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Dodgerench, ASE Certified Technician
Category: Dodge
Satisfied Customers: 3404
Experience:  30+ years Dodge/Chrysler exp., ASE Master with L1 certification. Driveability/ combustion specialist
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1975 dodge: engine turns..I had the ecu checked and they said it was

Customer Question

1975 dodge pick up. drive along and engine turns off like you turn off switch. I had the ecu checked and they said it was good. I have had to replace this item on the average once every two years.

Got it started this afternoon. I had to let it sit overnight though. (We had gone to a BSA (Scout) meeting in it and when I came out it would not start and I had to let it sit there.)
Since I was in a company vehicle I had to go home and have my wife take me back to the location to pick it up. But then it it would not start. It tries to start when I turn the ignition back after trying to crank it to start. Weird. Is ther any steps i can take to pin this problem down? I know the ballast has someting to do with it. Can I jump around it to see if thats the problem? Help.
Submitted: 7 years ago.
Category: Dodge
Expert:  Dodgerench replied 7 years ago.
HiCustomer welcome to Just Answer!.

I'm afraid I have no access to to the '75 electronic ignition wiring for specifics, but I understand the system quite well. It's the little things like dual or single pickup plate setups that complicate matters.

Which engine does your truck have?
The big differences would be small and big-block engines...

Is your ballast a two terminal or four-terminal unit?
Does the engine ever actually fire up as you let off the key as you described?
Or does it just tease you like it's gonna start?

Talk shortly,
Customer: replied 7 years ago.
Small block 318, has a four connection ballast and teases like its going to start. I know how to hot wire it . I have not done so yet. I may have do so to get it moved from where it broke down and save me a wrecker fee. Somebody mentioned the pick up coil in the distrbutor, but I do not know. just
Expert:  Dodgerench replied 7 years ago.

Thanks. I hope to be able to supply you with enough info to drive your truck back... tow truck rides get spendy.




This was the best I could do for a wiring diagram of the system and it's similar.... just not the same as your '75. The dual-ballast system went out of production some around the late 70s, so this one reflects the single type.


The ECU will be the same, however... as will be the pinout on the unit. Some ECUs are equipped with a 5-wire connector and I'm not sure what the 5th wire would do since everything you'd need to produce spark is shown here on the 4 wire setup.


The wire at about the 1:00 position is your coil negative circuit, the one that actually grounds and controls the process of spark production.


At 3 O'clock is the ignition power feed, which will come through your ballast resistor. More on that later.


Following around at 5 and 7 O'clock are the distributor pickup plate wiring circuits.


Grounding for the ECU is proved by the mounting bolts to the firewall. It's quite possible that the ground is weak or lost, so don't hesitate to shine up the connections or at least test the case for continuity to ground or for residual voltage (should show 0v).


Start by checking for voltage at your coil with the key turned on. Since power is being fed through a resistor, you'll see less than battery voltage... probably something around 8v or so. The negative side of the coil should show very little voltage because the ECU provides a ground circuit to the coil at all times with the key on unless it sees a peaked AC signal from the distributor pickup plate at which time it releases the circuit, allowing the "saturated" coil's magnetic field to collapse and produce spark. It operates on a 44 degree dwell cycle with the engine running if you remember your old points-n-condenser stuff. The coil will begin to feel warm to the touch after a few minutes of sitting with the key on.


With a dual ballast resistor setup, the ignition switch supplies an alternate path for voltage supply to the coil at the same time as you're cranking the engine to increase voltage that naturally drops with starter draw. Without this voltage shunt, you'll have a tendency to have difficulty starting the engine and it might fire only as you let off the starter.


What I'm not clear about here is just where that voltage source comes from. Later years took a feed from the starter relay... something that became energized at the same time as the starter solenoid. For now, check the feed side of the dial ballast at both levels to be sure you get power fed to the unit. Then check to see that the ballast hasn't burned out internally, especially on the bypass side. If ohmed out, expect to see something like 1 or 2 ohms of resistance on either resistor, which runs horizontally to the unit.


While the engine will run with straight 12v wired to the (+) side of the coil, the coil and ECU may overheat and fail after a bit of run time. If you have to do some jury-rigging with the engine wiring to drive it that way, give it a few minutes to cool about every 15 minutes or so.


Next up would be the distributor pickup plate if you still have problems with producing spark. I believe your unit will be a single pickup, but duals happened in some years. The run side pickup plate will have a connector shaped like the one I have highlighted above, where the start side pickup (if equipped) will have two bare male terminals on one end and two females on the other.


With the key on at this point, pull the coil wire out of the distributor cap and place it within 1/2" of a good ground. Then disconnect the pickup plate connector that looks like the one I highlighted.


Locate the bare terminal on the harness side connector and touch or scratch it to a good ground source. The coil mounting bracket works well for this. It should produce a storm of spark if the circuit to the ECU is good from that point. If this is the only way that you can produce spark, it pretty much means you're lacking a signal from the distributor.


