Low Voltage issues, new alternator too, and good battery, help, please.

[LT1drifter]

My battery was good, had it tested at Autozone, alternator was only reading 13.5 volts when i tested it on the car, and when the load was put on, with the AC on too, it dropped down to 10.5-11 volts, figured alternator must be going bad, so I replaced it int the parking lot, took 10 minutes. Tested it then, doing the same thing. It reads right above the red mark in the car, which looks to be around 11-12 volts, but on a test machine it shows 13.5volts consistently. My problem is now, it keeps dropping low enough to trigger the service engine soon light, I have checked the wiring, everything looks fine, any ideas on what it could possibly be?

Thank you for your help.

[Rob B (shoebox)]

Most often, the gauge can read almost a volt lower than if you take the reading at the battery. IMHO, this is because of a voltage drop across the ignition switch or the harness connection there (which is often poor due to excess heat).

You can easily compare voltage readings in the fuse block, between constant and ignition switched sources to see this.

[Injuneer]

Is the "SES" light coming on, or is it the "check gauges" light?

What year is your car?

Check for badly corroded wiring and terminals.

[Joe 1320]

is the alternator "NEW" or rebuilt? Heat will cause an increase in resistance, when the motor warms up so do the accessories. Any problems will likely get worse as heat builds. I've seen new parts that were bad. As was suggested, check over all the battery and laternator connections, especially the grounds.

[ConElite]

Mine drops below half ways when its about 90+ out and AC and sound system blasting. I just let it be. I checked everything. All good. If you got the alternators with the "Lifetime warrenty" be prepared to change it out a couple times. Im on the 3rd one now.

[LT1drifter]

It drops off low enough to kick the service engine soon light, its a 94 btw.

[Injuneer]

It drops off low enough to kick the service engine soon light, its a 94 btw.

If its low enough to turn on the SES light and set a code (DTC 50), its low enough to affect the fuel injectors, the fuel pump and possibly cause the PCM to malfunction. If you are certain the battery and alternator are OK, you've got to check the wiring, including the grounds.

[Trans Am]

has your car got an underdrive pulley?