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.
Removing the Distributor from the Vehicle
- Unplug the battery wire, tachometer wire, and pick-up-to-ignition-coil three-wire harness from the distributor cap.
- Remove the plug wires. On Corvettes it is sometimes difficult to see which cylinder each plug wire feeds without removing the wire completely. It is a good idea to mark the wires, so you will know where they go on the distributor cap.
- Remove the distributor cap.
- Crank the engine slowly until the rotor blade aims at a fixed point on the engine or firewall. This will allow you to mark exactly where the rotor button is pointing, to use as a reference point when reinstalling the distributor. As a result, you’ll be certain that your timing is accurate, and the oil-pump drive is in the correct location.
- Before loosening the distributor hold-down, mark the exact position of the vacuum advance. Once the hold down is loosened, the distributor base can be rotated. Again, put a mark on the engine or firewall as a reference point, so you’re sure to reinstall the distributor the base in the exact same position. This will ensure that your base timing is close.
- Remove the distributor hold-down bolt and clamp.
- Remove the distributor from the vehicle. Notice how far the rotor button moves in a counterclockwise motion when pulling the distributor up. This will be important when reinstalling the distributor.
Distributor-Shaft End-Play Test
It seems that most HEI distributors have excessive shaft-end play. This unnecessary clearance on the thrust bearing allows the shaft and gear to move up and down during operation. This movement can cause variations in timing, due to the position of the distributor drive gear at any given rpm.
To calculate the timing changes, simply apply one degree of timing for every 0.013 inches of end play.
The correct clearance for most distributors is 0.010 inches of shaft-end play. This would keep any variation of timing due to end play to less than one degree. You can use a feeler gauge to measure the amount of play present (Image D). Shim kits to adjust this clearance are available at most local parts houses.
Disassembling the Distributor
The first step in disassembling and rebuilding any distributor is the removal of the drive-gear roll pin from the distributor shaft (Image E). This will allow the shaft to be removed from the distributor housing. Use a wooden block to support the end of the drive gear, and a thin punch to tap the roll pin out of the gear. Then, pull the gear and thrust washers off the shaft.
If the drive gear seems to be stuck on the shaft after the roll pin has been removed—or the shaft assembly won’t come out through the top of the distributor—there may be creosote buildup on the shaft itself. This gummy buildup may need to be removed using a chemical cleaner, such as carburetor cleaner, before the gear can be freed from the shaft.
Notice that the distributor drive gear has an index mark. This mark should line up with the point of the rotor button on reassembly (Image F).
Whether you’re installing a distributor-curve kit or just replacing the distributor springs and weights, take care when removing the springs and E-clips from the shaft. Sometimes these parts will fly off, never to be found again.
When reinstalling the distributor weights, the numbers on the weights will normally face up (Image G).
After removing the E-clips, carefully pry off the advance cam with a small screwdriver. You may need to work back and forth between the ends, prying up a little at a time. After the cam has been removed, the reluctor/stator may be slid off the shaft (Image H). The reluctor/stator should be replaced if any of its teeth are broken, or if the component is rusty.
To test the vacuum advance install a vacuum pump on the assembly and apply vacuum. Make sure the vacuum-advance pushrod moves smoothly throughout its entire operating range. If the pushrod does not move, or the vacuum-advance pod will not hold vacuum, the component should be replaced.
Now remove the vacuum advance; it’s attached with two screws. Pry up on the rear of the vacuum advance (where it attaches to the distributor housing) until it loosens, then pry the pushrod down with a pocket screwdriver (Image I). Rotate the vacuum-advance assembly clockwise to remove the attaching arm from the distributor plate.
The pick-up coil assembly is held on the distributor with a thin snap ring covered by a fiber washer. Carefully pry out the snap ring (Image J). The pick-up coil may then be lifted out of the distributor housing.
The ignition-control module (Image K) and capacitor are secured to the distributor with three screws. Inspect the wiring harness for wear or cuts.
Next month we will cover checking the centrifugal and vacuum advances, installing your distributor, all of the timing variables, EST distributor testing, and converting a points distributor to HEI.
If you get started on rebuilding your distributor before next month’s segment, remember to purchase OEM or equivalent parts. You get what you pay for, and cheap distributor parts are just that. Remember, you only want to do this job once.
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