Make sure the distributor rotor turns first, then check your reluctor-to-pickup plate gap or replace the pickup plate. Either operation pretty much requires removal of the distributor by the way.


Instead of sweating the .006" air gap that's specified for this adjustment, I found a more practical method. Since the distributor shaft bushings will be worn (slop) and the shaft will almost certainly have some wobble to it, my preference is to set the air gap as close as is possible while pressing the distributor shaft toward the pickup plate. The important things here are to get it close as possible... but have no contact at any of the 8 reluctor tabs as the shaft rotates. When that's achieved... you should be all good.


I'll stop there for now, except to recommend taking some extra wire for jumpering purposes. You're gonna be a long way from Walmart and spare wire just plain comes in handy.


Good luck and we'll talk in a bit!


Customer: replied 7 years ago.

Well it would initially not start when I tried to hot wire it. No fire. It seemed to still start on removing the screwdriver after shorting across the starter relay.


I pulled the coil wire off and was also not getting spark when grounded out while trying to start it with the ignition. (I had replaced the ceramic ballast.) I then put the coil wire back in the distributor and turned the key and it fired right up. The hot wire was still hooked up. I drove it home . It is cold natured and died a couple of times because of that. The choke does not work on it. I started it each time using the ignition.


I then tried to turn it off at home and because the hot wire was on it it stayed running. I then killed it and restarted it 5 times with the ignition. I then let it sit for an hour and went out and hot wired it and it started and died when I pulled the Hot wire off. I then started it with the ignition a couple of times.


Could the coil be breaking down? It did not have fire when I hot wired it the first time.

Does fire go through the ECU? Could I still have a ECU problem? What would make it try and start on releasing the ignition? Would a bad ECU cause that? Would the ECU be involved in any way if I hot wired it? Am I bypassing the ECU and ignition when I hot wire it? If I am by passing the ECU and ignition when hot wireing then in my mind the ECU could very well be bad. Does that make sense? Could it be something as simple as my starter relay on the firewall? Bad coil wire?


Yet then again how come it then started with the ignition and hot wireing after I got home. Something is breaking down. I am tempted to just go out and buy a ECU and put it on it and see what happens. What do you think?

Expert:  Dodgerench replied 7 years ago.
Very very hard to tell at this point, dude.
I'd be interested to see what kind of voltage is present at the (+) side of the ignition coil at key on, during engine cranking and then with it running. If you hotwired 12v straight to the coil, you may be simply bypassing a low voltage situation from the normal electrical system... something that's too weak sometimes to produce spark.

Fire doesn't actually run through the coil, if you're referring to the spark that comes out of the ignition coil. The ECU deals only with battery voltage, switching power to the coil to saturate it and produce spark. It's certainly possible that it's responsible for your problems, but I'd be more comfortable if you could do some electrical tests first.

First off, there's the end-line voltage I'd like you to measure at the coil positive terminal.

Then if it shows to be less than 8v at any time, I'd like you to back up to the feed side of the ceramic ballast to see if voltage is close to being equal to that of battery voltage. There will always be some voltage drop as DC current flows through copper wiring, so expect to see up to a full volt of difference at the worst. The worst.

Testing the ECU for good grounding is needed as well. Your voltmeter should show 0v when the test leads are connected to battery negative and then the ECU case if everything is good. Any voltage shown on the meter when tested that way indicates a problem with draining system voltage off... bad ground basically.

The pickup plate is another thing to consider since it can cause sudden loss of spark. If you can catch the system in a state of zero spark, perform the "scratch test" I described earlier with the key on to see if it produces spark. This procedure tests everything but the pickup plate, using every piece of connection and switching except for the pickup plate. So if you produce spark with the scratch test... and it won't spark when cranking the engine... it looks bad for the pickup plate if system voltages check OK at the coil.

And then there are basic circuit tests that can be done to see that your pickup plate wiring isn't 1) open 2) shorted to ground 3) shorted between its individual wires 4) shorted to voltage. The two wires I highlighted that come from the pickup plate to the ECU need to be completely insulated from all outside influences between those two points.

If you'd like to test your pickup plate, only one method is used... the digital ohmmeter. Set it to 2K ohms and test through the disconnected connector at the distributor or at the ECU with everything else connected. Normal resistance can vary wildly, coming in between 600 and 1200 ohms according to the specs. Testing it at the pickup plate connector and then at the disconnected ECU connector would tell you if there is a significant circuit resistance problem between the two points.

I hadn't asked yet, but do you have a digital multimeter? If not, they're quite affordable these days and available at pretty much any Big Box hardware store, Sears, Harbor Freight or Radio Shack for $20 or less ($4 at Harbor Freight). All work magnificently for our purposes and the meter will come in handy for years to come on other projects including AC voltage issues at the house. Good stuff.