Diagnostic Code b448

[retiredarnold1]

I replaced my ipc and its working. While the ipc wasn't working my a/c started working again, kinda of weird. After putting in the new ipc I cleared the codes and the a/c light stayed off. Took it for a test drive and the a/c light came on again, checked the code it was b448. I've tried to find the code in the documentation but for the life of me can't find what it is. Anyone no what it is?

Thanks

[Guest Mc_Reatta]

Description of the code starts on page 8D2-60 of the 89 FSM. Guessing that's your year, but information would be the applicable for the other years as well.

The BCM tracks the pressure status of the AC system 3 ways. The first is via the low side temperature sensor. The second is via the low pressure switch. The last is via a high side temperature sensor which is not part of this problem.

The information from the temperature sensor will set codes B446 or code B447 when it detects that the low side of the AC systems gets too cold too quickly than it should.

When the pressure switch reports that the measured pressure on the low side drops below 10 psi for more than 3 minutes it will set code B448.

So if you are just getting code B448 without either B446 or B447, then you have a problem in the low side pressure switch. It will prevent the compressor from running even though there may not be anything wrong with the system other than this switch.

Check that the AC fuse #19 is good as this powers the circuit. Check that the connector on the pressure switch is clean and properly connected.

If you don't find any obvious issues, then a check of the switch needs to be done. Best way is with a DVM and at least an AC low side pressure gauge.

Do you have access to these? Need to verify that you have at least 12 psi or more in the system, and that the switch is open or closed and if those are OK, that the circuit is completed.

See attachment for location of the switch and sensors.

Beware! #s 2 and 3 are reversed in the attachment. #3 is high temp sensor, #2 is low pressure switch. (Thanks to Vincent Vega for catching this FSM error.)

A new pressure switch should not be too hard at a FLAPS and can be changed without having to purge the system.

Edited May 2, 2011 by Mc_Reatta
correction (see edit history)

[Ronnie]

Click the link below:

BCM & CRT Diagnostic Codes*-*ReattaOwner.com

[Vincent Vega]

......

See attachment for location of the switch and sensors. ....

If I recall correctly that diagram is incorrect. 2 and 3 are reversed.

[Ronnie]

Mc_Reatta, I follow what you had to say about how the A/C system operates pretty well but isn't the paragraph quoted below ignoring the fact that the system could be low on freon and the low pressure switch is working correctly?

If the pressure is borderline in the system to begin with, the pressure may not drop low enough to trip the low pressure switch warning until the car is driven with the RPM up for a while (3 minutes) such as during retiredarnold1's test drive. I may not be reading your post correctly or... I could just be be wrong.

So if you are just getting code B448 without either B446 or B447, then you have a problem in the low side pressure switch. It will prevent the compressor from running even though there may not be anything wrong with the system other than this switch.

[Guest Mc_Reatta]

If I recall correctly that diagram is incorrect. 2 and 3 are reversed.

Thanks Vincent for catching this and bringing it to my attention.

Didn't remember that one nor checked the FSM figure closely enough.

I have put that info into my earlier post.

[Guest Mc_Reatta]

Mc_Reatta, I follow what you had to say about how the A/C system operates pretty well but isn't the paragraph quoted below ignoring the fact that the system could be low on freon and the low pressure switch is working correctly?

If the pressure is borderline in the system to begin with, the pressure may not drop low enough to trip the low pressure switch warning until the car is driven with the RPM up for a while (3 minutes) such as during retiredarnold1's test drive. I may not be reading your post correctly or... I could just be be wrong.

The answer is you couldn't get a code B448 issued if you had a marginal system that would start up (no history 447 or 448 codes) if the switch was operational.

Sorry this is rather long, but here's why:

The pressures temperatures and times in this example are not be absolutely accurate, but are reasonable to describe how the system would react to a low refrigerant condition.

Say the target low side pressure of the system is 30 psi.

