AC-power problem

Strange thing, my truck loses power when I turn the AC fan switch to the highest position. Starts to stumble and won't rev to higher rpm. If I turn the fan speed down a notch, it regains power and acts normal.
Its an 87 SC 4x4, 2.9l 5 speed.
Any ideas?

That is really weird. The only thing I can think of is a problem on the high-speed wire, it being crossed with something else, or maybe an issue with the fan motor itself. If it is creating interference it could effect the computer.

The circuit operates differently in high speed, bypassing the speed control resistor completely and just putting power to the fan directly. Since the resistor is out of the loop in high speed it means the fan should actually pull less power in high speed, so a weak alternator is unlikely. If that wire is touching something else it shouldn't it could effect how the engine runs by interfering with a sensor signal.

Also, the fan itself is somewhat close to the computer, and could be emitting EMF.

I'd start by unplugging the fan and putting a light bulb in it's place in the circuit, go for a drive and see what happens. If the problem is gone I'd get a new fan motor. If the problem persists, then I would test the alternator, and if it checks out start tracing the high-speed wire.

What's the voltage at the battery when you turn the AC on max? Also did you try a full electrical load with everything on except the AC on?

adam08 I like your thinking but wouldn't just turning the fan on full cause the same problem or does it use a different set for AC?

Yes, I would get a volt meter and put it on the battery.
Good battery will be 12.3-12.8 volts
12.2 volts means battery can only hold 50% charge, lower is even less charge, batteries are usually at this voltage after 5 to 7 years of use, time for a new battery

Start engine
Battery should now read 14.6volts to 14.9volts, this is "recharge" voltage, it will last about 3 to 5 minutes, the newer the battery the less time it takes to recharge it.
After that it should drop down to 13.5-13.8volts, this is "maintenance" voltage, enough voltage to run the vehicle and keep battery charged but not "cook it"

Turn fan on to high, and turn on the head lights, voltage should drop then come back up to 13.5volts at least

If voltage drops down too low engine electrics start to "brown out", misfiring and stumbling

Alternators have 3 "Fields" that generate voltage, as they get older it is common for 1 field to fail, dimming head lights at idle is a sign of that, but 2 fields can run vehicle and keep battery charged as long as you are driving, above 1,500 RPM.
If a second field fails then there simply isn't enough voltage to go around

fan motor brushes are probably worn and arching, creates electrical noise.
swap out fan motor, or get a noise filter from an audio shop.
also the fan connectors corrode, reset that connector several times to wipe the pins.

RonD said:

Yes, I would get a volt meter and put it on the battery.
Good battery will be 12.3-12.8 volts
12.2 volts means battery can only hold 50% charge, low is even less charge, batteries are usually at this voltage after 5 to 7 years of use, time for a new battery

Start engine
Battery should now read 14.6volts to 14.9volts, this is "recharge" voltage, it will last about 3 to 5 minutes, the newer the battery the less time it takes to recharge it.
After that it should drop down to 13.5-13.8volts, this is "maintenance" voltage, enough voltage to run the vehicle and keep battery charged but not "cook it"

Turn fan on to high, and turn on the head lights, voltage should drop then come back up to 13.5volts at least

If voltage drops down too low engine electrics start to "brown out", misfiring and stumbling

Alternators have 3 "Fields" that generate voltage, as they get older it is common for 1 field to fail, dimming head lights at idle is a sign of that, but 2 fields can run vehicle and keep battery charged as long as you are driving, above 1,500 RPM.
If a second field fails then there simply isn't enough voltage to go around

Click to expand...

a few years ago the alternator crapped out on my '05 about 35 miles from home.
I turned everything off and drove home. battery was down to 9 volts and it was starting to stumble the last mile or so. at that point just hitting the brakes was almost killing the engine. too bad it was a 90+ day, and good thing it wasn't a 10 degree day!

I would think any load that pulls the battery down more than 2 volts would draw enough current that smoke would reveal the culprit.

adsm08 said:

That is really weird. The only thing I can think of is a problem on the high-speed wire, it being crossed with something else, or maybe an issue with the fan motor itself. If it is creating interference it could effect the computer.

The circuit operates differently in high speed, bypassing the speed control resistor completely and just putting power to the fan directly. Since the resistor is out of the loop in high speed it means the fan should actually pull less power in high speed, so a weak alternator is unlikely. If that wire is touching something else it shouldn't it could effect how the engine runs by interfering with a sensor signal.

Also, the fan itself is somewhat close to the computer, and could be emitting EMF.

I'd start by unplugging the fan and putting a light bulb in it's place in the circuit, go for a drive and see what happens. If the problem is gone I'd get a new fan motor. If the problem persists, then I would test the alternator, and if it checks out start tracing the high-speed wire.

Click to expand...

I disagree.
there will always be full voltage between the power source and ground, pursuant to the battery being capable of providing enough power.
a resistor simple divides the circuit, some of the power it used up pushing current through the resistor, the rest is used to push current thru the fan.
the resistance limits the voltages ability to push current/amps, thus less power to the fan.
with the resistor removed the fan gets full battery voltage, which pushes the maximum current/amps thru the fan. that means (quoting Tim the Tool Man)

MORE POWAH!!

Thanks to all, I will look at all your suggestions this evening and let you know what I find.
Did this once before and I assumed it was the fan motor pulling too much amps (although wouldn't it trip a breaker?) I changed the fan motor and it fixed it for a few years, now back. Maybe a cheap fan motor only lasted a couple of years? This actually started up again about a year ago, so I have been just never putting the fan on high and it runs fine. But it has been well over 100* here for the past few weeks, so full speed on the fan would help keep the sweat off faster.
Thanks again,

pjtoledo said:

I disagree.
there will always be full voltage between the power source and ground, pursuant to the battery being capable of providing enough power.
a resistor simple divides the circuit, some of the power it used up pushing current through the resistor, the rest is used to push current thru the fan.
the resistance limits the voltages ability to push current/amps, thus less power to the fan.
with the resistor removed the fan gets full battery voltage, which pushes the maximum current/amps thru the fan. that means (quoting Tim the Tool Man)

MORE POWAH!!

Click to expand...

In theroy yes. But like so many other things in life the theory only works with spherical chickens in a vacuum. In reality things don't always play out like they do on paper.

It's the same principle as a light on a dimmer pulling more power when being dimmed than it does at full brightness. The voltage in the circuit may still be the same, but the current draw goes up. Also, as voltage increases amperage decreases, and vice versa.

My first guess would be the alternator is not able to keep up with the amp draw when the fan (and other stuff) is on. On high that fans draws a bunch.

Like Ron said there are three windings, all putting out 12vDC. But, the amps are additive from the three. So, weak bat, weak alt equals problem.