Help 4.0l Killing Rotors (Pics)

What would cause my 88 Comanche to kill rotors like this? Both of these have less than 150 miles on them.

I'd check the ohms in my cables. Looks like the spark is finding it's way to the shaft for ground.
Do you have a stock coil? The spark thing can be overdone. Many after market coils require a ballast resistor to tone down the spark some.
Looks like something is causing excess resistance at first glance. Either the cables or the plugs. Though usually when this happens you will get a cross fire, that's when the spark jumps to the next closest lug on the cap and fires the wrong plug.
Some other things that might be the cause, is the rotor being wrong and having way to much clearance. I can't think of anything that would make the shaft hot enough to melt a rotor, but heck anything is possible, even if not probable.

Does that year motor have a ballast resistor?? If so, it might be shorted out and delivering a full 12 volts to the ignition system instead of 6 while running.

Maybe check that? Not too familiar with the RENIX motors, but I hat an old Dodge Dart and the ballast resister fused to zero resistance and i caused all sorts of cool things like you are looking at.

Just my .02.

Yeah check the ballast resistor..

Pic off of google for ref: its the long white thing with two orange wires and one bolt!

um, that orange and black wire coming out of the ballast resister goes to the fuel pump....?!?
you can bypass it and it only makes your fuel pump a little louder?
am i missing something?`

xcm said:

um, that orange and black wire coming out of the ballast resister goes to the fuel pump....?!?
you can bypass it and it only makes your fuel pump a little louder?
am i missing something?`

Click to expand...

Some after market coils come with a ballast resistor (just like the one for the fuel pump in the above picture). The ballast resistor for an after market coil is bypassed during starting, the input voltage during normal running, is reduced/limited by the resistor. Running full voltage to some after market coils can burn up a set of plugs pretty quickly and likely cause hi voltage shorts to ground anywhere the system is vulnerable.
I've seen the stock coil cause fairly impressive light shows when the cables are cracked or split. The inside of the distributor cap with lightning shaped scorch marks from moisture or dirt.
Checking the shaft to see how hot it gets, is likely as simple as pulling the rotor and (carefully) touching the top of the shaft with your finger, after the motor has been running for awhile. I've never seen it, but it seems possible, friction could heat up the shaft considerably if something was rubbing.

If the shaft is hot, it could very well mean that your distributor bearing is going/gone. Otherwise check all your resistances in wires/coil/plugs...

Is your spark plug gap correct? Too large a gap makes the spark try to take a short cut (dist. shaft).

It turns out that the cap was bad. The truck is running great now, expect that in every gear but 4th I hear a loud knocking sound that speeds up as rpms increase. I'm thinking that I might have fried the sync sensor, however, it is running better than ever.

BTW it only happens under a load not in neutral

xcm said:

um, that orange and black wire coming out of the ballast resister goes to the fuel pump....?!?
you can bypass it and it only makes your fuel pump a little louder?
am i missing something?`

Click to expand...

nope, your right. I posted that at 2:30am because my 7mo would not sleep.

i dont know what the hell a fuel pump ballast resistor has to do with a distributor.. :eye:

lol

ignore the picture

nw-xj-scott said:

Yeah check the ballast resistor..

Pic off of google for ref: its the long white thing with two orange wires and one bolt!

Click to expand...

That ballast resistor is for the fuel pump. There is no ballast resistor for the ignition. Usually a ballast resistor in the ignition is needed for a points setup to limit the current through the coil, and may be switched momentarily to ge a hotter spark while cranking.

Carone said:

It turns out that the cap was bad. The truck is running great now, expect that in every gear but 4th I hear a loud knocking sound that speeds up as rpms increase. I'm thinking that I might have fried the sync sensor, however, it is running better than ever.

Click to expand...

A good reason to replace both the cap and rotor as a matched set.

lawsoncl said:

That ballast resistor is for the fuel pump. There is no ballast resistor for the ignition. Usually a ballast resistor in the ignition is needed for a points setup to limit the current through the coil, and may be switched momentarily to ge a hotter spark while cranking.

Click to expand...

Agreed, the XJ ballast resistor is for the fuel pump.

Earlier Chrysler ignitions usually incorporated ballast resistors, and MSD requires them on some applications with their 6-series ignitions.