Please explain "start" circuit to me

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'74 Sport

'74 Sport

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Aaron's slanted '74 Dart Sport finally had its first major issue in the past three-plus years he has been driving it. Since completing a total rebuild in March 2006, he has used it for his daily driver and has put about 30,000 miles on it.

He called me after work about a week ago and said his car wouldn't start. We thought it might be flooded, but after letting it sit for a while, it still would not start, so I went to see if I could help him. We fiddled around for a while with this and that, not really doing much other than making sure no wires were disconnected or melted, vacuum lines were intact, carb was getting gas, jumper cables were boosting his system, and ignition wires were all on tight. No major diagnosing. Then it just simply started...

He has driven it this past week without any other issues... until yesterday morning. Wouldn't start again. Cranks and cranks, just no go. So, we got out the multi-meter and ignition tester and went to work on it yesterday afternoon. Still won't start. Here's what we have determined so far:

Installed ignition tester in-line on cylinder one... no spark
Same on cylinder two
Replaced coil with brand new one off the Duster project... nothing
Replaced ECU with brand new one off the Duster project... nothing
Installed ignition tester in-line on primary wire from coil... single spark

Here's what I need explained...
With the ignition tester connected to the coil's primary wire and grounded to the body, we get a single spark when the ignition key passes the "run" position, then there is no spark as it gets to the "start" position as the engine cranks. I understand the distributor is out of the loop in this scenario, so I would not expect the test light to "flicker" as it would if connected to a spark plug wire. Since there is no interruption of the circuit being introduced by the spinning rotor, why would the light not burn steady while the key is in the "start" position. I assumed there would be a constant supply of voltage as the engine tries to fire up, then once the engine starts and the key is released to the "run" position, the distributor takes over to "distribute" the voltage among the six plugs - hence the "flicker". What am I missing here?

I will admit, since it was getting dark outside, we buttoned everything up before pulling the distributor cap and checking it and the rotor's condition. I can't tell you if the rotor was spinning or not. I just realized that there would be no spark at any of the plug wires if the nylon gear on the distributor shaft has broken off. That's the next thing we will look for is a spinning rotor.

Any other comments or suggestions are welcomed,
Jerry
 
use the engine for a ground test and check the coil for power with the key in the run position. this will make sure you have a good ground to the engine.
 
One more thing Be careful for gas fumes and don't touch the car with your body and pull a spark plug wire off the distributor and see if a spark will jump to the plug wire about a 1/2 inch or less away from the hole on the distributor while it is being started, may be just a bad ground or ignition switch
 
The coil is supposed to get a full 12 volts when cranking and only 6 to 7 volts in run. When the cranking voltage is lost the result will be a single spark every time the key is released to the run position. Most common culprit is ballast resistor. Can be the ecu too. When changing a ecu it should be mounted for a proper chassis ground before the harness connecter is put on it.
 
I just remembered the chassis ground from the battery must be there.
I did that once and it drove me crazy then I looked at the battery and said Oops!! got to be grounded to hehe.
 
Thanks guys. As I said, the coil produces a spark when the key is in "run" position, so I'm guessing there is a good ground. It just loses voltage when going to "start". For some reason, I always thought that a bad ballast resistor would allow the engine to start, but would then die when the key was released. Oh, well... maybe need to investigate the ballast resistor.
 
My '74 has been doing that for a long time. Make sure the electronic control unit is free of rust on the mounting bracket and tight on the fender. Also, I have heard that if there is something on the passenger seat the switch may keep it from starting if the seat belt is not connected. I think this may be unique to '74's, but I am unsure if it is true or not. I just bounced up and down on the seat a couple of times, and my car seems to be starting all the time now. The good part is it costs nothing to try. Hope this helps, I can't stand electrical glitches.

OOps, I'm a minute late.
 
We replaced the front seat with one out of an earlier Duster and never reinstalled the wiring harness to the seat. Even though the '74 has the seatbelt interlock system, it has never been a problem for the past three years of daily use. Just for grins, we even pushed in the little red button yesterday.
 
magnumdart said:
My '74 has been doing that for a long time. Make sure the electronic control unit is free of rust on the mounting bracket and tight on the fender. Also, I have heard that if there is something on the passenger seat the switch may keep it from starting if the seat belt is not connected. I think this may be unique to '74's, but I am unsure if it is true or not. I just bounced up and down on the seat a couple of times, and my car seems to be starting all the time now. The good part is it costs nothing to try. Hope this helps, I can't stand electrical glitches.

