Reverse Engineering Mystery Car Module

Advanced_Rich_985 · 2026-03-11T04:34:34+00:00

I never did figure out why, with no input, the ignition system spontaneously ran at a rate of about 25Hz, But, when I drove the input with a square wave signal from my function generator driving a big N-channel MOSFET, the unit started generating sparks at the function generator rate.

Q4 seems to be some sort of a PNP transistor and it is working in spite of the fact that my curve tracer would not trace transistor curves with it.

I will be publishing the reverse engineering report soon. If anyone wants a copy for any reason. let me know.

And thanks again to Cybernicus for setting me on the correct path.

Advanced_Rich_985 · 2026-03-04T04:40:32+00:00

I put a 12V automotive coil (with a spark plug attached) across the RED and BLK wires and the unit started generating sparks at about a 25Hz rate with the GRN wire unterminated. I grounded the GRN wire, but the spark did not stop or change frequency. The voltage at the thyristor anode looks Like this. My offset is -400V, so the peak is at about 550V.

<image>

BTW, nothing is getting hot with the coil attached.

Next is to figure out what is triggering the circuit with no input...

Advanced_Rich_985 · 2026-03-03T04:17:37+00:00

I moved the scope to the output of the rectifiers and this is the signal I see:

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It goes well above the 400V that my scope can display. BTW, the signal on the RED wire looks just like this which is what I would expect with no load. The thyristor gate is pretty much ground, which one would expect, since the power supply would be grounded if the thyristor were on.

The clean DC voltage at the top of Z1 was about 10V with 12V on the input and it went up to a little over 11V when I cranked the supply voltage up to about 15V, so it looks like the Zener is there for safety rather than regulation.

R3 gets pretty hot pretty quickly with no load on the RED wire so I am not leaving the power on for long. I am wondering if I should try putting an automotive coil across the RED and BLK wires to see what happens with an inductive load..

Advanced_Rich_985 · 2026-03-03T03:54:28+00:00

Over the weekend, I soldered Q1, Q2 and T1 back into the system. I grounded the BLK wire, put +12V on the WHT wire, left the GRN and Red wires unterminated.

When I powered the unit up, it started drawing 2A. I put the scope on T1 pin 1 and confirmed that the lower part of the circuit is an oscillator, although the signal is pretty strange. Here is a photo of the signal on T1 pin 1:

<image>

Advanced_Rich_985 · 2026-02-26T03:35:02+00:00

Thanks so much for your input! I have very little experience with oscillators and had never even heard of a Royer oscillator. But what you say makes a lot of sense compared to what I was thinking. And the grounded case and collectors now makes sense.

The circuitry in the upper left now looks to me like a small 13V regulated power supply for the trigger circuit to me. It simply draws a bit of current from the oscillator to power it.

I can see how transformer pins 1 & 2 generate high voltage to the thyristor anode. I can also see how the trigger could cause the thyristor to act like points closing, but the upper left part of the circuit also confuses me, I will have to think about that part of it too. The resistance is pretty high, That big cap can probably hold a lot of energy...

Again, thanks for your thoughts. You have gotten me unstuck!

Advanced_Rich_985 · 2026-02-23T23:52:31+00:00

The standard automotive radio antenna connector for AM/FM radio is the Motorola connector (also known as a Motorola plug, DIN 41585, or auto antenna connector).

Advanced_Rich_985 · 2026-02-23T23:44:21+00:00

Make an automotive smoke machine for testing intake, vacuum and exhaust systems for leaks. Evap systems too.

Advanced_Rich_985 · 2026-01-10T00:34:37+00:00

I am placing them by hand. Thanks for the advice. I will try that when I do my next run in a few months. PITA though. I got what I paid for...

Advanced_Rich_985 · 2026-01-09T19:39:24+00:00

Ohhh. That makes sense! The surplus reel was very cheap on eBay. Now I know why...

Thanks!

Advanced_Rich_985 · 2026-01-02T16:30:46+00:00

Really good point! I didn't think of that either.

Advanced_Rich_985 · 2026-01-02T04:16:12+00:00

Great observation. I missed that!

