Since I have converted my Blazer from carb to TPI this summer, I have gone through 2 brand new coils in mabye 1500 miles (last one probably had less than 500 miles), anyone have any ideas why? This thing is a pain in the *** to replace, and I'd rather not have to do it in the middle of the woods

 

bad spark plug wires can cause that to happen. also a cheap coil, bad ground, and poor mounting (external coil) could also cause this

 

Shorted plug wire(s), bad ground on ign module, no heat sink compound under ign module, resistor or resistor wire in the 12v feed to the coil or ign module.... there are some places to look.

 

Both coils were bought new at the GM dealer. Where does the coil ground? The distributor, ignition module, wires and plugs all have less than 1500 miles on them. I am using the same positive wire for this coil that I had on my vac advance distributor back when it was carbureted

 

What do you have your plugs gapped at?

Kat

 

Did the original dist use points?
If so, there is probably a resistor in-line with the power wire.
The resistor was factory installed to help prevent the points from burning up if ign switch was on and the points were just starting to open or close.
Check the voltage on that wire with the eng running and see if it is the same as battery voltage. It should be pretty close.

 

The coil grounds through the back of the ign module, to the distributor body.

Wires being new doesn't mean they haven't burned thru on an exhaust header.

 

The plugs are .40" if I recall, I will check the next time I work on the truck. It had points originally, but I bought it with an HEI distributor, I have no idea if it uses the original wire. I remember checking voltage at the coil the 1st time this happened and it was the same. The wires dont touch the headers either

 

The coil ground could be bad.
The metal clip going from the ground plug also goes to one of the bolts that hold the coil down on an HEI cap. If that bolt doesn't have good contact with the metal it can start burning stuff up.

 

I would look for the ground strap, thats usually missing, on a coil-in-cap style HEI. It goes from one of the hold down bolts (mentioned in the post above) to the connector on the side of the cap. If this is gone, thats why the coils are burning up.

 

The blazer did have a resistor originally. On chevy trucks of that era its a resistor wire. I've changed many of these for people who have put in hei's then had problems. (myself included years ago) Many times it'll run fine for awhile but the resistor wire can't handle the load that hei puts on it, and eventually starts to burn up.

If the wire is still the resistor wire, it will say resistor on it. Also it will have a sort of cloth covering instead of the modern plastic. Dont just look at the coil end though, the resistor wire ran all the way to the plug in on the firewall!! I've seen this wire spliced in the middle before. If you still have this wire just disconnect the firewall plug, remove the wire and replace it with a regular one.

 

It's not a coil-on-cap HEI, the coil is seperate. I dont remember ever seeing a cloth-covered wire anywhere on the truck, but I'll look