'87 911 Air Conditioning questions - Yes, another A/C thread!

[llangston1]

Just a quick update on my rebuilt compressor. It was a Denso rebuild in a Denso box. It has a stock rear condenser, a stock front condenser, and Zimms rear fender condenser and a Rennair evaporator. since I will need to open up the system to fix it I am considering replacing the front condenser and possibly the evaporator as well.

[kuehl]

Since you had a Denso factory compressor I'd simply disassemble it and inspect the internals, if all is well then you just need a complete seal kit (and, naturally you'd liquid flush the system if you find debris, system o-rings and a new drier).

Why do you want to replace the evaporator and front condenser?
Is it performance issue or other concern?

[llangston1]

I want to replace those condensers primarily for more performance. The AC worked great when the Houston area weather was less than 80. I usually have to run the AC with the temp switch fully CCW to keep from freezing myself out but once the temps get about 90 it struggles to cool the car even. I have also had lots of evaporator icing issues. I was thinking that changing the front condenser might be the easiest way to get some extra performance. What is the best way to flush the system. Disconnect the hoses from all the components and flush them and them separately flush each component, condensers, evaporator, etc?

[wwest]

I would solve the icing problem before going deeper, to more expense.

"..fully CCW to keep from freezing myself out.."

That's a rather solid symptom that something is amiss.

Low, restricted evaporator airflow..??

Low refrigerant charge will sometimes result in that symptom.

Or the thermostatic switch seems to be a common point of failure but the easiest way to confirm is to (attempt to) calibrate the sensor.

Even fully CW the thermostatic switch should calibrated such that the compressor switches off just below freezing (~28F) and due to designed in hysteresis it shouldn't switch the compressor back on until the evaporator temperature has risen to well above freezing (~36F). At full CW the regulation, on/off cycling, should be such that the AVERAGE evaporator temperature is at least 2F above freezing.

[kuehl]

Flushing:

General overview.

System's refrigerant has been recovered; empty system.

1) Remove evaporator (if you intend to keep it), remove the TEV, flush the evaporator core; Interdynamics or your favorite flavor. When you put TEV back on, new o-rings on external balance tube and the base of the TEV.

2) Remove engine deck lid condenser, flush

3) Remove compressor, disassemble, dip in mineral spirits or blow out with carb cleaner. Reassemble with new seal kit

4) Remove front condenser, flush

5) Drier - get a new one

6) Hoses. Since it is difficult to flush 'up hill', the hoses need to be on a downward or flat plane. This means;

1) Disconnecting the evaporator to compressor line at the evaporator, and dropping it down away from the steering rack and disconnecting any hose clamps that hold that hose end upward. Flush the from the end in the engine compartment, have a catch can (ie. old 1 gallon empty bottle) at the evap end
and duct tape where it goes into the bottle leaving a small gap for air to escape.
The blow out with shop air, a lot of shop air, a few times.
2) Disconnecting the deck lid condenser to fender condenser (ditto alike the evap line).
3) Disconnecting the fender condenser to front condenser and dropping any section of the line (like in the front fender) so the line can drain downward.
Ditto on the flush and blow dry.
4) Disconnecting the front condenser to drier line at both connections, remove, flush it, blow dry
5) Disconnecting the drier to TEV at both ends, drop any section that gives you a "loop"...

All hoses must have a downward to gravity path with no up hills in between.

You might find by the time you disconnect and reconnect the hoses, add up the cost for the flush... a new hose set would seem easier.

Components you take off the car, after flushing and air blow, let them sit in the sun for a few hours.

New o-rings on all connections, and a new drier.

[Rawknees'Turbo]

Originally Posted by llangston1

Just a quick update on my rebuilt compressor. It was a Denso rebuild in a Denso box. It has a stock rear condenser, a stock front condenser, and Zimms rear fender condenser and a Rennair evaporator. since I will need to open up the system to fix it I am considering replacing the front condenser and possibly the evaporator as well.

Originally Posted by llangston1

I want to replace those condensers primarily for more performance. The AC worked great when the Houston area weather was less than 80. I usually have to run the AC with the temp switch fully CCW to keep from freezing myself out but once the temps get about 90 it struggles to cool the car even. I have also had lots of evaporator icing issues. I was thinking that changing the front condenser might be the easiest way to get some extra performance.

I would bet that, based on the info above, you have a less-than-perfect refrigerant charge situation (too little or too much refrigerant, air and moisture in the system, etc)., as I have almost the same componentry as you (except I have the small, half-bay Turbo tail condenser, a GM blower motor, and I heavily insulated the car's evaporator housing and smugglers box, blocked all points of air entry into the cabin, and so forth) and in upper 90s and high humidity here in Norf TexASS, I consistently get upper 20s vent temps, even with the evaporator fan on max, remarkably quick cool down from intense cabin heat soak, and no evaporator freezing.