If a little leaks out and the pressure drops to 25 psi this could cause the evaporator to freeze up, so would not be desirable. If the pressure drops to 20 psi the system needs to be disabled to prevent trouble, and if the pressure drops to 10 psi the compressor could seize up from lack of lubrication so you definitely want the system shut down.

If there is at least 10 psi in the system as reported by the low side pressure switch, the system will turn on. If not the system will not turn on and code B448 will be set after 3 minutes.

The BCM senses that what the pressure is on the low side of an operating system by noting that the temperature of the low side drops quicker that would be expected.

It knows what the ambient temperature is already. Lets say for example that the BCM is programmed to measure the time it takes for the low temp sensor to report a temperature that is 10 degrees colder that ambient. (starting temp of the low side will be higher than ambient because it is in the engine compartment, but that isn't critical to the example.) There is a reference time in the program somewhere of what this time should be for a properly charged system, and two other times that tell the BCM when to issue a trouble code.

For grins let's say that it expects the time to cool 10 degs is 30 seconds (at a normal 30 psi), and the first warning is issued if the time taken is 25 seconds (say 25 PSI), and the second warning is issued if the time taken is only 20 seconds (say 20 PSI) or less but (above the 10 psi switch cut off).

So if the charge in the system drops somewhat, the BCM will note that the time to cool 10 degs is between 21 and 25 seconds and will issue a B446 code low refrigerant warning, but the AC will still run with the BCM cycling the compressor off and on to prevent the evaporator from freezing up.

If the BCM detects that the time to cool was 20 seconds or less, and or the pressure switch opens for any length of time, it will issue a very low refrigerant warning B447, and shut the system down and latch so it won't try to run again until cleared.

If the BCM detects that the switch is open for 3 minutes or more, it will issue code B448 and latch so again the system won't run.

It is very possible to get multiple codes current or history depending on the problem, how severe it is, and when it occurs, but some combinations would not appear.

Rough rule of thumb:

If you get a B446 low warning by itself, either the ambient temp is below 50 degs and the system level is a little low and you probably just need a little refrigerant to be added to get back to optimal from a slight leak.

If you get B447 with or without B446, you probably have a significant leak in the system have lost 30 or more of the refrigerant, and need to have it fixed before recharging, or the pressure switch opened when the AC was operating either because it failed or a leak developed that brought the pressure down low enough to open a working switch. In this case, a B448 code would not appear as the system would shut off when the 447 code was issued.

If you get just a B448 code, you either have had a large leak that let all or most of the refrigerant out of the system when it wasn't on so the BCM didn't pick up on it before all the refrigerant ran out, or there is a problem with the low pressure switch or its circuitry. If there is enough pressure in the system for it to be allowed to start, you would not get to the condition for tbe 448 code to be issued because as soon as the pressure dropped sufficiently to allow a working switch to open, code 447 would be issued, and the system would be shut down.

I don't think you could get a current B446 code and a B448 together if the pressure switch was operating correctly, nor could you get a current code B447 and B448 at the same time either as the B447 would shut the system down before the time needed to set the 448 could take place.

If a major leak occurred when the car was not running, when the AC was next was turned on there would be no cooling since the compressor wouldn't be allowed to run, and after 3 minutes the lone code B448 would display as it would if the switch failed while a good system was running.

If the leak occurred after the AC was running for a while, say a stone hit the condenser, a code B446 probably wouldn't be set, but a code B447 would be issued or as the pressure dropped enough to open the pressure switch and the system would be shut down before code B448 could be issued.

Edited June 6, 2011 by Mc_Reatta (see edit history)

[Ronnie]

You are correct. It is a large post and I'm getting lost in all the BXXXs. Instead of sounding argumentative I will leave it at just saying in my experience it is possible to start a car with low freon and not have the BCM give the warning that the "compressor had been disabled to prevent damage" until I started down the road. That seems to go against what I think you are saying but I'm not sure.

