
11,004 POSTS

LET'S TRY "KEY ON" (NOT KEY AT "START")AND THIS TEST BELOW (SEE MY 1st PIC AT THE BOTTOM) YOUR RESULTS MAY VARY.

ON YOUR VIN, STARTS OUT "J" THEN A NUMERAL. MY 1977 CJ-5 STARTS OUT "J7" MEANING IT'S A '77. IS YOURS "J8"?

DO YOU HAVE PRESTOLITE IGNITION OR OTHER? NOT KNOWING WHAT YOU HAVE OR SHOULD HAVE MIGHT MEAN YOU HAVE SOME OF YOUR IGNITION SYSTEM WRONG.

MY 3RD PIC IS OF A PRESTOLITE DISTRIBUTOR FOR A 304, NOTE THE 2 PROTRUSIONS ON THE FRONT, THEY ARE PLASTIC, ONE SIDE HAS A VACUUM HOSE CONNECTION

IF YOU HAVE A MOTORCRAFT SYSTEM, THERE IS NO PLASTIC FRONT, THE SIDE WITH A VACUUM CONNECTION WILL HAVE A METAL BELL ON IT. IT WILL LOOK SIMILAR TO MY SIX CYLINDER DISTRIBUTOR IN PIC 4

GIMME SOME ANSWERS, WE'LL GET YOUR BABY RUNNING REAL SOON, I'VE GOT MANY MORE "HOW TO" PIC, INCLUDING SOME WIRE DIAGRAMS

IN FACT, IF IT WILL RUN WILL AT "STARTING", I CAN SHOW YOU HOW TO MAKE IT RUN (JUST TO MAKE YOU FEEL GOOD/ TEMPORARY). BUT WE WILL STILL HAVE TO FIX THE PROBLEM TO GET IT RIGHT!

THE MEDIC

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