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Discussion Starter · #1 ·
Just wondering if this knocking noise from the A/C compressor is common on the 2nd gen Rodeos when it gets old? My first A/C compressor started making a knocking noise so I replaced it with a junkyard one and this one makes the same noise, not as bad but it also makes a groaning noise. I'm thinking of buying a NEW compressor but it's very expensive...so before I do, just wanted to ask if anyone has had this problem?

It is louder with the truck in drive with the brake held and A/C button is switched off. But now I can hear it at idle.

It is definitely the A/C compressor. I can clearly hear it knocking with a stethoscope. More so the back part of the compressor. The noise goes away with the drivebelt removed and I run the engine for a few seconds. Also when I had the old A/C compressor out I ran the engine and no noise coming from the block or anywhere else. So I guess I will try out a NEW A/C compressor and see. For good measure also a new drier too.

You guys got any thoughts on this? I just put in a new motor in this thing and the knocking noise worried me there for a sec....

Think maybe I put too much oil or freon in it? I put in 2 ounces of oil and 1.45 lbs of 134a...
 

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I never knew that the old reciprocating-type compressors were factory on an Isuzu - sounds more like an add-on unit or what is commonly called 'dealer-air' to me.

The reciprocating compressors CAN have bad rods or pistons in them like an engine, after all, they are really a little two cylinder engine minus an ignition system and fuel delivery.

They used to be so tough that I put them into a few old tow trucks as an air compressor without any provision for oil at all and they'd live for years doing yeoman's duty in a climate and condition totally out of their original design to pump up tires and keep an air tank full on the service trucks for customers flat tires.

So to say you've had a couple now fail and knock is kinda odd to me - but not impossible.

Over charging with freon (R134a lends itself to this problem as people don't really understand a full charge isn't something that you can read on a gauge set - it's the temperature of the return gas and therefor the overall temperature of the compressor that's the key to a correct charge) is prolly the cause.

Either that or insufficient oil in the system - but that's kinda hard to do, really.

You say 'new' - do you really mean 'new' and not 'new to you' or 'remanufactured to be as new'?

New usually comes on a new car/truck and new isn't usually available outside that closed circle for many parts.

Then there's always 'new' from China - and you can use your imagination and pocketbook to test THAT theory in marketing if you want to.

One thing I don't understand is your statement here:

ibid: It is louder with the truck in drive with the brake held and A/C button is switched off. But now I can hear it at idle.
Can you elaborate on this? Since when you push the AC button and the compressor stops compressing, then you cannot be hearing the compressor itself, but the idler bearing on the input shaft of the compressor.

Either way - it is acting as a bearing when the compressor is OFF and it doesn't move or bear any weight when the compressor is ON - so that's a paradox.

It cannot make noise in both conditions; only one or the other.
 

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Discussion Starter · #3 ·
well what i mean is when it's at idle or the truck is in drive with the AC off, it makes a knocking noise.

when the AC is on (under load) it makes a groaning noise and becomes a whining noise as engine speed increases. i took apart one of the compressors and it is the vane type compressor. no pistons in it. it looked fine inside. maybe i did not have enough oil in there? there's supposed to be a sticker on it saying how much oil it takes but it's all faded. i just added about 2-5 ounces in there when i charged the system.

both compressors are old. one from the original old engine with 185k miles and the new 25k mile engine didn't come with one but i put in one that i got off an ebay seller (used/junkyard compressor).

i assume it might be a bad clutch bearing on both compressors?

the knocking noise i am describing is similar to this vid...minus the scraping noise...

http://www.youtube.com/watch?v=w5wWGRlO ... re=related

knocking noise only comes on when the ac is OFF and the ac pulley is freewheeling. judging by all the vids on youtube that i've seen, i'm leaning towards bad clutch bearings on both compressors. any thoughts?

i will try and make a vid of my ac compressor making that noise when i get the chance if that helps...

knocking noise also sounds similar to thiis vid...

http://www.youtube.com/watch?v=yURvs60e ... re=related
 

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OK - this is a paradox now. Thank-you for telling me these are not piston - reciprocal compressors.

