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Mike S.
Mike S., ASE Certified Technician
Category: Cadillac
Satisfied Customers: 6777
Experience:  ASE certified
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The aultornator is charging at intermediate voltage and is

Customer Question

The aultornator is charging at intermediate voltage and is very inconsistent, at times the aultornator won't charge at all then I leave the car for a couple days and it starts up and charges fine for a while and then it will stop charging I have had the rebuilt twice aultornator twice, not sure if it's not the ecm or something? This all started when my service battery charging system message came up and I seen that the voltage was low so I had battery checked, replaced the aultornator, still didn't fix it so I got a new battery and replaced the alternator again, still hasn't fixed it! Now I have a brand new alternator and don't want to fry it?
JA: How old is the battery? And have you tested it with a voltmeter?
Customer: Yes brand new battery
JA: What is the model/year of your Cadillac?
Customer: 2007 Cadillac dts
JA: Are you fixing your DTS yourself? What have you tried so far?
Customer: Yes, as I described above!
JA: Anything else you want the mechanic to know before I connect you?
Customer: Not that I can think of!
Submitted: 1 month ago.
Category: Cadillac
Expert:  Mike S. replied 1 month ago.

Hi, I am Mike, welcome. I'm reviewing your question.

Expert:  Mike S. replied 1 month ago.

According to the wiring diagram from which I have uploaded, the alternator is controlled by the ecm and the ecm communicates with the bcm it appears over data lines, so at this point I believe the next best step is to have the trouble codes retrieved from all of the modules and give these codes to me. These modules would include both the ecm and bcm. If you don't have a scan tool that can access these module's codes, perhaps trying a few auto-parts stores near you, they usually retrieve them for free or you may need to go to the dealer or anyone basically who has a scan tool capable of getting those codes, usually a simple code reader isn't enough.

Expert:  Mike S. replied 1 month ago.

You could also try this test, you may b able to just skip right to step 6.

Expert:  Mike S. replied 1 month ago.

The L term would be where the orange wire is. F where the gray wire is.

Customer: replied 1 month ago.
I will check that and get back with you,thanks!
Expert:  Mike S. replied 1 month ago.

ok

Customer: replied 1 month ago.
Am I checking the "regulator turn on signal" when checking the L-term?
Customer: replied 1 month ago.
Also I have the schematic here in front of me and I was wondering by looking at the schematic how can you tell it's 5volts?
Expert:  Mike S. replied 1 month ago.

Computer and sensor power leads are almost always 5 volts.

Are you referring to step 7? If so, what was the voltage reading?

Expert:  Mike S. replied 1 month ago.

keep in mind you said your problem was intermittent, so you might have to catch it in the act and be ready to test when it's not charging.

Customer: replied 1 month ago.
Haven't checked it yet going to be putting the new alternator in tomorrow and I will let you know! Can you tell me what can cause a alternator to "burn up"? From what I've read the battery current sensor sends a PWM signal which decides gen. Duty cycle and voltage output, so my theory is that if the batt. Current sensor is bad and is telling the aultornator to output at 100% duty cycle all the time,could that burn up a alternator?Is there a way to check the PWM signal with a meter?
Expert:  Mike S. replied 1 month ago.

I don't even know what a PWM is.

Customer: replied 1 month ago.
Pulse Width Modulation Signal
Expert:  Mike S. replied 1 month ago.

ok, I still have no test for it.

Customer: replied 1 month ago.
What do you think about my theory on burning up the aultornator?
Expert:  Mike S. replied 1 month ago.

it sounds logical.

Expert:  Mike S. replied 1 month ago.

Did you price a new battery current sensor?

Customer: replied 1 month ago.
Hey Mike , I got the battery current sensor and installed that, installed the new aultornator and measured the voltage at the L-term. I'm only getting 3.9 VDC when I test with my meter, is 3.9 acceptable?
Expert:  Mike S. replied 1 month ago.

Is that with the engine off or on?

Customer: replied 1 month ago.
4.41 is best reading I'v got.
Expert:  Mike S. replied 1 month ago.

Is that with the engine off or on?

Customer: replied 1 month ago.
ON
Customer: replied 1 month ago.
0 when off
Expert:  Mike S. replied 1 month ago.

I see where you may be confused and I am too a bit, it says at step 7 "is the voltage within the specified value" and it just says 5. usually when something says within it gives you 2 numbers to be in between.

Is the alternator still charging inconsistent?

Customer: replied 1 month ago.
last night I took the car for a 15 mile cruise and it was hovering between 14 and 14.5 and this morning when I drove to the store it was around 12.9 +/- .1.
Customer: replied 1 month ago.
Does the BCM determine the output of the generator through the battery current sensor or from the ECM serial communication?Is the F-Term on the ECM a Output from the ECM to the generator or a input to the ECM from the generator?
Customer: replied 1 month ago.
Before leaving the store I unplugged the
F&L-term connection from the generator because I read if there is a field control circuit failure the regulator would default to 13.8 VDC , after this it has continuously ran at 13.6 vdc so it seems the regulator functions properly! I think I would be better off running the vehicle with this disconnected until the real issue is resolved, do you see any issues with this?
Expert:  Mike S. replied 1 month ago.

Did you ever retrieve any trouble codes?

Expert:  Mike S. replied 1 month ago.

From looking at this schematic, I would say both the L and F are input voltages from the ECM cause they both go to ground at the alternator end.

Expert:  Mike S. replied 1 month ago.

I am reading that this vehicle has a boost idle thing, so unless the charging system message is still coming on the inconsistent charging may be normal. Here read for yourself,

Expert:  Mike S. replied 1 month ago.

Here is more from that section,

Expert:  Mike S. replied 1 month ago.

Do you have an actual voltmeter on your instrument cluster or are you hooking up a multi-meter or what?

Expert:  Mike S. replied 1 month ago.

Since the bcm and ecm communicate through computer data lines and it appears the battery temperature sensor gives information directly yo the bcm and the bcm relays this info to the ecm through data lines, the best way to diagnose this is by checking the trouble codes from both of those modules and actually all of the modules just incase the instrument cluster has it's own module, then if there is a trouble code, I'll look up the troubleshooting procedure for that trouble code.

Customer: replied 1 month ago.
No I will be setting up a appointment with shop to read the codes on monday.
Do you know a way to check the signal from the F-Term?
Expert:  Mike S. replied 1 month ago.

With a scan tool.

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