Why Oh Why HEI

HEI is probably the oldest and best-known GM electronic ignition system. Short for High Energy Ignition, HEI has been around for a couple decades now. Sad thing is it still has the same problems today it did 20 years ago.

Hey Sparky, Over here.

First some basic ignition theory... This is by no means a complete description but should help you understand how HEI works or any system for that matter. All ignition systems work pretty much the same way whether they are wholly mechanical like an old point system or electronic like DIS2.

In very short, something provides a switch with synchronization to the cylinders. This switch closes and opens to charge and cut voltage flowing to the coil.

The actual plug firing voltage is generated by the collapse of a magnetic field around the coil. When the previously mentioned switch is closed, current passes thru the coil Primary winding and builds a strong magnetic field. Later, when the switch opens, the field collapses almost instantly and generates a huge voltage spike in the coil. A key fact is that the field collapses far faster than it builds up. (This same factor is why many relays and the AC clutch have spike protection in them. If they didn't you'd have all sorts of things dieing in the car regularly from the spikes. AC clutches are well known for blowing things when the protection diode fails.)

The mechanics are much more involved but that's what it really boils down to.

Programs! Get You Programs Here...

You can't tell the players without a program.

Reluctor

The Reluctor, which the Pickup Coil is part of, is essentially a very small AC generator driven by the distributor shaft that provides a base timing signal to the Ignition Module and ECM/PCM. Each time the magnet and pole pieces pass each other, a voltage spike is sent to the module.

Ignition Module

This has three functions.

It takes the pulses from the Reluctor and cleans them into usable information for itself and the ECM.
In EFI systems, the Ignition Module controls ignition timing directly during cranking and early startup using the base timing that you set by physically turning the distributor. After starting, timing control is passed off to the ECM, which is why the ECM must receive a clean reference signal from the module. The ECM reads the base timing signal and advances or retards final timing based on tables stored within it. (Carbureted systems use a more mechanical HEI that has Vacuum and Weight timing advance systems just like point distributors.)
The coil driver charges and fires the ignition coil. It is important to remember that the module doesn't tell the coil when to fire.... It actually fires the coil. There is a "Power" transistor in the module functioning as a set of points would in a mechanical dist. This transistor is why the module needs good cooling and ground on the dist. If either is lacking you'll pop modules like Kettle Corn.

The coil

The coil is just that... a couple coils of wire often wound in/on some kind of metal core or frame. There is a Primary coil, connected between the module and battery voltage, and a Secondary coil, ultimately connected to the spark plugs. HEI coils are typically dry types encapsulated in plastic.

When the module's driver transistor is on, considerable current flows thru the primary side of the coil. The transistor can handle much more power than points so HEI can deliver more spark than point systems right out of the box so to speak.

The current flow doesn't fire the coil! Instead it builds a strong magnetic field in and around the coil. When the transistor cuts off, the current stops, the magnetic field collapses and that is what actually generates the plug firing voltage. You'll often see a specification for coils that indicates hundreds of volts in the primary winding. That voltage is generated when the coil field collapses.

Cap and Rotor

Many people think the rotor and distributor cap determine timing, they don't. These parts are really just a selector switch that routes secondary voltages to the correct sparkplug. Actual timing of the secondary pulse is always based on something more precise like a cam or reluctor that reads the distributor shaft.

Why does HEI have so many problems?

First off... It is still a distributor-based system. It still has many of the same problems that have always been common in distributors.

Sloppy distributor shafts are all too common and will mess with the whole system. Mechanical HEI used on most carbureted engines is really only slightly more advanced than a point system since this still uses mechanical means to control timing. EFI systems are less sensitive to a sloppy shaft but it is still a problem.

Normal Wear is trouble. Burning the terminals of the cap and the tip of the rotor eventually opens the gap between them. That increases the load on the coil while reducing power to your plugs. Bad plugs or plug wires can cause similar problems. Anything that increases coil load can be bad for the coil and the module.

A weak cap or rotor is also trouble, and not always very obvious. The cap and rotor blade may pass power to the plugs but weak insulation or even simple dirt on these parts can bleed high voltage to the rest of the distributor. This can fry off both the pickup coil and module. In some cases it could even damage the ECM.

Tach Filter

Step 1... Unplug the Tach Filter any time you work on Ignition problems. The Filter can cause a short circuit and kill the system.

See PFF and use search. Many problem/solution in the forum.

Mounting Hardware & Grounds

Mounting hardware is important beside holds the module on distributor, it's also a ground. The Module must have two grounds to work correctly. Not all grounds are the same! Signal Ground for module connects ECM and Power Ground, via mounting hardware, for module connects itself and coil to give you spark voltage. Bad power ground, mounting screws too loose, and signal ground can't handle it and no/weak spark. Either ground is bad, can cause Code 42. More Grounding issues...

