Changing To A Non-ESC Distributor

Written by: Mike Ervin

On my 85 C10 that came with a 305 (LE9) engine, it had what is called a Electronic Spark Control (ESC) distributor.  I would like to state that my truck is not computer controlled.  If your truck has an ECM, I would not advise doing this because you will need to change everything.  This is the only thing that is controlled electronically on mine.  I decided to change this over to just a regular HEI without the ESC module and knock sensor since I had already done just about everything else.  Below is a brief description of the ESC system.

Note!  This will not affect emissions.  If you are like me and have to pass emission inspections, changing the distributor will not keep you from passing.

Electronic Spark Control
An additional electronic control (ESC) is used on vehicles equipped with an LE9 engine.  The Electronic Spark Control (ESC) system is a closed loop system that controls engine detonation by modifying the spark advance when detonation occurs.  The amount of retard is a function of the degree of detonation.

The (ESC) controller processes the sensor signal and applies it to the distributor to adjust spark timing.  The process is continuous so that the presence of detonation is monitored and controlled.  The controller is a hard wired signal processor/amplifier which operates from 6 to 16 volts.  The controller has no memory storage.

The (ESC) sensor is a piezoelectric device, mounted in the engine block that detects the presence (or absence) and intensity of detonation by the vibration characteristics of the engine.  The output is an electrical signal that goes to the controller.  A sensor failure would allow no retard.

ESC Vacuum Switch
On LE9 engine equipped vehicles with automatic transmission a "tip in" vacuum switch is used.  Its function is to provide a momentary contact closure (signal) to the ESC controller during a throttle "tip in" condition which then briefly retards spark timing to minimize knock.  The switch contacts are normally open under steady engine vacuum conditions including no vacuum, and all brief increasing vacuum conditions.  Basically the switch closes only during rapidly decreasing vacuum conditions such as that encountered on rapid throttle operation.

Replacing the ESC distributor
My distributor had a lot of miles on it and in turn had more advance than it should have.  I thought about rebuilding it but I was going to have just about as much in it as I would a new one.  Then I started thinking, why not get rid of all of the above mess in the process.

It not hard to change it over to a regular HEI distributor without the ESC system.  Besides, I didn't want it retarding my timing without me knowing about it.  I bought a MSD Pro-Billet HEI distributor part number 8365 from Doug Herbert Performance Parts. Of course you can use a distributor from 74-79 cars and trucks if you can find a good used one.  The first thing you have to do is remove the distributor.  Once it is out the only thing you need is the hot wire (large pink) that plugged into the distributor cap at the BAT terminal.  If you are neat freak like me, you will want to remove all the wiring and related parts that went with the ESC. 

Old Parts Removal

First remove the glove compartment door.  Above the heater box you will see the ESC controller, which is held in place with four bolts.  Unplug the controller, and remove the four bolts.  Then you can take the controller out.  What I did was cut the controller wiring about a foot from this plug so I could pull it out of the firewall.  In the engine bay pull the wires out, you can see were they all go to.  One should go to the back of the head for a ground, two should go up to the Vacuum Switch (if your truck has the ESC Vacuum switch) mounted on the firewall.  The other four go to the plug that goes to the distributor.  Unplug the switch wiring and the vacuum hose that goes to it.  You can remove the hose from the carb and cap the fitting at the port.  Now you can remove the switch.  Remove the blue wire that goes to the Sensor (knock sensor) mounted in the block in the front of the starter.  If you want you can remove the sensor and replace it with a 1/4" pipe plug.  Just be careful when you take it out.  I have heard about some people who have twisted the sensor off in the block.  I haven't had this happen to me, but it is possible it could happen.  It would be a real pain to get out.  With the metal being soft, you won't be able to get a good bite with an ease-out.  You will need to drain the radiator, and because the knock sensor goes into a water passage in the block, be prepared to catch the coolant.  Don't forget to wrap some teflon tape around the threads of the pipe plug before you install it in place of the sensor.  Also, where the wires went through the firewall to the controller, I made a round metal plate to cover the hole after I removed the grommet.  That's all there is to removing everything.  Now it's just a matter of installing the new distributor and plugging the large pink wire (hot wire) into the BAT terminal of the distributor cap.


Here is a way to remove the ESC from the distributor without having to change distributors.  I recently found this quick fix on the 73-87 truck forum at Chevytalk.  It was posted by a member that goes by the name of 86Scottsdale.

I had a problem with my 86 C20 305-H engine. It could start and run fine and just suddenly die out, sometimes start right back up or take a couple hours to get restarted.  After several weeks of this nonsense (and getting very good at getting those half turn dist. cap latches off), I did some checking.  There is a power feed to the spark control unit under the dash that comes off of the pink (hot) wire to the distributor.  The pink wire clips into the connector for the feed, which in turn clips into the dist. cap.  What had happened was the feed for the Spark Control was shorting out somewhere, killing the spark.  I talked to some Chevy mechanics at my dealer about this and they told me how to bypass the spark control computer and keep the original distributor, something they claim is done quite often.

First thing to do is unhook the spark control computer lead from the pink wire and hook the pink wire directly to the dist. cap.  Make sure you plug it into the cap at the BAT terminal.  The second thing is to locate the 4 wire connector that feeds into the ignition module.  There's a brown (D), green (C), white (B), and black (A) wire there.  The letters in () are on the black clip that the wires come out of.  You cut the green (C) and black (A) wires between this clip and where they go into the distributor.  Then all you need to do is solder the 2 cut leads from the dist/ignition module together, i.e. the green and black ones, (make sure to tape the exposed wires) and now your HEI distributor will function just as a pre ESC model does.  I did this and it has been running fine for well over a year now.


Note:  Here is a photo I made of the plug which goes to the ESC control and feeds the ignition, which Dan mentioned above.  I labeled the wires which need to be cut and showed which wires are A, B, C and D.

That's it.  Of course you will need to adjust the timing when you get it running.  This mod converts your truck back to a regular HEI ignition, without all the electronic controls.  You will have to monitor your spark knock yourself since you won't have the ESC to do it for you.  If you have ever had an older vehicle you know how to do this anyway, why do you need the ESC to do it for you?