1978 Jeep CJ5 Starting problems

Tiny
JAC525
  • MEMBER
  • 1978 JEEP CJ5
  • 5.1L
  • V8
  • 4WD
  • MANUAL
  • 68,000 MILES
I have replaced the starter, coil, ICM, spark plugs (gaped to.035) and the ignition switch located on the bottom of the steering column. The motor turns but will not start. I took the coil wire to see if I was getting spark, it was not until the key was turned off and then I get a single spark. When the key is turned on with out starting there is a single spark and then when the key is turned off there is a single spark. The ign fuse is good.

Any help would be appreciated.

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Saturday, October 10th, 2015 AT 5:35 PM

3 Replies

Tiny
CARADIODOC
  • EXPERT
First think of a breaker point ignition system. There's only two states; the points are open, (off) or closed, (on). In the first condition there's no current flow, and in the second condition there's maximum current flow. An ignition coil operates by magnetism, and specifically it induces a voltage in the secondary winding by responding to a change in the current flow. What you're seeing is a change from no current flow to maximum when you turn on the ignition switch, but it takes some time for that magnetic field to build up. Since it does that relatively slowly, the induced voltage, and therefore the spark, is rather weak.

You're also causing a change in current flow from maximum to 0 when you turn the ignition switch off. In this case the magnetic field has no choice but to collapse instantly, but since it doesn't like to do that, the induced voltage is very high and you'll get a nice big spark. That's the one we use to ignite the fuel. Those two sparks you're seeing are the same as if you had a breaker point system and the points were closed.

Now, with your electronic ignition system, the control module takes the place of the breaker points, but only in terms of function, not location. The module acts like the closed circuit and lets maximum current flow through the ignition coil until it is told to open the circuit so a spark will be created. With the breaker points, the rubbing block riding on the lobes of the distributor shaft tell the points when to open. In your distributor, a magnetic sensor tells the module when to go to an open state. Based on the symptoms and the fact you already replaced the control module, which is a god suspect, the next one is the pickup coil in the distributor. It provides the signal to the module to tell it when to momentarily go to an open state.

The things that can prevent that signal from being generated are a break in the winding of wire around the magnet, a gap that's too large, or a distributor shaft that isn't turning. I know the gap on the '70s Chryslers was.018" but I don't know if that's true for other brands. The coil can be measured with an ohm meter. As I recall, 300 to 800 ohms is typical.
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Saturday, October 10th, 2015 AT 7:40 PM
Tiny
CJ MEDEVAC
  • EXPERT
YOU MIGHT ALSO GET JUICE TO THE COIL FROM "I" WIRE

IN 1ST 1st PIC/ DIAGRAM, THAT WOULD BE "WIRE 6" ON TERMINAL "D" (PAY ATTENTION ONLY TO THE #s AND LETTERS-I HAVE REVERSED THE STUFF ON "A" AND "D" POSTS ON MY JEEPS, DISREGARD MY CHANGES) I WILL BE EXPLAINING THE FACTORY SET UP FOR YOU

"WIRE 6" GETS REDUCED VOLTAGE (VIA THE RESISTANCE WIRE) AT ALL TIMES WITH THE KEY "ON"

WHEN THE KEY IS IN THE "START POSITION", THE SOLENOID DELIVERS "FULL BATTERY VOLTAGE" TO "WIRE 6" OR "I TERMINAL" (SAME THING). THIS OVERRIDES THE RESISTANCE WIRE VOLTAGE AND FEEDS THE COIL "FULL JUICE" AS AT THIS SAME TIME THE STARTER IS "ROBBING" JUICE TO TURN

ONCE IT STARTS UP, THE KEY IS RELEASED, AND THE "I" TERMINAL DIES OFF. THE WIRE STILL HAS THE REDUCED VOLTAGE- BUT THE SOLENOID "I TERMINAL" IS NOW SORTA IDLE )

TO GIVE YOU AN AID LOOK AT THE "TEE" IN THIS WIRE AT THE COIL, YOU WILL SEE WHY "I WIRE" HAS BOTH VOLTAGES

PIC 1) MAP FOR SOLENOID

PIC 2) EXPLANATIONS OF IGN SYSTEM, I HAVE ERASED OTHER CRAP

PIC 3) HAS NOT BEEN MODIFIED - ARE YOU WIRED LIKE THIS DIAGRAM ON THE SOLENOID?

OK

ALL OF THAT ABOVE MAY BE IRRELEVANT.
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**********HAVE YOU TAKEN THE DISTRIBUTOR OUT?***********
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NO MORE BUYING ANYTHING TILL YOU TALK TO ME FIRST!

HOW 'BOUT A CHEAP DIGITAL VOLTMETER (HARBOR FREIGHT HAS 'EM FOR $5)

DO THE TEST IN PIC 4

SEND YOUR RESULTS

HAVE YOU BOUGHT A COIL? SEE PIC 6

I HAVE LOTS OF TEST PICS- YOU MUST SEND RESULTS-DON'T JUMP AROUND-PLEASE RUN BACK THRU AND ANSWER MY QUESTIONS!

WE WILL FIND THE PROBLEM!

YOUR TURN

THE MEDIC
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Saturday, October 10th, 2015 AT 9:02 PM
Tiny
CJ MEDEVAC
  • EXPERT
OOOOPs!

YOU HAVE A '78

IT HAS PRESTOLITE IGNITION?

KEEP IN MIND, I HAVE SORTA DESTROYED THIS DISTRIBUTOR. LOOK AT THE FACE (FRONT MISSING), THIS IS A PRESTOLITE (PIC 1) THE OTHER IS A MOTORCRAFT

IF YOU HAVE THE PRESTOLITE- THE COIL NOT THE ONE I MENTIONED IN MY 1st ANSWER. THE ONE YOU WOULD NEED HAS INTERNAL RESISTANCE!

PRESTOLITE IS NOT A WELL DESIGNED SYSTEM, MY CJ-5 IS A 1977, I UPGRADED IT TO A MOTORCRAFT SYSTEM (LATE '78-'86)

BUT ANYWAY, IF THIS IS WHAT YOU HAVE, WE CAN MAKE IT WORK, MAYBE MAKE THINGS A LITTLE BETTER THAN BEFORE

THE DIAGRAMS IN THE 1st ANSWER ARE FINE AS FAR AS THE SOLENOID GOES, THE IGN SYSTEM IS A '79

STILL WANT THE TEST IN PIC 4

YOU ARE ALLOWED TO WRITE LOTS OF INFO BACK TO US!

THE MEDIC
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Saturday, October 10th, 2015 AT 9:20 PM

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