Ballast resistor

I have a 90 4l and I noticed the ballast resistor on the fender. My question is under WOT does the PCM allow the fuel pump full battery voltage? If not, are there any drawbacks to directly wiring the pump to the battery? :idea:

i dont think you can wire it directly to the battery you need a relay or are you asking if you can remove the resitor and jump it out , yes you can.

remove the resistor and use a proper guage wire to jumper it.
the only side effect will be increased noise from the pump.
87's didn't even have them, there was a TSB about retrofitting them if the customer complained about pump noise.

Slo-Sho said:

I have a 90 4l and I noticed the ballast resistor on the fender. My question is under WOT does the PCM allow the fuel pump full battery voltage? If not, are there any drawbacks to directly wiring the pump to the battery? :idea:

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Yes it does via the ballast resistor bypass relay.

langer1 said:

Yes it does via the ballast resistor bypass relay.

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Actually, it uses the O2 sensor relay for this job, at least on a Renix setup, but it does bypass during WOT operation, according to the Mopar Performance book. It is interesting to note that even for hotted up racing Renixes with performance ECU's and camshafts and the like, bypassing the resistor is not mentioned as a performance aid, so I'm guessing that doing so, while harmless, will also be without benefit if all components are working properly.

Matthew Currie said:

Actually, it uses the O2 sensor relay for this job, at least on a Renix setup, but it does bypass during WOT operation, according to the Mopar Performance book. It is interesting to note that even for hotted up racing Renixes with performance ECU's and camshafts and the like, bypassing the resistor is not mentioned as a performance aid, so I'm guessing that doing so, while harmless, will also be without benefit if all components are working properly.

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it only increases pump voltage by 2.3 volts acording to my 90 and 3.2 for my 93, it made no differnce in fuel pressure when i checke that though. My thought on this is the pump doesnt work as hard with more voltage and could possible extend its life a little but its no proven. I was toying with this stuff a few weeks ago and bypassed the resistor and I have seen zero change prformance wise and surprisingly very little extra noise from the pump

Thanks guys for all the input. I'm just thinking that a DC motor is sensitive to voltage input and it's output is usually proportional to voltage supply. I used my DMM and measured the voltage after the resistor and it got about 8.3 volts at the front of the jeep...it's probably worse at the pump itself. Looking at this from a performance perspective that 8.3 volts is dismal. As mentioned above the pump resistor gets bypassed at WOT, but what about 3/4 throttle or 7/8 throttle...my pump would still be 'underdriven'.:doh:

Also, is it true that supplying a DC motor with less voltage than it was rated for (12v?) will naturally cause the motor to draw more amperes at the reduced voltage?

Actually the pump works hardest at slow engine speeds because very little fuel is passing through the injectors, the pump is fixed pressure and volume the resistor only limits current that the motor draws.

Also, is it true that supplying a DC motor with less voltage than it was rated for (12v?) will naturally cause the motor to draw more amperes at the reduced voltage?

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Thats true for AC motors not DC motors.

dzraces said:

it only increases pump voltage by 2.3 volts acording to my 90 and 3.2 for my 93, it made no differnce in fuel pressure when i checke that though. My thought on this is the pump doesnt work as hard with more voltage and could possible extend its life a little but its no proven. I was toying with this stuff a few weeks ago and bypassed the resistor and I have seen zero change prformance wise and surprisingly very little extra noise from the pump

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Yah - but I've seen the TSB and the recall, and they put the ballast in for reasons of NVH - not safety issues. That's why it was a "voluntary" recall.

My 1987 doesn't have one, never has. My 1988 doesn't have one - it got wet, and I removed it (jumpered with wire one size larger - 14AWG, I think.) Both of my 1989's have them - but I haven't removed them yet.

5-90

Since the pressure regulator is a bypass which sends fuel back to the tank, as long as the pressure remains at or above that value, it doesn't matter how low the voltage to the pump is or how underdriven it seems. You won't get any benefit from removing the resistor unless demand outruns the pump, and the WOT bypass anticipates that. I'd ignore the whole thing unless it malfunctions.

A shunt wound permanent magnet motor like the one in the fuel pump will run more slowly and at the same time use less current when the voltage is reduced. If you have an older XJ with a voltmeter (which is highly sensitive to heater fan load), turn the fan on high, and notice what happens to the voltmeter when the resistors are taken out of the circuit.