Saturation Time: Checking GM HEI module dwell.

Saturation Time: Checking GM HEI module dwell.

April 7, 2020 1 By Ray Bohacz

In 1975, General Motors, thru their Delco Electronics Division, completely rethought how an ignition coil was controlled.

This was accomplished with the advent of its High Energy Ignition (HEI) system. Subsequently, almost every modern electronic ignition system has roots back to that GM design.

Due to their quality, many farms still have an old Chevy/GMC gas engine around that is being used. It may be in a grain truck, on an irrigation pump, or powering a piece of equipment such as a haybine or welder.

If you want to keep those old engines in service, it is essential to understand how to check the module dwell.

The HEI difference

This electronic ignition did away with the breaker points, distributor cam, condenser and resistance wire that dropped the voltage to the coil.

Whereas a breaker point system coil was only fed full battery voltage on crank and ran on 6 to 7 volts, HEI employs full battery voltage to charge the coil.

This made for higher potential energy to be stored in the coil. GM took advantage of the electrical power by increasing the spark plug gap from the previously customary 0.035 inch to 0.060 to 0.080 inch.

The ability to work on full battery voltage was made possible by the ignition module and its circuitry that used transistors. HEI uses no ballast resistor or resistance wire.

In a GM HEI system, the module replaced the contacts of the breaker points, and the pick-up coil was in substitute for the rubbing block and distributor cam.

The ignition coil fires when the module shuts off the voltage to the coil primary. This is akin to the opening of the breakers.

Instead of having a fixed coil saturation time (dwell) as breaker points did (usually 30 degrees on a V-8), the hallmark of the HEI module was a patented expanding dwell period. From idle to 2,500 rpm, the dwell should expand from approximately 5 or 6 degrees to 30 degrees.

This is important since if the module fails in one area of the circuitry, the dwell will be fixed and not expand and contract with engine speed.

The engine will still start and run but will not run properly. Depending on where the dwell becomes fixed, the engine may stall, run rough, have no power or backfire.

The module is often not considered since it is falsely believed if there is a spark to the plugs, all is fine.

Simple to check

To confirm the expansion of the dwell, simply connect the green lead of an old tach/dwell meter to the TACH terminal on the HEI distributor cap. Start the engine and take a dwell reading at idle and then slowly raise the engine speed to around 2,500 rpm. The dwell should expand (increase) in a linear fashion until it reaches 30 degrees. If it does not, the module is faulty.

Perform the same check on a new module. Many cheap, offshore produced units expand the dwell but in a limited fashion, and the engine then does not run correctly.   

More than GM

Ok, you may be saying to yourself this is great, but on our farm, we have no old GM gas engines. Well, the time spent reading this primer is not lost. Most, if not all, electronic ignition systems have some DNA linked back to the original Delco design.

The modern ignition system on a myriad of engines found on the farm will share the use of some style of pick-up coil that acts like the breakers and rubbing block of a point system. It then works in conjunction with a module to control the charging and then collapsing of the field in the coil. This is used to multiply the input voltage to a level that can arc the electrodes of the spark plug under load.

The specifications may be different, but if the engine has an electronic ignition system and does not run correctly, do not fall into the trap thinking that the problem cannot be ignition related.

I bet you are now happy that you never threw away the old dwell meter!