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Discussion Starter · #1 ·
I have the 1990 2.5L YJ and the problem I have is that the Jeep runs fine but as soon as you switch on the headlights the RPMs will slowly bog down and the engine will die. I've read somewhere else that someone fixed a similar problem by installing a new O2 sensor and I did the same. The result was that it was fixed for about 1 day, but the problem returned the day after (but less extreme). Does anybody have an idea what this might be?
 

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Discussion Starter · #3 ·
Here are my findings:
Off the battery is at 12.6 V
Using the max/min function while starting, the min is 9.6 and max is 15
Voltage while running is 15 V.

I've also found that the problem is inconsistent- the jeep didn't die with the headlights on this morning (although it did last night). The Voltage measured at 14.9 with headlights on
 

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I would say u have a bad ground somewhere... seen this before where a bad ground reduced the voltage to the computer then it shuts down.. check all your grounds out.. make sure the headlight wires dont have any cuts causing a short..
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everyone blames a faulty ground... and although it is common, Im not sure that is the issue right now... not with the info given so far...

it would be nice to know the voltages when the jeep dies...maybe hook up a voltage gauge full time so you can see the voltage when you encounter the problems...

Im kinda leaning towards an alternator problem, or internal diode, (same issue, bad alternator).

it should be noted that alternators are super easy to rebuild... I think it's about $10 in parts, and about 15-30 minutes of rebuild time...

basically you are replacing brushes, bearings, and diodes...

pulley puller is about the only extra tool needed other then regualr hand tools
 

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Discussion Starter · #7 ·
it would be nice to know the voltages when the jeep dies...maybe hook up a voltage gauge full time so you can see the voltage when you encounter the problems... Im kinda leaning towards an alternator problem
If it does it tonight I'll put a multimeter on it and post the results. Is there any way to test the alternator to see if it is causing the problem?
 

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Yes, and no.

The problem is intermittent. . So testing it when is working doesn't tell us anything.

And from the results you posted, it's working fine
 

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ok, sounds like the alternator is absolutely ruled out...

when it 'dies', does it 'turn off', like when you turn the key offf, or does it cough a little bit kinda like running out of gas. or is it instant, like flipping a switch...
 

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Discussion Starter · #11 ·
It is more like running out of gas. As soon as I switch the headlights on, the RPMs starts to slowly go down and eventually it dies. It takes probably less than 30 sec. I've noticed that sometimes driving with the headlights on it will "hesitate" when I push the gas. I'll push the gas and nothing will happen and then all of a sudden it jerks forward. Only with the headlights on
 

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Try running a ground wire jumper from the headlight ground at the grille direct to the battery negative. See if the problem disappears.
 

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Discussion Starter · #13 ·
Reviving an old post here...

Hey all, still haven't managed to get this thing figured out... I ran the ground wire jumper that DREDnot talked about to no avail and have replaced the ignition coil, 02 sensor, MAP sensor, throttle position sensor, ignition control module, and ICA motor. Can't think of where else to go from here, maybe a fuel pump problem? Again, the problem ONLY manifests itself with the headlights on, and it's almost immediate (as soon as you turn the headlights on, it starts to bog down and die and will hesitate on acceleration). The PO had also installed a pushbutton start, I'm not sure if this would effect the situation. Any help is definitely appreciated.
 

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I would like to know how you are performing the electrical tests.

are you using a volt meter? or is it an electrical gauge from a dashboard?

are you connected directly to the battery, and while the jeep is running, and you turn the lights on, are you inside the cab, and where is the volt meter?

when you turn the headlights on, and the motor starts to drop RPMs, is it like the jeep is running out of gas, and the motor just slowing down, or does it sound like the motor is under strain, and getting 'bogged down' until the motor finally dies, turns off.

And again, are you reading the volt meter while you are turning the headlights on.

not before, or not just after. but during, while the motor rpm is slowing down.

how long does it take from when you turn the headlights ON until the motor starts to slow down, and then after it starts to slow down, how long does it take.

getting some video with sound, with the volt meter in the frame would be awesome.


replacing the wiring for the headlights is kinda easy, and suggested so you can install high power headlamps. that new wiring harness will have heavier gauge wire, and will be independent of the motor wiring.


also, thinking out of the box a little.... is ur fuel pump an electric fuel pump, or mechanical. the older, and carburated motors are not electric. at least not the stock setup... my previous owner disconnected my mechanical pump and installed an inline fuel pump on my motor, and I hate that....
 

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Do Yjs have distributors? Could it be a distributor or spark plug related issue? When was the last time spark plugs and/or distributor cables changed. I had a similar problem with my mustang. It turned out to be this. Also the battery's max amp load rating may not be enough. Did you try a battery rated at higher amp?


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