Hope someone has run into this kind of situation before...

I have a 351HO with the Duraspark (blue grommet flavor) ignition module (IM) running in a custom application (Jeep YJ). Everything was fine until the other day, when the IM died. We put a new one in and while doing so, verified the coil, dizzy pickup and every wire involved (as the result of initially being sold the wrong part and troubleshooting why we had no "fire in the hole" with the new IM). Anyway, once we put in the first new IM it fried dead within 10 minutes of being installed. I cleaned all of the connections to eliminate any high resistance, re-cleaned the ground inside the dizzy and put in IM #3 (second new one). I ran it for about 10 minutes and while it's still alive, my infrared thermometer shows the hottest part of the heat sink at 200F. I don't have the confidence that anything is actually fixed, even though this one hasn't died, and don't know how hot the little Chinese-made heat sink can or should go.

For those curious, I've verified all operating voltages (e.g. input to the IM while engine running, coil + and - side during "run" (engine off) and "start", etc. etc.) and all wire(s) resistance. While I don't think any of these is contributing, I can share a reading(s) if it will help.

The only thing I'm aware of that will fry an IM is high voltage and/or high resistance, but neither ain't it.

Thanks!
- IH

 

What is your coil + voltage while running? And what type of coil are you using? You should have something like 8-9v on the coil + while running, and the original duraspark II coil will put the correct load on the module. The duraspark II coil is the one that takes the horseshoe connector.

 

Thanks Dave. I didn't know the coil put the load on the IM - good to know. Yes, the voltage on the coil input drops to ~9VDC when running. We took the Jeep out last night for ~20 minutes of drive time and so far the new IM hasn't died. There is no horseshoe, but there are two male tabs on the coil (+ and - sides) that we've mated to female connections (i.e. it's not the old round button style connections coil). It could be that IM #2 was just a POS or that cleaning the ground and wires again strengthened the circuit - but time will tell.

 

You should be ok as long as it is not one of those aftermarket "hot" coils, and not one of the later model square coils.

That is the module's main job, to turn the coil on and off by grounding and ungrounding the coil negative. All the current going through the coil primary goes through the module also.

If you hand wired all this, and you drive it in cold weather, you may also want to make sure you have the resistor bypass wired in. The 9v is coming from the resistor or the resistance wire in the + wire of the coil. Most older cars and trucks with this system had a fender mounted starter relay with 2 small terminals. The one small terminal was for cranking, the other one had 12v on it during cranking only. They would run a wire from this terminal (usually labeled "I") over to the coil +. This gave the coil a full 12v during cranking, and helps it start quicker, especially in cold weather. The battery voltage can get pretty low while cranking the engine over, the bypass gives it hotter spark.

Most of the 80-up Fords had this bypass built in to the ignition switch, not on the solenoid on the fender. If your vehicle originally had this system from the factory, it's probably already wired in somehow.

 

The start post of the starter solenoid switch, mounted on the passenger side fender well, is wired to the white wire on the IM to provide voltage during the starting process. I don't think the wiring includes a resistor and it was wired when we purchased the Jeep. I just went through most of the harness troubleshooting earlier, although I did re-wire things I didn't like (e.g. the red and white wires were spliced at the IM, which I understand is "okay" but isn't the way it's supposed to be..., etc.).

 

The white wire wired into the IM is the start retard feature of the IM. When the IM gets 12v on this wire, it electronically retards the timing, the same thing as twisting the dist around. This makes the engine easier to turn over in hot weather.

I am assuming you have a resistor or resistance wire somewhere between the keyswitch and the coil +, that is why you only get about 9v on the coil + while running. Don't worry about it, but if you do use this vehicle in the winter, and you notice that it just cranks and cranks when it's cold and has a hard time firing off, you may need to investigate the resistor bypass further. You can't just tie the start wire from the fender solenoid to the coil, because the 9v from the coil would keep the starter turning even after you let off the keyswitch. That is why the used the other terminal on the solenoid, some of the later solenoids don't have this terminal because like I said, they had a extra terminal in the keyswitch/ignition switch.

