Troubleshooting the HEI coil in cap ignition

Troubleshooting a faulty or inoperative ignition should always use a logical approach.  Overlooking the basics will leave to frustration and unnecessary time and expense. The first thing to remember that the system needs to have power. Make sure the power connection to the distributor cap or the coil is good, no blown fuses, no broken, loose or corroded connections.  A good 12V powered test light should always be one of the basic troubleshooting tools you need, along with a reasonable priced digital-volt-ohmmeter.


A 12V test lamp and Digital Volt-Ohmmeter should be your basic troubleshooting tools for HEI-EST systems. Use a DVOM for testing, calibrate meter scale (some do automatically) to read zero when the leads are touching each other. Do this before testing coil to insure greatest accuracy.  If meter will not calibrate to read less than 0.2 ohms, check leads or replace battery.

This is the terminal for the coil-in-cap HEI ignition power. With the key turned on (engine off), there should be full battery power here. If not, you will need to check for loose connections by tracing back the wire as far as you can. Also check the fuse that protects the circuit from overload.  If the fuse is blown, you will need to do further troubleshooting to find and correct the reason why the fuse blew. Short to ground is the most common cause.  The connection (TACH) to the left is the tachometer output.

Connect the one  side of your test lamp to the battery POSITIVE cable.  Probe the TACH terminal on the dist. cap while an assistance cranks the engine.  The test lamp should blink repeatedly as the engine cranks.  No blink will mean either a  bad module or pickup coil. If the light blinks but there is  no spark points to a  bad ignition coil. Further testing is required to pinpoint the problem. 



As you pull the coil cover off, these are the connections you are looking at and how they are identified. Besides the Tachometer and Power connectors, the next important connector is the three-way that is marked on the coil cover as "C-", "GRD" and "B+".

In the picture on the right, see how this correlates with the ignition module.  Study the picture below of the full diagram with the ignition coil.

Now with the diagram broken down to just the ignition coil, we can get an idea of the connections on how to troubleshoot for a faulty coil.

The first thing to remember that there is no physical connection between the coil windings. The "B+" and "C" winding is separate from the ground/sparkplug wire coil.  The resulting turning on and off the voltage on the "B+", "C" coil windings, causes a magnetic field to be generated. When the voltage is turned off  through the trigger signal, the collapsing magnetic field induces a very large voltage and current into the primary winding which is the feed for the sparkplug.  This cycle is repeated as long as there is a functioning ignition module and coil, and a good ground connection, which means it is imperative that the engine grounds are solid and clean. 

It must be noted that the secondary (battery/trigger coil) windings will have considerable less turns of wire on the core than the primary (sparkplug) windings.  The voltage and current is inversely proportional to the number of windings on the core. So a small voltage/large current on the secondary windings will translate into a large voltage/small current at the sparkplug.

TEST 1:Remove and invert the cap, Measure between the "TACH" and "BAT" terminals , using your DVOM, Set to OHMS scale, RX1 and be sure the scale reads zero ( 0.00) It should read less than an ohm, typically around 0.5 to 0.6 ohms. Anything less or more, replace the coil.

TEST 2: Set your DVOM scale to RX10k or higher. Touch the probes between the "BAT" and the carbon pickup for the coil. This reading should read between 6000 and 30,000 OHMS.  Outside of that range the coil is bad, replace. 

Also using test 2 as a reference, check for shorted to ground connections, by probing the "BAT" connection  and the center terminal with your DVOM leads. There should be infinity resistance or OL on your meter. You can also check the "TACH" connection as well. Either way, if the reading is anything less than OL, replace the coil.

Testing the Pick Up Coil

If the ignition coil checks out good but you were not getting a "blink" with your test light as described at the top of the page, then the pick up coil needs to be checked. This test should be done before moving on to the ignition module. 

With your DVOM set on the highest scale, touch one lead of your meter to the distributor body and the other lead to one of the terminals for the pick up coil (either the green or white lead. Your reading should be infinity or OL. Test the other terminal. Any reading less than infinity means the pick up coil is junk. 

With your DVOM set on RX10K scale or equivalent, touch each terminal with your leads from your meter. The reading you see here should be around 500 to 1500 ohms and the reading should be steady as the wires are wiggled. 

If the coil fails either test, replace the pick up coil. 

Testing the Ignition Module

If the pick up coil and the ignition coil checks good, the next thing to move on to is the ignition module. There are two ways to do this. One is with a module tester, such as shown in the picture below. This is also handy to check the pick up coil as well, using the reluctance mode.  If you don't have a tester, you can use the method below. 


The method below will work for both styles of the 7 pin module. Terminal identification is identical. 

You may be asking, "if the ignition coil and pick up coil checks okay, why waste time, testing the module?". You're right, but this will get you familiar with the procedures, if this is the only thing you have to work with.  It is best not leave anything to chance and work in a logical, methodical approach. 


When  re-installing your old one, do not remove the white pasty substance from the bottom of the module or from your distributor. If you are installing a new module, you may clean the old paste from the distributor but, ALWAYS, use the paste when installing the module, and follow the instructions for application.

Failure to use the paste, which is a thermal transfer that allows heat to be conducted away from the module to the larger surface area of the distributor to dissipate heat, will mean almost instant destruction of the ignition module. 

Thermal transfer paste must be used.


                     Page 1: HEI-EST basic Fundamentals

                    Page 3:  Troubleshooting basics for the HEI-EST system (remote coil.)