Electronic ignition diagnostic

[Dart/Valiant]

The patient is my 1975 Dodge Dart. Since about three weeks, the engine is intermittently difficult to start, no matter hot or cold. This week, the car stalled in traffic. Restarted just long enough to let me pull on the side and stalled again. Hood was open for a few minutes, seems to cool down, and started again. I thought about setting aside the carburetor, it's new from last year and all is well. So I did electrical tests, especially since I'm suspicious of the ignition system since I had to change two coils in the last three years.

I religiously followed the manual and did the prescribed tests.
- ignition coil 2.5 ohms primary and 10580 secondary (blown coils went under 10000 ohms)
- ECU box properly grounded, ohmmeter tested also
- distributor pickup coil 286 ohms, should be between 150 and 900 ohms

So far, so good, but testing the ecu wiring harness cavities... (battery voltage 12.87v)
- available voltage at cavity #1 should be within 1 volt of battery voltage, got 11.30v
- available voltage at cavity #2 should be within 1 volt of battery voltage, got 11.35v
- available voltage at cavity #3 should be within 1 volt of battery voltage, got 11.37v
Is these low readings serious? The manual says nothing. Something bad is going on?

 

[AJ/FormS]

IMO those readings are all fine,that box in my experience works fine down to 10.5 volts and sometimes even to 9.5.
Now you just gotta make sure nothing changes with temperature and especially the ballast resistor and it's connectors.
And that the reluctor gap is stable,
And that the spark goes from the coil thru the the rotor/cap and on to the plugs

If I was a betting man, I'd say the problem is in the fuel itself, like water in it. I would remove the carb and dump all the fuel into a clear glass casserole pan. The fuel better be clear, and if there is water in it,you will see it by tilting the pan on one corner, and it will all collect there at the lowest point.

 

[krazykuda]

Check the reluctor gap in the distributor, should be .007"

 

[halifaxhops]

all Good info also keep in mind the distributor pickups sometimes fail when they get hot and when they cool down work for a bit.

 

[teringer]

I would also go with the dist pick up I just went through the same problem and it was the pickup if you have a heat gun just heat up dist and see if problem occurs

 

[67Dart273]

Do this:

Remove all ignition connectors, ballast, ECU, distributor, and work them in/ out several times to "scrub" them and "feel" for tightness. Use a light and examine for corrosion. Be ESPECIALLY critical of the distributor connector

Examine the pickup/ reluctor for rust, strike damage, and shaft play. As mentioned, set the reluctor gap

Examine cap and rotor for dirt/ grease/ moisture/ carbon tracking. Especially look at the rotor for punch through

Measure resistance of the coil wire and make sure it is not "open."

Hook a meter to coil + and turn key to run. You should have about 6-10V.
Crank the engine using the KEY and read meter while cranking. You should have "same as battery" and should have at least 10.5V, 10V absolute minimum

MAKE ABSOLUTE CERTAIN that the ECU is grounded. Same for regulator

With key in "run" position, disconnect distributor connector. Take the engine harness end, and repeatedly tap the bare terminal of that connector to ground. Each time you do it should generate a nice "snap" spark through a test gap on the coil

 

[Dart/Valiant]

Many thanks. Today I replaced the pick up coil and the reluctor.

Also, someone told me to do the voltage tests again, but instead of the voltmeter use a test light, looking for an egal brightness eveywhere. Done, everything looks fine.

 

[Dart/Valiant]

Hook a meter to coil + and turn key to run. You should have about 6-10V.
Crank the engine using the KEY and read meter while cranking. You should have "same as battery" and should have at least 10.5V, 10V absolute minimum

There is a problem! Between a good ground and the positive side of the ignition coil, got 5.95V While cranking, got only 8.5V How this can be explained...

 

[67Dart273]

That might just be your problem. The Mopar ignition switch has TWO ignition voltage sources. 1...is "ignition run" and is hot ONLY in the run position. This goes through the bulkhead, and supplies the "key" side of the ballast, the VR, the alternator field, and smog doo dads if present, electric choke, etc

THAT VOLTAGE GOES COLD in "crank" or "start'

The ONLY voltage serving the ignition system during cranking is "ign 2" which is a separate contact on the ignition switch. This is brown, goes from the key/ switch through the bulkhead connector to the coil + side of the ballast. Either it came unhooked, the bulkhead connector terminals are corroded, or the igniton switch connector or switch is going bad.