Mounting hardware must be clean and bright. Use dielectric on screws but be careful, mounting screws will strip if they are too tight.

Coils

The Coil needs good ground! Make sure the Coil's Frame is grounded or it will mess with the Ignition Module and can cause Code 42.

Bad coils can overload the module and fry it long before the coil completely dies. It may never die completely. If you are blowing modules frequently, you should probably also replace the coil. There isn't any easy way to home or even shop test the coil for problems. Even if the coil wiring seems OK thru an Ohm test, you could have weak insulation causing shorts at high voltage when the thing fires.

Aftermarket coils can be trouble in any car but especially with an ignition that is already heat stressed like many HEI systems are. Any coil that increases load on the module will make the module run even hotter than normal.

Aftermarket coils tinker with one or both coil windings as well as the rest of the coil in an effort to enhance the secondary voltage. There are several things you can change about the coil that affect secondary voltage such as change the number or gauge of windings, the design of the frame/core, etc. If any of these changes increase the load on the module, the module will run hotter and probably not last long.

Coil On Cap

This is usually used only on V8 engines and presents a unique set of problems. One being that many COC setups are missing parts. There is a metal strip/tab that connects between the frame of the coil and the coil electrical connector. The strip provides a ground terminal and unless you want weird problems that can even kill the system, it better be there.

Another is the rotor terminal from the coil... If you use stock rotor buttons with aftermarket coils, they often won't last very long. You can get replacement buttons for this use.

The last one is that you are putting a pretty good amount of mass, the coil, on a plastic part, the cap. This has been known to crack caps over time.

Heat

Keep in mind that the HEI coil primary winding draws 6-7 Amps. That is quite a bit of power, about the same as the AC clutch and the various electric fans in most cars. That much power is going to heat the coil and the module's driver transistor quite a bit.

As noted above, heat is a problem for HEI, Ford TFI and most other electronic systems because of the driver transistor in the Ignition Module. The problem is the heatsink for the HEI module, the distributor, is also being heat soaked from normal operation of the engine. Any exhaust leaks, missing heat shields or engine cooling problems can make the distributor even hotter, frying the ignition module.

Some people have remote mounted HEI modules. While it can solve the heat problem, if you mess it up you'll have other problems from poor electrical connections and electrical noise. I'm not going to even try to explain how to do this since there are several kinds of modules used with HEI. (Ford actually did move the TFI module off the distributor in many models but it can still suffer heat problems.)

Heat will also affect the coil. It affects the coil directly by increasing resistance in the wire. This in turn affects load on the ignition module and spark voltage. Depending on the car... Simply moving the coil away from the engine block can help allot. A simply shield to reflect heat away from it will also help. Don't try to insulate it... The coil generates its own heat that it has to be able to unload.

Testing

Note: Disconnect the Tach Filter anytime you are working on HEI system. A bad filter = a short, often an intermittent short, circuit on the Coil. You do not need the filter to drive etc. Only thing dead is the Tach with out the filter.

Coil testing without special equipment is pretty useless.

Like DIS coils HEI coils have very low primary coil resistance, typically under 1 Ohm. This means testing with a common Ohmmeter is unlikely to show anything but the grossest problems like a complete open or high resistance. They won't be able to show a primary short.

Secondary resistance is normally in the thousands of Ohms but again an Ohmmeter will show nothing but the worst problems. You won't be able to see things like some shorted windings or weak insulation that is leaking when the coil fires. Insulation testing requires a device called a Megger that is rather expensive and not common outside the manufacture of high voltage parts.

Modules on the other hand can be tested at many parts stores. The thing to watch out for here is that the test machine won't heat the module like normal operation. Modules that pass on the tester often fail once they get hot but it's better than no test at all and can still help you isolate a problem.

Pickup coils be tested on the car but you have the same sort of issue as the modules. They can still flip out when they are hot. These things will show 1-2 AC volts during cranking.

Aftermarket Ignition Boxes

Products such as MSD change things in the system considerably. One of the selling points of Capacitive Discharge systems is that they generate multiple sparks. What they don't tell you is that the system must generate multiple sparks to operate the engine properly. The problem is the CD ignition doesn't have enough spark duration (dwell) without the multiple sparks under many sets of running conditions. There is more to firing the cylinder than simple spark voltage. The spark has to last a certain amount of time as well.

Are these products bad? No but don't expect miracles from them either. They will likely help people with boosted or high compression engines much more than stock engines.

One advantage of these systems is that the added control box unloads the power transistor in the ignition module. The aftermarket boxes typically use their entire huge case for heat sinking the driver transistors in them.

When you mount these things, make sure they will get some air. They need it to stay cool. An aftermarket control box can die just as easily as the main ignition module if it can't get enough cooling.