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2003 TJ
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I'm two weeks into owning a new to me 2003. I'm excited to learn the basic mechanics, did not expect it to be this early on into the relationship. Last Sunday it through the check gage light, and code P1954, over charged battery. It has not shown the light again. Last night it would not start. It turns but won't catch. If you press on the gas, the motor runs but as soon as you left off it dies. Felt like it could be fuel related. Replaced the fuel pump relay, tried starter spray in the air horn, banged on the gas tank while trying to start to eliminate fuel pump. Battery is fully charged. Disconnected the battery and let it sit for 15-20 mins to reset the computer. Fired right up. Went to have the alternator checked, reading a little high. Disconnected the battery while running, and it did not die. Is it possible the alternator is simply at the end of its life? Guy at Advanced Auto was great. He said it's most likely that the code from last week was causing the start issues as the computer was trying to readjust to high battery voltage. I have a new alternator sitting in the passenger seat.

Now wondering if I bought someone else's head ache. She has 158k miles, real main seal leak and needs the front end diff replaced/rebuilt. Electrical short with the blinkers (randomly come on and engage the hazards). After this, i'm concerned that there is major electrical issues. Not a spot of rust on her, she's only lived in the south. Suspension is in top shape, transmission has been overhauled. I'm the second owner, why would someone give up one after 18 years??

Thanks in advance for any/all input.
 

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Never pull the old car guy trick of disconnecting and reconnecting battery while engine is running as it produces a voltage spike that can fry electronics like the pcm(aka computer)

First check battery temp sensory in battery tray

Next verify both ends both battery cables clean and tight

Next check the back of engine to firewall ground strap - both ends clean and tight

Next check alternator wiring

Then if all above good pull alternator and have it tested

Then if good recheck alternator wiring

Only after all that consider failure of voltage regulator that is built into pcm


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Discussion Starter #3
Never pull the old car guy trick of disconnecting and reconnecting battery while engine is running as it produces a voltage spike that can fry electronics like the pcm(aka computer)

First check battery temp sensory in battery tray

Next verify both ends both battery cables clean and tight

Next check the back of engine to firewall ground strap - both ends clean and tight

Next check alternator wiring

Then if all above good pull alternator and have it tested

Then if good recheck alternator wiring

Only after all that consider failure of voltage regulator that is built into pcm


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Thanks Digger! How do I check the temp sensor? Batter cables are clean and tight, zero corrosion, they look new. Roger on disconnecting the cables, I'm learning as I go. I have read on the forum that you can put an external voltage regulator and not have to replace pcm, is this correct?
 

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Make sure it is there intact/Undamaged and plugged into Undamaged wires

Yes external voltage regulators have need used but often result in check engine lights sometimes even after a resistor is added and a cel can make you fail emissions


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Thanks Digger! How do I check the temp sensor? Batter cables are clean and tight, zero corrosion, they look new. Roger on disconnecting the cables, I'm learning as I go. I have read on the forum that you can put an external voltage regulator and not have to replace pcm, is this correct?
Welcome to the Forum, from Cave Creek AZ.
 

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When I had my TJ repainted in the summer '18, shortly after that I had the code for low battery temp. That's when I discovered that there was a sensor under the battery. Paint to get to, you have to remove the battery and the tray. That had all been undone when the Jeep was painted because the right front fender was replace due to removing the PO's snorkel. I ignored the light and drove it without issue. I loaned it to a friend who is a mechanic and while he had it, he removed the battery and tray and hooked the sensor back up.

Point is that a bad or disconnected battery temp sensor will give you a MIL light but that's all. IIRC the code was P0517.
 
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After being snowed in for nearly a week with a dead jeep, I replaced the alternator yesterday. She fired right up and sounded great. Today, back to the original problem. It'll turn but not catch. If I apply gas, engine will rev but as soon as my foot is off the gas peddle, it dies.

I'm going to look into the battery temp sensor tomorrow. Fingers crossed the PCM isn't on the way out.
 