OOps, I'm a minute late.
Click to expand...

Yeah I keep forgetting about that seatbelt interlock which is 74 model only. If I owned a 74 I'd be taking that out of the loop one way or another.
 
Check for power on the brown wire at the ballast resistor. This is the start circuit from the ignition switch. You can disable the engine from turning by pulling the yellow wire at the starter relay. If there is no power with a test light at the brown wire with the key in start position, trace it back to the bulkhead, then inside to the ignition switch. You can also check power at the ignition connector coming from the column. Find where power stops.
 
Try everything else first. You dont want the ignition switch to be the problem simply because it is a real chore to R&R. Plus some of the aftermarket switches are wired wrong at the harness connector.
A few owners have found very odd electrical problems causing the same condition. One had a short in the blue wire to the altenater which was grounding and drinking the strating voltage. Another had a short in the added electric choke heater on a 73 model that also grounded away the starting voltage. Bottom line... 12 volts is required and you dont have it. Good luck !
 
Point taken...
Yes, tearing into the column to replace the ignition switch is not a picnic. Three or four of them have provided me a lifetime of fun and excitement I could very well do without.
 
About 2 years back I had a problem with my 74 swinger same thing spent hours playing around and then it ran fine for about a month and the same thing so I went and changed coil cap rotor ecu ballast wires etc. and the car ran perfect for over a year then after some messing around under the hood it wouldn't start same problem I was completely lost and swapped everything from my runny 74 custom to my 74 swinger to no avail then I started playing with wires the car starts after some more playing I had a broken wire running to the ballast about 2 inches from the connector so I cut the connectors off and added new wire and new connector no problems now you may be looking at the same thing try cranking while someone slowly moves the wire around near the ballast where they go into the main engine bay harness. Just a thought have 2 identicle cars didn't even help me much on that one except elimination
 
Jerry, you're aware of the problems I had with getting my 74 Dart to crank. I spent months tracing wires, making sure there were no breaks in the wires, changing coils, ballast resistors, and 4 ECUs. I even took apart the distributor and changed the pick-up coil. After all that headache, it turned out to be that damn plug sitting on the driver side engine compartment between the washer fluid bottle and the ballast resistor. That plug connects to the 74 starter interlock switch. (I read above where you tried pushing in the button on the switch.) I also know you don't have Aaron's seats or the interlock set up to function on his car.

The mechanic who got my car running disconnected the wire plug (two or three wires) from that interlock switch and told me to never reconnect it. I think you should try the same. This may not be your problem, but it will not hurt to unplug it.
 
tedsweet said:
THE INTELOCK prevents cranking
Click to expand...
Quite true...
Only twice have we ever had to press the reset button, and that was to get the starter to crank the engine over. This problem is not "cranking", it's "firing".
 
Check what Redfastback said.

The ballast doesn't do anything when the car is cranking, it's bypassed by the cranking start wire to the coil side of the ballast.

My Dart does that every once in a while and it's always been the bulkhead connectors.
 
tedsweet said:
THE INTERLOCK prevents cranking
Click to expand...

I agree. That's what the little reset button was used for. However, the mechanic that finally got my 74 running told me that interlock somehow was shorting the ECU. No matter how many ECUs I bought, none of them worked until that INTERLOCK was disconnected at the driver side inner fender.

As I said, this may not be Jerry's problem, but it cannot hurt to disconnect a potential electrical problem caused by a device that Aaron's car is not using.
 
So, you just drive it home, park it, and the next morning it won't start for love nor money? And it's all because of what? The bulkhead connector developed a little corrosion?

Dang those Chrysler engineers. Every one of them should be horse-whipped with a battery cable.
 
I had a similar problem on my 74 Duster and it turned out the ignition module plug connector was corroded and loose fitting, I cleaned the female terminals with a pipe cleaner and gave them a little crimp with the needle nose and it cured it.

I could be a couple of the other suggestions, just tossing out one from my experience. It also depends on where the car is from, we get different electrical issues out in CA than you guys in the cold/damp states.

Good luck! John
 
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