Thursday, November 12, 2020

Replace Distributor Cap and Rotor on a '76 Corvette

I want my Corvette's engine to idle smoothly so I decided to inspect the distributor cap and rotor to see the condition they were in.

And it was a good idea since there was a lot of corrosion buildup on every contact, plus lots of carbon dust everywhere. It really looked like the 40-plus-year-old part it is.

I have no way of knowing if either the cap and rotor were replaced at some point, but they were overdue. And while they can be cleaned and reused (the car was running okay with them), the cost of new components is only $25 for ACDelco parts.

This is another DIY project that the average home-mechanic can accomplish in a few hours and save quite a bit of money.


REMOVING THE DISTRIBUTOR CAP

Start by removing the air cleaner assembly, followed by the chrome distributor shield if so equipped. The shield is secured by two thumb (or wing) screws, one on each side.


To make your life easier, mark each spark plug wire so they are reinstalled correctly when you install the new distributor cap. You can use clip-on wire markers, something I may add in the future, or do as I did here and use a piece of masking tape with the numbers on it.

As far as I know, the firing order for most Small Block Corvette engines (and Big Blocks, for that matter) is:

1, 8, 4, 3, 6, 5, 7, 2

This is their location on the distributor cap.

Next, unplug all wires and move them out of the way. You can then disconnect the ignition, tach, and battery connectors located in the protruding portion of the cover.

Above: Carefully unplug all electrical connectors by unhooking the lock tabs.

Above: The ignition connector has two locking tabs.

Above: The tach (brown) and battery (white) plugs have one locking tab each.

With the electrical connectors out of the way, you can proceed to loosen the distributor cap by releasing the four, spring-loaded, "J" hooks that secure the cap to the distributor base.

Above: Use a flathead screwdriver and turn counterclockwise as you press down
to release each one of the four "J" hooks.

Above: The photo above shows one of the "J" hooks after it has been released.

After all the "J" hooks are loosened, you can pull the distributor cap up and remove it.



DISTRIBUTOR CAP INSPECTION

At around $15, the cost of a new distributor cap is low, which makes replacing it an easy decision. And the same is true for the rotor.

You can visually inspect the cap for damage and corrosion on the contacts. Corrosion can be caused by a hairline crack on the case which allows water and/or condensation to form, bad grounding, or an overcharging alternator.



As the photos above show, my distributor cap was ready to be replaced. I have yet to determine what caused all the corrosion, and I will address that in the future if it continues to occur.


ROTOR REMOVAL AND INSPECTION

The rotor of the distributor is secured in place by two screws. They usually remain attached to it so just loosen them and lift the rotor off the base.




Like with the distributor cap, the rotor can be cleaned and reused. Having said that, at $10, a new rotor is inexpensive to replace and highly recommended.


DISTRIBUTOR BASE CLEANING

A combination of compressed air and an electronic's cleaner will properly refurbish the distributor base components which will extend their service life.

I removed the ignition module by loosening the two screws that secure it in place, followed by the capacitor and all associated wires. I wanted to clean and inspect all components to ensure optimal performance. 

Above: The ignition module is secured by two screws.

Above: Ignition module, capacitor, wiring harness, and related hardware.

Above: I took all connectors apart to properly clean the terminals.

Above: The distributor ignition wiring harness.


Above: I used a small wire brush to clean the terminals.

Above: All components clean and ready to be reinstalled.

Above: Reinstalling some of the components.

Above: I do not have a compressor, but this CRC spray cleaner and a rag worked great.

It is crucial to always double-check installation with the factory manual. In this case, the manual emphasizes the importance of applying a "liberal coating of special silicone grease" when installing the ignition module.


Above: Dielectric grease must be used when installing the ignition module.
I applied a nice coat to the contact surface of the module.

Above: And I applied dielectric grease to the base as recommended by Chevrolet.

Above: The base of the distributor ready for the new rotor.


Installing the New Cap and Rotor

I received my new ACDelco distributor cap and rotor and, after a quick visual comparison to make sure they were identical to the parts being replaced, I started the installation process.



The rotor can be installed right away, and that's what I did. All that's required is a Phillips screwdriver. Just remember where the old one was pointing and install the new one facing in the same direction. Having lots of photos of the disassembly process helps quite a bit.


Next, I prepped the distributor cap. Unless you bought a new coil, you will have to use the old one, and the same applies to the coil cover.



The two photos above show how the distributor contacts are supposed to look. Quite a difference from how the old cap looked.

The new ACDelco cap included a new insulating grommet and a carbon brush.


Place the carbon brush in the opening at the top of the cap, in the area where the coil will sit. The coil has a contact in the bottom that connects it to the brush.



Before installing the insulating grommet, I applied a generous amount of dielectric grease to both sides of the grommet. However, I did not grease the carbon brush hole.



With the grommet in place, I installed the coil. The new distributor cap had a new ground wire and terminal, so I used that one. Both ground wires must be installed correctly for the car to run properly.


With the coil in place and properly seated, I tightened the four screws that secure it to the cap and then installed the coil cover.


Before you drop the new cap in place, make sure all four "J" hooks are pointing outwards so they do not interfere with the installation. You want the cap to sit properly on the distributor base. You will have to push the spark plug wires and any other wiring out of the way. 

Take your time and make sure the cap sits flush on the base, and when it does, lock it in place with the four spring-loaded "J" hooks. You will need a flathead screwdriver for this.


Next, plug the ignition connector into the slot marked C- GRD B+, followed by the BAT connector (pink wire with a white connector), and the TACH connector (brown wire with a brown connector).


Then you are ready to reconnect the spark plug wires. A small dab of dielectric grease in the rubber portion of the caps is a good idea.

Your wires may look different than mine, but their location on the distributor has to match the 1,8, 4, 3, 6, 5, 7, 2 firing order unless you are using an HEI-corrected cap.





The photo above shows the distributor ready for a test start. Mine did not have a retainer ring and I don't really know if one is necessary or if it would fit under the chrome distributor cover with one. If yours has a retainer ring, reinstall it to keep the plug wires securely in place.


I did take my Vette out for a 30-minute shakedown run and I am very happy with how well the car is performing. It now idles very smoothly at 700 RPM.

Lastly, after some research, it appears the corrosion buildup in my cap was caused by ionization, which is what usually happens to battery terminals. I am still investigating what could have caused it to such a degree and will make the necessary repairs if it happens again.

I hope you find this article helpful if you decide to replace your Corvette's distributor cap and rotor.

Thank you for following my '76 Vette Blog!





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