[kuehl]

Originally Posted by llangston1

I usually have to run the AC with the temp switch fully CCW to keep from freezing myself out

Get yourself a glass filled with ice and water.
If you have a good digital thermometer you can put it the glass
simply for observations.
If you want the cold mixture to last a bit longer and a bit colder you can put a few table spoons of salt in the ice and water.

Carefully remove aluminum thermostatic probe from the evaporator core.

Turn the ignition key on to 'accessory' position, just before start position.
Turn evap fan speed switch 'on', to speed 1.

Turn thermostat knob on to max cold, fully CW to the right, then back it off about 5 clock degrees to the left, ccw.
The front condenser blower motor should be running and the compressor clutch engaged. Insert it in the glass about 3" into the ice/water.
The condenser fan and comp clutch should turn off
in less than 30 seconds. If not, you need a new thermostat.

If the condenser fan did turn off, back off the thermostat knob CCW to the left
1/2 way between "off" and "max cold". Remove the thermostat probe from the glass and warm the end in you hand until you hear the condenser fan and clutch turn on again. Then reinsert the probe in the water. The condenser fan and comp clutch should turn off in less than 30 seconds. If not, you need a new thermostat.

If the condenser fan did turn off, back off the thermostat knob CCW to the left
about 5 clock degrees just before "off". Remove the thermostat probe from the glass and warm the end in you hand until you hear the condenser fan and clutch turn on again. Then reinsert the probe in the water. The condenser fan and comp clutch should turn off in less than 30 seconds. If not, you need a new thermostat.

If the condenser fan did turn off, back off the thermostat knob CCW all the way to to the left, "off". Remove the thermostat probe from the glass and warm the end in you hand until you hear the condenser fan and clutch turn on again. Then reinsert the probe in the water. The condenser fan and comp clutch should turn off in less than 30 seconds. If not, you want a new thermostat.

Otherwise the issue could simply be the location of the thermostat probe and how well its making contact with the aluminum fins in the evaporator.

[llangston1]

Thx for the tips. I had used the cup of ice trick when I first rebuilt this AC system a few years ago and the thermostat switch worked OK then. Also I finally got my gauges on the system today and got this reading without the motor and compressor running. It seems like both the hi and low sides were indicating about 80 psi. I have not tried to get a reading with the engine running yet. I need to get the compressor to engage before that. Here is a pic of the static gauge reading.

[llangston1]

So if my system still has freon and decent pressure then what are all the items that could prevent the compressor from engaging? I have checked the main relay in the smugglers box and its fuse was still good. I will check the thermo switch again but if it still works correctly then what's the next place to check since it would seem to be an electrical issue. Sorry I am being so dense but I last worked on my system back in 2010 and 2011 and I have just about forgotten everything I did back then. I have never really had any electrical issues with the system before.

[Rawknees'Turbo]

^^^

In addition to the fuse, the relay in the smugglers box has contact points inside it; if the points get burnt for some reason, or develop corrosion from non-use, etc., they can stick together and prevent the system from operating. You could try tapping lightly on the relay case with something like a screwdriver handle to see if they free up, and/or you could take the cover off and clean the points with fine sand paper.

Also, unless you added a low pressure cutoff switch, the absence of refrigerant would not keep the clutch from engaging (it looks like you have more than enough static pressure in there, regardless, though) - no high/low pressures switches originally came on 70s-80s 911s.

[kuehl]

Originally Posted by llangston1

and my hi/low pressure switch is keeping the system from engaging. I have checked all the fuses and they are fine. If the hi pressure switch was working correctly what would cause the nose seal to blow. \

Your aftermarket AC mfg'r should have provided you any electrical schematics that modified or added to the original factory ac circuit. Absent that:

Assuming you have a aftermarket pressure switch in the system and the switch is located back near the compressor, and your primary AC fuse in the front fuse panel is good.... you checked with a test light or VOM for voltage on both sides of the fuse and the electrical wire in the bottom section of the fuse holder under the wire screw is clean without corrosion with good strands. Just a note, some mid 80's and onward Carrera's with electrical motor seats can blow out this fuse when seat motors or switches fail.

Refer to this common 911/930 AC Schematic

Ign key in 'accessory' position just before engaging starter. Evaporator blower motor switch set to first fan speed. Thermostat set fully CW (right) to max cold. The front condenser blower motor should be operating. If not use a test light or VOM and trace the circuit backwards starting at "Relay AC" in smugglers box (relay is located behind evaporator box passg side (LH drive vehicles).

1) If you have power to the "Relay AC" but coming out, the issue could be the relay or starter ground circuit.

2) If you have power coming out of the "Relay AC" but not out of either of the 3 evaporator fan speed switch's output terminals its the fan speed switch.

3) If you have power coming out of the "Relay AC" but no power at the evaporator blower motor's positive wire its the Evaporator Fan Resistor Unit.

4) If you do not have power to the thermostat its the evaporator fan speed switch.

5) If you have power to the thermostat but not out is the thermostat

6) If you have power out of the thermostat but not at the original ac clutch power spade terminal back by the oil filter you have a break in the wire leading to it.