 

Dave, my son was able to limp his Jeep home tonight after multiple engine stops. I've asked him to take it around the block and watch the tachometer to confirm a "zero spark" event (or not) and will let you know what we find out. In the meantime, I put my heat gun on the IM (200 degrees) and a voltmeter on the coil when he got home and verified ~9VDC when starting but about 13VDC when the engine is running (i.e. it's running on what the alternator is sending to the electrical system). So, it appears that there is no resistor - sorry. Does this change anything?

 

Yes, you need to wire in a resistor inline to the coil + to drop the voltage. I usually tell people to go to the store and get one of those big white resistors made for a Chrysler, and they seem to work fine. Mount it somewhere out in the open, usually on the firewall, and keep other wires and hoses away from it, it will get hot.

Once you do this, you can think about the resistor bypass, now that you have a resistor. If your solenoid does have two small terminals, the one labeled "i" will get a wire hooked to it, and just run it over to the coil + or the side of the resistor that goes to the coil +. That will put the full 12v on the coil during cranking only. If your solenoid only has one small terminal, you can buy one with two. I hesitate for you to do this, the new solenoids they are making in China now are lousy and like to stick on. You can't buy anything anymore.

 

Dave, will look into the resistor line if this is what is shortening the life of the IMs. Yes, the starter solenoid relay switch has the "I" terminal with a wire from the ignition switch on it and a "S" terminal with a wire to the start / white wire on the IM.

In the meantime, we've had a related(?) issue. First, the starter solenoid relay switch stuck in the circuit closed position causing the starter to continue turning over with the key in the off position. We assumed the relay switch was the culprit by pulling the "I" wire (i.e. killing any voltage signal to the relay switch) and the engine stopped turning over. We installed a new relay switch and the car started and the relay switch opened and closed the circuit like it should.

Here's the new symptom... now the engine will turn over but won't start using the key & ignition switch. It will turn over and start if we jump the solenoid relay switch battery / hot terminal and "S" terminal. We bought a new ignition switch (GM-style, located on the bottom of the steering column) and have the same results; we used a screwdriver to move the switch to the start position to eliminate any key cylinder or linkage issue. When we try to start the engine using the ignition circuit, the voltage at the coil and to the red IM wire drops to .4VDC (that's zero point decimal four). The white IM wire receives 12VDC when the engine starter turns over, but doesn't seem to be passing this voltage to the coil (is it supposed to?). Notably, the PO had the white and red IM wires spliced together.

UPDATE: I assume there is another wire that is part of the key ignition switch circuitry, that "isn't doing it's job and isn't necessary when we use the starter solenoid relay switch to start the engine"(?).

Obviously, I don't fully understand how all of this is supposed to work. The engine started and ran the day after we split the white and red IM wires. I don't understand why we fix one thing and another stops working (unless it's just BAD coincidence). I appreciate your assistance and please let me know if I left any important detail out of this post.

IH -

 

First off, you seemed confused with the starter relay over there. The "S" terminal is the one that goes to the windings of the relay, and this is the wire that comes from the neutral start switch and then the ignition switch start or crank signal. 12v on the "S" terminal should make the starter relay (solenoid) click and the starter should turn over.

The "i" terminal is just a aux output. When the relay is activated and the starter is turning, the "i" terminal will have 12v on it. When you release the key and drop out the 12v on the "S", then the 12v will drop out of the "i' terminal. The only purpose of the "i' terminal is to send 12v directly to the coil during cranking.

I would look at the Jeep wiring diagrams, and see what wire you should be using for the coil + from the ignition switch. Apparently the one you are using now only has power when the ignition switch is in the on position, but does not have power when in the cranking position. Since you have changed the relay(solenoid), see if you have 12v on the "i" terminal when the starter is turning. You may have been relying on this terminal to supply 12v to the coil during cranking, and then relying on the ignition switch to supply the coil in the on position.

