GM Century/Lumina/Grand Prix/Intrigue 1997-2000

Diagnosis and Testing


Due the sophistication and complexity of the systems on these vehicles, the testing a non-professional can accomplish is limited. The factory recommends their special tools and test equipment for even the most basic testing. These tools, such as a Tech 1® or equivalent scan tool, take advantage of the On-Board Diagnostic capability built into the Powertrain Control Module (PCM), reading out Diagnostic Trouble Codes (DTCs) that will point the technician in the right direction. A scan tool also taps into the PCM's data stream, giving the technician a wealth of information on voltages, resistances, engine speed, temperatures and much more. Their data capturing capabilities can greatly assist in detecting difficult-to-find intermittent problems. With labor rates in most areas well over a dollar-per-minute, quick, efficient and accurate diagnosis is in everyone's best interest.

Early in the factory testing procedures is a requirement to check for factory-issued Technical Service Bulletins (TSBs), service newsletters and factory training material. This allows the factory to get up-to-date service and testing information to their technicians, much of it based on warranty claims which may show a trend, such as a weak or defective component that should be checked first. Such information and the factory testing tools are just not available to the non-professional.

One factory recommendation that makes particularly good sense is to Verify The Complaint. Is there really a problem or a misunderstanding on the part of the vehicle operator of how a system should operate- Another recommendation is to start with a thorough Visual Check. Some of the items to look for include:

For example, although wiring connectors have a good reputation for high reliability, on occasion, a connector could become loose, especially if that connector or component was recently serviced. Are all connectors properly mated, with no broken locks or damaged terminals-
Vacuum lines crack and fail with age and heat or even get disconnected in the course of other service work and so they should be checked carefully.
Was other major work recently done on the vehicle- Most W-Body vehicles will have at least four ground straps commonly used to provide an electrical connection to the following: body, frame, engine, transaxle. Always reinstall any disconnected ground straps and replace any broken ground straps to make sure that all electrical components have adequate ground paths to the battery. A forgotten ground strap disconnected during major service work could affect the operation of a number of systems.
Has the vehicle recently had major body repair- In some cases, components may not be restored to their pre-accident condition.
Is the fuel of acceptable quality and proper octane-
Has the vehicle seen regular service so that it can be reasonably assumed that the internal engine components are in good condition-
Are there any unusual sounds or odors- A sulfur-like smell at the tailpipe when the vehicle is running could indicate a fuel delivery problem.
Is the battery in good condition, properly connected with clean, tight terminals, and fully charged-
Are there any fuel, coolant or oil leaks-
Have non-factory components been installed- Over-size or under-size tires, for example, will affect the Vehicle Speed Sensor (VSS) which was calibrated for that particular vehicle and original tire size. Changing the tire size affects the VSS output, a major input to the vehicle's PCM. Some add-on electrical accessories can affect the vehicle's electrical system.
Are all of the fuses in good condition-
Are all of the relays in the underhood power center properly seated- Are there signs of corrosion or damage-
Is the air intake duct plugged or air filter dirty-
Are any wires or hoses misrouted too close to high voltage wires such as the spark plug leads-

All of these are things that should be checked on a regular basis and especially if there is a problem.


Except 3.5L Engine

One test that can be performed is checking for spark at the plugs. This is an old test, especially when checking on a Will Crank But Won't Start condition. The question has always been, 'Is the trouble in the fuel system or in the ignition system-' If there is a healthy spark at the spark plug, it can be assumed (but not guaranteed) that the problem is in the fuel system. The new twist is that with the high power ignition systems in use today, great care must be used. An older electrical system, if allowed to, might give you a shock as a reward for inattentive work. Today's electrical systems with 40,000 volts or more available to the spark plugs, could cause cardiac arrest, if mishandled.

  1. Locate and remove the fuel injection fuse from the underhood electrical center. This will keep the fuel system from spraying fuel into the engine when cranking the engine over with the starter.
  3. Install a spark plug tester. GM recommends their tester J 26792. This is similar to a spark plug with a wide gap and an alligator clip fastened to its metal shell. Similar spark plug testers are available at most auto supply stores. Connect the alligator clip to a good engine ground. Disconnect one spark plug wire and plug it onto the tester.
  5. Crank the engine while observing the spark tester. A crisp, blue spark should be observed.
  7. If a healthy spark appears, reconnect the spark plug and install the fuel injection system fuse. It can be assumed the ignition system is in good condition.