HEI modules die

I have a Cadillac HEI that kills module if a tach is connected to the tach terminal. Snap-On Tach and Dwell or Autometer. Engine dies instantly.
Modules also die intermittently with 1000 mile life max.
Two other Cadillac HEI's in different cars that look and are wired the same work trouble free for tens of thousands of miles.
AC Delco and Standard Ignition modules have been tried.
About ten modules have been destroyed in this vehicle.
Engine has heavy duty woven ground strap from starter to frame. Battery is grounded to frame in typical Cadillac fashion.
Any ideas?
Thanks

 

many times the cause of failure is a bad coil.Coil shorts internally and overloads the module.Some brands of new coils fail in weeks....

 

Anyone else had this problem?

 

My experience is not anywhere near yours, and I've been working with them for 20 years. As the other guy mentioned, swap coils.

Only other thing I could think of is that the tach is shorting the circuit from possibly being hooked up incorrectly, but I surmise from your post that its not likely.

 

First thing that came to me might be the ground, but looks like that's covered.

For what it's worth, I worked on a Pontiac 4 cyl once that had the wrong rotor button putting #1 in a location that let it arc or something that would take out ign modules.
Rotor was for the same motor in a different, earlier model car. Almost never figured that one out, and still don't completely understand it. But turned out that was the problem.
Check rotor to another correct one exact for your Cad, but I'd put a coil on also like everyone said.
Like to know what you find. Tom S. in Tn.

 

Also make sure it has the correct coil (one has a yellow wire, the other has white), and make sure there's some heat sink grease on the base of the module where it contacts the distributor body.

 

you have to make sure you use the dialectric grease as well or it won't transfter heat to the distributor housing properly and you will fry it that way.

 

I missed a ground at the coil once. Failed to get it under the hold down screw.

 

Be wary of some aftermarket parts............. Just because it fits......

4TTRUK

 

"New"..... Only means shiny.............

4TTRUK

 

"dialectric grease" and "Heat sink grease" aren't the same animal.

 

Make sure you have the little brass coil ground strap installed correctly and the heat sink grease is a must.

 

The HEI can be very troublesome based on polarity of parts and parts working together.
However i would try another tach before i condemed the HEI.....l

 

heat sink grease and under the coil ground strap connected to a ground wire.