Bad Alternator? Dies with AC on!

alternator output varies with RPM, so it makes sense that voltage drops when RPM drops. You should consider getting some Throttle body cleaner (not carb cleaner, there is special throttle body cleaner that is different and needs to be used). Use the throttle body cleaner to clean your IAC thoroughly and see if that helps.

What sounds like is happening is that when your A/C is turned on, it creates extra drag on the engine, so the IAC tries to increase the idle speed and it's creating a surging condition instead.

See "Surging Idle Checklist for help with your idle problems. The first two posts contain all the updates to the fixes. I continue to update it as more people post fixes or ask questions.

R12 & R134 are types of refrigerant gas used in automotive A/C systems. R12 was the original refrigerant gas used in A/C systems, but it no longer being manufactured in the USA or Canada. It is also illegal to sell it to anyone who does not have an EPA 609 A/C certification. Many Mustangs have been converted to R134 for that reason.

The A/C clutch is on the front of the A/C compressor. It has a big electromagnet that pulls the clutch hub against the pulley when electrical power is applied to the clutch.

The A/C compressor has a two wire pigtail harness that goes to the A/C clutch assembly. The pigtail has a connector plug right at the top of the A/C clutch assembly.

Go through the "Surging Idle Checklist one itenm at at time. Be sure not to skip any steps: you will find and fix the problem. Jumping around the checklist means you are more likely to jump over the exact thing that is causing your problem.

Igntion timing advanced too far (more than 14-16 degrees) will cause an idle surge. I found this out after replacing the harmonic balancer. The balancer had spun, moving the timing marks. Then I had to advance the timing to line up the marks using a timing light. That gave too much advance, resulting in an idle surge.

Be sure to set your timing with a good timing light and remove the SPOUT connector when you set it. Put the SPOUT connector back in when you finish setting the timing.

A bad O2 sensor would likely set code 41/91.

Code 41 or 91 - O2 indicates system lean. Look for a vacuum leak or failing O2 sensor.

The computer sees a lean mixture signal coming from the O2 sensors and tries to compensate by adding more fuel. Many times the end result is an engine that runs pig rich and stinks of unburned fuel.

The following is a Quote from Charles O. Probst, Ford fuel Injection & Electronic Engine control:
"When the mixture is lean, the exhaust gas has oxygen, about the same amount as the ambient air. So the sensor will generate less than 400 Millivolts. Remember lean = less voltage.

When the mixture is rich, there's less oxygen in the exhaust than in the ambient air , so voltage is generated between the two sides of the tip. The voltage is greater than 600 millivolts. Remember rich = more voltage.

Here's a tip: the newer the sensor, the more the voltage changes, swinging from as low as 0.1 volt to as much as 0.9 volt. As an oxygen sensor ages, the voltage changes get smaller and slower - the voltage change lags behind the change in exhaust gas oxygen.

Because the oxygen sensor generates its own voltage, never apply voltage and never measure resistance of the sensor circuit. To measure voltage signals, use an analog voltmeter with a high input impedance, at least 10 megohms. Remember, a digital voltmeter will average a changing voltage." End Quote

Testing the O2 sensors
Measuring the O2 sensor voltage at the computer will give you a good idea of how well they are working. You'll have to pull the passenger side kick panel off to gain access to the computer connector. Remove the plastic wiring cover to get to the back side of the wiring. Use a safety pin or paper clip to probe the connections from the rear. The computer pins are 29 (LH O2 with a dark green/pink wire) and 43 (RH O2 with a dark blue/pink wire). Use the ground next to the computer to ground the voltmeter.

Testing the O2 sensor wiring harness
Most of the common multimeters have a resistance scale. Be sure the O2 sensors are disconnected and measure the resistance from the O2 sensor body harness to the pins on the computer.

Note that all resistance tests must be done with power off. Measuring resistance with a circuit powered on will give false readings and possibly damage the meter. Do not attempt to measure the resistance of the O2 sensors, it may damage them.

The O2 sensor ground (orange wire with a ring terminal on it) is in the wiring harness for the fuel injection wiring. I grounded mine to one of the intake manifold bolts