IF the AC is off and the clutch is NOT energized, engine on (natch!) - then any noise the unit makes is not from the compressor - but it IS likely from the internal idler bearing on the clutch.

That big wheel HAS to be running on some sort of anti-friction device, and they put a bearing inside the center hub of the clutch to allow the compressor to sit and not work while the belt still goes around and around keeping whatever else is on the belt in motion.

So - it seems NOW that you don't primarily have a compressor problem - just a clutch internal/idler bearing problem.

HOWEVER -- when the compressor is energized and the clutch is hooked up, they any noise you hear will be from the internals of the compressor - NOT the clutch idler bearing as it is at that time stationary and not being a bearing at all.

SO - maybe you appear to have two problems.

1) A bad clutch idler bearing in the belted wheel in front of the compressor where the engine drives it.
2) A noisy compressor - but that may or may NOT be normal as I have no idea of the amount of oil/freon in the system or if the compressor's actually gonna grenade or not.

Do this:

A} When the AC is running and you've been on a run - stop and quickly look at the expansion valve area on the aluminum pipe going into the evaporator. Is it icy or frosted up? Icy is not too good, but frosted up is OK - preferred even.
B} Is the receiver/drier frosted? SOME condensation is preferred - NO ICE HERE at all.
C} Take the temperature of the compressor outer case. If you can - and be careful here - put your hand on it gently with the AC-ON and the engine running. Is it a little too hot to be comfortable? It should be just getting into the too-hot-to-touch temperature zone or about 160-170ºF.

If it's cool to the touch (below 140ºF) - cooler than the other things around it - the freon charge is low and expansion is happening INSIDE the compressor and that's not good.

If the compressor is egg-frying hot - you are overcharged or the compressor has a problem.

What all the pressures and heats and cold absorption theory comes down to, is the compressor will tell you if the system's correctly charged or not.

All the seminars and classes I took on the 'NEW R134a" systems (when they were new) - came down to the actual temperature of the compressor.
 

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Discussion Starter · #5 ·
SurferJoe said:
OK - this is a paradox now. Thank-you for telling me these are not piston - reciprocal compressors.

IF the AC is off and the clutch is NOT energized, engine on (natch!) - then any noise the unit makes is not from the compressor - but it IS likely from the internal idler bearing on the clutch.

That big wheel HAS to be running on some sort of anti-friction device, and they put a bearing inside the center hub of the clutch to allow the compressor to sit and not work while the belt still goes around and around keeping whatever else is on the belt in motion.

So - it seems NOW that you don't primarily have a compressor problem - just a clutch internal/idler bearing problem.

HOWEVER -- when the compressor is energized and the clutch is hooked up, they any noise you hear will be from the internals of the compressor - NOT the clutch idler bearing as it is at that time stationary and not being a bearing at all.

SO - maybe you appear to have two problems.

1) A bad clutch idler bearing in the belted wheel in front of the compressor where the engine drives it.
2) A noisy compressor - but that may or may NOT be normal as I have no idea of the amount of oil/freon in the system or if the compressor's actually gonna grenade or not.

Do this:

A} When the AC is running and you've been on a run - stop and quickly look at the expansion valve area on the aluminum pipe going into the evaporator. Is it icy or frosted up? Icy is not too good, but frosted up is OK - preferred even.
B} Is the receiver/drier frosted? SOME condensation is preferred - NO ICE HERE at all.
C} Take the temperature of the compressor outer case. If you can - and be careful here - put your hand on it gently with the AC-ON and the engine running. Is it a little too hot to be comfortable? It should be just getting into the too-hot-to-touch temperature zone or about 160-170ºF.

If it's cool to the touch (below 140ºF) - cooler than the other things around it - the freon charge is low and expansion is happening INSIDE the compressor and that's not good.

If the compressor is egg-frying hot - you are overcharged or the compressor has a problem.

What all the pressures and heats and cold absorption theory comes down to, is the compressor will tell you if the system's correctly charged or not.

