AC squealing noise question

[bevans6]

When I tried the AC on my MCI 5C, the voltage drew down to 22 volts and the alternator started squealing like a pig.  Needless to say I turned it off again.  What are the obvious things that could be wrong?

What I know about bus (or any other) AC systems can be writ large on the head of a pin...
Brian

[RTS/Daytona]

not sure about MCI 5C or what engine you have

but

are you sure your alternator is not gear driven and it's your A/C compressor belts that are squeaking

maybe loose belts (that would be nice) - or frozen compressor (not nice)

Pete RTS/Daytona

[wildbob24]

Sounds like the alternator belts are loose. You need to make sure all the belts are still there and that the air cylinder that maintains belt tension is functioning properly.

[bevans6]

the engine is 8V-71, the alternator is a belt driven DN-50, four belts, all quite equally tight, turn-buckle adjusted.  No question it was the alt. belts that were squealing, they also got quite hot.  It seemed to me like the alternator was taking a very significant load to draw it down to 22 volts from about 28, which would be normal running voltage (given the accuracy of the dash gauge, obviously).  the only electrical load that I can see that is AC related is the condenser fan motor and the evaporator fan motor.  is there anything else?

Is a current draw of that level to almost try to stall the alternator normal?

Brian

[Dallas]

Yup, if the compressor is locked, the clutch will keep trying to stay engaged and burn itself out with all those wonderful associated aromas.
It's possible you have  too much freon in the system, and that will also cause the condition. As Pete said... hopefully it's not a frozen compressor.

  the only electrical load that I can see that is AC related is the condenser fan motor and the evaporator fan motor.  is there anything else?

Is a current draw of that level to almost try to stall the alternator normal?

Brian

[bevans6]

thanks for you reply.  if the compressor was locked, that's the big pump to the curb side of the engine, is it not?  It seems to turn OK.  What does it have to do with the DC voltage drop I'm seeing?  how are the two related?

A little more research in my manuals shows that the evaporator motor is the same as the heater blower motor, only operating on high speed rather than low speed.  The heater blower motor seems to work fine.  My condenser fan motor is an "axial fan" type, jut a big old motor with a fan blade on the end, not a squirrel cage set up.

thanks again to all!

Brian

[Nick Badame Refrig/ACC]

Hi Brian,

That big pump curb side is the A/C compressor. It freewheels until you turn on the A/C switch. The switch ingages a magnetic clutch on the pully

of the compressor, and also your 2 blower motors. At this point the compressor should be turning internally with a load. You can hear a distint sound change.

Did you have your high idol on when you heard the belt squeel? You need to but, if you also have blower motors going bad they could also draw

a big load on the alt.

Listen for the internals of the compressor turning. They should sound free but pumping like a car a/c compressor. Listen to both your blowers, they should

sound powerful and no bearing noises..

Let us know
Nick-

[Hartley]

Sounds like one of you fan motors may be stuck causing a higher than normal
amp draw on the electrical system.

Typical fan motor draw runs 40 to 80 amps and the blower motor is about the same.
This means normally you have about 140 amp loads from the blower and fan.

If either is not running, shorted or jammed the amp draw will go crazy and overload
the alternator causing the belts to shriek...

Smoke will happen shortly so don't keep trying until you have made sure the fan and blower
motors turn freely.....

Dave...

[Len Silva]

Since the heater blower seems to work OK on low speed, it seems to me that the problem is most likely the condenser fan.

Can you see it operating?  Can you turn it by hand?

If not a seized motor, then most likely an electrical short in one of the fan circuits, though they should trip a breaker pretty quickly.

I agree, it should not be dragging the voltage down that much for more than a couple of seconds.

If you have schematics, it should be fairly simple to operate the blower and fan solenoids with a jumper (engine off) for just a few seconds to see what is happening.

[bevans6]

thanks for the suggestions.  it's been some years since I read schematics that have relays in them, but I think I have traced out a couple of things. The condenser fan rotates freely, no drag at all, and I can easily power it up after I organize a 24 volt battery cart to see what it's doing. The heater and evaporator fan are the same fan, but are powered from totally different circuits and run at two different speeds, low for heater and high for AC, so it is still a possibility.  I'm installing roof air, so may in the end go the route of taking out the coach air, but not without a fight!

Current project is ripping out (done) all the PO wiring and replacing it with romex (half done).  He had no box grounds, multiple outlets in the same box fed from different non-interlocked breakers, no GFI anywhere, ground bonded to neutral, run with cheap extension cord wire, and all of the  DC wiring is red - positive and negative both!

[Len Silva]

Is this a new condition or the first time you have tried it?  Based on your description of the PO's wiring, I wouldn't rule anything out.  I do think that you are correct that it is an abnormal load of some kind, shouldn't be too hard to trouble shoot.  If not a locked motor, then a dead short is suspected.

[buswarrior]

Let's get simple first.

Tighten the alternator belts some more. By already being tight, what do you mean? Those belts are fairly short, there shouldn't be any droop to them, not quite pluck them and hear a twang, but they need to be tight. No automotive droop or deflection.

If nothing changes, put them back.

The reason I say try tightening them, 22 volts sounds just right for a couple of horsepower worth of fans running off the batteries... the heavy load stalls the alternator, the loose belts slip, no power being made at alternator, big fans are running on the battery, low voltage shows on the gauge.

You have already confirmed everything else that is electrical is free to spin.

On fast idle, the headlights will dim on AC start-up, there are a lot of amps flowing!

happy coaching!
buswarrior

[Sojourner]

Your post already explain that it not a compressor lock-up. However hot alternator belts and voltage drop from 28v to 22v is an indication of one of two things....either the belts are still not tension tight enough ...or....it is over max alternator output of 270 ampere. If it the latter....have someone who has a 500 amp carbon pile load tester unit and draw the voltage  from 28v or whatever to 22v to read the ampere meter for ampere load result. If it over 270 ampere....you have a severe short or way overload circuit.

Now you know which one of two needs to troubleshoot further.

About who has a 500 amp (at least 300 ampere) carbon pile load tester...most all truck alternator & starter rebuilder has them. You can drive it to them to do a load test at 22v. It usually a free service.

Example of what a 500 Amp Carbon Pile Load Tester look like. It must have volt and amp meter. If it a single meter (volt)...it a wrong one....it for testing battery only.

Wish you well.

Sojourn for Christ, Gerald