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Old 09-05-2014, 05:51 PM   #1
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Dash A/C blowing fuses

My Evans Tempcon dash A/C unit isn't happy. It just started blowing the 7.5 amp fuse, and when I replace it, the unit works for about 20 or 30 minutes, then blows again. Importantly, when it is working, it doesn't seem to be cooling, just blowing warm air. It's a PITA to change the fuse, it's on a block with the HVAC relays and fuses tucked up on a shelf accessed by removing the dash and fishing through all the electrical stuff and A/C ductwork. I've not checked to see if the compressor is kicking in when the switch is on, but will as soon as I remove the dash again to replace the fuse that blew today.

Any ideas as to what's making the fuse blow?

Thanks.
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Old 09-05-2014, 06:00 PM   #2
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Al,
My first guess would be that the compressor is trying to pump out more cold air than it is capable of. If it were mine, I would be looking at checking the Freon in the unit. I would also be looking at the Notorious electronic flapper valve stuff in the Generator compartment.
Do you have the manual coolant shut off valve? If not, I would look at putting one in.

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Old 09-05-2014, 07:00 PM   #3
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A fuse blows when the circuit is drawing too many amps. This could be caused by a short circuit or a device drawing too many amps. Since your fuse doesn't blow immediately, I suspect the second reason. Is the 7.5 amp fuse in the circuit that powers the fan or the circuit that energizes the electromagnetic clutch in the compressor? If it's the fan, it could have something impeding the fan and causing the motor to try and turn against more resistance than just air. In that case remove fan and check if it needs replacing.

If it's the fuse that is in the clutch circuit, it's either energized or not. How hard the compressor works wouldn't cause more amperage draw in the clutch circuit. Check for a bad insulation or a bad connection in the wiring.

An A/C compressor doesn't "pump out cold air" it compresses R-134a gas into a liquid which gets very hot and flows through a condenser to give up the heat. Then the liquid is sprayed through the expansion valve to change it a gas. That 'change of state' (a physics term, not a geographic one) going from a liquid to a gas requires energy. That energy is absorbed from the air passing over the evaporating coil and the cooled air is blown into the vehicle.

I have no idea what OldHatt45 is referring to with his "Notorious electronic flapper valve stuff in the Generator compartment." comment. His mention of, "the manual coolant shut off valve?" is about a valve to stop engine coolant from circulating through the heater core, but that has no bearing electrically on blowing a fuse, unless your heater valve is electrically controlled and malfunctioning, but you said nothing about a heat issue, just a fuse blowing problem.
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Old 09-05-2014, 07:32 PM   #4
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Bob,
My change of state is mostly from confusion to bliss. LOL

Generally speaking with the Evans Tempcon dash air units there is a known (by at least some) problem with the electronic control in the generator compartment which in my mind after reading more than one thread on the same problem, makes it notorious to me. That control operates the flapper that allows for mixing hot coolant air with the cold air from the proper operation of the compressor.

You are 1000% correct that it doesn't "pump out cold air" and does do all the technical physics related stuff. Thanks for the technical description!

My thought there was that if the Freon (aka R134) is low, it could be a case of the compressor having to work harder than it should have to, and perhaps the electrical load going up causing the fuse to blow after the compressor has been working too hard for 30 minutes or so?????

I would suggest doing a search on the forum about "dash air" and you'll undoubtedly find a number of threads and may possibly agree with my "notorious flapper valve" comment. And in addition, a couple of weeks ago, I posted the Evans Tempcon manuals to a thread, including the electrical schematics. If you don't find any threads, then sorry for wasting your time.

As far as Al's problem goes, my observation stands as far as what I would check.

If Al provides more information, or finds out other things about his problem, then my thoughts about further troubleshooting will likely change.

But hey, Thanks for the physics lesson.

Drew
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Old 09-06-2014, 11:37 AM   #5
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Thanks Drew.

My first thought was that it might be low on freon and working too hard, causing blown fuses. That will be the first place I look. And, yes, I have the manual valve installed to eliminate the notorious flapper situation. Will report back when I know more....just wanted to get some ideas of where to start. You've confirmed my first suspicion. Thanks.
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Old 09-06-2014, 12:04 PM   #6
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Thanks, Bob.

The 7.5 amp fuse that's blowing is not in the blower circuit. Thanks for your technical explanation of how the system works. I've been of the opinion, heard it or read it somewhere, that if the system is low on refrigerant it can cause erratic operation, including compressor failure. Just wondering if it would also cause a blown fuse by working too hard.

Will report back with more info as it becomes available.
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Old 09-06-2014, 12:17 PM   #7
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Quote:
Originally Posted by alfetch View Post
Thanks, Bob.

The 7.5 amp fuse that's blowing is not in the blower circuit. Thanks for your technical explanation of how the system works. I've been of the opinion, heard it or read it somewhere, that if the system is low on refrigerant it can cause erratic operation, including compressor failure. Just wondering if it would also cause a blown fuse by working too hard.

Will report back with more info as it becomes available.
If you are low on R134a, the low pressure cut out will shut the compressor down by cutting power to the magnetic clutch. The clutch circuit is either energized or not, it can't work harder due to conditions. The compressor itself is mechanical, no electric power used, so it doesn't use more electrical power when it's hot out or low on R134a. When the compressor is 'working' it is using energy from the engine by way of the pulleys and belt.

The fuse blowing could be in the dash control or the wiring between the dash control panel and the compressor. A bad contact could cause the magnetic clutch or other electrically controlled components to draw excessive amps, blowing the fuse. I'd try to trace all the items on that circuit. Then run A/C until the fuse blows again and immediately feel everything I could reach for excessive heat, showing where the resistance is drawing too many amps.
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Old 09-06-2014, 12:26 PM   #8
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Sometimes the electrical coil on the compressor clutch can break down when it gets hot, and short between windings or to ground, causing excessive current draw. One way to pinpoint if it is this is to put an ammeter in the circuit and watch for a current spike. It would pay to isolate the problem component by doing the following:

At the wire connector for the clutch (near the compressor), install an inline 7.5 amp fuse. At the fuseholder in the front (the one that's now blowing), install a 10 amp fuse (just temporary for diagnosis). The fuse at the compressor should blow first if the problem is there.
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Old 09-06-2014, 12:27 PM   #9
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Good troubleshooting strategy!!
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Old 09-06-2014, 02:30 PM   #10
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Len and Bob.....

More good advuce and troubleshooting ideas. Thanks.
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