Q:My 1978 Corvette is cutting out under acceleration. I took the car to the dealership, and they told me the distributor was defective and would need to be replaced. I would like to keep as many original parts on my car as possible, so I asked them if they could rebuild it. They told me that service was not performed anymore but offered to replace the distributor with a rebuilt unit. So…distributors can’t be rebuilt anymore, but they want to replace mine with a rebuilt one?
I checked at my local parts store, and most of the components for my distributor are available. I have a good knowledge of mechanics and am wondering if this is a DIY job I could accomplish at home. If so, could you give me some pointers and tell me how to test the individual components inside the distributor?
A:Let’s start by looking at the evolution of the GM High Energy Ignition (HEI) system. This system became available as an option in 1974 and was standard in all 1975-and-later vehicles, with the exception of the inline six-cylinder engine used through 1977, which utilized an external coil. Otherwise, the I6 ignition systems were virtually the same as the one that housed the coil in the distributor cap.
In these HEI ignition systems, the points were replaced with an electronic ignition module and a magnetic pick-up assembly. This eliminated the need for adjusting or replacing points at scheduled maintenance intervals.
The 1981-and-later HEI distributor employed Electronic Spark Timing (EST). The difference between this system and its predecessor is that all timing changes in the later units are controlled electronically using the Electronic Control Module (ECM). The ECM monitors information from various engine sensors, computes the spark timing required, and then signals the distributor to change the timing accordingly. With this system there was no need for vacuum or mechanical advance systems, since the ECM could now control timing using a computer.
You most certainly can rebuild your distributor and test most of the individual components. I will take you through a step-by-step process to help you get your Corvette back to smooth cruising. Let’s get started by checking out some of the parts.
Pick-Up Coil Test
If your vehicle cuts out on acceleration, look for splits in the white and green wires running from the pick-up coil to the ignition module. As timing is adjusted while driving, using the vacuum advance, the wires move, and over time they can become frayed. This is common, and you may well find it to be the cause of your problem.
The following test is best performed before removing the distributor:
- Inspect the white and green wires for wear.
- To test the pick-up coil, first disconnect the white-and-green connector that attaches the pick-up coil to the ignition module.
- Using your Digital Volt Ohm Meter (DVOM), connect the leads between the battery ground and either the white or green lead going to the pick-up coil. Any resistance measuring less than infinity indicates a defective pick-up coil (Image A).
- Now perform a pick-up-coil continuity test. This is done by connecting your ohmmeter between the white and green leads. Normal resistance is between 500 and 1,500 ohms (Image B).
Ignition-Control Module Test
Testing the ignition control module is a little trickier. When the module is exposed to a specific heat range—which can vary—it tends to fail. Therefore, the test may not be accurate. To make matters worse, testing for intermittent failures is also inconclusive, since it is difficult to reproduce the heat range that caused the initial failure.
The rule of thumb is, When in doubt, replace the module. It’s also a good idea to replace it any time you’re rebuilding your distributor, as these modules have a high failure rate. When doing so, be sure to thoroughly clean the module and its mounting location, and coat the surfaces with a silicone-based heat-sink compound.
A visual inspection of the coil should be performed to see if there is any evidence of carbon tracking, a burned contact, or a white, chalky film around the coil. If any of these are present, the coil should be replaced. If the distributor cap or rotor appears to be chalky or burned, they should also be replaced.
This test is best preformed before removing the distributor.
- To test the ignition coil, connect your ohmmeter between the TACH and BAT terminals at the distributor cap. The primary coil resistance should be less than one ohm (Image C).
- To check the coil’s secondary resistance, connect an ohmmeter between the carbon pick-up in the top of the distributor cap and the BAT terminal at the distributor cap. The resistance should be between 6,000 and 30,000 ohms.
- Replace the coil if the readings in Step 1 and 2 are not within specification.
These resistance checks will not disclose shorted coil windings. This condition can be detected using an oscilloscope or a suitable coil tester. If these instruments are unavailable, and the coil is in question, replace the coil during the distributor rebuild.