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Checking GM’s High-Energy Ignition Module Dwell

This primer will help you check the dwell on all engine types.

Way back in 1975, General Motors, through its Delco Electronics Division, rethought how an ignition coil was controlled with the advent of its high-energy ignition (HEI) system. Subsequently, almost every modern electronic ignition system has roots traced to that GM design.

Due to their quality, many farms still have old GM gas engines around that are in use today. To keep those old engines working, it is important to understand how to check the module dwell.

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 caused greater potential energy to be stored in the coil.

To take advantage of this energy, the spark plug gap was increased from the previous 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 pickup coil was substituted for the rubbing block and distributor cam. The ignition coil fires when the module shuts off voltage to the coil primary. This is akin to the breakers opening.

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

It is important to note, 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 it won’t run properly. Depending on where the dwell becomes fixed, the engine may stall, run rough, have no power, and backfire. The module is often never considered since it is falsely believed if there is spark to the plugs, all is fine.

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.

Next, slowly raise the engine speed to around 2,500 rpm. The dwell should expand (increase) in linear fashion until it reaches 30°. If it does not, the module is faulty. Perform the same check on a new module. Many cheap, off-shore-made units expand the dwell but in a limited fashion, and the engine then does not run properly.    

I bet you are glad you never threw that old dwell meter away! 

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You may be saying to yourself, this is great, but on my farm, I 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 myriad engines found on the farm will share the use of some style of pickup 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, to multiply the input voltage to a level that can arc the electrodes of the spark plug under load. 

Though the exact specifications will be different, if the engine has an electronic ignition system and does not run properly, don’t think the problem cannot be ignition related. 

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