A/C Noise In Big Blue

[Gary Lewis]

I have a noise in the A/C system that I'd like some help finding/diagnosing.

 

But I need to tell the backstory. Big Blue has a 1990 system in it with an FS10 compressor. It worked last summer but then as things warmed up this spring it wasn't cooling. Turns out the compressor had gone bad. After the shop installed a new compressor in late March I don't think I used it much as it was kind of cool out.

 

But on the trip to NM we used it some and it wasn't working well at all. So yesterday I took it back to the shop and they found that the condenser had a leak. After replacing it and recharging the system it cools fine but now there's a noise that can be heard in the cab but not in under the hood, as demonstrated in the videos below.

 

The mechanic and I are at a loss as to what it could be, but we fully agree that it only happens when the compressor is running and goes away when the A/C clutch is turned off. Janey and I drove it about 30 miles and at the end the noise was less pronounced, or we'd gotten used to it.

 

Any ideas what could be causing a noise like that?

 

 

 

[85lebaront2]

It definitely sounds like compressor noise. Have either you or the shop tried the old screwdriver stethoscope trick? I was looking to see if the idler/tensioner was bouncing in the second video, it isn't.

What do the pressures look like? Are they reasonably steady or noticeably pulsing at lower rpm?

[Gary Lewis]

It definitely sounds like compressor noise. Have either you or the shop tried the old screwdriver stethoscope trick? I was looking to see if the idler/tensioner was bouncing in the second video, it isn't.

What do the pressures look like? Are they reasonably steady or noticeably pulsing at lower rpm?

Haven't tried the screwdriver/stethoscope but may do that tomorrow and can put gauges on at the time time to answer that question. But I think I have a stethoscope that has a solid pickup so might use that instead of a screwdriver.

At times we do hear a little bit of a squeal under the hood that sorta sounds like the clutch may be slipping, but haven't pin pointed it. Maybe the stethoscope will.

Thanks.

[85lebaront2]

Haven't tried the screwdriver/stethoscope but may do that tomorrow and can put gauges on at the time time to answer that question. But I think I have a stethoscope that has a solid pickup so might use that instead of a screwdriver.

At times we do hear a little bit of a squeal under the hood that sorta sounds like the clutch may be slipping, but haven't pin pointed it. Maybe the stethoscope will.

Thanks.

Three items:

(1) pressure too high will cause a fair bit of compressor "hum"

(2) pressure too high can cause the clutch to slip and squeal or chirp on engagement

(3) pressure too high can and will result in poor cooling

FWIW, I can darn near hang meat in Darth with twice the cabin area BB has, but, I did a fair amount of insulating under the carpet, on the firewall, and the 1996 doors have insulated cards or panels whichever term you prefer.

[ArdWrknTrk]

Haven't tried the screwdriver/stethoscope but may do that tomorrow and can put gauges on at the time time to answer that question. But I think I have a stethoscope that has a solid pickup so might use that instead of a screwdriver.

At times we do hear a little bit of a squeal under the hood that sorta sounds like the clutch may be slipping, but haven't pin pointed it. Maybe the stethoscope will.

Thanks.

I watched both of your videos Gary.

It definitely sounds like the pump rather than the clutch but I suppose you'll know better if you listen closely with the stethoscope

[Gary Lewis]

I watched both of your videos Gary.

It definitely sounds like the pump rather than the clutch but I suppose you'll know better if you listen closely with the stethoscope

Bill - I asked the guy if it could be overfilled. He didn't think so as they use a machine that reads the ambient temp, pressures, etc. But I'll check it tomorrow.

Jim - Thanks. It sounds like the pump to me as well, but I'm puzzled that you hear it in the cab and not under the hood. But maybe listening to it with a 'scope will tell the tale.

[Gary Lewis]

Bill - I asked the guy if it could be overfilled. He didn't think so as they use a machine that reads the ambient temp, pressures, etc. But I'll check it tomorrow.

Jim - Thanks. It sounds like the pump to me as well, but I'm puzzled that you hear it in the cab and not under the hood. But maybe listening to it with a 'scope will tell the tale.