[Guest 88atta]

For what it is worth, when I first purchased my vehicle, the prior owner was having problems with the A/C and the B448 diagnotic code. The problem was being caused by the Schrader-Type valve on the accumulator allowing freon to escape while being parked for an extended period of time (as Mc Reatta pointed out above) . It seems that he had the R-12 system converted to 134 but no one ever checked the new accumulator for proper fit of the factory Schrader-type valve installed so he always had an on-going problem when the vehicle was not in use; old valve removed by A/C tech, accumulator thread corrected, new valve installed, system serviced and recharged- problem resolved.

Edited May 16, 2011 by 88atta (see edit history)

[Guest Mc_Reatta]

Your experience is correct Ronnie, but the code that will be issued in that case will be B447 not B448. The low side pressure probably dropped to less than 10 psi when the compressor kicked in one time which caused the switch to open for a short period which will immediately issue the 447 code rather from the rapid temperature fall which also can cause a 447 code to be issued. The BCM is not going to sit there and let the compressor continue to run for 3 minutes in order to get to the point where a 448 code would be issued, it shuts the compressor right off. Both of them will deliver the "very low refrigerant compressor has been disabled to prevent damage" message on the CRT, but when you pull the code, you will see whether it was 447 not 448 which will lead you to a slightly different troubleshooting path.

Edited May 3, 2011 by Mc_Reatta (see edit history)

[Ronnie]

Makes more sense now that you made it clear for me. I didn't remember which code was set. Every year up till this year I had to recharge the system on my car. Last year I discovered that when it was converted to 134a (before I purchased the car) the wrong cap had been used on the low side port. The cap had been holding the valve open slightly allowing the freon to escape. I replaced the cap with the correct one last year and this year I still have freon and no warning messages so far.

[Guest Mc_Reatta]

Ronnie, what made you suspect and change out the cap?

Now that you explained what your exact circumstances were, you might have gotten a B448 code, but your statement that the system ran would be somewhat inaccurate. You turned the AC on, but the compressor never ran due to the lack of refrigerant, so only ambient air came out of the vents, and 3 minutes down the road the error message lit up.

To me, in order to claim the system ran, the compressor needs to run and cooler than ambient air needs to come out of the vents.

May think it's just semantics, but makes a difference as to what code is set and how to troubleshoot the problem.

[2seater]

Mc, very informative. I have been considering having my original system evacuated and recharged due to similar circumstances. I have owned my '90 for over 17 years and am pretty certain it is untouched. The system will work normally at temperatures above about 70* ambiant or so, but will trip out at lower temperatures, like for defrost operation. I don't remember the exact code but I remember it said something like "low side pulls down too rapidly". I have attached my gauges and the system has about 90psi static and I watched it cycle for several minutes, (compressor on-off) and the numbers were similar but slightly lower than when last checked about five years ago. Once tripped out, it will only reset by battery disconnect and then may work normally for weeks or longer as long as temperatures are relatively warm. Am I wasting my time looking at a recharge or maybe just a top off? I can't find my numbers right now but I am pretty certain the low side pulled down into the 20's for psi.

[Ronnie]

Ronnie, what made you suspect and change out the cap?

Now that you explained what your exact circumstances were, you might have gotten a B448 code, but your statement that the system ran would be somewhat inaccurate. You turned the AC on, but the compressor never ran due to the lack of refrigerant, so only ambient air came out of the vents, and 3 minutes down the road the error message lit up.

To me, in order to claim the system ran, the compressor needs to run and cooler than ambient air needs to come out of the vents.

May think it's just semantics, but makes a difference as to what code is set and how to troubleshoot the problem.