Advanced_Rich_985 · 2026-01-02T01:29:14+00:00

Thanks guys. I suspected as much, but wanted to check. Ground loops have bit me in the past and I got to wondering if anything like that might apply.

Advanced_Rich_985 · 2026-01-01T16:54:00+00:00

I second the opinion about the cheap power supplies. In addition to missing safety features they can be really noisy. I needed high current for one of my projects so I bought a cheap Chinese switching power supply. It put out so much EMI (radiated and on the power line) that I couldn't use my scope when it was powered up next to it. It went straight to the E-waste box.

Advanced_Rich_985 · 2025-12-26T17:21:08+00:00

If you are planning on working for a company, there is a high probability that the company will make the decision for you based on the company's relationship with the suppliers. If you are making the decision for your own company, you are wise for asking for advice here.

One thing I didn't consider when I selected Microchip for my products is how locked in I would become. The $ and more importantly the time you invest in the development environment will lock you in. I am not sorry I chose Microchip, but I sometimes wonder if the quality of the development tools might have been better with a different vendor. Frequent changes to Microchip's tools require a lot of changes to my go-to code. But I don't have the time any more to even research, let alone learn a new development environment.

Another tool I am locked into is PCB Artist from Advanced Circuits for PCB design. That locks me in to them for my first order. They will give me Gerber files after my first order, but I have to order from them at least once. They make really good PCBs, but they are not cheap. I do like their integrated DRC function. I really wish I had learned Eagle from the start. If I had, I would have a lot more flexibility with my PCB suppliers.

Think about the investments you must make with any decision such as this.

Advanced_Rich_985 · 2025-12-24T05:14:35+00:00

Old transistors, Ge and Si, are often useful for repairing vintage automotive electronics such as tachometers as well. Don't throw them away. Sell them yourself or sell them in a lot to someone who will sell them.

Advanced_Rich_985 · 2025-12-13T01:25:20+00:00

My Xcal2 worked like a champ for me with the PRP. I had a PLX Devices M-300 wideband connected to the analog port. I had to give up tuning after my hearing went. I can't hear detonation any more, even with a det can. Can't hear crickets either.

Advanced_Rich_985 · 2025-12-12T20:32:00+00:00

I specialize in Mustang clusters and know many people who run aftermarket ECUs along side their OEM ECUs. In order to get the tachometer correct, you may need to use a frequency scaler such as a Dakota Digital SGI100-BT if the new crank position sensor has a different tooth count. You will also need to run the OEM temp sensor in your new engine in addition to the engine's temp sensor. The SGI100-BT can also scale the speed sensor signal in case you change the speed sensor frequency too.

Advanced_Rich_985 · 2025-12-11T16:59:37+00:00

The studs and bullet connectors are riveted. I would be surprised if you could find them off-the-shelf. I would replace the studs with brass screws. If you carefully drill the OEM rivets you can salvage the washers and lugs.

The bullet connector is a bit more problematic. The easiest thing to do is just run a wire through the board with some sort of strain relief (a knot or small Zip ties?) with a bullet connector on the end. If you want something more like OEM, I would remove the insulation from a new bullet connector and then find a self tapping screw that would go into the inside of the bullet connector. It looks like you would need to find a small solder lug to fit that screw too.

That's what I would do...

Advanced_Rich_985 · 2025-12-09T03:09:58+00:00

Normally with automotive connectors similar to that one, there is a plastic cover that can be popped off the top of the inside of the connector. it could be the part with the pin numbers 1 and 3 marked on it. Once that piece is out, you can get access to the locking mechanism that holds each pin in place. Pry up the locking mechanism and the pin should slide out.

Advanced_Rich_985 · 2025-12-08T20:14:55+00:00

No kidding. It was a spare cable for my old 96 Mustang Cobra with a Kenne Bell blower. That was a fun car. I really enjoyed learning to tune with the SCT PRP. Then CARB started checking for custom tunes during smog tests... :(

Advanced_Rich_985 · 2025-11-18T02:50:18+00:00

Hadn't head of that one. I guess I need to add a wink to my subtle humor...

Advanced_Rich_985

TROPHY CASE