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jeep wont idle with weak battery

B

Berrill

NAXJA Forum User
Location
Cape Town
My jeep won’t idle when the battery is weak.

Are all jeeps like this, in my experience cars with weak batteries either don’t start, or if they start the alternator will keep the electronics working and the car running. I.E. if you jump-start a car with jumper cables you can get home.

However if I jump start my jeep, it will start but I have to keep the revs up as it will not idle. Is this normal?

My jeep is a 4l auto

Thanks Antony
 
Your alternator does not generate "full rated power" at idle - it usually does so around 1000-1200rpm crankshaft speed. This would be why you can rev it up, and it will stay running.

If your battery is fully drained, it will "sink" all of the current output, which will leave nothing to run the fuel injection or the ignition. This can cause the vehicle to not idle (I've seen this before, actually...) and may indicate that you'll want a new battery.

I'd test the battery, or at least put a trickle charge (<2A) on it overnight. The lower, the better...

5-90
 
I have replaced the battery and its perfect, its just that I thought if the battery is still capable to start the jeep it should be capable to keep the jeep ideling.

This makes it very dificult to know when the battery is getting old.

so is this normal jeep behavier
 
Not necessarily - I haven't noticed it (but I don't have a weak battery, either...)

Have you also checked all your mains cables, including (especially, if you're 1990 and earler!) the ground from the cylinder head to the firewall? That could also cause trouble...

5-90
 
Lots also depends on whats wrong with the bad battery. A battery thats just used up but not shorted will not effect the idle, one that has a shorted cell will.

The newer systems with Pulse Width charging will not even fully re-charge a good but dead battery.
 
Have you ever looked at the size of the fuse for the ELECTRIC fuel pump ? That baby draws some power and draws it consistantly...
 
Mine didn't idle either when it was low or almost dead. So, I'm with you Berrill and Whacker..

It's all theory until you experience it first hand.


Elias
 
I've had this problem with batteries that were very dead, started with a boost. A stone-dead battery will usually not take a charge at first. If you put an ammeter on it, you'd see almost no charging current. I was told somewhere now forgotten that the problem is not low voltage, but high in this case, because when the battery does not accept a charge, the system is running essentially on the alternator output, which is not only too high, but also is "dirty" DC, AC chopped with rectifiers but not filtered in any way. In a statonary power supply, you'd have capacitors to smooth it out but in a car the battery serves that function.
 
Ok so I guess this is normal jeep behaver as my jeep is a 2001 model in excellent condition. This is the second time this has happened and changing the battery sorted it out both time.

I just find it strange and not a clever design.
 
Gone are the days of the mechanical fuel pumps, carbs and being able to jump start with a bad battery. Too much electronics drawing too much power. One year. ~84, with my 82 S10, 2.8L 4 speed I drove all the way back from the ranges at Ft Dix NJ to my house about 120mi away in north New Jersey with a dead alternator, just on the almost brand new diehard battery. Almost dark when I got home and the headlights were barely glowing when I pulled in the driveway. An impossibility to do today with all the computer controlled systems.
 
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