Wednesday, July 23, 2014

The Optispark distributor, the best and worst idea GM ever had

Ever get that sinking feeling? 

Gearheads do, all the time.  For many it's almost like ESP.  We just know something's off even if we can't quite put or finger on it.  Maybe it's subtle.  The occasional misfiring ignition, a hard start condition or maybe the not so subtle hint that's something gone wrong when your car just stops.
Any of those symptoms can indicate a problem with the ignition system but when it's a car equipped with the GM Optispark optical distributor you're going to be questioning the "excellence" in the "GM mark of excellence"

What's an Optispark?

The Optispark is a unique ignition system found primarily on GM's venerated LT1, LT4 and L99 V8's from 1992 to 1997.  This was the second generation of the GM small block V8 motor and the first major design change since 1955.  Unlike its predecessors, the Optispark distributor is an optically triggered ignition system much like those seen in high performance and race vehicles. 

Optisparks showed up in everything from Corvettes to Cadlillac Fleetwoods and they all have the same design flaw.  That being, putting it in the absolute worst place you could possibly locate a component sensitive to heat, moisture and oil contamination.

Put yourself in the following scenario.  Imagine having a job where you were forced to work next to a blast furnace every day with the threat of a huge water pipe bursting over your head.

That's pretty much the life of an Optispark distributor.  Buried deep under belts, hoses, pulleys and a water pump, the unit can be barely seen and hard to service. Replacement is always a major undertaking involving the removal of the aforementioned bits and pieces as well as assorted brackets, wiring harnesses and other assemblies depending on the vehicle being serviced.

So just what's different about an Optispark?

Other than the look of it, not much really.  It's still a distributor that requires a rotor, Ignition module and coil to provide spark to the spark plugs at the right time.  The primary difference is that timing is determined via an optical signal instead of the position of a reluctor or cam follower.  Normal distributors usually have shaft with a rotor on one end and a gear on the other that meshes with its counterpart on the camshaft.

Where the only indication of incorrect timing on a regular distributor is a poorly running engine, the Optispark ignition provides feedback to the engine's PCM.  Using data from other sensors at its disposal, the PCM can then adjust the ignition timing to optimize engine operation.  

It utilizes an optical pickup assembly that reads a thin rotating disk with two rows of perforations punched into it.  One row is comprised of 360 slots on the outer ring with another row of 4 cutouts of differing size on the inner ring corresponding to 90 degrees of engine rotation.  The disk rides on a bearing assembly mounted into a base plate that engages with the timing assembly on the front of the engine.  As the engine rotates the optical pickup reads the position of the disc and reports that information back to the PCM where it can adjust the ignition timing according to current conditions. The major components of the distributor are fairly simple consisting of:
  • ·         A cap
  • ·         Rotor
  • ·         Plastic separator
  • ·         Metal separator
  • ·         Optical pick up assembly
  • ·         Timing wheel
  • ·         Wheel support disc
  • ·         Bearing assembly
  • ·         The man body or base plate

Aside from some sealing gaskets and the wiring harness there's not much more to it.

Depending on the year of the engine, the distributor engages the crankshaft timing gear with either a splined shaft or on later engines (95+) a large bearing resembling a hockey puck with cutouts to engage a pin on the engine's timing gear.  An issue with condensation and ozone buildup in earlier models of the Optispark (92-94) caused premature failure.  GM redesigned the distributor for later models in an attempt to eliminate the issue.  The easiest way to tell the difference between the two is the presence (or absence) or two rubber hoses connected to the distributor.  One is connected to vacuum while the other usually connects to the intake ducting of the engine.

The Optispark distributor allows for both fine and coarse tuning of the engine timing and has a certain amount of redundancy because of the presence of those two timing rings.  Should something short of catastrophic failure (which usually happens anyway) interfere with either timing signal, the engine can continue to run at reduced power levels.  It usually sets a code in the engine's PCM that will trigger the Service engine light. 

Most common codes are:

P0372 - Loss of the high resolution timing signal ( not reading any of the 360 slots) 
* 92 to 95 engines will set an OBD1 code of 36

P1371 - Loss of low resolution signal ( Not reading the 4 cutouts) 
* 92 to 95 engines will set an OBD1 code of 16

The design allows finer control of the timing by reading not just what cylinder is firing in relation to mechanical engine timing ( what valves are open, where the pistons are in their stroke etc.) but exactly what degree in rotation the engine is currently in.  In this way the Engine's control computer or PCM knows exactly where the timing sequence was in relation to a specific condition and how long it lasted.  For example, If a condition such as a misfiring plug were indicated, the PCM would be able to either correct the condition or set a trouble code.

Sounds great, so what can go wrong?

Oh wow, let me count the ways...

I remember when optical pickup distributors were cutting edge.  Using light to time an engine might as well have been rocket science to most shade tree mechanics. 

Usually, optically based ignition systems are among the most accurate and reliable ignition systems available.  Even modern so-called "Distributor less" ignitions use "flying magnet" or Hall effect sensors that operate similarly to optical triggered ignition systems but use magnetism instead of light to determine timing.


Unlike most ignition systems that are generally designed to keep the most sensitive components away from harm the Optispark stands in stark contrast.

It sits low on the front of the engine between the water pump and crankshaft pulley and sandwiched between a rats nest of cooling hoses, brackets and engine accessories.  It's at just the right height to get drowned by the undercarriage spray at the local car wash and sit's below a notoriously leaky water pump.  Lest we forget seals for both the distributor to timing cover and water pump shaft whose inevitable failure are sure to coat the Optispark in a greasy mess. 

There's usually some degree of warpage on most Optispark distributor caps which allows gaps in the seal between cap and distributor.  That makes it even easier for infiltration of water and oil.

Then there's the issue of varying quality of replacements.  I've yet to find any source for replacement of just the optical pickup assembly which is the most critical component of the Optispark.  That means failure of it requires replacement of the whole distributor.  With prices ranging from $50 for a unit guaranteed to leave you stranded to $900 for a GM aftermarket unit that may be no better you could go broke before you found a reliable source.  Nothing like doing the same job two or three times before you finally get one that works.
We've talked a lot about failure of the Distributor itself but don't forget about the wiring.  Mid 90's wiring harnesses tended toward the flimsy and after 2 decades they can get a bit crispy.  It's always good policy to check the wiring  before going to all the trouble and expense of replacing an Optispark only to find out a $20 Distributor harness would have fixed the problem. 

A good source for troubleshooting Optispark ignition issues can be found at  While focused on 93-97 Camaro and Firebird models the troubleshooting steps will work on LT1's installed on other models.  If you have one of these engines, this is a bookmark you'll use often.

So if you've made it this far you'd probably like to know how to fix your Optispark problems.  Aside from the link above we'll leave that topic for the next article.

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