'87 GM A/C Issues

[miamibob]

Passenger side (just) firewall on the vacuum tree.

I'm sure I'll find it. Will post the results when replaced.

[gadget73]

if the blower speed changes when not in auto, bad connections or a dying motor would be my guess, though it would be worth listening to the motor under the hood to see if it is actually changing speed or if its just a matter of airflow inside the car changing. You should be able to hear the motor itself changing speed if thats whats going on. Shifting the air doors around will make it blow more or less out of the dash without being electrical at all.



. The resistor is bypassed at high speed, so if it varies then, the resistor isn't involved. That connector does get really crappy though, so its worth looking at just for good measure. check the connector at the blower motor itself, and at the fuse in the box. Its a high amperage circuit and any sort of questionable connection will cause a lot of voltage drop and a lot of heat as well. Just wouldn't do to melt something for a dirty connection.


Is the compressor cycling in and out? If so, that would explain this too. Low refrigerant charge makes for poor cooling and it will usually show up with compressor cycling too. Might have to bump the engine speed above idle to see what it does.

[miamibob]

if the blower speed changes when not in auto, bad connections or a dying motor would be my guess, though it would be worth listening to the motor under the hood to see if it is actually changing speed or if its just a matter of airflow inside the car changing. You should be able to hear the motor itself changing speed if thats whats going on. Shifting the air doors around will make it blow more or less out of the dash without being electrical at all.



. The resistor is bypassed at high speed, so if it varies then, the resistor isn't involved. That connector does get really crappy though, so its worth looking at just for good measure. check the connector at the blower motor itself, and at the fuse in the box. Its a high amperage circuit and any sort of questionable connection will cause a lot of voltage drop and a lot of heat as well. Just wouldn't do to melt something for a dirty connection.


Is the compressor cycling in and out? If so, that would explain this too. Low refrigerant charge makes for poor cooling and it will usually show up with compressor cycling too. Might have to bump the engine speed above idle to see what it does.

I pulled the connector to the resistor and it looked fine. So did the inside of the resistor (I know that is where it gets like burnt toast if there is a connector issue). Will have to check the connection to the blower motor, itself in addition to listening to the blower and see if it is changing speeds. As I stated, it changes speeds in HIGH as well so resistor should not be the issue. Am wondering about the vacuum, though so will check out the check valve, especially since I have a new one on hand. Don't like being a "parts replacer" but at this point it won't cost anything to try it.

[sly]

If the variance in high is major... yeah, definite connection issue or resistor issue or motor issue. If it's minor, it's probably alternator drop.

[tjc78]

Are you sure the fan speed difference is not the system changing from fresh air to recirc? In recirc you'll feel more airflow even though the blower speed hasn't changed.

[miamibob]

Are you sure the fan speed difference is not the system changing from fresh air to recirc? In recirc you'll feel more airflow even though the blower speed hasn't changed.

I keep the slider for MAX A/C always in the same spot so don't know. Also keep the temp slider in the same spot, as well. So if it is changing, it is doing so on it's own accord.

[tjc78]

Auto temp in an 87 doesn't have "max AC" it controls the recirc door depending on how much cooling is needed. For the hell of it, to try and narrow things down put the control to defrost and blower on high and see if things change. The system won't go on recirculate (max Ac) in defrost. If the fan still changes speed its somethimg with the blower if it stays constant it's probably the sensor as mentioned above.

[miamibob]

Auto temp in an 87 doesn't have "max AC" it controls the recirc door depending on how much cooling is needed. For the hell of it, to try and narrow things down put the control to defrost and blower on high and see if things change. The system won't go on recirculate (max Ac) in defrost. If the fan still changes speed its somethimg with the blower if it stays constant it's probably the sensor as mentioned above.

Good test and CORRECT - NO MAX A/C. I call it that from the usage of newer cars. and since I set it all the way cold. I think I set it to PANEL and the temp all the way COLD.

[miamibob]

Today's Update - Replaced the check valve referenced above with no result. I noticed it has an input and two outputs. One goes to the thermal lockout (I think from the routing) and the other to the "coffee can".

What I noticed - took the car out today and the fan speed cuts out when accelerating (under load) and comes back on high when coming to a light or stop sign (idle). Actually it could be air flow as opposed to spped to need to find that out. The receiver/dryer, however STILL does not get real cold. Merely "cool". Appears to be a vacuum issue so where are good places to look? Haven't checked the blower motor or ambient sensor. Just don't want to spend almost $90 on the sensor and waste my time replacing it if it is not the problem. Will check the hose that feeds it to make sure something is not stuck in it! Thoughts???

[gadget73]

The check valve's outputs go to the soup can and into the car. Inside the car, there is a tee that has the line from the check valve, the connection out to the TBL, and the vacuum supply to the climate control unit itself. If the TBL is bad, it will act like the soup can is rotted or the check valve is bad. Ditto if the soup can leaks. Temperature and vent position change when you accelerate if there is a problem in that vacuum system.

Unhook the check valve from the vacuum tree and try to pull vacuum on it. If it leaks, unhook the line to the soup can and see if that holds. If that works, it has to be on the hose into the dash. The TBL is very often the problem here. Plug that off and see if the problem goes away.

If all of that is good, unhook the line feeding into the blend door from the air sensor. Pull vacuum on it and hold it. This will force the blend door to full cold, and should force the fresh/recirc door to recirc. It won't do anything about where air comes out of the dashboard but it will isolate the air temperature sensor as being a problem. A vacuum gauge on the outlet of that will also work to diagnose it. It should hold vacuum when set to cold. If it floats around, either the air in the car is cold enough or the sensor is bad.

If all of that works, and its still not cold, then the AC system just isn't working right and you'll need a set of refrigeration gauges to tell you what ails it.

[miamibob]

Seems like two separate issues. The "up and down" of air flow and the receiver/dryer not being cold. Vent position doesn't seem to change - only air flow or speed.

[sly]

I'm thinking the vacuum line to the controller in the controls is clogged/kinked or the controller is being stupid.

If you have a vacuum pump to test with, you can try pulling the vacuum harness off the controller and the solid black line will be the feed line. If you pull the skinny lines off the check valve under the hood and are able to pull a vacuum on that black line, you'll have to replace that line (or run another line like I did in the Mark I sold).

[miamibob]

So, one of the lines from the check valve feeds the ATC controller? I know one goes to the "coffee can" and I "thought" the other went to the thermal lockout?? It's like a "rat's nest" behind the engine so perhaps I got confused (happens a lot lately!)?

[sly]

yeah... it is a rats nest in there. I'll try to find a diagram of all that and post it. can't find it right now though. I know I have it somewhere.

[miamibob]

yeah... it is a rats nest in there. I'll try to find a diagram of all that and post it. can't find it right now though. I know I have it somewhere.

Look forward to seeing it!