Trouble starting bad alternator or starter?

mattyice2

mattyice2

TJ Enthusiast
Joined
Jan 22, 2018
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Location
York County, PA, United States
All of a sudden, I have a starting problem with the Jeep. Still starts but really struggles to. After I run it for a while it starts up fine. Has me suspecting that it might be a weak alternator. Lights are a bit dim. I know it's not that battery because I just put a new one in.
 
mattyice2 said:
All of a sudden, I have a starting problem with the Jeep. Still starts but really struggles to. After I run it for a while it starts up fine. Has me suspecting that it might be a weak alternator. Lights are a bit dim. I know it's not that battery because I just put a new one in.
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New battery doesn't mean good battery. You'd be surprised how long those batteries sit around before they hit the US ports. Most auto part stores will test it for you. Put a voltmeter on the alternator or battery. Should put out around 13-14 volts while running. Starting issues not 48 hours after a new battery is pretty damning.
 
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IF the engine is hard to start in the morning, but starts properly throughout the day; there is a good chance one of the fused circuits may be draining the battery sitting overnight.
Before starting the engine in the morning you need to test the battery voltage; should be around 12.6 VDC after sitting overnight.
If the voltage is around 12.1 VDC; then there is a good chance the battery had a power draw on one of the circuits.
The circuits AMP draw can be tested by removing the ground cable from the battery, setting the meter on AMPS and measuring between the ground cable and the battery terminal; acceptable amp draw should be less than 100 mA. IF the amp draw in more than 100 mA; you have a circuit that is draining the battery.
To find the circuit that is the problem; you can one at a time remove a fuse and test the Amp draw until the measured amps drops to an acceptable measurement of less than 100 mA.
 
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Any auto parts store can check all three FREE. 🤫
p.s. I replaced my 8 year old battery and 20 year old alternator not long ago, after having similar issues. Faulty multi-function switch was causing my problem. Driving lights were randomly coming on when parked.
 
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mattyice2 said:
Unfortunately, I'm starting to suspect a near failing fuel pump. Now it never starts on the first try, but always starts the second. I'll put it on my future repair list.
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Cycle the key 5-6 times every time you go to start the Jeep for the day. If the problem goes away, then yeah the check valve is bad.

The check valve is at the top of the assembly. 3 things can fail in a fuel pump. The pump, sending unit, and check valve. If you drop the tank, buy an assembly and do all 3.

Did you get the battery tested? I'd have either fixed fairly soon. 2 months after my check valve went out, the pump did as well.
 
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5632 said:
While running disconnect the battery and if it continues to run it is not the alternator.
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Or just put a volt meter across the battery posts while it's running and see if you have more than 14VDC.
If you have anything less (yes, even 13+), then the alternator is likely the problem.
 
mattyice2 said:
Unfortunately, I'm starting to suspect a near failing fuel pump. Now it never starts on the first try, but always starts the second. I'll put it on my future repair list.
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Turn the key to ON for about 5-6 seconds before cranking it.
This gives the old pump a chance to build the necessary pressure.

Our '03 has this issue at times and this is how we get past it.
I'm not a fuel system expert, but it seems almost like there's either a check valve that's not holding pressure in the line like it should, or as you suspect the pump itself is just going south.
 
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