• When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.

Ignition woes...

P Dubya

Well-Known Member
Local time
1:53 PM
Joined
Jul 5, 2021
Messages
46
Reaction score
30
Location
50601
Well.... It looks like I have a very common problem from doing a bit of reading in past posts. Here is what I can tell you has happened and what I have done...Car is a 1969 Road runner, 440/6 pack. It has been updated to electronic ignition before I purchased it but I do not know what the under cap components are... Best guess is Mopar but could also be aftermarket Pertronix, etc... I think I have been lucky so far as the crank/no start issue has only happened at home. I think we can rule out any kind of heat induced electrical gremlins as the car is always in a 72 degree garage. When this issue started a few days ago I started the engine and no more than backed out of the garage and it shut off as if I had turned the key off. No spark at plugs. I tested ballast resistor for continuity between blades (there are 4 of them) and there seems to be good continuity. I did not know what value to use for resistance so did not try. I found one spade terminal that may have been a bit loose so did a little work on that... Still no start. Used a jumper wire directly from battery to + terminal of coil. No start... Pulled harness off of ICU, no corrosion or anything out of the norm visible there. Still no start... Walked away from it for a while and came back to do some more testing. This time I had spark at the plugs so I started it up...It idled happily in the driveway for at least a 1/2 hour before I headed to town for a drive, gassed it up for the weekend and put it away for the rest of the day. Evening came around and the wife and I headed out... Guess what, it didn't start! It has been a long weekend so I have tried to read up as much as I can on the ignition system. Can anyone suggest any other tests I can do before I look into purchasing parts, first and foremost a new ICU Mopar part # P4120505 and possibly a new ballast resistor? Thanks!!
 
If you have an extra coil try that. This year I had a similar problem did hot wire to coil and no start. Replaced coil and started right up. Is you ecm have a good ground ? Good luck
 
Usually if the ballast resistor is bad, it will start but then die when you let off the key. You could still check it by ohming it out. One side will be .5 - .7 ohm, and the other a bit higher. Also check the condition of the bigger black and red wires going through the bulkhead connector, this is a weak spot on these cars. Make sure your control box case is grounded well.....
 
Usually if the ballast resistor is bad, it will start but then die when you let off the key. You could still check it by ohming it out. One side will be .5 - .7 ohm, and the other a bit higher. Also check the condition of the bigger black and red wires going through the bulkhead connector, this is a weak spot on these cars. Make sure your control box case is grounded well.....

YES.....one half of the ballast resistor is is 0.5 ohms, which feeds the Orange ECU to the coil to generate the spark. The other half of the ballast resistor is 5.0 ohms which operates ECU's internals. if you are using an origional P4120505 ECU, you DO NOT NEED THE 5.0 ohm section of the ballast resistor as the internal circuit was revised to eliminate that requirement. The 0.5 ohm section of the ballast resistor LIMITS the coil's primary winding current and the switching transistor's on the ECU case to prevent premature failure. The ECU TURNS OFF, the coil's primary winding current to generate the spark. As noted previously, THE ECU case MUST BE SECURELY GROUNDED. IMO....use the origional MOPAR coil NOT something after market as they usually NOT compatible and will likely cause ECU failures. Just my opinion of course.
BOB RENTON
 
Well.... It looks like I have a very common problem from doing a bit of reading in past posts. Here is what I can tell you has happened and what I have done...Car is a 1969 Road runner, 440/6 pack. It has been updated to electronic ignition before I purchased it but I do not know what the under cap components are... Best guess is Mopar but could also be aftermarket Pertronix, etc... I think I have been lucky so far as the crank/no start issue has only happened at home. I think we can rule out any kind of heat induced electrical gremlins as the car is always in a 72 degree garage. When this issue started a few days ago I started the engine and no more than backed out of the garage and it shut off as if I had turned the key off. No spark at plugs. I tested ballast resistor for continuity between blades (there are 4 of them) and there seems to be good continuity. I did not know what value to use for resistance so did not try. I found one spade terminal that may have been a bit loose so did a little work on that... Still no start. Used a jumper wire directly from battery to + terminal of coil. No start... Pulled harness off of ICU, no corrosion or anything out of the norm visible there. Still no start... Walked away from it for a while and came back to do some more testing. This time I had spark at the plugs so I started it up...It idled happily in the driveway for at least a 1/2 hour before I headed to town for a drive, gassed it up for the weekend and put it away for the rest of the day. Evening came around and the wife and I headed out... Guess what, it didn't start! It has been a long weekend so I have tried to read up as much as I can on the ignition system. Can anyone suggest any other tests I can do before I look into purchasing parts, first and foremost a new ICU Mopar part # P4120505 and possibly a new ballast resistor? Thanks!!
Any luck?
 
Luck? I just need to find a little time to see how my luck goes! I have a new resistor and ICU ordered and should be here in a day or so. I am hoping the ICU decided to part company and it will be a simple fix. I have not had time to ohm the resistor as outlined above....
 
Any luck?

New ICU installed and now have spark at the plug I pulled. Also went ahead and replaced ballast resistor. Pretty sure I have flooded it now. New plugs if I can find them today....
 
check ignition power while wiggling the key to check if the ignition switch is intermittent power.
Check cap/rotor and reluctor to pickup clearance.
 
First thing to check is make sure the ECU is well grounded. Typical is by the mounting screws, not good enough. Sand the mount holes on the box & find a really good ground location & a wire to the bare spot on the box. The case of the box needs a good ground or it will be intermittent.
 
Would anyone have a schematic of the ignition wiring circuit?
 
Loose or weak fusible link maybe?


I know very little about this car as I recently purchased it so not even sure where to look for or how to test link when found...
 
Just an update so possibly others might not have to go down the same road.... Someone on another BB sent me a link to Daves Place - Chrysler Electronics or something like that. Very informative and detailed step by step process of testing the ignition. I had replaced a few parts but honestly the link was what led me to the cause of the entire problem. The problem was all under the cap and related to the gap between the reluctor and pickup. It calls for .008''... It was close to .020'' when I checked it. With a quick adjustment it started up and has run flawlessly for several hundred miles..... :)
 
Auto Transport Service
Back
Top