Service A/C light code B447c

[Steve2150]

Have 1990, the service a/c light comes on and off. When on the a/c will not work, goes right to econo setting. Had 134 checked, right amount, when light not on compressor turns on. Diagnostic code is B447c, there was some history codes also but cleared them before writing down.

[drtidmore]

B447 is the code for VERY LOW A/C Refrigerant Problem.  This condition is computed by the BCM based on high (condenser temp out) and low temp (evaporator input temp) sensor readings.  B448 is set when the low pressure switch is tripped.  Since you are getting B447 but not B448, the issue is pretty much one of the charge level in the system (i.e. too little R-134a)   R-134a while acceptable for our R-12 designed A/C systems is not an exact replacement.  First off for a given amount of refrigerant, R-134a does NOT remove as much heat as R-12.  It is also a smaller molecule.  Also, the typical guidelines for swapping over to R-134a is to use 80% of the recommended R-12 levels.  The problem with our Reattas is that the A/C is controlled by the BCM and it is expecting R-12 performance, not R-134a.  The best way to set the optimal charge level is to monitor BD028 in the BCM (evaporator input temp sensor).  Under optimal conditions of charge, the value will be a more or less constant 0 to -1.  If it is constantly cycling between -2 and 9 and back again then there is TOO LITTLE R-134a in the system.  If it does NOT drop to around zero (these are in celsius degrees BTW) then there is TOO MUCH R-134a in the system.  We are not talking about massive changes in the amount.  I suspect the techs that checked your R-134a levels based it purely on pressure.  BD028 is the best way to set it and I recommend that you take it out on the highway after each adjustment of the charge level and monitor BD028 while at highway speeds (I know that is possible in the 88/89s). 

Edited June 13, 2017 by drtidmore
I just caught that I had reversed the conditions when I made this post (see edit history)

[drtidmore]

Another thought is that the evaporator input sensor have been known to soft fail (incorrect value) which will throw things off.  The way I verified the sensor on mine was to use an infrared thermometer pointed at the low temp sensor junction to the evaporator tube immediately after the expansion orifice tube (underneath the black plastic trim piece that runs along the engine compartment just forward of the windshield.  The infrared temp vs the BD028 should be very close to one another.  

4 minutes ago, 89RedDarkGrey said:

 

Even easier- place a box fan in front of grill (to help evaporator) and hold idle at 1.5k while checking.

Quote

I have found that highway readings are more accurate for tweeting the R-134a levels.  Using a box fan and a fast idle works to get it into the ballpark but if you really want to hone in on that sweet spot of a constant zero degree evaporator temp, highway speeds are the best.  The condenser out front simply gets more airflow at highway speeds than you can push thru it with a fan. 

 

[drtidmore]

1 hour ago, 89RedDarkGrey said:

 

 

https://www.amazon.com/Air-King-9723-20-Inch-3-Speed/dp/B0007Q3RMA = 2,140 CFM

 

http://www.scienceforums.net/topic/25944-cfm-to-mph/ = 55.73 MPH ~ 1,900 CFM

This is one of those case where theory and reality didn't quite line up.  I used the box fan pointed directly at and in front of the lower grillwork opening and the high speed idle combo when initially charging the A/C and it worked pretty well, but once on the highway, I found that it was actually just a tad overcharged as it got into the -2 to 9 cycling mode within a few miles of constant highway speed.  I bled off just a tiny bit of refrigerant at a time until the system settled into a stable 0 to -1 range.  I suspect the difference has to do with the aerodynamics that the grillwork and lower air dam create on the highway vs the smaller footprint box fan.   

Edited March 21, 2017 by drtidmore (see edit history)

[Steve2150]

My mechanic replaced the low pressure switch today and charged the system 134a which it was previously. He said it was taking too long for the cans to charge the system, finally it did and was cold for around 5 minutes then the service a/c light came on and compressor off. Checked the codes B446H and B448C. He thinks something with the low pressure side. The orafice was changed with the new compressor 6 months ago but the old one was really dirty and clogged if that helps.

[Steve2150]

I have the car for 4 years so the R12 to R134a conversion was done by the previous owner, in the 4 years I have had it the a/c ran great, replaced the compressor, orafice and accumulator 6 months ago as the compressor was leaking slightly, ran great after that till now, noticed at idle originally the ac service light did not come on but when accellerating it did, now on permanently

[drtidmore]

I totally agree with 89RedDarkGrey on the contamination probability.  If the system was NOT thoroughly flushed using the appropriate solvent when the orifice was found to be clogged and a NEW accumulator, new orifice installed along with the compressor, the system was left full of contamination.  

 

But back to symptoms.  The A/C service idiot light sets when ANYTHING in the A/C system is out of limits or malfunctioning.  Going back to your original post, If the system is setting code B447 that pretty well exonerates the power steering switch AND the LOW PRESSURE switch. If the low pressure switch is being tripped, the error code will be B448.  

Remember B447 is NOT a specific sensor or limit switch but rather is computed by the BCM based on the high and low temp sensor readings.  As you can easily see what the BCM is reading for both sensors (BD027=high temp & BD028=low temp) and it is pretty easy to get a good infrared temp gun on the location of both and that should be the next steps.   They both should agree within a couple of degrees between the BCM reported value and the infrared temp gun measured.  The low temp sensors have been particularly known to soft fail in that they still report, but the value is off.  Finding a new or NOS low temp sensor is very problematic so you may have to resort to salvage.  

 

BTW, it is easy to verify the power steering pump switch.  Go into the ECM inputs and monitor EI78 while turning the steering wheel back and forth through it normal travel limits.  The power steering switch ONLY trips when there is high back pressure on the high pressure side of the pump. 

 

 

Edited March 23, 2017 by drtidmore (see edit history)