I don't have a sniffer so I was using a spray bottle to check the valve. I noticed I could spray the valve with the cap off and there were no bubbles. However, I noticed that bubbles would form in the excess liquid around the base of the cap when it was screwed down. Then I noticed the bubbles stopped as the cap was unscrewed. I should have known from the beginning that the cap was wrong because it was Red in color instead of black. I don't know what the cap was off of but it had a dimple inside that made contact with the valve. Replaced the cap, so far so good.

What I had done in the situation I was talking about was reset by disconnecting the battery cable, then starting the car and heading down the driveway. I didn't test for cold air but I did hear the compressor kick in. The system was running but it probably had not timed out to cut out the compressor.

[Guest Mc_Reatta]

Your pretty lucky it you have had your AC run for that long without a major problem. Must have a climate that allows for a balanced use of the AC every year. Not used to much to wear the seals out too fast, and not used too little to allow the seals to dry out.

Most likely your compressor seals have worn to the point that there is some leakage going on there. Your at a crossroads where you are going to have to make a decision to keep adding R 12 to your system on an ongoing basis to compensate for that leakage which will continue to get worse over time, or bite the bullet and replace it along with the accumulator and orifice tube. If you do that then do you stick with R12 or switch to 134a or go with a propane/butane mix along with comparable oil? Decisions, decisions, decisions!

If you still have 90 psi static I think I would top off with some more R12 and go another season if you can. Your leakage is still rather small. You want to get the low side pressure up to 30 psi or just a little higher.

Trick is you really want to replace your compressor before it eats a seal and creates the black death that will make fixing the system harder and more expensive. Problem is knowing when this will occur. Richard D had a good post on how he made this decision a while back.

If you can watch your high side pressure, that will give you insight into the state of your compressor. It's hard to do since that pressure is dependent on charge state, the airflow thru the condenser, RPMs, and the ambient temp. But if you can control those variables you can see if the compressor can produce the same pressure output which will tell you the health of the seals.

An easier but more subjective way to try to monitor this is to measure your vent temperature under repeatable driving circumstances. When the system is charged properly, can you get the same vent temps when you drive under similar circumstances over time? You might even be able to get a feel for this by the seat of your pants if you notice that the AC doesn't seem to cool as well as it used to and you check the charge status and it is OK. Good indication that the compressor is not capable of the performance it used to have.

[DAVES89]

2 seater if you need some r-12 I have some in my basement. email if you are interested...

lemke1044@aol.com

[2seater]

Thanks for the offer. I will see what it costs for a "start over" system recharge to get a baseline for the expected system performance. It still works almost perfectly but I know age will eventually catch up with even a sound system. The temptation is to try adding R12 until the system cycles in the pressure range desired but I may have a specialist give it a once over for issues as mentioned previously. I didn't intend to hijack this thread as I thought my symptoms were somewhat similar to the original post. Thanks to all.

[2seater]

Update on a/c system charge. After making arrangements to have the system drained and recharged @ one service facility they called back later and asked if I was doing a 134A conversion, which was not an option. So, I took it to a second facility and spoke with the service tech there. He pretty much echoed the advice from MC and stated my numbers were not too far off, and was likely just a little low on charge. The catch is, they cannot add Freon without doing a full leak check and essentially doing what I originally requested. He suggested to just run it until such time as a conversion or replacement was necessary. I do have access to R12 so I simply topped off the system to get the low side at 30-31 psi minimum. The numbers look very good as related to temperature, high and low side, so I guess I have established my own baseline to keep an eye on the system over time. Now only time will tell and I greatly value the tutorial on error codes and the advice.

[padgett]

B446, 447, and 448 all mean low pressure (temperature) reading. Think 446 is a little low, 447 is more and 448 is A Lot low but not certain. Is in section 8D2 of the FSM.

[Guest Mc_Reatta]

2seater, given your options a top off is definitely the way to go.

Hopefully your leak is very small and you can go all season without any more trouble.

As long as you have access to R 12, I'd stick with that. Keep topping off until it won't hold the charge long enough to be practical anymore. Then figure out the best way to go at that time.