Ignition Control Mule Failures 77 Spider

[REHolder]

Recently upgraded ignition on my 77 Spider to electronic with proper coil, module and heat sink as supplied by Fiat Vendor. Engine performed flawlessly for 125 miles then the Ignition control module failed. I replaced 5 modules in succession before noticing that the electronic tachometer was "dancing". After consulting with the vendor he suggested I disconnect the brown wire from the tach to the negative side of the coil. Voila, module number 6 is performing well now for 100 miles. Does this make sense that a short in the tach or the wire from it to the coil was blowing these modules? If so can someone explain why? Just curious!

Thanks

[18Fiatsandcounting]

I think the "dancing" tachometer needle is a symptom of a failing Ignition Control Module, not the cause of it. The ICM is just a switch to connect the negative post of the coil to ground when triggered by the pickup coil inside the distributor. The brown lead for the tachometer just "counts" these trigger pulses over time. A short in the tach lead to ground would just ground the negative side of the coil, which is what the ICM does anyway.

Five ICM failures tells me that it's likely overheating (or just very low quality), which could be an inappropriate ICM or the wrong coil. Did you use any thermal paste between the back of the ICU and the heat sink? It really helps keep the ICM from overheating, and I use Arctic MX-4 or the equivalent. It's meant for computer processing or graphics chips where heat dissipation is the key to longevity.

-Bryan

[REHolder]

I am using a GM heat sink and have been very careful with applying the conductivity paste and getting a tight bond. I now have over 200 miles since disconnecting the tachometer lead to the coil with no issues. The module i'm using was purchased off Amazon for $9.95. I fried 6 more expensive ones from various suppliers, including an Excel High Performance that cost $69.00. Anyway I carry a spare at all times. Could be an extreme coincidence I guess.

[Nut124]

I have never had one fail in over 25 years.

What coil are you using? Coil resistance determines the amount of current thru the module. Could it be that the coil has too low resistance -> high current, high heat, failure?

[REHolder]

Using coil supplied by Vicks for this application. Vicks even sent a replacement in case the first one was bad. I swapped it out upon receipt and the modules still failed until I disconnected the brown wire from the tach to the negative terminal of the coil.

[Tcamp]

I had this same issue. Tach bouncing. I got info from this site. You need to put a capacitor in line between tach and coil. Can't remember the UF rating of capacitor but it is in this site somewhere. Repaired this problem instantly and has been working for years. Tach's are set up for point ignitions systems not electronic.

[85redpini]

I had this same issue. Tach bouncing. I got info from this site. You need to put a capacitor in line between tach and coil. Can't remember the UF rating of capacitor but it is in this site somewhere. Repaired this problem instantly and has been working for years. Tach's are set up for point ignitions systems not electronic.

i had a tach wire melted into the ignition wire on a spider once