Automotive Air Conditioning Information Forum (Archives)

Provided by

We've updated our forums!
Click here to visit the new forum

Archive Home

Search Auto AC Forum Archives

compressor draws too much voltage

jpeter17 on Mon June 15, 2009 6:55 PM User is offline

Year: 1996
Make: Bluebird
Model: bus
Engine Size: deisel
Country of Origin: United States

This AC unit is in a bluebird bus. The AC unit works fine, however, when I turn the AC unit on my volt gauge goes down to about 9volts
on the gauge. The AC unit draws all the power from the battery and I end up with a dead battery.
I have had the electrical system checked out and the alternator is working fine, the voltage regulator is good. The mechanic was checking the electrical system not the AC unit. When he kicked the AC unit on, the voltage went down to 9 or so volts The mechanic says it could be a bad compressor, clutch or something else. Any ideas?

Chick on Mon June 15, 2009 8:40 PM User is offlineView users profile

did you ever change the altenator? If so you might have a lower amp altenator in there.. They come with different amp ratings..Just a thought..

Email: Chick


Freedoms just another word for nothing left to lose

ChevyMan on Mon June 15, 2009 8:44 PM User is offline

I am not familiar with busses, so I need to ask you this. Is the compressor driven off the engine, or is it an electrically driven compressor. If it is engine driven, as is normal on most vehicles, then the compressor can only draw very little power. It has an electromagnet that engages the clutch. The wires going to the clutch are only 16 gauge or maybe 14. Therefore I don't see a shorted coil as drawing all the power available from the alternator, or the battery for that matter. In order to pull a 12 volt battery down to 9 volts takes a lot of amps. I would be looking at a bad connection somewhere.

Just a thought as I'm writing this. Is this a full size school bus? Does it have the condenser mounted away from the engine compartment (like under the bus with a grill through the side of the body)? If it does, then there must be at least a couple of big electric fans to cool it. Combine that with the probability of multiple evaporator fans, the system could pull quite a few amps. Are you sure that the alternator is working right?

mk378 on Tue June 16, 2009 10:57 AM User is offline

Maybe the engine is slowing down too much when the compressor engages. If you can keep the voltage up by revving the engine slightly with the gas pedal, check or adjust the system that increases the idle when the A/C is on. Otherwise it seems, like the others said, the electrical system is not up to the task. If all the wiring is OK, consider upgrading to a higher amp alternator. Electric evaporator and condenser fans can draw 30 or 40 amps each.

Compressor clutch coils draw less than 5 amps normally, and if they get internal shorts so they draw more, they will soon either blow the fuse or go up in smoke. So it is not that.

bohica2xo on Tue June 16, 2009 12:23 PM User is offline

9 volts looks like a wiring issue. Busses have lots of "assembler induced" wiring deffeciencies. If your battery voltage was actually crashing to 9 volts, the battery & alternator are very suspect.

First check the battery voltage at the battery posts with a volt meter, engine off. This is the resting voltage of the battery. Write this number down as "A"

OK fire it up, and check the battery voltage at the battery posts with a volt meter & the vehicle running - all acessories off. Write this number down as "B"

Next, check the voltage at the alternator output stud if you can reach it safely with the engine running. Write this number down as "C"

Now turn on the A/C, and check both voltages again.


"A" should be above 12.5 volts. If it is less than that, the battery is undercharged or bad.

"B" should be about 2 volts higher than "A". Someplace between 13.8 & 14.9 volts is ok. Anything below 13.5 is a bad alternator / regulator.

"C" should be very close to "B". "C" may be a half volt (.5v) higher than "B". If greater than .7v difference, you have a wiring problem.

I suspect you will see an actual battery voltage much higher than your panel meter when the A/C is on. It sounds like all of the electrical loads are fed through a single wire at some point, and it is too small.


"Among the many misdeeds of the British rule in India, history will look upon the act of depriving a whole nation of arms, as the blackest."
~ Mahatma Gandhi, Gandhi, An Autobiography, M. K. Gandhi, page 446.

jpeter17 on Tue June 16, 2009 6:46 PM User is offline

jpeter17 on Tue June 16, 2009 6:55 PM User is offline

Hey thanks for the info. A couple of things to answer here. Yes it is a full size bus with the Ac unit mounted on the side. And a double whammy the alternator had been replaced about a year or so ago. The place where I brought the bus deals in busses, big rigs and all kinds of diesel power equipment. They said the alternator is ok. ??? I'll have to call the guy who changed out the alternator to see if he remembers what size unit he installed. Thanks again for all the responses, lots of good info.

Back to Automotive Air Conditioning Forum

We've updated our forums!
Click here to visit the new forum

Archive Home

Copyright © 2016 Arizona Mobile Air Inc.