Evaporator control unit

E28 technical advice asked and given! Troubleshooting, modifications and more.
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RossDinan6
Posts: 85
Joined: Jun 24, 2006 11:23 AM

Evaporator control unit

Post by RossDinan6 »

Cross posting from my Big Coupe post.

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.
Blue Shadow
Posts: 9759
Joined: Feb 12, 2006 12:00 PM
Location: SE PA

Re: Evaporator control unit

Post by Blue Shadow »

What refrigerant are you using? For 134a the high side doesn't reach 300 psi until ambient is 95 F. High pressure can be overfilled, all else working correctly.
gadget73
Posts: 903
Joined: Nov 22, 2017 10:30 PM
Location: New Jersey

Re: Evaporator control unit

Post by gadget73 »

might also want to check the temperature drop across the evaporator to get an idea whats going on there. Not sure what design would be on these but typical is somewhere in the 10-15 degree range between the outlet of the expansion valve and the outlet of the evaporator. Excessive drop is an indicator of under-charge or an expansion valve problem. Not enough drop usually means its over-charged or there isn't enough airflow across the evaporator coil. This is the superheat measurement just in case you want a new word to add to your pretend AC tech vocab. (I'm not an AC tech either, I just have old cars and dislike sweating)

if you can't measure temp directly on both sides of the evaporator coil because of where the expansion valve is, you can usually figure out what the temperature is on one side by the refrigerant pressure and converting that to temperature.

don't know anything about this system but if its cycling there must be an electrical cutout. Usual suspects are a low pressure switch, a high pressure switch, and sometimes an evaporator temperature sensor. Not sure what you've got but you'll want to verify none of those things are bad and giving false trips, or verify there isn't actually some condition thats causing them to trip out. If the controls just break the 12v supply to the compressor coil, a test light wired across them will tell you whats opening. No light = switch closed, light on = switch open.
Federico
Posts: 453
Joined: Sep 16, 2013 6:38 PM
Location: Argentina / California

Re: Evaporator control unit

Post by Federico »

I would start by testing resistances for the evap temperature sensor and the cabin sensor as well . There should be value tables in the bentley manual.

You can also try disconnecting the evap sensor and running the a/c to see what happens.

The compressor will likely stay engaged permanently if you unplug the interior temperature sensor (unless you crank it to max heat).
This might help if you need to drive the car.

Also, you could have an issue with bad grounds: viewtopic.php?f=3&t=152458

I did the whole troubleshooting thing under the dash years ago, and the basics are as far as I recall:

The temp knob is the actual climate control unit. It sends variable voltage to the compressor relay (control unit).
This voltage varies with knob position and sensor inputs.
The compressor control unit receives this signal and trips the relay when it goes over/under a set voltage.

The above explains why, in a pinch, messing with the sensors may throw the signal into the "engaged" value ( but you may lose temperature regulation).
RossDinan6
Posts: 85
Joined: Jun 24, 2006 11:23 AM

Re: Evaporator control unit

Post by RossDinan6 »

Blue Shadow: 134a and I'd be thrilled if it was under 95 here in SW FL. It was high 90s and probably 80%. I had a shop fan blowing through the condenser at the time.

Gadget 73: I know the system is still under charged as the evaporator out line isn't sweating yet. I wanted to correct the short cycling issue before finishing the charge. It is being shut down by the controls. The high pressure switch is not changing state and there is no low pressure switch in this system. This has me thinking temp sensor(s) or control unit.

I'll use your advice on superheat when I get to finalizing the charge.

Fredrico: I believe this is the type of information I am looking for. I have found no details about how the compressor control circuit operates. I find understanding how an electrical system functions makes troubleshooting much easier. I'll look into the sensors tomorrow.

It looks like I am opening up the console again if I can't reach the evap sensor through the radio hole.

Thanks all for the input.
RossDinan6
Posts: 85
Joined: Jun 24, 2006 11:23 AM

Re: Evaporator control unit

Post by RossDinan6 »

After work today I dug a little deeper. Fortunately slipping the radio out of its DIN mount I am able to reach the evap temp sensor. I pulled it and the interior temp sensor and tested them. Both are within spec. Then I checked the temp regulator by monitoring the voltage while sweeping the temp knob from cold to hot. It measured .25v full cold to 7.4v full hot. This could be a plausible voltage range based on the datasheet I downloaded for the op amp on the evap temp sensor board. Anyone have the voltage range for the temp regulator?

I saw one pin on the op amp chip in the regulator was not soldered at all and had an AHA moment. Inspecting the board and datasheet I now doubt it. The op amp is a dual input/output and the #1 in/out is all common to ground. That implies they are only using I/O #2.

So, It appears the temp adjustment is good. The sensors appear to be good. The temp regulator relay appears good since it is cycling. Earlier testing and the gauges suggest the high pressure switch is fine, and new BTW.

Why is the compressor short cycling based on the above info? While I hate to be a parts replacer, I would if troubleshooting pointed at a potential defective part. I can't see one.
Federico
Posts: 453
Joined: Sep 16, 2013 6:38 PM
Location: Argentina / California

Re: Evaporator control unit

Post by Federico »

One thing I would try Is to backprobe the compressor control unit at the wire coming from the temp knob, for voltage. Start at max heat and slowly turn it colder to find the voltage at which the compressor engages. Bring it full cold, and if the voltage stays in the "engaged" range but the compressor cycles, then the knob side is probably ok and the issue lies at the compressor control unit / evap sensor side of the system.

I strongly suggest you dig up the splice I mentioned in the link above and solder it. Pretty much all of your HVAC controls are grounded through it and then on to ground G200 (under the steering column in an E28). Clean the latter.

You can rule out the high pressure switch by bridging it temporarily.

The first rule of auto repair is: Never assume, TEST :laugh:
RossDinan6
Posts: 85
Joined: Jun 24, 2006 11:23 AM

Re: Evaporator control unit

Post by RossDinan6 »

Oh, test I did. Electrically everything tested good. Plugged everything back in today and fired up the car.

Blowing cold and pressures look good. I suspect I didn't have enough air across the condenser the other day causing the evaporator to ice up. It was Africa hot here at the time. Today I had a little over 50F vent temp. I turned down the fan and raised the temp a bit for the fist time in two decades. The project was worth the effort in the end. I just hope it holds.
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