All the seminars and classes I took on the 'NEW R134a" systems (when they were new) - came down to the actual temperature of the compressor.
yeah the compressor DOES seem to run hot to the touch. i put in 1.45 lbs of freon in there and about 2-3 ozs of pag oil w/ dye and it calls for 1.43 lbs according to the sticker under the hood. i didn't think the extra .02 lbs matter. i will try and recharge it again properly and see how it sounds.

now i saw online that it takes 1.32lbs and 5 ozs. of ac oil with the system completely dry of oil, 2-3 ozs. if just replacing a part (i.e. compressor)....in that case, i overcharged the system and the sticker under the hood is wrong? can you tell me the proper capacity of the ac system? and thanks again for the replies Joe.
 

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That sticker is the best reference.

Do NOT trust gauges to tell you the pressure of the system as R134a is very expansive and doesn't tell the truth all the time unless the system is extremely well thermally balanced and not seeing any thermal changes from moment to moment.

It's THAT critical!

Again, it's all about the heat inside the compressor. It really comes down to that in the end after the correct amount of freon and oil and there's not much else to do from that point.

Too much freon and/or oil will make the compressor knock and that might be the internal noise you hear.

The idler making noise in the drive pulley is a second problem I think.

  • ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

I just charged a system from dry today and I jump out the low-pressure sensor in the receiver/drier with a paper clip bent into a 'u-shape' to get into the connex - to keep the compressor running while I charge it.

Once I see the slightest frost on anything, I plug the sensor back in and continue recharging until the compressor gets to 165ºF.

Then I let it HIGH IDLE for 15 minutes, shut it all down and restart after the R/D vents off and recheck the compressor temp again.

WARNING!!! MAKE SURE YOUR COOLING FAN IS WORKING RIGHT FOR THIS!!!


This time it will usually be 170º or so - and if the system is gonna run, this is a very good temperature for the compressor for a curb test. (It is also gonna be very much in the correct range +/- a tiny bit) for the ram-air effect on the highway at speed too.
 

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Yeah - that's a good one - Fluke is top-notch gear.

You don't need one that expensive, and I think HF has them for considerably less - just test it on an ice cube and then point it at some boiling water to check the calibration once you get it home.

You can assume it's gonna be fairly accurate if those two extremes are OK.

As far as the idle - that's engine speed - not compressor RPM. I use a tool called a 'Tac-Right' to hold the engine speed where I want it - a cool tool - but maybe it isn't available any more (I think).

Here's what I do: I put my hand on the outside of the compressor as I dump the first can of freon into the system to see if I'm in the ballpark for the case temperature and continue with the next can - that's until/unless it gets too uncomfortable then I switch to the thermometer.

But you need to know when and if you can remove the jumper on the Low Pressure Cutoff switch, either in the suction line or on the R/D somewhere.

Once the system is 'seeing' pressure, it should hold the connection OK and you can fine-tune the total amount of freon from that point on.

And remember - a can of freon is NOT a pound of freon. The cans are only 12 ounces, whereas a pound is 16 ounces!

Nasty trick in packaging there by the manufacturers!
 

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Thanks Joe. Mine seems to be working ok and the pressures are in the range but I think this is an interesting way to verify/validate. With the temps the way they have been here in OKC I will take all the A/C efficiency I can get.

I got the truck used and it has UV dye in it so I know at one time either it had a leak or equally likely someone bought a can with the dye in it and added some. The compressor cycles about like I would think it would in that it engages and stays that way unless it is fairly cool outside in which case it cycles some.
 

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Yeah - I hate the compressor-cycling systems. They're hard on clutches when it comes in at 80 MPH against some superheated head pressures.

Give me a modulated or feedback EPR system any day!
 

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Joe,

If I can bug ya just a bit to make sure I have this right. I've borrowed an IR gun as I wanted to wait for a sale to actually buy one.

As it is the low side at idle is about 43psi. I think this might be a bit on the high side.

So what I think you are saying I should do is run the motor at about 1500rpm , windows down, fan on high, recirc off and measure the temp of the compressor. Ought to be 160-170. If it is higher than that I need to look at letting off some of the charge?
 

