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Functional Schematic

Troubleshooting Automotive Ignition Systems – Part 2

The intermittent stalling problem was solved by replacing the distributor. This is the sequel to troubleshooting my ‘86 Dodge Aries that defied logic and baffled my mechanic. To be sure, electronic engine controls like this are difficult to troubleshoot.

Typical Schematic

The long awaited Haynes repair manual arrived… and yes, it had nice schematic/wiring diagrams in the back. However upon checking my system against the connector pin-outs and wire colors, my enthusiasm quickly abated. Then I found the notation “Typical fuel injection system wiring diagram.” So Haynes pulled a fast one by publishing only one of many thus leaving me in the dark. Apparently no schematics are published web either. Regardless, I still recommend that anyone who maintains his own vehicle obtain a repair manual.

Haynes Repair Manual

Tracing out the circuit

The next best approach was to trace out the ignition portion of the module interconnects. To do so, I identified many wires (ground, +12V etc). To locate the wires in question, I did the wire-wrap wire break-out trick: this involved stripping back the insulation on pieces of #30AWG wire-wrap wire and inserting them into the female side of the module connectors before inserting the male side. Since these automotive connectors have such large contacts, the fine wire-wrap leads fit with ease. This I did on both the distributor and ECM connectors. I then documented what I learned in an abbreviated functional schematic. One confusion factor is that the ECC and ECM are often confused with each other – the terms often interchanged.

Functional schematic diagram

Functional Schematic

How it works

The distributor is connected directly to the computer that is located behind the right kick panel. It is supplied approximately 9V by the computer and outputs a 5V square wave signal to the ECC when the engine is running. The interface type is open collector that is standard for connecting peripherals of this type. As soon as pulses from the distributor cease (1sec), the ECC drops the Auto-Shutdown Relay via turning off the open collector driver – this both kills the ignition without generating a misfire, and turns off the fuel pump, thus increasing fire prevention safety.

Identifying the problem

In order to run down the location of the problem, I connected an LED to the Auto-Shutdown Relay driver break-out, and also connected an analog DVM to the distributor signal break-out. At length, I was able to identify the problem during a road test – as observed on the analog multimeter, the output of the distributor ceased to pulse, with its output remaining high (+5V). I replaced the distributor and it has not stalled since. Note that I could have replaced simply the hall sensor device for half the price, but the distributor already had other mechanical issues.

The usual, brain-dead repair method is to replace one module at a time – If I had pursued this approach, the distributor would have been dead last and I would have spent an additional $150.

Post mortem thoughts

I believe that the failure mode is simply a weakened magnet in the hall sensor assembly. How does such repair itself temporarily? My guess is that during operation with alternating flux paths, the magnet tends to weaken – when the motion stops, the magnet tends to realign itself and strengthen sufficiently to resume operation. I cannot believe that this is an unusual problem, but I was unable to find any information on the web.

Retiming the ignition

When retiming the ignition, my timing light failed so I timed it by ‘feel.’ To do this, the distributor is rotated until it is so far advanced that there is pre-ignition (ping), and then backed it off until there was no obvious ping – it takes a little trial and error to get it right – sometimes engines do not respond well to this method, so in that case a timing light is necessary. Fortunately there is no emission test requirement for such an old vehicle – it might not pass.

Conclusion

The conclusion is that I now know more about my particular ignition system than most experienced mechanics. As a result, I am not afraid to tackle other problems, should they occur. The bottom line is that the engine now has about 20% more power and 20% better fuel economy – not bad for an already quite efficient engine. I again have a reliable vehicle having logged over 1500 trouble-free miles.

Photos

Undocumented words and idioms for our ESL friends

pull a fast one –idiom –trick or short cut, sleight of hand, sucker punch

One Comments

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  • krokkenoster

    I did find this way of setting the timing sometimes more often than not to be the best for a particular engine. The timing setting that the manufacturers give, I use AS THE STARTING POINT when working on engines. In the ’70;s we had the Ford Essex 1600 crossflow engine fitted to road sweepers and the like as well as in the Cortina 1600. On the Cortina I discovered that I had also more power than the factory settings Here I discovered that if I drive the car at 40 Km/h in top gear then I would “Floor” the excellerator and it must pink but still be able to pick up speed then that car would go like the proverbial bomb!

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