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I'm two weeks into owning a new to me 2003. I'm excited to learn the basic mechanics, did not expect it to be this early on into the relationship. Last Sunday it through the check gage light, and code P1954, over charged battery. It has not shown the light again. Last night it would not start. It turns but won't catch. If you press on the gas, the motor runs but as soon as you left off it dies. Felt like it could be fuel related. Replaced the fuel pump relay, tried starter spray in the air horn, banged on the gas tank while trying to start to eliminate fuel pump. Battery is fully charged. Disconnected the battery and let it sit for 15-20 mins to reset the computer. Fired right up. Went to have the alternator checked, reading a little high. Disconnected the battery while running, and it did not die. Is it possible the alternator is simply at the end of its life? Guy at Advanced Auto was great. He said it's most likely that the code from last week was causing the start issues as the computer was trying to readjust to high battery voltage. I have a new alternator sitting in the passenger seat.

Now wondering if I bought someone else's head ache. She has 158k miles, real main seal leak and needs the front end diff replaced/rebuilt. Electrical short with the blinkers (randomly come on and engage the hazards). After this, i'm concerned that there is major electrical issues. Not a spot of rust on her, she's only lived in the south. Suspension is in top shape, transmission has been overhauled. I'm the second owner, why would someone give up one after 18 years??

Thanks in advance for any/all input.
MAYBE it's HER TIME OF THE MONTH !!
 

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If I apply gas, engine will rev but as soon as my foot is off the gas peddle, it dies.
This is typical of a bad battery. You may have charged the battery, but have you had it load tested? If not get the battery properly tested. The high voltage from the alternator could just be the PCM trying to get some charge into a poor battery.
 

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After being snowed in for nearly a week with a dead jeep, I replaced the alternator yesterday. She fired right up and sounded great. Today, back to the original problem. It'll turn but not catch. If I apply gas, engine will rev but as soon as my foot is off the gas peddle, it dies.

I'm going to look into the battery temp sensor tomorrow. Fingers crossed the PCM isn't on the way out.
With 150k, replace crank and cam sensors. Ask me how I know. MOPAR parts Only for this!

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Discussion Starter #14
This is typical of a bad battery. You may have charged the battery, but have you had it load tested? If not get the battery properly tested. The high voltage from the alternator could just be the PCM trying to get some charge into a poor battery.
Good to know!! I have not had the battery tested, but will. Thanks!!
 

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With 150k, replace crank and cam sensors. Ask me how I know. MOPAR parts Only for this!

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Awesome. It fired right up this morning. She's at the mechanic to get the front diff rebuilt. I will check into the sensors when she's returned. Thanks!!
 

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Replacing sensors randomly is really lame

Nothing wears in a hall sensor so unless bad do not randomly replace as
Not all new parts are good parts
And
On the 4.0 the crank sensor is a pain to get to


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A dead battery doesn't keep a vehicle from running, it keeps it from starting. A bad alternator keeps it from running. If that weren't the case you'd only be able to start your car once...

If you had a bad alternator your battery would be dead and the vehicle wouldn't run. Given your symptoms this is a possibility, but also super easy to check. 1. Battery voltage with vehicle off should be able 12.6V+. 2. Voltage at battery with vehicle on should be 13-14V right after starting. Use a multimeter and check to see what happens to the voltage when you let off the gas.

My guess is vacuum line.

d-
 

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Discussion Starter #18
Replacing sensors randomly is really lame

Nothing wears in a hall sensor so unless bad do not randomly replace as
Not all new parts are good parts
And
On the 4.0 the crank sensor is a pain to get to


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Good Morning Digger,
Jeep as been at the shop for almost 2 weeks. The PCM is bad. I've looked a little online and it seems like it's pretty plug and play. Provide the VIN to a vendor, and it comes pre-programmed. Would you advice trying to take it on or pay for the mechanic to replace? Thanks!!
 

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Good Morning Digger,
Jeep as been at the shop for almost 2 weeks. The PCM is bad. I've looked a little online and it seems like it's pretty plug and play. Provide the VIN to a vendor, and it comes pre-programmed. Would you advice trying to take it on or pay for the mechanic to replace? Thanks!!
If you are even remotely handy then replace it yourself. It is simple enough to get to which will save you the labor, and it will save you the markup that a mechanic will charge you to get the parts.

d-
 

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Discussion Starter #20
If you are even remotely handy then replace it yourself. It is simple enough to get to which will save you the labor, and it will save you the markup that a mechanic will charge you to get the parts.

d-
Everything I'm seeing looks almost too easy. Growing up I always heard about the computer going out and how the vehicle is pretty much done at that point. Thanks! I'm going to order one and expect the best.
 
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