Q: "Drastic" voltage drop when A/C is on and stopped...

[Zim Hosein]

Recently I've noticed that when I'm stopped at a light and the A/C is on, the voltage gauge starts dropping down below 12V, sometimes close to the 10V line. The battery light never comes on nor does the check engine light. Any ideas on what might be wrong or is this normal? :confused: Any assistance would be appreciated! Thanks in advance! :beer:

[fastblackmerc]

Recently I've noticed that when I'm stopped at a light and the A/C is on, the voltage gauge starts dropping down below 12V, sometimes close to the 10V line. The battery light never comes on nor does the check engine light. Any ideas on what might be wrong or is this normal? :confused: Any assistance would be appreciated! Thanks in advance! :beer:

Do you have under drive pulleys installed? If so the idle speed needs to be about 200RPM higher.

[justbob]

That's odd. The battery light is designed to start flickering on at around 13.2 at a high draw. Other than that your PCM output varies its signal to the alt between 9.0V to 14.4 under normal operation factoring in load, speed, TPS, MAF, and ECT input. Check your voltage at the battery and the battery wire at the alt. idling with everything off and again with everything on. Don't go off your interior gauge, use a multi meter.

Secondly if you have an Xcal, data log the same test through the PCM port. Keep in mind there can be small variables in your findings on all four tests, that is normal due to wire lengths, however they shouldn't vary anymore than .2 or so.

Third, confirm good ground with your red probe on the Alt wire and your black probe at various grounds, (battery- and engine block.)

Final test, KOEO (key on, engine off), disconnect the three wire terminal at the alt. and test for 12V at the big yellow wire, 0V at the center Green wire, and 9V at the small yellow wire. Plug it fully back in and have someone monitor your PCM readings while idling under load as you gently wiggle the three wires and push in on the three wires.

If everything passes I would load test the battery.

I just went thru this and it was the 9 volt PCM signal wire at the three wire terminal. I barely pulled on it and it came right out. My battery charged the alt just fine, but when tested through the PCM it was only seeing 13.0 - 13.1 causing the battery light to stay lit.

Hope this helps. Only tools needed are a $5 multimeter and anything that can monitor voltage thru the PCM. (Xcal, OBD11 scanner. ECT)




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[Zim Hosein]

Do you have under drive pulleys installed? If so the idle speed needs to be about 200RPM higher.

No fastblackmerc, I do not have under drive pulleys installed, she is bone stock. How does one adjust the idle speed? :confused:

Thanks in advance! :beer:

[RF Overlord]

One doesn't, unless one has a hand-held tuner with that option, but it isn't relevant if you don't have U/D pulleys.

Do what justbob said above... ^^^

[Zim Hosein]

That's odd. The battery light is designed to start flickering on at around 13.2 at a high draw. Other than that your PCM output varies its signal to the alt between 9.0V to 14.4 under normal operation factoring in load, speed, TPS, MAF, and ECT input. Check your voltage at the battery and the battery wire at the alt. idling with everything off and again with everything on. Don't go off your interior gauge, use a multi meter.

Secondly if you have an Xcal, data log the same test through the PCM port. Keep in mind there can be small variables in your findings on all four tests, that is normal due to wire lengths, however they shouldn't vary anymore than .2 or so.

Third, confirm good ground with your red probe on the Alt wire and your black probe at various grounds, (battery- and engine block.)

Final test, KOEO (key on, engine off), disconnect the three wire terminal at the alt. and test for 12V at the big yellow wire, 0V at the center Green wire, and 9V at the small yellow wire. Plug it fully back in and have someone monitor your PCM readings while idling under load as you gently wiggle the three wires and push in on the three wires.

If everything passes I would load test the battery.

I just went thru this and it was the 9 volt PCM signal wire at the three wire terminal. I barely pulled on it and it came right out. My battery charged the alt just fine, but when tested through the PCM it was only seeing 13.0 - 13.1 causing the battery light to stay lit.

Hope this helps. Only tools needed are a $5 multimeter and anything that can monitor voltage thru the PCM. (Xcal, OBD11 scanner. ECT)

First off, I can't thank you enough for the write up on what to check justbob! :bows:

That being said, I don't currently have a multimeter nor a scanner that could monitor voltage thru the PCM :o

As I said the battery light never comes on so I thought it might be something simple like maybe it was a sign of my alternator going bad, bad serpentine belt, dying battery, etc. :confused:

Since I still have a warranty, I'm going to play it safe and take her to the dealer next week; I'll post my results so that this thread might assist someone in the future.
Thanks to everyone that offered suggestions! :beer: