Ignition Distributor & Distributor Belt Testing & Servicing
The 993 has two spark plugs per cylinder driven by a pair of Ignition Distributors and high voltage coils. All these components are over 25 years old and can be the source of a Check Engin Light or in the case of a broken distributor belt can result in engine damage.
One distributor is directly gear-driven off the engine the other is belt-driven off the gear-driven one. The belt becomes brittle with age and should be replaced every 60K to 80K miles.
If the belt is broken it can cause an out-of-time spark that can damage the engine. It doesn't always throw an OBD Code or a check engine light (CEL) as a warning. For this reason any time, there is a change in the smoothness of the engine's running, a misfire CEL, an apparent backfire or at least at the beginning of every driving season the belt should be checked.If the belt is found to be broken on your car and you intend to take it to the shop for repair unplug the left-hand distributor cap's center wire before driving the car to avoid engine damage from an out-of-time spark.
Testing the ignition system & distributor belt for spark:
- With the engine off and the key out of the ignition unplugging the directly attached distributor's center wire, and see if the engine runs. The directly attached distributor is the one on the right closer to the engine. If the engine starts and runs at all the belt is intact and the high voltage system related to the belt-driven distributor is working.
- With the engine off and the key out of the ignition reattach the center wire and pull the center wire on the belt-driven distributor on the left, furthest away from the engine, and see if the engine starts and runs on the directly driven distributor alone. If it does one knows this leg of the high voltage ignition system is also functioning.
If the car won't start with the center wire on the right-hand distributor detached the distributor belt is most likely broken. Verify this by removing the key from the ignition and removing the left-hand distributor cap and see if the rotor can be rotated with gentle hand force. If it moves at all the belt is broken.
The belt can break from becoming brittle with age or exposure to ozone trapped inside the distributor housing or some of the bearings in the distributor failing and putting too much force on the belt or both.
Some like to replace the belt in the distributor on a Do-It-Yourself basis. For me, the problem with this approach is a distributor on a car with over 80K miles on it probably needs some additional parts such as a specialized bushing, bearing, or washer. Identifying much less sourcing these parts once the distributor is out of the car and on the bench can present challenges, one being one's 993 is now inoperative until one can find the part(s) they need.
In any event, the distributor needs to be pulled and sent out to have the belt and any other failed mechanical components replaced. Proven sources of repair are listed on this page.
Removing the distributor for servicing:
Below is how to remove and reinstall the dual distributor preserving its correct rotational position and in doing so also preserving the engine ignition timing when sending one's distributor out for service.
Read the below instructions carefully in their entirety before attempting this procedure.
- Chock the wheels of the car, apply the hand brake, and put the transmission in neutral. The car should not be moved until the refreshed distributor is reinstalled. This is to avoid the unintended rotation of the engine between when the distributor is removed and when it is reinstalled.
- Remove the center coil distributor wires on each distributor cap and tie the caps with the spark plug wires attached clear of the work area.
- Remove the lower duct from the heater blower and the upper duct part that vents out of the engine compartment grill leaving the remaining duct that attaches to the blower in place.
- Take a strait-down picture of the directly attached distributor on the right (closest to the engine) with its cap removed and its rotor in place to document the rotor's rotational position.
Rotor Installed Position - Use care to make sure your camera's lens is exactly centered over the rotor so no off-axis picture distortion of the rotor's position is recorded in the picture. See the picture to the right.
- Remove the Distributor's hold-down nut that locks the distributor to the case at the base of the directly driven distributor.
- Then use a crowbar to pop the distributor loose without removing it. To do this the crowbar's claw is positioned
on the underside of the right-hand distributor body next to its center shaft and the crowbar's elbow positioned as a fulcrum on the engine case below the distributor. The picture to the right showing the crowbar position is with the distributor cap on. In the actual procedure, the distributor cap has previously been removed.Crowbar Position
in Red
- With the hold-down removed the distributor is only held in place by its O-Ring seal's resistance and should pop loose from the engine case with very little force. If you are using more than very little force, stop what you are doing as something is not right. Once the seal breaks loose do not remove the distributor just yet.
- Take a 2nd picture as above after slightly lifting the distributor just to the point where its drive gear inside the engine case disengages while not completely removing the distributor. As the distributor slides off its gear engagement the rotor's position will move slightly. With careful observation, one can see the rotor move slightly as they lift. If it stops moving while lifting the gear is fully disengaged.
Rotor disengaged
- When reinstalling the serviced distributor use:
- The 2nd picture taken to preposition the rotor before reinserting the serviced distributor into the engine case and;
- The 1st picture taken is used to validate the rotor's correct final position once the distributor is fully seated.
- By doing the above procedure one will avoid the top-dead-center alignment shenanigans by simply reinstalling the distributor back in the exact rotational position that it came out with.
- Use some hose wrapped in electrical tape to protect the dual distributor's exposed drive gear during shipment to the rebuilder. Also, package the entire assembly tightly in its shipping box as it has considerable mass and otherwise it may slide around in the box damaging it.
- Once removed the distributor can be sent out for servicing. It seems the availability of parts and other issues have made getting a properly executed servicing a challenge for some. One source of rebuilding that has proven to be reliable:
- Mark Cohen at markc123@bellsouth.net, phone:770.365.1601
An interesting anecdote from Mark Cohen who services these is he finds many times when he is asked to replace a belt or rebuild a distributor and once opened up it becomes clear that he is addressing a previously botched rebuild by others.
- Once serviced reinstalling is the reverse of the removal process using the earlier taken photographs to make sure the distributor's rotor is returned to the same rotational position as when it was removed.
The gear that engages with the engine at the bottom of the distributor shaft has rather widely spaced teeth. So when reinserting the distributor the incremental rotational position of the fully seated rotor is large as well. For this reason, it will be very clear if the proper position is achieved when the distributor is fully seated upon reinstallation. If it is inserted one tooth clockwise or counterclockwise in error it will be visually apparent and can be pulled out and reinserted again until the rotor position matches the pictures take before it was originally removed.
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| Distributor with its gear wrapped |
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#ignition #timing #distributor #dual #bosch
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