7) If you have power at the at the original ac clutch power spade terminal back by the oil filter but not coming out of the aftermarket pressure switch (assuming it is between the compressor clutch) the pressure switch may have failed.

8) If you bypass the pressure switch and connect the compressor clutch to the original ac clutch power spade terminal back by the oil filter and the compressor clutch does not engage its either the compressor clutch coil or grounding of the compressor body to the comp bracket.

9) You can use a separate power supply (12vdc) with a minimum of 3-5 amps supply and power the compressor clutch's coil wire (insure your power supply's ground is grounded to the compressor body) and try to engage the compressor clutch. If it does not engage check the resistance of the compressor clutch coil to the compressor body, should be 3 ohms nominal.

Too bad Eric and Ron aren't here to help

[llangston1]

Well I believe that Rawknees wins the prize for the correct answer to my problem. It just took a little rap on the main smuggler's box relay to get my AC up and running again. Everything appears to work like it used to so hopefully I have dodged a bullet this time. I drove it around for 20-30 minutes this afternoon in our 85 deg afternoon and got 35 deg center vent temps in the picture with the temp switch fully CW and the fan on low speed. At high fan speed the temps were just a little above 40 deg and at the middle fan speed the temps were just below 40 deg. Not sure I believe the digital gauge. It's a HF and exhibits some strange temp readings at times before it settles down. I will try to get some AC manifold gauge readings soon and post them. Thanks everyone for their help with this. If my relay is gonna need a smack every now and then is it time to replace it? Also after my drive I did check my deck lid and saw no oily streaks so I am not sure what was on the lid when I thought the compressor had failed. Maybe some road grime from a drizzling day?

[wwest]

Relays can be quite sensitive to applied voltage, check that you have >12 volts across the relay armature.

The compressor clutch coil draws a high level of current at the same instant that the relay is initially energized, momentarily causing a substantive drop in source voltage on the SAME switched circuit.

[kuehl]

Original AC relay years back was metal can, 20+ amps.
Get 30-40 amp, its common.
Clean all the spade contacts.

[wwest]

Looking at the schematic diagrams it appears that the ONLY purpose for that relay is to power the front condenser blower motor....

If so that would make it extremely unlikely that that was/is the cause of the non-operational status of your A/C. Lots of idling along, no airflow from forward motion, YES, but on the highway, cruising, NO.

If that 30.4dF reading of digital thermometer is anywhere close to correct (I tend to have more faith in digital vs analog) then you most definitely need to evaluate the thermostatic switch's condition and/or calibration. The thermostatic switch's electrical contacts may be sticking closed, common fault for switching highly inductive loads.

[Rawknees'Turbo]

Originally Posted by kuehl

should be 3 ohms nominal.

Too bad Eric and Ron aren't here to help

A couple of chowderheads right thar, I tell ya'!!! Almost as bad as that Mighty Pieholio/Bob K dude (at least the former two don't own Targas, so I hear )!

Originally Posted by llangston1

Well I believe that Rawknees wins the prize for the correct answer to my problem. It just took a little rap on the main smuggler's box relay to get my AC up and running again. If my relay is gonna need a smack every now and then is it time to replace it?

Glad to help!

Like Griff said, it wouldn't be a bad idea to replace the relay, but if you're feeling adventurous, you can gently remove the cover from it (probably need to do some prying with a tiny screwdriver, and so forth) and clean all the contact points with fine sand paper.

Originally Posted by wwest

Looking at the schematic diagrams it appears that the ONLY purpose for that relay is to power the front condenser blower motor....

If so that would make it extremely unlikely that that was/is the cause of the non-operational status of your A/C.

Incorrect.

[wwest]

Not all model years included a damping diode at the compressor clutch connection. That diode was/is used to suppress the HUGE inductive voltage spike when the thermostatic switch opens, so it didn't cause RFI/EFI interference, nor contribute to WELDING the thermostatic switch contacts closed.

Initially the contact WELD may only be intermittent, contacts sticking closed, requiring more and force to open them, tilting the hysteresis "span" to well below freezing.

[wwest]

Rawknees'Turbo: "...Incorrect..."

How so...??

[kuehl]

llangston1, I'd suggest replacing the AC Relay.

If you review the 911 AC schematic in the hyperlink I provided you will see the relay supports primarily the compressor clutch and the evaporator fan motor, it provides just a tad of power (probably milli amps) to the front condenser blower relay. If you have wired another device off the primary AC power supply wire back in the engine compartment by oil filter that will put more demand on the relay as well as the thermostat.

If you measure the length of those circuits, their wire size and the actual load of the components it is easy to understand how the AC Relay can get "beaten up" over the years. One easy check is grab that relay in the palm of your hand after the AC has run for 1/2 hour or so, it does get mighty warm and I've seen plenty of them toasted.

Hence, do not put in a 15 amp relay, use a 30-40

[wwest]

Puzzling...

The schematic diagram I posted is for an '85 911..

Kuehl's link is to an '82....

llangston1's 911 is an '87....