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Tech / General EngineIs your car making a strange sound or won't start? Thinking of adding power with a new combination? Need other technical information or engine specific advice? Don't see another board for your problem? Post it here!
ok, so ive done some searching around and cant seem to find an answer, for sure. i have an 87 z28 with a 305, an edelbrock carb(5 speed car also) i do have a serpentine setup on it, which came off a v6 s10 iirc so it uses a truck belt.
the problem im having is the compressor vibrates between about 1000-1500 or so rpm and when the rpm drops back to idle with the a/c on, sounds like im grinding the gears. everthing is fine when the a/c is off. i have replaced the condensor, accumulator, and the compressor with a reman one. the old one was makeing the vibration noise as well. everthing is tight as far as i can tell
not sure where to look or what i should do, any help would be awsome...thank you in advance!
There is a similar type of brace that goes from the back of the compressor
to an exhaust manifold bolt. If you don't have one I think I have a couple
around somewhere. At least that's how the stock 5.0 serpentine systems work. Don't know if yours
is the same
i may have the brace around somewhere....should be close to stock 5.0's, ill be looking around to see if i do have them tomarow...however it has gotten better, found that the compressor mount was loose as was the idler wheel, tightened them up. still seems to do it though just not as loud.
i have also noticed that if you rev the engine up a little the tensioner moves which obviously puts less tension on the belt, is this normal? or sign that i need a new one?
belt does vibrate like a guitar string a little bit next to the crank pulley as well
ok vibration is fixed, just needed a brace. i found one around the house put it on and its all good now.
only issue now is that compressor cycles constantly. read that i should have about 27 psi on the low side and 225 at the high side when its 80 degrees...its been 90 plus lately so i put more in...hangs out around 50 psi on the low side and seems to cycle when it hits 252 on the high side.
i bypassed to high pressure switch to see what would happen and sure enough compressor stayed on...this also made the low side pressure go down to 40ish psi and high side get to 300ish, will have to check that again for more accurate numbers. my question is: is the high pressure swich starting to go or do i have too much in the system?
fyi i have replaced the compressor, condensor, the accumulator and the high pressure tube if that affects anything
Where did you by pass the switch...the LOW is on the Accumulator. If its low, your compressor is going to cycle. Sitting in the garage while charging, it probably wont cycle due to no air across the condenser. Feed it slow and watch the low side. The high will go up until it cycles, then come back down. Never Never bypass HIGH side Pressure Switch on compressor....that's trouble!
If you pressures are sitting at those numbers, take it out on a sustained cruise and put a temp probe in center vent. You are in the ballpark of 40-45* air.
Ok so crusing vent temps get down to 60-62 and the compressor is cycling constantly...its about 95 degrees with hunidity right now, system has about 50 psi low side
When you pulled a vacuum on the system did it hold the vacuum for an extended period of time? Such as 15 mins or so? Not really important to the pressure and cycling unless you are leaking refrigerant.
Do you have any idea how much r134a you have put in the system? Your car calls for 3 pounds/48oz of r12 and when you retrofit r134 into a r12 system you only need 80% as much refrigerant, if I am remembering right "Edit i referred back to the ASE training text, Auto Heating and Air Conditioning third edition by Chris Johanson, and 70-80% is best* So that is 36.4 oz or a hair over 3 of the little 12oz cans. Overcharging the system can reduce how cool the air is just as much as undercharging.
Of course it should only cycle whenever the low side pressure is lower than the switch is set for and the high pressure is higher than what the switch is set to trip at.
Snap ons service manual program says your low side switch should cycle on at 46 psi and shut off at 25 psi. If low side pressure is never getting below 25 and the compressor still cycles with the switch bypassed than it is probably a bad high pressure switch. As was previously mentioned you should never bypass a high pressure switch for any type of diagnosis except maybe for a system that will never engage the compressor. With that said scrap on says the high pressure switch is only supposed to cut off the compressor if high side pressure exceeds 410 psi and then resume operation when pressure drops back to around 150-250 psi. That high pressure cutoff seems pretty high to me so the data might not be 100% accurate.
I don't know if 87 does but my wife's 86 has a switch in the power steering line right above the steering box that is supposed to cut off the compressor when the power steering pump is under a heavy load in order to prevent stalling. Like when parking. Might check that switch out.
Hopefully this helps, if it doesnt help you then maybe it will someone else. It felt like it took hours to type this up on my tablet. Lol
Last edited by Blk91Firebird; 08-04-2017 at 11:50 PM.
07-18-2017, 11:55 PM
