First of all it does it when the AC is off. It seems to do it when the clutch does it's cycle every few minutes. Now deuce in one post you say it's normal and the next you say it's not. I know damn well it's normal for it to spin every so often. I tried to video my lights last night and of course I didn't notice them do it at all. I haven't done anything really to the truck except change a couple fuses. Two of them for my blue lights that wouldn't have anything to do with it cause they only get power when the switch is on. The other fuse was for my air compressor setup. Which thinking on it now was the fused all time hot wire from the battery. It was in a "water tight" fuse holder that took on water and blew the fuse. I suppose it's possible it was causing a short of sorts and could be a cause of the problem. I'll keep an eye on it

 

That's because in response to one post, it was "cycles every few seconds", and the next it was "on for 2, off for 2" (short cycling). "A few" is a pretty ambiguous definition. 3? 5? 12? I took it to mean that he considered the compressor cycling to be indicative of a problem. It certainly isn't, as you know.

The duration for which the clutch is engaged depends on many factors; system design, charge percentage, system saturation, ambient temperature, etc. A cycle that occurs 3-5 times per minute is pretty normal, though. That could mean on for "a few" (11 seconds) and off for "a few" (7 seconds), for example.

So no, I'm not contradicting myself. I'm changing my answer in light of previously unstated information. Though I'm skeptical that he actually sat there clicking the button on a stopwatch whilst watching the compressor..........

At least he didn't get into decimal point division of seconds. Then I'd be citing average human reaction times and such

ETA:

OK, figured I'd hit this one more time before going off to bed to kinda summarize.

Basically, you're getting a voltage fluctuation that is enough for noticeable change in headlamp brightness.

Step one is a charging system check. If your battery is weak, or the alternator has somewhat low output at idle or excesive diode ripple, that needs to be addressed. The most common cause I've found for your symptoms is one of those two things. If the issue lessens or vanishes with some RPM increase, that really points to those causes. Also, check your alternator output for A/C voltage. Should never see more than .5 VAC, preferably much less.

Now, even more true if this is happening at idle, when the A/C kicks on, it's dragging engine RPM down initially until the IAC compensates. Obviously, lower Alternator RPM=lower output. Grab your DVOM and digital tach/timing light, and watch the voltage drop at the moment the A/C compressor kicks on. .2-.4 V is about right, and will cause a visible dimming of the headlights. It is due to engine RPM decrease and subsequent decrease in alternator output. It has nothing to do with the compressor clutch coil. ~5 amps won't be enough to cause a noticeable change in lumens; your brake lights draw more than that.

And, of course, you know that the auxiliary fan draws quite a bit. It wasn't until very recently that engine management and electrical system calibration and isolation became good enough to largely mitigate headlight dimming when the cooling fans kick on. a 20-50 amp draw is big, and the draw spikes even higher on on start-up for most car's fans, since they're usually a simple circuit without start capacitors. And because idle increase takes longer than relay switching, this is just a fact of life with electric cooling fan motors.

 

I was going to let this one go.

1. because I'm not an expert on Computerized Engine Controls.
2. Just don't care enough to argue differing opinions online.

But, I wanted to know why and if my experiences and theories had any merit.

1st of all, I am notorious for being ambiguous in more than a few statements. 99% of the time I'm talking about "Not normal" operation.

So as not to be ambiguous....When the operator turns on his (or her) A/C via the climate controls inside the cab of their vehicle, a signal (this is a voltage) is sent to the PCM to energize the A/C clutch. The PCM will then send the signal to the A/C Clutch relay (and some other controls) to energize the relay, thus energizing the A/C compressor clutch.

The A/C clutch switch provides a signal back to the PCM to adjust idle speed to compensate for load variations. (There may also be a signal line connected to the battery to monitor for low charge voltage. I haven't looked yet so I don't know if my 98 has this signal wire or not.)

If your electrical system is already near max load because of any after-market additions and you turn on the A/C the PCM will attempt to adjust idle to compensate the additional load. If Idle speed is not increased or the voltage is not increased to compensate for the drop, the PCM will de-energize the A/C Clutch.

Unfortunately, you still have the A/C switch turned on on the climate controls, so now the PCM follows the signal to turn on the A/C compressor clutch back on. And the A/C cycle now enters an infinity loop and cycle the clutch on and off. According to the signals being received. I guess eventually a code will be thrown and the A/C clutch will stop energizing, but I don't know.

SO, it is my opinion that if you are in a pinch or do not have resources to do, or have a proper load test on your electrical system and you want to know if your system is near max load. Turn on your A/C. If it does not function normally you are at max, or near the max load for your electrical system. OH, yeah, make sure your after-market crap is also turned on, or you won't be at max load and it won't matter if your A/C is running or not.

In my case the Taurus fan setup is too big for my 96A alternator. I don't know if a 136A alternator will take care of this or not, in theory, it should.