GM S-Series Pick-ups and SUV's 1994-1999 Repair Guide

Diagnosis and Testing


The symptoms of a defective component within the DI system are exactly the same as those you would encounter in a conventional or HEI system. Some of these symptoms are:

Hard or no Starting
Rough Idle
Fuel Poor Economy
Engine misses under load or while accelerating

If you suspect a problem in the ignition system, there are certain preliminary checks which you should carry out before you begin to check the electronic portions of the system. First, it is extremely important to `make sure the vehicle battery is in a good state of charge. A defective or poorly charged battery will cause the various components of the ignition system to read incorrectly when they are being tested. Second, make sure all wiring connections are clean and tight, not only at the battery, but also at the distributor cap, ignition coil, and at the electronic control module.

  1. Check the cap for tiny holes and carbon tracks as follows.
    1. Remove the cap and place an ohmmeter lead on the cap terminal.
    3. Use the other lead to probe all the other terminals and the center carbon ball.

  3. If the readings are not infinite, the cap must be replaced.


It is imperative to check the secondary ignition circuit first. If the secondary circuit checks out properly, then the engine condition is probably not the fault of the ignition system. To check the secondary ignition system, perform a simple spark test.

  1. Remove one of the plug wires and insert some sort of extension in the plug socket. An old spark plug with the ground electrode removed makes a good extension.
  3. Hold the wire and extension about 1 / 4 in. (0.25mm) away from the block and crank the engine.
  5. If a normal spark occurs, then the problem is most likely not in the ignition system. Check for fuel system problems, or fouled spark plugs.
  7. If, however, there is no spark or a weak spark, then test the ignition coil and the camshaft and crankshaft position sensors. For testing the camshaft and crankshaft position sensors, refer to Engine Controls .