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Remove this! It blocks airflow and makes a noticeable difference in air volume after it is gone. It can be removed without removing the lower cover.
Slowly pull the plastic temperature sensor out from between the tubes. (It takes some effort to pull out.) The screen just slides out. Push the sensor back into place.
I cannot tell a difference between any setting on the min-max adjustment. I am considering deleting the fancy relay and bridge terminals 4 (blue wire) and 5 (black/red wire) on its plug to have the solenoid always be open when the fan is running.
Adjust the front blend doors! If the front feels warm after turning on the rear, this may be the reason.
Originally Posted by PorKen
Adjusted the heater core block-off doors. I have often read about this adjustment but embarrassingly have never done it. The foam seal on the doors falls apart over the years so the doors need to be closed more to compensate. Now with the AC off on a cool day there is actual (mostly) cool air coming from the vents, and the AC is genuinely frigid.
The parcel trays need to be removed but otherwise the adjustment is simple. Set the climate control temperature slider all the way cold with the key on. Give it a minute to make sure the laboriously slow servo motor (left side) does its thing. The servo motor control arm should be pointing straight down. Turn off the key.
On the left side, pop the rod out of the arm with a small screwdriver and rotate it free. Shorten by turning clockwise until the arm just fits into the plastic keeper. (It will be hanging at least 3mm too low if it has never been adjusted.) Push the rod back into the keeper.
On the right side, loosen the bolt with a 8mm wrench. Turn the bolt out enough the the lower arm can be adjusted but do not remove it. Move the arm up and down to verify that the clamping "teeth" are disengaging. Pull the arm downward as much as possible and tighten the bolt. Boom, done!
It seems obvious, but I forgot about turning on the rear AC when I checked the hi/lo pressures the first time. Best to at least run the rear for a bit to equalize the system before turning it off to check the front pressure only.
I observed about a 10"hg difference in suction pressure when the rear air was turned on (from about 18"hg to 28"hg at 80F).
I cannot tell a difference between any setting on the min-max adjustment. I am considering deleting the fancy relay and bridge terminals 4 (blue wire) and 5 (black/red wire) on its plug to have the solenoid always be open when the fan is running.
I take it back. Driving in the evening with cooler humid outside temperatures I was able to discern the difference between minimum and maximum adjustment. But, the rear air is almost always used during the day when the sun is baking the interior so the variability is moot.
So, unlimited, frosty cold rear air it is! (Unlimited meaning there is no longer an electronic freeze switch.) After a few minutes of driving I turn both the front and rear blowers down to low speed. Can't beat a cool breeze on the back of your neck! (Side note: I also zip-tied the heater valve closed for the season.)
Relay bypass jumper installed (between blue wire and black/red wire) and no-longer-used temp sensor removed from evaporator.
Relay electronics. I always thought the solenoid open time might be controlled by a variable pulse width, but it is just a temperature threshold on/off.
There is a diode for the relay coil obscured by the spring.
Did you know that the same unit is used in the BMW 635 E24 Highline, but it has a fridge!
Is this the most little known fact about our 928s? OUR cars have a fridge!
it’s described in the (86.5) manual and you can check it for yourself. There is an AC duct that drafts AC air into the dash glove box !
Look into the glove box and in the right side is a small port to duct AC air in.
I take it back. Driving in the evening with cooler humid outside temperatures I was able to discern the difference between minimum and maximum adjustment. But, the rear air is almost always used during the day when the sun is baking the interior so the variability is moot.
So, unlimited, frosty cold rear air it is! (Unlimited meaning there is no longer an electronic freeze switch.) After a few minutes of driving I turn both the front and rear blowers down to low speed. Can't beat a cool breeze on the back of your neck! (Side note: I also zip-tied the heater valve closed for the season.)
Relay bypass jumper installed (between blue wire and black/red wire) and no-longer-used temp sensor removed from evaporator.
Ken, don’t be surprised if you get giant puddles under the car, under the rear AC unit on some days. That’s even possible when the rear air is all recirculated air.
Until I set my front anti freeze switch probe correctly, I would get frigid air and then ice reducing air flow and then large puddle instances on the garage floor, up front.
Ken
After my AC refresh, replacing most everything except evap and condenser coils, the front antifreeze switch replacement was part of the upgrade. The setting on the switch was left at factory setting.
Everyone is looking for a temperature situation for their environment. I suspect the rear temp sensor has a compressor interrupter capability like the front antifreeze switch. You would know better. After finally getting my front antifreeze switch probe set correctly down the guide tube, here are the differences, with temperature set to max cold:
Before getting the antifreeze switch probe all the down it’s guide tube
The front vent temp would get down to 40 quickly and slowly edge towards freezing. Yes, you could at times see your breath vapors. It was great, until after an hour or so, the air flow from the front vents would greatly reduce and the interior temp would uncomfortably rise. Yes, the front evap would ice over.
After getting the antifreeze switch probe set correctly down its guide tube
Then the compressor would noticeably cycle on-off, pretty often like changing state every 10 seconds. The temp from the vents locked onto 48 degrees (as per the bargain analog vent thermometer) and hung right there, the entire drive. I’m in the mid-Atlantic. Yesterday the temp was 93 and ridiculously humid. The interior was almost uncomfortably cold. I could never feel a difference in air temp that would indicate a compressor shut off interval.
I kind of suspect that the rear temp sensor is redundant if it performs the same purpose as the front evap temp sensor. The front temp sensor may cycle the compressor enough to keep the rear evap from freezing. But I’ve never tested that scenario. You might try periodic checks of air flow thru the rear unit during long drives.
Thanks for being a hero for the rear evap unit. I’m still working on rear resistor pack issues on mine.
I suspect the rear temp sensor has a compressor interrupter capability like the front antifreeze switch.
The rear air relay compares the resistance of the temp sensor to the dash **** rheostat and switches the solenoid under the passenger seat open and closed. I imagine the relay also serves as an anti-freeze switch at the maximum cold **** setting, but it does not switch off the compressor, it just stops refrigerant flowing to the rear evaporator.
I think it is unlikely that the rear evaporator would ever freeze up. If the compressor is being regularly switched off by the front evaporator freeze switch, then the rear evaporator refrigerant flow is also getting cycled off. Also, the rear evaporator is stealing some of the pressure to the front evaporator so the front is less likely to dip into super low temperatures?
On my car, with the switched rear air with AC button modification wiring and external relay, the front freeze switch will also switch the rear air completely off.
My rear air works so well now I always keep it on fan speed #1 with the front at #2 or #1.
07-01-2024, 09:40 PM
Anti-water-droplet screen
