<E28> Re: AC death- troubleshooting procedures-long

[bmw4curt@xxxxxxxx (Curtis Kiser)]

Tom Jarlock's suffering in the Florida heat [I'm a close second in the
Virginia muggy air].
My 85' 535i is doing the exact same thing as his car.  It used to be
intermittent but it's 100% dead now.

My symptoms: AC switch operates (replaced it anyway), the blower comes on
but no cold air.  The temperature control setting seems to 'thump' when
you turn it to warm settings indicating that something is trying to
operate properly with temp control changes.   No compressor.  The freon
was checked & filled when it was working last month.  

I've been trying to fix it myself and have learned a lot about the E28
A/C system.  Just haven't fix it yet but I'm bearing down on it.  Some
things to do if your the mechanic:

1.    Check the fuses

2.    Check the freon level.  If it's low it will cutoff the compressor. 
Bubbles at first is ok but they should clear when you rev the engine.

3.    Check the deicer sensor [4" long] in the evaporator.  You can get
to it by pulling the radio and extract it out of the evaporator.  There
is a two wire connector [white] that you disconnect.  You can measure the
resistance while in place and the values are in the Bentley.  I pulled
mine out and stuck it in a glass of ice/water and measured the ohms while
the temperature changed.  It checked out ok but they DO get intermittent.
 Major pain to debug.  If the probe is plastic BMW has a new replacement
part number for it.  If it's metal it should be more reliable.  *Major
tip*....if you remove the probe/sensor, disconnect the plug , pull it out
and yank on the wire.  It's held by a small metal clip to the right side
but if you pull hard enough it will come.  Otherwise, pull the center
console (been there & they don't hand out T-shirts for it either).

4.    Check the A/C switch [snowflake].  They go bad sometimes.

5.    Check the compressor out.  You can do that by activating the inside
AC switch (snowflake), then wire battery voltage to the single wire that
goes to the compressor (engine running).  That should activate the
compressor/clutch and get things cold if it is good and you have freon in
the system.  ****WARNING****   This tests overrides all pressure cutouts,
deicing and temp control circuits.  Do NOT connect 12 volts in a manner
that sends it back into the dash/control circuits.  If squeamish about
this....don't do it because it could damage the system if done improperly
or something is seriously wrong!  If this works and blows cold air you
may be lucky and have a good compressor.  The problem then either points
toward the electronic controls/wiring, maybe a low-pressure cutout switch
& drier problem or a leak in the system.

6.    The interior temp control, snowflake switch, and deicing probe
feeds 
a *control module* that hides above the gas pedal and to the 
right [up behind the knee panel].  This is what makes the 'thump' sound
when 
you turn on the A/C and or vary the temperature control from warm to
cold.  
This module is what sends the 12 volts to the plumbing part of the system

so it will run!  The connections can be bad or the unit can be
shot too.  If it 'thumps' the relay should be working.  It could have bad
contacts but the solenoid is working.  This module costs $58.  
The guts consist of a 8 pin chip, 1 transistor, 1 cap, 1 resistor and the
solenoid.

I haven't been beyond this and have yet to crack open the pressurized
system.  I've seen some digesters replacing the driers this year and
that's not a bad idea on the older cars.  If I have to open the plumbing
up the drier is getting replaced.

My situation is that all seems to be working but no 12 volts at the
wire/connector feeding the compressor.  Tommorrow the wiring/connectors
and pressure cutouts gets checked.  The compressor runs great and blows
very cold so it's the control system.  I'll get it soon....I'm close,
very close!

Good luck on your A/C journey Grasshopper.....you will need that stone
you snatched out of the Master's hand to honor the ice-cube gods.

Curt Kiser
SVR - E28 division of ///Team Bubba Sports
BMW CCA #118507 Tidewater Virginia Chapter
85' 535i 
86' 325

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