Engine Driven A/C Noise

When I bought this MH last March the A/C didn?t cool. Although there was pressure in the system it was low on freon. I recharged and it cooled great.

However sometimes when the engine has been reving above 2500 rpm something makes a hissing / screeching sound that sounds like it?s coming from the evaporator area. After the rpm?s drop to my normal cruise of 2000 to 2200 rpm the sound goes away.

Lately it?s been taking longer and longer for the noise to stop. Sometimes I have to turn the compressor off and back on to stop it. It will then be ok until the engine has to rev up again.

Is there an expansion valve at the evaporator coil that could make this sound?
Is it a sign of over or under charge?

How do I troubleshoot and fix this?

Tom

HIssing could be the expansion valve, which is located near the evaporator. Screaching sounds like either a beraring (Blower motor) or drive belt (Engine) .. But beyond that.. I have no idea.. Unless the sound is NOT A/C related.

Get a $3.99 mechanic?s stethoscope at Harbor Freight to isolate the source of the noise.  https://www.harborfreight.com/mechanics-stethoscope-63296.html

It?s more of a hiss than a screech. I have a mechanics stethoscope but of course it?s at home. I don?t mind buying another but I don?t think I can get it to make the noise while stopped.

Doesn?t seem like any belt or bearing noise I?ve heard before. It doesn?t instantly stop when I hit the compressor switch. It takes a second or two to quit. Like the pressure has to bleed off a little (?).

Tom

I think they normally have a high pressure switch that shuts the compressor off if head pressure gets to high, but I wonder if it might have a high pressure bypass.
Have you cleaned the condenser coil lately?

Something you might be able to try, Tom.  Many years ago when I worked on automotive A/C, I would sometimes "go for a ride" with my (extra long) gage hoses hooked up and the gage head laying against windshield.......all with only the "second" hood latch keeping the hood from coming open (whilst keeping the hoses from getting pinched).  I felt I got more realistic data that way.  Maybe you could ck out the high pressure readings at 2500 rpm in a similar fashion...... but your ride would be r-e-a-l-l-y noisy w/o a doghouse in place.  Ouch.

BTW- How did you accomplish your re-charge?  Did you add refrigerant by wt up to the spec amt or use another method?  (Not likely the problem; charging does not have to be super, super precise normally.)

Agree with Neal....... a look at condenser cleanliness is probably in order.

Good luck with your troubleshooting........... and safe travels............ ed s

Let us know what you find....

I cleaned the condenser coils a year ago, but have put on 12,000 miles since then. Will try that first.

I don?t have adapters for my old R12/22 gauges to attach to R134a fittings. I?m usually more precise, but if memory serves I added Freon until the compressor stopped cycling on and off due to the low pressure cutoff.

I won?t be able to deal with it until I get back home in May. I?ll follow up here with what I find.

Tom

Just my guess, Tom:  Your system is a bit low on refrigerant; at 2500 rpm, you have a "starved" evaporator I'm thinkin' ........... even with the TX valve calling for "wide open".  Undercharged systems will usually have refrigerant that is in 2 phases in the liq line, i.e.,  liq refrigerant and refrigerant vapor (AKA bubbles).  Those bubbles could make significant noise as they go by the seat in the TX valve.  Maybe that adds up ................ dunno for sure.

Just some of my 'sperience: When charging a TX valve system (back in the day, for me) it was always faster to charge until the "bubbles" disappeared from the sight glass (if no sight glass, I would add one to the customer's system) then add more refrigerant (approx 8oz for a 3-Ton residential system .......... maybe 4 oz for an automotive system.)  [You may not have a sight glass; 'twould save 5 bucks in the production of an expen$ive MH, right?]  :^(  Then I'd allow the system to come to equilibrium whilst putting away tools or whatever; and then go back and check pressures and superheat (you can adjust the superheat on many [but not all] TX valves).  Very often the system needed no more "tweaking" and I could get on my way to my next sweltering Houston customer.

I would suggest putting another 4 oz or so in the system and check the "noise" results.  If you overcharge an expansion-valve system slightly, the only real drawback is that you reduce the heat transfer ability of the condenser a teensy bit because you have liquid refrigerant "stored" in an additional  condenser tube or two.  Not "ideal" but not a big deal either.

Oh, and if you don't know the history of recharges on your system, I would suggest you add about 4oz of oil to the system first.  Auto parts places have the precharged cans for R-134 systems with instructions (it's easy).  The product that does not have dye in it would be preferred.  The justification for the addition of oil  is that "freon" type refrigerants have a great affinity for oil (in antiquity, low-vapor-pressure refrigerants were used to clean parts.....but then you would have to protect them from ambient air cuz they would rust overnight.)  When freon leaves your system, some oil goes along for the ride .... just the physics of it.

We have several forum guys who have significant knowledge/experience in refrigeration stuff.  Maybe they will chime in re this riddle of yours.  This is, obviously, just my rambling on and on $.02 worth. 

Many readers will be looking fwd to your follow up, after May.

Best .................... ed s