But they do have a weakness. It is pretty much pointless to upgrade coils, if you can’t heat it up some! A recent project where I basically just used the MegaSquirt as an ignition module replacement, drove this point home. Starting off with zero map adjustment and letting the distributor retain vacuum and mechanical advance control, we took a badly running Jeep and turned it into a perfectly behaved one. The only change was getting programmable dwell control to increase the spark output. Of course we then locked the dizzy out and went to town... But JUST increasing the spark output solved all the issues that lead to the upgrades!
Anyways, it got me thinking... A little idea stewing in some far back burner for a while, gets brought to the front. Opening up and repairing an ECU on the side of the road, is not my idea of fun. But swapping out an ignition module (even where mine is buried) is cake.
So the project is simple. To gut, and then modify, a Bosch 139 type ignition module. In essence turning it into a Bosch 124. Simple enough to just go get a 124, but I have several 139's in my piles, and handfuls of ignition drivers to choose from. The conversion alone, is easy. But why bother if I can't delve further into it? So I want to employ a couple other tricks I have used in the past.
1) In the past to protect the fragile VB921's, I have replaced R43 with a fuse. It is easy to run two jumpers out to a fuse holder right outside of the DB connector. Simple and elegant. Worked well too popping a pile of fuses in testing mode and not a single VB921! But this time I want to use this to get some other info, coil amperage usage. Using a Fuse Buddy in place of the fuse, I can quickly see what the coil is using.
So staring at the V3.0 schematics, this fuse would go on the right leg (pin 3) of the TO-220 package. This is also where R43 would be placed when the package is removed from the ECU. On a Modified 139 module, this would be attached to Pin 2 (of the module, not the driver) and then grounded.
Is pin 3, or the emitter, the best spot for a fuse intended to directly protect the IGBT driver?
Of course the power feed to the coil will also be protected with a breaker. I want slightly faster protection for the IGBT driver though... 2) I LOVE LED's! Like there may be a mental illness condition involved here... I buy them in bulk packages from China. If it turns on, or I might want to know its condition at a glance, it is indicated with a LED. I have LED's programmed to tell me when coolant and IAT temps exceed normal. Sequential shift lights, of course. And all auxiliary lighting, also has LED indicators. I was in heaven when the GLOW fuses came out! So of course I must use some here also!
Indicating power to the 139 is almost pointless as it no longer really “needs” to be powered up. But as it is already wired in and if it is seeing power, then the rest of the mission critical systems there are also online. I have had a couple moments of confusion where the breaker was tripped. So while useless for the module, not so useless as an indicator for the other systems. Simple that one.
But I would also love visual indicators of actual module function. Input to the “139” would be redundant (still I am sure I could justify it...) but the actual output would be useful.
Is this as simple as a LED from the power side over to the output (with like a 470 resistor on the power side)?
I do not want to interfere with the coil operation. 3)
I am unclear on using R43 to scope Dwell and coil saturation. I have some sitting around, so incorporating this into the module would be nice. But everything I find out there seems to skirt actually using this. I have spent some time looking too. 
It would be simple to send this info out via one of the unused module pins, if I knew exactly how to hook the scope up, and to where. Me and scopes have never gotten along to well... 
4) I grabbed a fuse tap holder to use for this. It takes one circuit and branches it into two. I choose this, as it will hold solidly after potting it in epoxy. The single holders would not have been physically secure when incorporated into the module.
I would love to make use of that second one, but really can't come up with a viable idea to do so. If I was fusing the power side, sure. But we will have it on the ground side of the circuit with it on the emitter. If I could use this to add some small protection to the scope leads and hook them both up directly, that would be golden.
5) Hopefully I have something appropriate in my piles of bits... But icing on the cake would be a fast way to change burnt out drivers. Specifically something that slides over the leads and lets me swap out the drivers without soldering. As this all needs to fit inside the 139 module packaging, space is at a premium.
I have a bunch of VB921's, GB14C40L's, and of course the awesome Bosch BIP373! I am convinced the weak spark is getting blown out under high boost. So as I am going through this trouble to get some testing flexibility in my hands, I might as well make it simple to destroy and replace a few of these drivers.
Well some of this may be answered with a simple test, to go build it and see what happens!
So I think I will give it a shot as I have enough bits to risk killing a couple in the process. But any input would be greatly appreciated!