 

Thanks Dave and yes... shade-tree mechanically with enough electrical experience to be dangerous - but a little confused, especially because all of this worked before our original symptom (the dead IM). I'll try to comment inside your quote for clarity and to see if it changes any advice - thank you. I also hope another can learn from this and save a little head-against-the-rock feeling.

So should I now assume that the problem with voltage dropping to the coil when the starter is engaged lies between the "I" terminal, where 12VDC is being applied (by the relay switch?) and the coil positive side. Let me know if I'm off? Starting to wonder if the problem is with the new IM (it's what has changed, aside from the starter solenoid relay switch...).

 

Sounds like you have it right. I do not know why the 'i' terminal goes all the way back to the ignition switch. That is going to make it difficult to add a resistor inline to the coil +, since the "i" wire needs to be after the resistor, so it can supply the coil + directly, and then when the starter is released, the coil + is supplied voltage by the resistor, which get's it's voltage from one of the ignition switch terminals. If they are using the same ignition switch terminal to supply 12v to the module (red wire), then where it is spliced in is also important, the module needs 12v any time the key is on or in start, it can't be supplied with that lower 9v from the resistor.

That 'S" terminal on the solenoid should have a splice in it also somewhere. It's signal should originate from the keyswitch, and usually goes through a neutral safety, and then to the "S". Somewhere along the way there will be a splice that taps another wire off, and goes to the white wire of the IM for the spark retard feature. However they do it, if the "S" wire goes to the white IM wire, it may have a splice very near there and it goes on to the ignition switch.

I do not know how you are at wiring diagrams, but on page 26 in this link is a factory Ford wiring diagram for the duraspark II ignition system.
Start & Ignition - Gary's Garagemahal (the Bullnose bible)

Starting at the left, you can see the R/LB is the start from the ignition, and it feeds the solenoid and has a splice and feeds the spark retard of the IM.

The large grey box that says "solid state" is the IM

You can see the red wire coming off the top of the grey box, and going up directly to the ignition switch. You can also see this feeds the resistor, going to the right from splice s401. That goes to the right and then goes down to feed the coil, but you can see also the coil is fed a direct 12v in the start position by the right most part of the ignition switch. So during cranking, the power to the coil + is straight down to the coil, and when the key is released to run, power is fed from the middle section of the switch, through the resistor, and then to the coil +.

 

UPDATE #2: we took IM#3 out and had it tested; it failed. At this point, I'm concluding: 1) the module was failing to give 12VDC when the key was in the start position, and; 2) that because we may or may not have a resistor in place, our IMs keep cooking. We have the Jeep at a mechanic's shop now (I'm ready to get out the dynamite) and will advise him what needs to be changed so we can move forward.

UPDATE: Planning to just run a new wire from the "I" terminal to the coil and will know once I test starting the engine with a temporary jumper wire from same. I really don't see any advantage continuing to dig through the harness for a bad connection, when we know what the starting circuit is supposed to do and the engine will run otherwise. I will plan to open up all of the conduits in the near future to reduce / eliminate future problems.

Thank you again, Dave. Yes, I can read basic electrical schematics and have seen other typical Ford wiring diagrams, but this one is very good also. Given that the vehicle has a painless-type aftermarket wiring system in it, I'll need to trace each wire and see where / if it's spliced, then try to figure out what the person wiring it up did in lieu of what's supposed to be in there; for example, I don't think there is a NSS circuit wired up to the "S" terminal wire, if I recall correctly. I'll also verify the exact route / splicing of the "I" terminal wire, as I used an ohmmeter to tell what was connected to it; it could be that the wire is direct to the coil or the fuse-box bus that branches to the ignition switch. I was hoping for something not involving tearing apart the wiring loom, as everything did work before it didn't. I will ohmmeter test the "S" terminal wire to see if / where else it connects, aside from the IM. I'll probably also test the new IM at Napa, just to make sure it isn't failing some kind of start-circuit logic test.