Help with A/C problem

[padgett]

In diagnostic mode hit No for ECM? then Yes for BCM? and Yes until you get to BD25 and start recording values.

With engine off it is a good sensor check since it has nothing to do with pressure. Just drove 88 so checked 'vert- BD25, 26,& 28 read 30C, 27 read 31C. Close enough for government work.

If the car has been sitting overnight and the engine is off, all should read close to each other.

BTW my experience is that the AC will cycle when the LO temp drops under 0C (+32F). If you are reading -40F or C, are the same, something is rong.

Edited September 4, 2011 by padgett (see edit history)

[Guest luvmy89riv]

Great Ronnie. I will try to read tomorrow morning.

Thanks,

Ray

[wws944]

Slightly off topic, but I didn't think you had to replace the BCM to get a Riv compass working. Just enable the right option in diagnostic mode.

[Guest luvmy89riv]

You must also have the compass module. In the Riviera, it mounts in the trunk between the speakers and then the wires that run to the BCM.

[padgett]

Have had one for my 88 for a long time just never a round tuit. When I looked into it the compass module becomes another device on the ALDL (essentially a serial LAN). The BCM already has the needed subroutine so it is just a matter of enabling the option.

Also have a RKE. Same-same.

ABATR Coupe reads BD25 & BD28 29C, BD26 & 27 30C

Edited September 4, 2011 by padgett (see edit history)

[Guest luvmy89riv]

Here are my numbers.

BD25 37

BD26 29

BD27 -37 (yes, that is a negative number)

BD28 12

Please tell me what this really means. I am guessing that there is nothing wrong with my BCM, right? Can anyone out there help me fix this?

[wws944]

Have had one for my 88 for a long time just never a round tuit. When I looked into it the compass module becomes another device on the ALDL (essentially a serial LAN). The BCM already has the needed subroutine so it is just a matter of enabling the option.

Gosh - if I had a '88-'89, that would be one of the first things on my upgrade list. Just too kewl, and easy, not to have it.

[Ronnie]

Here are my numbers.

BD25 37

BD26 29

BD27 -37 (yes, that is a negative number)

BD28 12

Please tell me what this really means. I am guessing that there is nothing wrong with my BCM, right? Can anyone out there help me fix this?

Are these the readings you got first thing in the morning before starting the engine?

[Guest luvmy89riv]

Yes, these are the readings during morning before starting the car.

[padgett]

The -39 reading is just plain wrong. Actually none look right, 37 is 98F is it that hot there ? BD26 reading of 29C (84F) is the only one that sounds right.

Are you sure you have the right BCM PROM for your car ? Did you just switch the whole silver box from something else ?

[Ronnie]

The BD27 reading is so far off I really don't know what to make of it. I'm guessing the sensor is shorted internally or the wire going to it is shorted (or maybe open)... I don't know. Maybe a bad BCM is causing it??

The BD28 reading is the one that is making the compressor cycle too often. You have the same 12*C reading I did when my compressor was cycling too often for the car to get cool. I find that odd that your BD28 reading and mine were exactly the same. Anyway, you are facing the same choices I was to correct the problem.

1. Find two good used temperature sensors to replace the defective ones you now have. New ones are not available as far as I know. To replace the sensors you will need to discharge the system and refill after you get the sensors changed.

2. Install a resistor in parallel with the low temp sensor that will fool the BCM into turning the compressor on and off at a lower temperature. If you will read back through this thread you see the steps I took to correct the same problem with my car.

Hope this makes sense and helps you with your problem.

[Guest luvmy89riv]

right now, I am in Huntsville AL. The temp this morning outside at 9 am was about 72F degrees. I parked the car last night at about 8 pm. I did change the whole silver box (BCM). It is located behind the glove box. Also added the compass module in trunk between the speakers. I got the BCM, compass module and wiring for such from a wrecked 89 Riv. It has worked great for 7 years until about 2 months ago. What is the BCM PROM? The service technician said the numbers looked odd and thought possibly the BCM could be bad. I do have the original BCM from that car. Except it is in Kansas. I will have it shipped to me next week.

