Evaporator control unit

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RossDinan6
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Evaporator control unit

Post by RossDinan6 »

I am nearing the end of my AC system upgrade. New Sanden compressor, parallel flow condenser, expansion valve, HP switch, hoses etc. Flushed out the evaporator. Assembled it all and did a nitrogen pressure test. Evacuated it a couple hours this morning with my $85 Gast vacuum pump from ebay. All is good to this point.

Started charging the system and it is short cycling. I know I am still low on refrigerant as the suction piping isn't sweating yet. It is Africa hot here in FL as one would expect. I suspect I may have an issue with my Evaporator control unit. Without a low pressure switch a low charge wouldn't cycle it, correct? The high pressure switch is new and did not open when I checked it and the HP is below its setpoint (385 psi IIRC). The compressor cycles frequently enough I doubt an iced up evaporator.

Is there any troubleshooting a guy can do to the control unit? Would a bad evap temp sensor cause this symptom? Supplying the compressor with 12v directly from the battery I am seeing about 50 psi/340 psi and mid 60s F vent temps. Ambient was probably 95F and 80%(if not more).

Appreciate any input. I'm impersonating an AC tech at this point.
Ross
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Jubilados
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Re: Evaporator control unit

Post by Jubilados »

What is an evaporator control unit?
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RossDinan6
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Re: Evaporator control unit

Post by RossDinan6 »

It controls the compressor with input from the evaporator temp sensor.
Ross
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TimU
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Re: Evaporator control unit

Post by TimU »

Hi Ross,
I've just completed a full AC system rebuild (R12->R134a) with same/similar components to you, and I don't recall reading or touching anything to do with an Evaporator Control Unit.

Having said that, the compressor clutch is a binary switch: It's either on or off, not a variable switch.
You said that your compressor cycles on and off, so I would expect that the electronics are doing their job just fine.

My suggestion would be to gas it up properly and see what (if any) issues remain.

For what it's worth, here is the output result after my system rebuild.
IMG_20220611_205415~2.jpg
IMG_20220611_205415~2.jpg (93.4 KiB) Viewed 957 times
One more thing: You didn't mention if you have a single zone or dual zone system in your car.
It's not related to the symptoms you described, but it's always good to highlight your setup because some components (like the front TX Valve) are different between the two setups.
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RossDinan6
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Re: Evaporator control unit

Post by RossDinan6 »

Mine is a single zone with a new expansion valve (along with everything else). I am well aware the compressor is on/off. The issue is 4 seconds on and 10 off will not cool the car. With no low pressure switch it can't be the charge causing it to short cycle. You may be right that the electronics are fine, but why the short cycle? If I supply the compressor with 12v through the handy alligator clip jumper I get similar vent temps and it was hot yesterday 100f/80ish% in the garage.
Ross
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Re: Evaporator control unit

Post by Jubilados »

“ It controls the compressor with input from the evaporator temp sensor.”
That makes sense, but I don’t know what does that. Would be interested to find out.
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RossDinan6
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Re: Evaporator control unit

Post by RossDinan6 »

It is an electronic control module. It has some type of electronic circuit that monitors the evaporator temp and decides when the compressor turns on and off. Technical version: Electricity goes in a little magic happens and the cabin cools off.
Ross
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Re: Evaporator control unit

Post by Jubilados »

Ok, here it is.
71ABE752-683D-481D-A83B-C95A8DC0D7BB.jpeg
71ABE752-683D-481D-A83B-C95A8DC0D7BB.jpeg (776.31 KiB) Viewed 924 times
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86_6series
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Re: Evaporator control unit

Post by 86_6series »

Ross, if your running the car in the garage there's a good chance the condenser
cannot cool down enough. Try outside with water from a hose spraying on the condenser.

That's what we did in the old days, helped the condenser cool the Freon.
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RossDinan6
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Re: Evaporator control unit

Post by RossDinan6 »

The water is a good idea, didn't think of that. I did have a shop fan blowing through the condenser and radiator. The temp gauge sat mid scale as usual. Still looking for input on the short cycling compressor.
Ross
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Re: Evaporator control unit

Post by Jubilados »

Use the wiring diagram above.
Put your AC settings on full cold and use a multimeter or test lamp and see if the output from the Evaporator Temp Regulator (the black “compressor control” wire) is cutting on and off.
Jubilados
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Re: Evaporator control unit

Post by Jubilados »

Check for 12v on that wire.
See if the 12v stays constant or cuts on and off with your compressor.
You can also do the same at the main ECU.
From the wiring diagram, it’s pin 29.
Then do at the high pressure switch etc.
It’s just tracking it down until you find the component in the circuit which is causing the cycling.
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