Depotting An Ancient Car Computer

Carburettors were king for decades, until the onward march of technology brought electronic fuel injection to the fore. During their final years, a handful of automakers experimented with computer control of the humble carb, trying to squeeze out every last bit of efficiency and reduce pollution as much as possible. [NeXT] happened to own a vehicle fitted with AMC’s Computerized Engine Control system, and decided to see what made it tick.

This was easier said than done due to choices made by Ford, who manufactured the engine computer for AMC. Unlike modern ECUs which usually feature a metal case fitted with rubber gaskets, the CEC computer was potted in epoxy. [NeXT] was able to de-pot the circuit board by placing it in a stock pot of boiling water, and then slowly peeling the epoxy away.

With the potting removed, it was possible to begin reverse engineering the board. The main microcontroller is an Intel 8049, of the MCS-48 family. The board uses through-hole technology, and only features a handful of other small ICs.

It’s always interesting to look back at forgotten technologies and see how things were done in decades past. [NeXT] hopes to keep working on the project, intending to dump the ROM from the CEC module and build a replacement computer with an Arduino. It’s possible to build your own ECU from scratch, so we’re looking forward to seeing [NeXT]’s AMC Eagle running on modern silicon real soon.

25 thoughts on “Depotting An Ancient Car Computer

  1. When I was MUCH younger, I jump started a 1956 Mercury (Step Fathers) with a piece of gum wrapper. (Mostly aluminum)
    No such thing a a computer.
    I miss the “Good Ol’ Days”.

        1. Absolutely. I just wanted to mention that “jump starting” means “starting one car with the aid of jumper cables and another car’s battery”. And that would call for A LOT of bubblegum wrappers.
          Bubblegum wrappers saved my life lots of times, though. Mostly used to “repair” blown fuses.

  2. These systems were pretty simple. Basically they read an oxygen sensor and would vary the duty cycle of a single solenoid that modulated a trim circuit in the carb. For several years Holley even offered a similar add-on for their carbs. If I remember correctly one version had an oxygen sensor like the OEM solutions, and another was manually controlled and targeted hot rodders and drag racers by eliminating the need for manual jet changes – just twist a knob!.

  3. When I peeked inside my fuel-injected 1973 VW Type 3’s “computer box”, all I saw was passive, power transistors, and NE555’s which were brand new back then.

    1. A ECU in essence just does math based on its inputs which outputs signals to the ignition coil(s) and injector(s)
      So I’d actually believe that it could be controlled by logic components.
      But doesn’t those have a old fashioned distributor and only a monopoint injector?

      1. BOSCH KE-Jetronic had two ECU versions – completely analog and digital 8051 (IIRC) based. Both versions took engine temp, oxygen sensor voltage and in some conditions air volume signal and calculated fuel pressure regulator current. I have Audi100 C3 with PH engine with analog ECU variant. Still runs without any problems.

          1. When it fails or if you disconnect it you dramtically loose power. Jetronic ECUs just assisted fuel distributor (fluidic computer) to prepare optimal air-fuel mixture. So, without ECU you will get subobtimal mixture, but still will be able to drive home. Modern ECU’s also have some kind of emergency mode, it turned on when ECU detects some sensor failure f.e. However if modern ECU itself fails, you have no options to drive home. :)

          2. You can drive home, but at least in the version VW used, unplugging the electronics would leave the frequency valve at one extreme of mixture, where normal mixture would be around a 50% duty cycle.

          3. The “limp home” function of the ECU in my ’94 Suzuki Sidekick (Rest in Pieces), would kick in when a fuel injector died. MPG would drop from ~26 to ~12. The fail was triggered by the O2 sensor in the exhaust pipe.

            The exhaust smelled like raw gasoline, so once I held a Bic lighter about a foot from the tailpipe while it was idling.
            The resulting flame was about a foot wide and 2-3 feet long!

          4. I own a bmw e46. Never had an issue with ECU but if somehow a sensor fails, the car lose so much power. It sends rich fuel air mixture in order to protect the engine, which is quite unpleasent feeling. However it is a life saver, you can still drive quite long distances.

  4. Here in Germany in the late 80s they sold catalytic converters for retrofitting in carburetted cars. The lambda-regulation consisted of a oxygen-sensor-controlled bypass-valve in the vacuum-line to the brake booster and a slightly richer carb tune.

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