[Ronnie]

Your readings are so far off that changing the BCM would be a good idea, especially since you already have one. Then address the sensor problem if it is still there.

PROM stands for Programmable Read-Only Memory. It is installed in the BCM (in this case) and can be changed as a separate part. It contains programed instructions on how the BCM should control certain functions of the car, including the AC.

Edited September 4, 2011 by Ronnie (see edit history)

[Guest Mc_Reatta]

The negative 37 deg is indicative of an open circuit.

Check the connector on the high side temp sensor, if it looks OK, disconnect it and measure the resistance of the sensor. If it doesn't give a reading then its the cause of the negative reading.

If it reads around 2500 0hms, then the problem is in the wiring or BCM.

I doubt the BCM is bad.

If you find some sensors, measure the resistance of each.

They should read about 2500 ohms at around 80 deg F ambient.

[Guest luvmy89riv]

ok. Ronnie. I read your earlier posts. Brilliant!!! I will first try changing out the BCM. Mainly because that is easy and I have done it before. If that is not it, then I will try the resistor. I'm not much of an electrical guy. I usually do pretty good with directions. Can you please explain where to get and how to install a resistor in parallel? should it be 5 OHM or 25 OHM?

@ Mc_Reatta - Thanks. I will see what I can do to do those tests too.

Can anybody out there help me find these temp sensors? Junkyards with Buicks or Cadillacs?

Thanks!

Ray

[Ronnie]

The resistor I used is a standard resistor that was in an assortment of resistors that I purchased at Radio Shack. It is nothing special... probably a 1/4 watt resistor. I believe the resistor I used was 25 ohms. You may need a slightly different resistance but that should be close. The exact resistance you want to use is whatever will bring the reading of BD28 in line with the other readings.

I wanted to install the resistor in a way that would be easy to change and easy to remove if I ever decided to replace the the defective low side temp sensor with a good one. A while back I took some photos to help explain to a friend how I installed the resistor. He was having the same problem with a bad low temp sensor, like I wrote about in this thread, and he wanted to know how to fix it.

The first photo shows where I installed the resistor just below the connector. The second photo is just a mockup with a scrap piece of wire to demonstrate how I installed the resistor in the wires. I didn't want to un-tape my wires again since everything was working good.

What I did to install the resistor was use the long straight pin to put a small hole through each wire going to the sensor connector and then push the leads of the resistor through each wire as shown in the demonstration. Then I wrapped the leads around the wires and used tape to hold in place.

Ignore the last resistor and the yellow text in the second photo. It was included to explain something else.

There are probably better ways to do the same thing but the way I did it is simple, it works and the resistors can easily be changed or removed if needed without modifying the wires any more than necessary.

Hope all this makes sense.

[Guest luvmy89riv]

Ronnie. Thanks a lot! Your explanation is wonderful. I'm sure I can do that. I wonder if my issue may be different since the High temp sensor had the -37C reading. Or can I a resistor on the High temp connector too? Are you still running with the resistor in now? Did you ever find any temp sensors to put in? After putting in the resistor, do you still put the connector into the temp sensor or just let it hang out there?

In the winter when I need heat, what should I expect from the system? Does any of this change?

Thank you very much for your help here.

Ray

[Ronnie]

Ronnie. Thanks a lot! Your explanation is wonderful. I'm sure I can do that. I wonder if my issue may be different since the High temp sensor had the -37C reading. Or can I a resistor on the High temp connector too? Are you still running with the resistor in now? Did you ever find any temp sensors to put in? After putting in the resistor, do you still put the connector into the temp sensor or just let it hang out there?

In the winter when I need heat, what should I expect from the system? Does any of this change?

Thank you very much for your help here.

