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1990 Reatta blowing A/C fuse #19


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Yesterday I had an experienced A/C repair shop in San Jose, CA evacuate and recharge the R12 (it was only down 0.4 lbs from full at 2.375 lbs).  The A/C worked fine for about an hour of driving [Note air temp is record heat wave at 96], then the 20 amp fuse #19 blew and compressor clutch stopped operating and Service A/C light came on.  Any ideas of what could be causing this problem?

Chuck [husband of NASCARLady

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If the fuse blew first then it may have just been an old fuse. Back in the day GM used a 30A fuse on the AC clutch line. Id replace it first then monitor BD27 while driving but a high back pressure will trip the high pressure switch and not blow the fuse.

 

In general R12 runs at lower pressures than R134A and I have no problem with mine in the 90s.

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Thanks for the quick replies.  Within the family we have two Reatta's 1989 and 1990.  The 1990 is having the A/C problem.  I will check on the items that you have mentioned. One question -- The 1990 doesn't have the CRT (like the 1989) so how can we monitor the BD27?

 

Also, the schematic shows a diode across the clutch coil to reduce the inductive kickback. Has anyone had a problem with that diode failing [shorting] at high temperature and causing the clutch fuse to blow? Notes say the diode is inside the connector on top of the clutch coil.  Is it possible replace the diode without having to remove the clutch or clutch/compressor?

 

Chuck

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Yes, 90-91 have full diagnostics built in. Push off and warm in the HVAC at the same time and hold until display blinks. Values will appear in the odo. Select mode in fan up (yes) fan down (no). It is in the FSM.

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Thanks much for your help.  I checked BD27 engine cold, reads 27.  Then with engine running and AC off, reads 31.  Then with AC on, reads 54.  Then keep engine running and turned off AC, went to 33 but started inching up to 44 over about 3 minutes.  Then turned on AC again, went back to 54 but soon after fuse blew. 

I'm wondering if the high side temp sensor is not working?

 

Chuck

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Updated information -- Found that the fuse blowing resulted from the suppression diode was breaking down with higher heat.  Replaced the diode and that problem was solved.  Then noticed that compressor was not cycling and system was getting very cold.  So I replaced the low pressure switch. But now the low pressure switch does not turn on so the compressor does not come on at all and the diagnostic throws the B448 code.  So I removed the connector from the low pressure switch and jumpered the connector.  This causes the compressor to come and stay on and their is no B448 code. 

Any suggestions of what I should check next? Could it be the low side temp sensor or high side temp sensor?

Note: I also confirmed that the A/C relay output from the ECM going to pin 5 on the clutch relay does pull down to 0 volts.

 

Thanks       Chuck

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I'm not an AC expert but I will tell you what I know from working with my Reatta AC system. Unlike standard GM AC systems, the Reatta climate control system does not rely on the low pressure switch to cycle the compressor on and off under normal operation. It only functions as a safety switch to signal the BCM that the system is low on Freon. The BCM will turn off the compressor if it gets a signal from the low pressure switch for a short period of time or it will set a code B448 and lock the compressor out if gets a signal from the low pressure switch for an extend period of time. Since you have a new low pressure switch and you are getting a code B448 I would bet your Freon is low.  This will probably explain it better than I can. Thanks to Mc_Reatta for providing the information.

 

Quote

Then noticed that compressor was not cycling and system was getting very cold.

 

That is normal operation on my car if the doors are open and the AC is set to MAX cool on a really hot day. The low temperature sensor monitors the temperature of the evaporator and cycles the compressor off to prevent freeze up. Otherwise the compressor will run contentiously to try and cool the car down to the temperature you have set in the climate control system. As the car cools down, the BCM will adjust fan speed and cycle the compressor to maintain the correct temperature based on the feedback it is getting from a number of sensors that monitor interior temp, sun load, etc..

 

If I were you I would check the Freon first. Then go to this section of my website and go through the AC sensors troubleshooting tutorial. It will tell you if your sensors are working properly.There are a lot of articles there that might help you.

 

My low temperature sensor failed causing the BCM to cycle the compressor off before the evaporator got cold resulting in the car never getting cool enough. Replacing the sensor corrected the problem.

Edited by Ronnie (see edit history)
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Thanks for the good suggestions.  I ran two tests, each test starting with resetting codes by disconnecting the battery.

Test 1 -- Low pressure switch in place

Results -- Now seeing B448 code; compressor not running; B027 reads 20,  B028 reads 20

 

Test 2 -- Low pressure switch disconnected and jumper across connector

Results -- No B448 code; compressor is cycling with approx 5 seconds on and 5 seconds off.  B027 cycling from 23 to 31 and then back down to 23.  B028 cycling from plus 9 to minus 2 and then back up to plus 9

 

Do these measurements show that I may have lost to much freon while changing the low pressure switch or is there still something else wrong?

I don't have gauges to measure actual system pressure, so I'll need to go back to the A/C shop to get an R12 refill.

 

Chuck

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27 sounds low, suspect you need more Freon. At idle and with a strong fan in front of the grille should be more like 20 seconds on/5 off to on continuously. Actual values depend on your air tempersture.

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Thanks much for all the help.  The rest of the story ......

 

Took the car to the A/C shop and owner Grant [B&M Radiator in Fremont, CA] pulled about 1.4 lbs of R12 out.  That's my bad because the new low pressure switch that I put in had the plastic thread. I was afraid that I would over tighten but I didn't discover that until the car had lost about 1 lb of R12.  Grant took out the new low pressure switch and put in a new O-ring and re-installed.  Grant evacuated the system and recharged. He disconnected the battery to reset codes.  When I turned on the car the compressor did not come on -- Oh No! Grants A/C test machine said all the pressure readings were fine now.  Since I brought the old low pressure switch with me I suggested we put it back in. Grant put it, I restarted the car and the A/C came on working like a champ!

 

Lessons learned --

1. If the A/C fuse blows, suspect that the A/C clutch suppression diode could short out at higher temperature.  Replace the diode.

2. Since the replacement low pressure switch was a brand new switch intended for our model car and purchased from a well-recognized independent parts dealer  maybe was set for the wrong hi/lo pressure readings.  Maybe the use of plastic instead of metal helped cause the problem.  In any case, double check that the replacement pressure switch is really the right one.

 

Thanks again      Chuck

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