Ballast resistor is red hot

My '92 Cherokee has been losing power at wide open throttle. It has the 4.0 with Aw-4 transmission. At idle and normal driving it seems to run fine. However, if I give it full throttle and wind the engine out, it starts to lose power around 3500 rpm and I can't even get it to 4000 rpm. Not only does it start to lose power but it shakes and vibrates like the engine is going to fly apart. As soon as I let it come down to normal driving rpm's it runs fine. There is never any engine knock or ping at wide open throttle. I took it to a mechanic and he said it was the fuel pump and crank position sensor and wanted 1000.00 to replace them. I said no thanks and did it myself. I replaced the fuel pump, fuel filter, and crank position sensor. No difference. This morning I replaced the fuel pressure regulator, drove it around the block for 5 minutes and no change. However, when I opened the hood, I placed my hand on the inside of the drivers side fender and touched the ballast resistor and almost lost fingerprints because it was so hot. I thought the Jeep would catch on fire. My question is what does the ballast resistor do. Can it have an impact on fuel delivery if it is bad. Is it supposed to be so hot. Any ideas on the funtion of the ballast resitor would greatly help. Thanks. Randy

i had a problem with my jeep dying and narrowed it down to that piece but could not figure it out, I replaced it but no help, so I just elmininated it. That only negative so far is my fuel pump is a bit noiser but hardly noticeable with aftermarket exhaust and such.

The ballast resistor is in effect a primitive voltage regulator. The nature of a resistor of this type is the hotter it gets the higher the resistance, that's how it regulates the voltage.
That 3500 hundred number makes me think O2 sensor. Especially if you get an occasional stumble around the mid RPM range also.
You can measure the in and out voltage of the resistor, there us usually around a 25% drop in voltage. Though this number is far from exact (more of an average), the ballast resistor is rather primitive old school technology.

on a car with 'points' in the distributor, the ballast resistor limits the current flowing thru the points during normal driving. on startup, the ignition switch bypasses the ballast resistor to give more current thru points.. i assume because of the draw of the starter, so that on startup there is enough current to produce a decent spark. i rewired a car once and decided to skip the ballast resistor. it ran for about 30 minutes and then the points were burned to a crisp. why they have a ballast resistor in cars with electronic ignition(no points) is a mystery to me.

skyblue said:

on a car with 'points' in the distributor, the ballast resistor limits the current flowing thru the points during normal driving. on startup, the ignition switch bypasses the ballast resistor to give more current thru points.. i assume because of the draw of the starter, so that on startup there is enough current to produce a decent spark. i rewired a car once and decided to skip the ballast resistor. it ran for about 30 minutes and then the points were burned to a crisp. why they have a ballast resistor in cars with electronic ignition(no points) is a mystery to me.

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LoL I remember the days when Mopar folks who knew a thing or 2 or had been left stranded by a Mopar ballast resistor failure would carry a spare in the glove box.

ISTR the ballast resistor reduces voltage to the fuel pump to cut its speed.
At WOT there is a relay to bypass the resistor so the pump can flow more fuel. Check the resistance of the resistor to make sure it is normal. Around 1ohm. Try temporarily bypassing the resistor to see if that cures the WOT problem

Ive had my share of trouble with the ballast resistor. I was told to bypass it but not too fond of that idea. I ALWAYS carry a spare and also a jumper wire just in case. I went through a few in 18 months once so I left the bolt off that holds it to the side wall in my 93. That was probably 18 months ago. I havent had a problem with it since I left it free and loose, I think having more air circulating around it and not bolted on has helped. Just my opinion.