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vanduker said:
Joe,

If I can bug ya just a bit to make sure I have this right. I've borrowed an IR gun as I wanted to wait for a sale to actually buy one.

As it is the low side at idle is about 43psi. I think this might be a bit on the high side.

So what I think you are saying I should do is run the motor at about 1500rpm , windows down, fan on high, recirc off and measure the temp of the compressor. Ought to be 160-170. If it is higher than that I need to look at letting off some of the charge?
Not totally correct.

Any AC testing should be at the middle fan speed, MAX or RECIRC ON, and windows open and the engine idling.

Internal temps should be tested in the passenger's side vents, preferably with a probe-type thermometer.

When you are charging the system, that's when you run at high idle, jump out the LOW CUTOFF at the R/D, watch the temp of the compressor and have the fan on HIGH.

It requires three observations::
  • 1) Watch for ice building on the tube where the expansion valve is located
    2) Watch for sweat/condensate building on the R/D
    3) Keep your hand on the compressor until it becomes too uncomfortable, and then IR-test it with a non-contact thermometer.
If the system's already driven and you want to test the freon for correct volume or if it's filled or overfilled, then you need to get the vehicle to ambient temperature - usually in the shade or a garage.

Then start the engine with the AC ON, windows down, blower on MEDIUM, RECIRC and idling.

Put your hand on the compressor and feel for it's temperature change.

Then quickly observe the following:
  • 1) It starts out ambient-cool and slowly builds to too warm to touch
    2) It starts out ambient-cool and rapidly heats up to untouchable.
    3) It starts out ambient-cool and never becomes uncomfortable to touch, kinda pleasant really all the time.
#1 = normal
#2 = overcharged
#3 = undercharged

EDIT: In all cases, the compressor should be about 160º-170ºF without the intrusion of underhood temps on the compressor.
 

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Discussion Starter · #14 ·
ok i finally got down and spent more time with the truck....turns out it was a loose bolt behind the compressor for the compressor bracket, holding the bracket to the block. how it got loose, i don't know. perhaps i forgot to tighten it real good when i put this engine together last december and the vibrations shook it loose. but that was the fix! good thing i don't need a new compressor.
 

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98HPasSport said:
ok i finally got down and spent more time with the truck....turns out it was a loose bolt behind the compressor for the compressor bracket, holding the bracket to the block. how it got loose, i don't know. perhaps i forgot to tighten it real good when i put this engine together last december and the vibrations shook it loose. but that was the fix! good thing i don't need a new compressor.
So - it had nothing to do with putting your hands on it?

I'll accept the healing of the system you did if you say 'No' - but I really think a hands-on approach would have been best.

lol
 

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Joe,

I hooked up the low side as a reference and measured the temp.

At idle with fan on speed 2 (is 4 speeds so no middle) the psi stayed right at 35 and the compressor varied some (hotter at the front by the pulley) from upper 160s to (cooler at the rear) mid 140s.

When I raised the fan speed to 3 the pressure and temp went up some. Pressure was about 38 and temp was right at 170 and about 150 at the rear.
 

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Sounds about right.

See the relationship with the temp of the compressor and the engine speed?

This is what I found to be universally true on all R134a systems, original or retrofit.

If the compressor is in that temperature range at that engine speed, then you can be pretty sure that you've got it right.
 

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Discussion Starter · #18 ·
SurferJoe said:
98HPasSport said:
ok i finally got down and spent more time with the truck....turns out it was a loose bolt behind the compressor for the compressor bracket, holding the bracket to the block. how it got loose, i don't know. perhaps i forgot to tighten it real good when i put this engine together last december and the vibrations shook it loose. but that was the fix! good thing i don't need a new compressor.
So - it had nothing to do with putting your hands on it?

I'll accept the healing of the system you did if you say 'No' - but I really think a hands-on approach would have been best.

lol
lol yeah i swapped compressors never noticing it was the ac compressor bracket all along. it wasn't really that loose to tell by touch. it was like 1/4 turn. but obviously it was enough to cause a loud knock. i resecured the bolts with loctite.
 
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