What tells the computer when to fire on a TBI w/no crankshaft sensor


Asked by Feb 20, 2014 at 10:54 AM about the GMC Sierra 1500

Question type: Maintenance & Repair

Distributor has no points. Timing light shows the firing is all over the place,It looks like a
two inch band from about zero to 8 deg. plus BTDC. It's a 1994. It's the model before you
could use the hand held diagnostic computer.

3 Answers


K here is the rundown. In the distributor there is a pick-up coil under the plate on the shaft that has two wires and is plugged into the ICM (Ignition Control Module). The ICM is plugged into the coil and back to the ECM. The pick-up coil detects the engine rotation. The ICM takes this signal and uses it to control the spark -> it also sends signal back to the ECM as the RPM input. The computer uses the data for fueling. The ECM sends the spark advance/retard signal back to the ICM for proper spark under the current engine condition. On a side note the ECM will note spray fuel untill it sees ~400 rpm!


You can scan codes and live sensor data from this truck and most other GM OBD1 vehicles with the right equipment. Of course shops and GM scanners can but with a 70 dollar cable and free software anybody can with a laptop. Look-up ALDL cable.

1 out of 1 people think this is helpful.

I would change the ICM first. #5 on my '92. Make sure you use the supplied heat sink compound between the module and the baseplate or get some to use. Or it will cook quickly, yes one of the 5.

2 out of 2 people think this is helpful.

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