Vehicle - 1990 F350 diesel

It is still R12 - has never been changed

It blows 'cool' at best.

A friend I work with has the gauges and equipment and several tanks of r12 . I took it to him today to check and possibly charge.

I know nothing about A/C systems...I will post here with i THINK he said to me.

He placed the gauges on....without the engine running and pressure equalized, the gauges read around 100psi.

We started the engine, put the ac on max and idled the engine up. Pressure read 30-40 on the low side, and close to or over 300 on the high side. He thought that was too high and something is blocked.

He said it took very little R12.

He thinks the problem is a stuck expansion valve.

What and WHERE is the expansion valve? Do you think this is the problem?

 

Every 1990 Ford Truck has a CCOT AC system. There's NO expansion valve.
That 300+ psi on the high side should have told him (your friend with more R12 than brains) that there is little, or no airflow over the condenser. It's very likely that the 17 year old fan clutch is bad.

Clean the crud from the outside of the condenser and radiator first. On an F-350 work truck, Easy-Off oven cleaner is probably the best. Hose it down with the oven cleaner, let it set for 10 minutes, then blast the condenser with a garden hose. Blast it from the Fan side of the radiator too, if possible.
If the AC still doesn't work right, replace the fan clutch. It's done for as far as the AC is concrned.

Yeah, I know, the motor wasn't overheating. The engine will tolerate a weak fan clutch with no problem. The AC system will wheeze out at the first sign of a fan clutch going South.
That Fan Clutch will be about $150-175 for a 7.3 Diesel, but the AC won't work again until you replace it. The engine will also start to overheat at idle and in traffic really soon if you let it go much longer.

 

I don't think this is the case ...but what do I know?

#1. The condenser is clean. I counted two moths in it. The radiator was replaced last winter and all components were cleaned at that point. Actually, that was my friends first thought and he did inspect this, and was suprised to see how clean it was.

#2. I think the fan clutch works fine. In fact, when the fan clutch kicks in i can hear a howl that is reminiscent of the fan screaming in a Cummins powered Freightliner.

#3. The engine doesn't come close to overheating in city or on the highway. The temp gauge never passes N on NORMAL. Even towing 6000lbs.


But to entertain the weak fan clutch theory, why does low airflow cause a pressure spike on the pressure side of the system? I guess I'll look at this closer.

Fill me in so I can more intelligently discuss this!

Thanks

 

Cleaning the condenser was a nearly $0 option that could possibly solve the problem. You would be surprised at how much stuff builds up between the radiator and condenser, esprcially on "work vehicles"
( I just reread my reply and it sounded rather grumpy. My apologies if it sounded that way to you).

The best way to check the fan clutch is to watch the AC High side pressure. It's in the ~300psi range now. Mist water from a garden hose over the condenser. If the pressure drops like a rock, the fan clutch is bad.
You will probably find that , with good airflow over the condenser, the system is actually undercharged. Good airflow is critical to AC operation. It's what carries heat away from the condenser.

I'm really surprised that the fellow who worked on it didn't know this. It's the very first thing that should be checked if the head pressure is too high. At that pressure, he should have certainly known better than to try and add refrigerant.

If the pressure doesn't plummet, there is an internal blockage in the system, likely debris in the orifice tube from a failing compressor. This means a major repair. The low side pressure doesn't seem to indicate this though.

The fan clutch may be locking up at the wrong time, or at random. It may not be pulling enough air when it does lock up. There is no good way to test a fan clutch, except by watching the AC pressure as I described.
The general rule of thumb is : If you suspect a bad fan clutch, and it's more than about 8 years old, replace it. By 8 years, all fan clutches are at least getting weak. They are a wear item that is almost always ignored. Every summer I see a whole slew of bad fan clutches on cars in the 8-15 year range. Your symptoms are classic for this failure. Misting that condenser and watching the head pressure will verify it.

 

If I might add this point. Checking pressures at idle isn't the best either. You should bring the RPMs up to 1500-2000 RPMs. That way you are getting air flow. 300 PSI R-12 high but not that high. Outside temp & humidity play a part in high pressure readings also. lsrx101 is right running water over condensor will drop high side pressures. Something else to consider is if you haven't serviced this A/C unit maybe now is a good time. Service, what do you mean? I change my filter/dryer out every 8-10 years. On your Ford it is called an accumulator. It is the tank looking thing by the heater, A/C unit. If your friend has all the A/C equipment then you can recover & filter your R-12 and reuse it. So the only cost is an accumulator which is about $35-$40.

 

Good points kermydog. I don't often hear of folks actually "servicing" the AC system. It's one of those "should do" things than nobody ever does. Most people don't know the accumulator is actually a wear item.

I will disagree with one point, though. In this case, checking the pressure at idle is appropriate. The job of the fan clutch is to provide good airflow at idle. If you have to rev the engine to get good airflow, the fan isn't doing it's job.

Of course, system performance readings should be done at ~1500-2000 to get the compressor up to optimum speed.

 

The pressures were taken at idle and 1500-2000 rpm. I don't recall what the pressure was at idle. 300 was taken at higher rpms.