Looks like the pressures might be high. The video I took shows really steady needles, but the pressures look high to me.

The ambient temp was 77F and the humidity HIGH. Both doors were open and the fan on high. Didn't measure the discharge air temp but it was pretty cold.

On the left are the pressures soon after starting, about 46 and 250, and on the right are the pressures as the system got fully up to temp - 54 & 320. My table says at 75F we should be seeing 35-45 & 150-170 PSI. And at 100F we should be seeing 50-55 & 315-325 PSI, which is right where we are.

I also used my stethoscope with a solid probe and cannot find that sound under the hood. The compressor has noises but not really that one. And the receiver drier and evaporator have noises but they don't change with RPM and the one in the cab sure changes with RPM.

So is it pressure?

 

[85lebaront2]

Looks like the pressures might be high. The video I took shows really steady needles, but the pressures look high to me.

The ambient temp was 77F and the humidity HIGH. Both doors were open and the fan on high. Didn't measure the discharge air temp but it was pretty cold.

On the left are the pressures soon after starting, about 46 and 250, and on the right are the pressures as the system got fully up to temp - 54 & 320. My table says at 75F we should be seeing 35-45 & 150-170 PSI. And at 100F we should be seeing 50-55 & 315-325 PSI, which is right where we are.

I also used my stethoscope with a solid probe and cannot find that sound under the hood. The compressor has noises but not really that one. And the receiver drier and evaporator have noises but they don't change with RPM and the one in the cab sure changes with RPM.

So is it pressure?

Gary, the low side definitely seems a bit high. First what condenser are you using? Darth has a 1994-1997 gas engine one. Second, what orifice tube is in it and what evaporator core (again I have the 1994-1997 one). Third, accumulator, when Darth is running the accumulator frosts up on low fan, but will go to just wet on higher speeds. Discharge temp is around 40° F at low fan (remember humidity here can hit 100%) climbing to maybe 65 at high fan. I test with a large fan blowing on the front of the vehicle to simulate driving speed.

[ArdWrknTrk]

Looks like the pressures might be high. The video I took shows really steady needles, but the pressures look high to me.

The ambient temp was 77F and the humidity HIGH. Both doors were open and the fan on high. Didn't measure the discharge air temp but it was pretty cold.

On the left are the pressures soon after starting, about 46 and 250, and on the right are the pressures as the system got fully up to temp - 54 & 320. My table says at 75F we should be seeing 35-45 & 150-170 PSI. And at 100F we should be seeing 50-55 & 315-325 PSI, which is right where we are.

I also used my stethoscope with a solid probe and cannot find that sound under the hood. The compressor has noises but not really that one. And the receiver drier and evaporator have noises but they don't change with RPM and the one in the cab sure changes with RPM.

So is it pressure?

Glad to find out it isn't the new compressor!

Have you tried climate control 'fan' without the compressor on?

It seems odd to be the sound would be in sync with engine rpm, but I guess it's possible.

[Gary Lewis]

Gary, the low side definitely seems a bit high. First what condenser are you using? Darth has a 1994-1997 gas engine one. Second, what orifice tube is in it and what evaporator core (again I have the 1994-1997 one). Third, accumulator, when Darth is running the accumulator frosts up on low fan, but will go to just wet on higher speeds. Discharge temp is around 40° F at low fan (remember humidity here can hit 100%) climbing to maybe 65 at high fan. I test with a large fan blowing on the front of the vehicle to simulate driving speed.

Bill - The receipt shows "7-4150" which I take to mean they put in a Spectra Premium 7-4150 and Amazon says that's the right one for a 1990 system. And I have a red orifice tube in. The evaporator is from a 1990, and the accumulator is for a 1990.

Perhaps I just didn't have enough air flow? I don't think the fan clutch had engaged at that point, but I'm not sure. I don't have a large shop fan for that but the hood was open.

Jim - The fan doesn't have that noise by itself, and the noise comes and goes with the A/C compressor's clutch coming in/releasing regardless of fan speed.

I have a call into the mechanic and we'll see if he wants me to bring it in.