HEI troubles

[mike ramirez racing]

Ok so I have a friend who says he keeps burning out the modules on his HEI in his 72 chevy truck. He's replaced modules with stock AC Delco, Standard/blue streak, pertronix, etc...you name it. He's changed coils, even replaced the whole distributor 3 times. He runs the ign power source thru a relay just to see if it does anything. All his grounds are good. Charging system never over 14 volts. He says the wires inside the coil the red yellow and black one of those 3 will be cooked.

Any suggestions would be greatly appreciated. Thanks.

[Shopboss]

Make sure the ground strap in the dis. cap is in place. If It is missing then he will have this problem. I'm not talking about the ground wire coming out of the coil. There should be a seperate metal strap or some times a black wire that goes from one of the 4 hold down screws to the three wire plug on the cap.

[mike ramirez racing]

that was the first thing I asked him and he said its there. So I will double check again. Thanks.

[BirdMan]

Quote: There should be a seperate metal strap or some times a black wire that goes from one of the 4 hold down screws to the three wire plug on the cap.

Coil hold down screws? I am having the same problem with a 75 Chevy Van 350 c.i. I will have guy check or I may visit him and look myself. Interesting coincidence? thanks, Dale

[Shopboss]

The strap that I'm referring to goes to the center of the three wires on the cap. This is the ground for the coil. BTW Make sure that there is plenty of the white heat sink past between the module and the dis. base.

[Speedbump]

Strikes me HEI wasn't stock in the 72. If he used the stock power wire from the orig setup for the HEI power connection, the HEI might not be getting enough voltage due to chevy's use of a resistance wire in the "run" portion of the ignition circuit. HEI wants all of the primary voltage available with no drop. Seems there was a GM tech bulletin out some years ago talking about a poor crimp and/or corrosion at the connection of the HEI connector that plugs into the cap. This also caused a voltage drop and the module saw less than optimum voltage. The low voltage condition caused module failures. Might look in that direction in addition to the "too high" voltage causes.

[mike ramirez racing]

HEI's didnt come til 75. He ran a 12 volt constant source fromt he ign key and even put in a relay. He checked voltage to it and its getting no more than 14 volts when running.

[Schurkey]

He says the wires inside the coil the red yellow and black one of those 3 will be cooked.

Is it the same wire each time; or have all three cooked at some time? Red and Yellow are primary circuit; the black is secondary along with the terminal that leads to the spring and carbon button.




1. The HEI should be safe to about 17 volts; at least short-term.

2. What is the plug gap? Anything over .045 is just heating the ignition system. Yes, some Olds came with a .080 gap; but they were firing an EGR-polluted lean fuel mixture. The big gap is hard on the entire secondary system.

3. How much resistance in the plug wires? I've seen engines run decent at idle and part throttle with 50K ohms in the plug wires--but--at heavy throttle they're misfiring; and again--very hard on the coil.

4. Install a low-resistance button under the in-cap coil. Available from MSD and Accel; I used to get 'em from Crane but I don't know if they're still available there. While you're in there, verify that the cap isn't melted around the carbon button; and that the rotor conductor isn't eroded at the button or at the end that fires to the plug terminals.

5. Might be worth verifying that the 3 wires leading from the three-wire connector on the cap to the module area are OK--in particular the ground (middle) wire.

[mike ramirez racing]

He hasn't gotten back to me. I will ask him again about the wires being cooked. He says charging voltage never goes over 14.2 v And I will tell him about the low resistance coil button. Stock HEI wires and stock plugs gapped at .045"

[Shopboss]

Check your cap to rotor indexing. If this is to far off the secondary voltage will have to be extremely high to jump the distance from the rotor tip to the plug wire post in the cap. Since your primary voltage is close to a constant 12 volts then the amperage that it takes to allow the spark to jump the extra distance in the cap will go up. This will over heat the small primary wires at the coil.
The easiest way that I’ve found to check the phasing of the rotor is to cut a hole in the side of a cap so you can see the spark as it jumps from the rotor to the cap.
It should sweep several degrees across the plug wire contact point. But like I said before if this is to far out of phase then the spark will have to jump so far that the secondary voltage will always be too high. This over heats the small wires going to the coil.
I had one engine that came in with a sever detonation problem. It had gone through two sets of bearings. The timing would check normal and dead on at lower RPMs. But while running it on the dyno as the RPM got to a certain point it would start detonating like crazy. To make a long story short, the distributor cap and rotor was so far out of phase that it would start firing the next plug in the firing order. Thus the detonation. A screwed up distributor cost the customer several thousand dollars in engine work.
You can adjust this phasing by modifying the vacuum advance.
Make sure you have the right advance pod for the application. Some are longer than others.
I’ve also seen some distributors that have several notches in the housing for the location tab on the cap.
If you are using the wrong notch then your cap will be way off. Make sure your shaft is free and turns smoothly. Also make sure your distributor isn’t down to far and bottoming out on the oil pump shaft. This can cause spark scatter which puts an extra load on the system.
Make sure you have atleast a #12 wire for the B+ and that your ignition switch is in good shape.
Sorry I got long winded. Hope this helps.

[mike ramirez racing]

Shopboss that helps out alot thanks. Never considered that being its a stock motor and stock distributor. We check that on race cars, especially when we're using nitrous.

[Tuner]

Reversed polarity kills HEI modules. If diodes in the alternator are damaged it is possible to have 14V DC with 28V AC at the same time. Measure at the alternator output terminal, DVM set on AC volts, with all lights and accessories turned on and the engine at 3000 RPM. If there is more than .1V (one tenth) volt AC, there is a problem.