[Vwdiesel] TDI AC debugging

[Val Christian]

Well, the furnace craps out on the coldest days of the year, so
the auto AC should die on the hotest days.

Last night the bugs had a feast on my skin as I tried to find out why
my 03 TDI ac died yesterday.

Symptoms:  System cooling anemic, topped off the R134a charge a couple 
of weeks ago.  Worked better for a day then backed off in cooling power.
Yesterday, total loss of cooling while running.

Diagnosis: Charge OK, by guages.  Compressor not engaging.  VAGCOM 
apparently is of no help...the module for HVAC doesn't respond.  I
assume that this car doesn't have one.  Pulled lower "skid plate"
plastic, and measured power to compressor.  4.8 volts, sometimes 
cycling quickly to 7.8 volts.  Since it's just a two wire connector,
I assume that the power should be 12V when energizing the AC compressor
clutch.  The somewhat worthless Bentley CD shows J293 (which other 
literature calls the fan controller) as powering the clutch.  The Bentley
CD also points me to a 13 relay cluster, which apparently my car lacks.
I pulled lots of dash looking for it.  Apparently a vestige of a bygone
era.  

So, it seems I may have a failed J293.  However, judging from a few posts
(the forums on the Bentley website thus far seem to be the most useful)
people have swapped J293 only to find that is not the problem.

Tonight, if I get home at a reasonable hour, I'll check out the 134a
pressure sensor, which apparently drives the J293.  It's not clear to
me why the voltage would be low and pulsed, going to the clutch.
The clutch wear seems to be high, and I'm wondering if it did allot of 
slipping in recent times.

Interim conclusion:  Still learning.  Bentley forums promise to be the
most useful.  Manual is of marginal use.  Older, printed Bentley manuals
are marginal in their usefulness on AC jobs.  Other manual vendors 
for other vehicles have them beat.  My guess is that the J293 module,
or the pressure sensor will be found the be the problem.  It is not
clear to me how the ECM comes into play.

Any pointers are appreciated.

Val