Ripps

I have an '89 C4 with the Manual HVAC A/C controller converted to
R134a.

A/C Blows Hot on ALL settings.
•Compressor will engage and run
•Pressure is showing 40 psi. (w/no leaks).

Any Ideas on what I should be checking next?

TIA - ds

rodj

Whether the door on the ducting in pass footwell moves to close off the air through the heater core.

Quick fix is to put a valve in the heater supply line so you can cut off hot water to heater core

SunCr

40 psi means your Evaporator is 50 or better so it's not going to make real cold air. In the interim, remove the Resistor Pod on the left side of the Evaporator Case and look inside to view the Temp Door. Have someone operate the Controls and make sure it's completely closed when the Slider is at Cold. If not, Manual controls it with a Cable and it probably needs a new one or the one it has isn't attached. Access by removing the Passenger's side Hush Panel.

Recharge and aim for a low side in the high 20's/low 30's if it's 80 to 85 at the Condensor.

Ripps

The instruction I have on R134a is 40 psi/low side. Is that incorrect?

I'm in the middle of pulling the Dash and Bread Box.. let you know how it goes.

Thxs!!!

SunCr

R134 is 32 degrees at 28 psi/45 degrees at 40 psi. After vaporization, the design adds heat which raises the temperature and though the pressure remains the same, the refrigerant becomes a superheated gas so that it won't flash back into a liquid and destroy the compressor. Vapor/compression a/c works best around freezing - if the system is clean and the compressor is good, it's got too much gas if the low is only pulling down to 40 psi (assuming a condensor temp of 80 to 85 degrees). R134 has a smaller molecule than R12 and it's easy to cram too much in - try 80% of the R12 charge. Further, the Low Pressure switch should be adjusted to compensate for the R134 charge. For R12, it cuts power to the compressor at 25 psi. For R134, it can tolerate a threshold of 22.5 psi. You accomplish that by pulling the blower motor connector with the system on Max while observing your low pressure gage. Then note the pressure when the compressor cutsout. There's a screw between the Low Pressure Switch terminals. Turn it clockwise to raise the threshold - usually a half turn - counterclockwise to lower it. Recheck.

You don't need to tear into the dash to check the Temp Door.

Ripps

Well, If got the dash and bread box off, also removed the upper air duct on the passenger side so I could see down below.

I'm trying to locate the exact point where the Manual Cold-to-Hot cable is connecting at. I thought that with the dash and bread box gone I'd be able to find that easy enough... but no such luck!

I can see that the vacuum actuated door at the center near the firewall is closing with the Manual control set to Cold and AC on MAX. Does that scenario sound right?

SunCr

Temp door is cable - no vacuum involved. I'd guess what you're seeing is the Defroster Vent. By default, it's always sucked shut (so the vacuum source is always put to work). Cable/Temp Door connection should be on the heater plenum, passenger side and towards the floor. Removing the ECM often helps.

65Z01

Exactly. You can see the little motor and the mechanical rod from under the dash.

When you move the temp setting from max to 60deg you should see the motor arm & control rod move. A common problem is for the little plastic fitting holding the rod to the motor arm to disintegrate.

SunCr

Member has Manual Air - no motor involved.

MBDiagMan

For meaningful a/c diagnosis you must get low AND high side pressures and post them along with ambient temp and vent temp.

Ripps

Ok, I found the cable connection on the Manual (Cold-to-Hot) line. It IS connectect at the other end and is in working order... The door is opening and closing with the Manual Slider Control!!

So now where I see myself at it is here:
A/C Blows Hot on ALL settings.
•Compressor engages and runs
•Heater Door Works (opens and closes)

So I suppose I am back to looking at Pressures and Temps?

***I have been assuming that you check the Low-Side from the Accumulator Dryer.***
Is this Correct?

SunCr

Yes, Low Side fitting is on the Accumulator/Dryer. High side fitting is on the smaller line from the Condensor to the Evaporator. Disconnect the Fan Switch on that line so that the Main Fan is running (it's next to the High Pressure Limit - whichever harness when disconnected turns on the Main Fan is the one you want disconnected). Turn the a/c on max. Block the throttle so it's running at 1500 rpms and read your operating pressures. If the compressor starts to cycle on/off (it shouldn't with a low side of 40 psi), disconnect the Low Pressure Switch (Evaporator Outlet) and jumper the harness connector with a paperclip so that you can get a steady reading of the low side. Note the outside air temp and post what you get.

Ripps

OK, I did as you said...

Disconnect the Fan Switch on that line so that the Main Fan is running - Check

Turn the a/c on max - Check

Block the throttle so it's running at 1500 rpms -Check

Read your operating pressures. Lo = 38 Hi 205

If the compressor starts to cycle - No It Did Not

Note the outside air temp and post what you get - 111 F (ouch)

SunCr

At 111 and low humidity those numbers are great; maybe even a tad low (and I'd probably go a bit low if I lived in the desert). I'd expect high 50's maybe even 60 out of the vent - much above that, something is blocking air flow (plugged up Evaporator - is the air flow out of the vents good?) - or heat is getting around the Temp Door - or the duct work isn't sealed up (is there air blowing past the ECM towards the Floor with the Passenger Side Hush Panel removed?

Ripps

Air flow out of the Vent is good! Just Hot.

Here is another thing though which is making me think I may be back to the heater Temp door...

I see no noticble change in temp weather it is set to AC MAX or VENT!

Blocked Airflow Question...

Should the lines at either side (Hi/Lo) cool down during operation?
(If the answer is No, I am about to bark up the wrong tree here...)
And if not, is this an indication of a blockage?

Just trying to get my head around this?

SunCr

Inlet (at the very bottom after the Orifice) and outlet from the Evaporator should be cool and colder than ambient. If you had any humidity, the outlet would/should be sweating. No difference between a/c and vent (with those pressures) would lead me to believe that the conditioned air isn't getting into the passenger compartment or your compressing a mixture of air/water with not much gas. Remove the Resitor Pod on the left side of the Evaporator Case and stick a thermometer next to the coil - that will give you the temp of the conditioned air and if it's not 58 - 62 degrees then the charge is probably contaminated. If that's the case, you'll need to get rid of what's in there; replace the Dryer, flush the Compressor and pull a vaccuum so that you can charge it right. A shop might also check it out for a minimal charge. For more ideas, you can ask on the Board at www.ackits.com

SunCr

PS: You might take a look at the static pressure (a/c off). R134 at 110 degrees should be in the low 90's. Thinking about it, given that the high side at this air temp, suction side looks restricted. If the Dryer wasn't swapped out with the changeover, they'll fall apart.