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Discussion Starter · #1 ·
New to the forum, and looking for some insight / help on a problem I'm having. My daughter tells me the battery light is coming on periodically on her PT Cruiser, but not consistantly. Sometimes at idle, sometimes driving, lights on lights off, etc. At idle I check the voltage at the battery and show about 15.9 VDC, plus I notice moisture on the battery top, so likely gassing due to overcharge. I put my scanner on the car, and in live mode, it shows about 14.5 VDC, which is pretty much dead on spec, while my DVM still reads about 15.9 VDC. I pul all the grounds, clean them off, voltage is now about 15.6 VDC, but not enough to matter. I've ohmed out the battery cable to the alternator, with about 0-2 ohms of resistance. So I'm left with 2 things I can think of. 1. I have high resistance on the sensing lead from the alternator to the PCM or 2. A bad alternator. Any thoughts or guidance on typical issues, or am I missing soemthing else here that could be the problem? Thanks in advance.
 

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Welcome to the forum.

I would take it to an auto parts store, most of them can check the starting, alternator and battery in just a few minutes, all from the vehicle.

It should give a readout something like this:

 

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What is the battery voltage with the motor off? Should be around 12.4 - 12.6 volts.

Have the auto parts place also pull the codes. There should be a code stored or pending due to charging lamp turning on. How old is the battery? A weak or failing battery will cause codes to appear randomly.
 

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. . . . My daughter tells me the battery light is coming on periodically on her PT Cruiser, but not consistantly. Sometimes at idle, sometimes driving, lights on lights off, etc. At idle I check the voltage at the battery and show about 15.9 VDC, plus I notice moisture on the battery top, so likely gassing due to overcharge. I put my scanner on the car, and in live mode, it shows about 14.5 VDC, which is pretty much dead on spec, while my DVM still reads about 15.9 VDC. I pul all the grounds, clean them off, voltage is now about 15.6 VDC, but not enough to matter. I've ohmed out the battery cable to the alternator, with about 0-2 ohms of resistance. So I'm left with 2 things I can think of. 1. I have high resistance on the sensing lead from the alternator to the PCM or 2. A bad alternator. Any thoughts or guidance on typical issues, or am I missing soemthing else here that could be the problem? . . . .
Your scan tool shows the voltage 14.5 V that the PCM (powertrain control module) is sensing on the charging circuit. Measured at the battery voltage is 15.9 V for a difference of 1.4 V. This is excessive voltage drop and implies a poor connection or excessive resistance in the sensing circuit between the alternator and PCM. Check the generator sensing circuit and field control circuit. See attached image.

A failing alternator would tend to undercharge and you would NOT see 15.9 V at the battery. So you can eliminate a failing alternator as a possibility.

Charging System Wiring.gif
 

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Discussion Starter · #5 ·
Your scan tool shows the voltage 14.5 V that the PCM (powertrain control module) is sensing on the charging circuit. Measured at the battery voltage is 15.9 V for a difference of 1.4 V. This is excessive voltage drop and implies a poor connection or excessive resistance in the sensing circuit between the alternator and PCM. Check the generator sensing circuit and field control circuit. See attached image.

A failing alternator would tend to undercharge and you would NOT see 15.9 V at the battery. So you can eliminate a failing alternator as a possibility.

View attachment 73365
Bob,

Thanks for the diagram, that really helps. My bad alternator thought was a stab in the dark, casue there just isn't much else to look at. I'll update when I get back under it with what I have found.
 

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Bob,

Thanks for the diagram, that really helps. My bad alternator thought was a stab in the dark, casue there just isn't much else to look at. I'll update when I get back under it with what I have found.
You'll be glad if it's something other than the alternator, they're not easy to replace.

Here's another thread about the battery light:

 

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Discussion Starter · #7 ·
You'll be glad if it's something other than the alternator, they're not easy to replace.

Here's another thread about the battery light:


Bob,

Thanks again for the diagram. With the battery disconnected measured for continuity C1 24 to P2, dead short, pin to grounds and + battery cable; open, C2 19 to P1 dead short, to GND open, but to the + battery cable 5M ohms, which leads me to believe enough voltage can seep across to causing the charging system to be fooled. No codes, which makes sense since the PCM is getting faulty info. Assuming I am correct in my line of thinking, is there a "known" trouble area for this wire / cable bundle, or a simpler fix then tearing out the wiring harness? If I'm off base, happy for any other input or ideas.
 

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. . . With the battery disconnected measured for continuity C1 24 to P2, dead short, pin to grounds and + battery cable; open, C2 19 to P1 dead short, to GND open, but to the + battery cable 5M ohms, which leads me to believe enough voltage can seep across to causing the charging system to be fooled. No codes, which makes sense since the PCM is getting faulty info. Assuming I am correct in my line of thinking, is there a "known" trouble area for this wire / cable bundle, or a simpler fix then tearing out the wiring harness? If I'm off base, happy for any other input or ideas. . . .
By "dead short" in your testing I believe you mean the circuit checked had continuity??? You need to do a dynamic test with the engine idling and check for excessive voltage drop.

You have already checked voltage at the battery with engine running. You indicated 15.9 volt between positive and negative battery posts. In a similar fashion back probe the 2 wire connector at the generator. Meter positive lead on terminal #2 at the generator connector and other meter probe on battery negative post. Note reading.

Then check the voltage between connector C1 pin #24 and battery negative post terminal. If there is more than a 0.1 volt difference then there is excessive resistance in the sense circuit between generator #2 terminal and connector C1 pin #24 at the PCM. I would suspect the 2 splices shown in the diagram: S103 and S141.
 

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Have you checked all of the ground connections inside the driver side front fender? There are approximately 7 ground connections here IIRC. And yes, this has been a known problem area on the PT Cruisers. Those ground connections are prone to exposure of fumes from the battery and can corrode or break loose from their connectors. You'll need to remove the air box in order to access them, but it's not that difficult. Take you time and check each wire and connector for corrosion and breaks. Clean the connections and ground lugs and reconnect. Even if this doesn't solve the problem, you'll have the peace of mind knowing that you eliminated a possible cause.
 
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