Ray

Your issue is different from mine in the fact that your high temp sensor is completely off the chart. I don't know how that will effect the operation of your system. I don't think you will be able to compensate for a reading that far out of line with what is normal by adding a resistor. Someone here with more AC knowledge than me might be able to shed some light on how the high temp sensor effects operation.

Yes you must connect the wires back to the sensor, and yes, I'm still running the resistor in my system. The AC is working great... the best it has since I've owned the car. I have not tried to find more used sensors but I would pick up some used ones if I have to go to a salvage yard for something else. If I had them I would not install them until I need to open the system for some other reason. I rarely repair things that are working good.

When you turn on the heat it should work as it normally would. I can't say for sure because I haven't tried it.

Keep us posted on how you resolve your issue.

[harry yarnell]

Are you sure about 25 ohms? Shouldn't it be more like 2k (2,000) ohms at 80°?

Edited September 5, 2011 by harry yarnell (see edit history)

[Ronnie]

Are you sure about 25 ohms? Shouldn't it be more like 2k (2,000) ohms at 80°?

BTW, the low temp sensor is what is preventing the system from cooling properly. I paralleled a 25 ohm resistor with the sensor and the system is cooling great and the compressor is cycling as it should. Vent temp is ~50* at 94* ambient outside. I will check in the morning and see how close I came to getting the BCM to see the correct temperature. The resistor should get me by until I can change out the sensors. Hey, if the sensor can fool the BCM so can I! LOL

Harry, I'm not absolutely certain it is 25 ohms but I believe it is based on my post back in June quoted above. Keep in mind that I didn't replace the sensor with a resistor I only added a resistor to the sensor circuit. By adding the resistor, I changed the resistance that the BCM sees to bring it more in line with what the BCM would see with new sensor installed.

I want everyone to know that I realize this is not the best way to fix this problem. It just a workaround to get the AC to cool without having to open up the system. I will save opening the system for something big that stops the system from working like a compressor failure and try to install a good sensor at that time. Right now we don't have a source for new sensors as far as I know. To me it would not good sense to open the system, which is working fine with the resistor, to install a used sensor that might fail next week. I only offered this workaround as an alternative to driving around in a hot car that could be cooling well with the addition of a 25¢ resistor. I hope I didn't mislead anyone.

Edited September 6, 2011 by Ronnie (see edit history)

[Guest luvmy89riv]

Ronnie,

I just found a guy that has many of the A/C temp sensors. I just ordered a few to fix mine with a spare. He still has several left. He mainly deals in Reatta and Riviera parts. If you want to fix yours. Let me know.

Ray

[Guest 66chevy]

I am confused. Can low pressure be change with out losing freon. I already lose freon when I lossen high senor. 1990 coupe

[Ronnie]

I am confused. Can low pressure be change with out losing freon. I already lose freon when I lossen high senor. 1990 coupe

Yes, the Low Pressure Switch can be changed without losing the freon charge. Only a small amount of freon should escape because there is a Schrader valve that shuts it off similar to the one in the port where the gauges are connected.

[Guest luvmy89riv]

The Low Pressure switch is different from the A/C Low temperature sensor. There is also the A/C High temperature sensor too you may be referring to. When changing the A/C High and/or Low temperature sensors, you need to evacuate the Freon from the system.

[Guest luvmy89riv]

Thank goodness! The A/C pressure sensors I bought fixed my A/C issue. Upon Inspection of the replaced high side pressure sensor, the lead solder was broken away from the center piece. Walaaaa... A/C blowing very cold again!

[Ronnie]

Thank goodness! The A/C pressure sensors I bought fixed my A/C issue. Upon Inspection of the replaced high side pressure sensor, the lead solder was broken away from the center piece. Walaaaa... A/C blowing very cold again!

That's good news. Glad you let us know.

I think a lot of complaints we hear about poor AC cooling, that is due to failing temperature sensors. Maybe a tutorial on how to troubleshoot the sensors is needed on my website.