rangerruck

after getting a used 88' 4rnr, I have it at the mech shop getting the whole thing
checked out from top to bottom. The amazing thing is, it was 1 owner, and in 20 years, all he did was bottom end engine work, change 1 set of tires, and change out the clutch cylinder, that is it!!!! Mechanic says everything else seems to be hunky , except one thing.
When it goes into hi revs, or maybe he said 5th gear-hi revs, the a/c has a tendency to cut off for a bit of time, then cut back on, etc., etc.
I have no idea, almost sounds like alternator's electrical pull is so great elsewhere, that is does not have enough oomph to keep up with a/c demands, or maybe something else like ignition coil, or something else in the regular electrical/flow circuit of items.
Ideas would be great. thanks

myyota

Thats normal on some cars and trucks for the a/c compressor shut off while the engine is under a heavy load, i didn`t know toyotas did it, but it just reduces drag on the engine like if your getting on the freeway and your on the gas hard.

rangerruck

okay, it looks like after a little searching, it is going to be the A/C amplifier.
good to know.

toy power

there's nothing wrong with it. It is cycling on and off in order to keep a constant pressure

iamsuperbleeder

does it blow cold still?

if so, I wouldn't sweat it

As toy power said, the compressor will cycle on and off while the motor's running; it's normal, and if it's the normal cycling of the compressor that the tech was talking about not being right, then I don't think I'd take my Toy back to him

just my $0.02

abecedarian

there are two safety switches just before the evaporator: a high pressure switch and low pressure switch. the high pressure switch prevents the head pressure from going too high by shutting off the a/c compressor and blowing the expansion valve / orifice, hose from the compressor to the evaporator, or compressor itself, then turns the compressor back on once the pressure drops low enough. the low pressure switch turns off the a/c if the head pressure is two low.

At speed in 5th gear there's quite a bit of air blowing over the condenser. that causes the refrigerant to return to a liquid state more easily. when it does that, the high side head pressure (pressure / hot side of the compressor) goes up. Low ambient (outdoor) temperatures can also cause the system to cycle since the condenser is more efficient at cooling the refrigerant, causing lower evaporator temperatures. There is a thermocouple that regulates the discharge temp from the cabin fan / evaporator. Using low speed on the cabin fans can cause the system to cycle- this is due to the lower airflow being able to get cooled more quickly and if the compressor was not cycled off, the evaporator would get too cold and begin to ice up.

Also, if the refrigerant charge is too low, that can cause the a/c to cycle as well and is not a good thing since the oil in the system will not be circulating properly and could cause compressor damage

The fact that the compressor cycles tells me the switches and amplifier are working properly. a check should be done to make sure the a/c is not overcharged or undercharged though, because that will also cause a similar problem.

If the vents are blowing cold, I'd suspect you have at least a sufficient charge to avoid the low pressure switch being actuated. Then it comes down to the other variables mentioned above- ambient temp, refrigerand charge, evaporator discharge temp, fan speed.

like mentioned- if the mechanic has no idea how air conditioning works, particularly a 20 year old system, maybe you should find another mechanic.

rangerruck

hmm, that is interesting. I have used this guy for 10 years, on 3 of my rides, and he has been in the same shop, same location for 60 yrs, and all his techs are ase certed. it was his mom/pop's shop, then they retired , but his ma still answers the phones; I can't see him possibly messing up the rep that his parents built, he is a good Texas boy by nature. I will def ask him , and let him know what some of the dudes here have said.
Origionally , I thought it a bit low on refrig; since the dude I bought it from never did any a/c work, and figured it was low. so I know he boosted the refrig first. It was then that he called me and said it is still cutting off, but he didn't mention that it was coming back on or not.

04tacomatrd5speed

It is normal for the ac compressor to cycle on and off when the heater controls are set to defrost.

rangerruck

turns out it is the a/c amplifier. even though this compressor is designed to cut on and off, at higher revs, like your a/c in your house, what was happening here, is the compressor-a/c was losing any electricity alltogether, apparently a common problem as well with this model a/c unit. I asked my mech about the a/c unit comes off and on regularly due to it's design with the hi and low pressure sensors, and he assured me he was very familiar with it's design.

abecedarian

for whatever reason, when the a/c compressor is shut off, it loses all electricity.
sorry, but... duh.
that doesn't tell me it's the amplifier, other than the amplifier controls the signal to the compressor.
but whatever. who am I to question the mechanic? Just because my 88's a/c does the same thing and the amplifier is fine... oh well.