2004 Ford Ranger AC or alternator problem?

The other day when I was driving at night all of a sudden I noticed that when the AC compressor kicked on my headlights and dash lights would start to get very dim. When I got home I looked took a look under the hood and whenever the AC compressor would kick on a slight squealing/squeaking would start. I thought maybe the alternator was going bad so I took a volt meter and hooked it up to the battery terminals. Whenever the compressor kicked on the voltage would steadily drop and then come back up when the compressor turned off. You could also visually see the belt and the fan get slower when the compressor kicked on. Well I thought the alternator was the culprit so I took the old one out and had it tested. It failed the test at the auto parts store so I purchased a “new” remanufactured one from NAPA and put it in. Same thing still happens. Whenever the compressor kicks on the lights dim, and you can visually see the belt slow and the engine fans slows (mechanical belt driven fan). But if you watch the tach when this happens the engine speed does not seem to be decreasing. I’m just trying to figure out if it’s the AC compressor that’s the problem, or if it’s a problem with the alternator still. Fuses and relays seem to be fine also.

Unplug the mag coil on the compressor and then turn on the A/C full blast on the ‘Max A/C’ setting. If the lights do not dim, the mag coil is suspect. Check the resistance on the mag coil. It should be higher than 2 ohms. The mag coil is replaceable on these trucks without de-gassing the system.

I unplugged the mag coil and did what was suggested. The voltage stayed very close to the same with the A/C on full blast (14.2 at the battery). Measured the resistance on the mag coil itself in I got 3.5 ohms which is within the acceptable range. Plugged the coil back in on the compressor and the voltage dropped again (11.2 at the battery). Could it still be the mag coil even if the resistance is fine?

I think/know the belt is slipping. You say things slow down, except the engine.

Agree with Elly, new belt and possibly tensioner if applicable.

EllyEllis and Barkydog may be right. But if a correctly tensioned new serpentine belt does not cure the problem, check to see if the vibration damper is slipping. Put a mark from the outer rim to the inner hub; run the engine with the A/C and head lights ‘ON’; shut the engine ‘OFF’; and see if the marks are still alligned. If not the outer rim has become unbonded from the inner and the vibration damper needs to be replaced.

Oops, It looks like the drive pulley is mounted to the center hub. If the outer rim were slipping the Crank Position Sensor would be giving an erronous signel and the PCM would be objecting with a DTC.

Sarry – a red herring

Compressor may be getting ready to sieze.

Have you measured the voltage at the battery when this happens, with a DVM? What is it before, and after you turn the AC on?

Researcher was right. Harmonic balancer pulley was slipping and about to come off. Thanks for all the input though.