AC Woes...

[nUcLeArEnVoY]

Yay, another AC thread.

So I have a multifaceted problem here with my '79 400 4-speed equipped with factory AC, and I've been on-and-off managing it for a year now.

Bought the car in May of last year, got to enjoy AC for a couple weeks before the blower motor went on it. Compressor and AC system still seemed to work (cycled on and off, the accumulator got cold and sweat like normal, etc), just no blower. Another thing that happened at the same time is that whenever I turned on the AC, anytime the compressor engaged, it bogged down the motor big time. I know that it's normal for that to happen, but usually the purpose of the AC Idle Up solenoid is to compensate for the compressor load, which it was no longer doing. It no longer had the ability to move the throttle lever under its own strength unless the AC compressor was disconnected, in which case it did have the power to move the throttle lever. So both of these things happened at the same time: blower motor went (all speeds, just completely dead), and AC idle up solenoid no longer changed the engine RPMs to compensate for compressor load.

Didn't really focus much on it, I usually drive the car in the evening and even here in South Florida, you get used to it.

Fast forward over a year to the past couple weeks, I've finally started throwing parts at it. Things I've replaced: blower motor resistor, I changed out the compressor extension harness, and installed a new AC idle up solenoid. In regards to the harness I replaced, I did NOT replace the red wire that goes between the hi-speed blower connection on the alternator and the corresponding wire coming off the hi-speed blower relay, because for some reason a PO SOLDERED the connection between the current extension wire and relay connection, when normally there is a black plastic male/female connection joint there.

Some things changed...

• Blower started working again on all speeds, although pretty weak and sounds groany... it obviously needs replacement, regardless. I plan on doing that.

• New AC Idle Up Solenoid still doesn't have the ability to move the throttle lever unless the compressor is disconnected. Now, I've read that these idle solenoids are deliberately designed that way in that you have to goose the throttle once you activate them, and then they instead "hold" at the adjusted raised idle, and are not meant to actually move the throttle lever under their own power. But then why is it able to move the throttle when the AC compressor is disconnected, but not able to when it's connected and engaged? The AC compressor is robbing THAT much voltage? So I took a multimeter to find out. When the compressor is DISCONNECTED, and I turn on the AC to energize the solenoid, it gets around 14.1-14.2 volts as expected and is able to push the throttle lever to around 820-840 RPMS (my base idle is around 700). Then when I reconnect the compressor and then turn the AC on, the solenoid's received voltage goes down to around 13.1-13.2, and it no longer has the ability to move the throttle lever. Is this normal?  I don't ever remember feeling the engine heavily bog down when I first got the car and everything was working as it should, so that must mean the AC idle up solenoid was working correctly without me ever knowing. Maybe I just wasn't paying attention to the RPMs...

• And then finally, now when I turn the AC on, the compressor stays engaged and doesn't cycle off, even after two or three minutes... which could be normal for hot weather like here in FL, but then why was it cycling on and off last summer when this whole mess first occurred and it was just as hot outside? Does it have a leak? It blows cold air in the cabin, FYI, so it's working right, but it doesn't cycle off and it bogs down the engine really bad and ends up raising the engine temp pretty darn quickly due to the strain. I have a 160 degree stat and a Champion aluminum rad; my engine temps barely ever pass 185, and so when I had this AC compressor continuously running and I saw my temp hit 205, 210, and then 215, I immediately shut it off and sure enough my temp went right back down to 185. I know an AC compressor will jack up the temp, but really? That much? And blipping the throttle to see if the solenoid will "hold" at the higher speed, it still sounds like it's running terrible.

So how are all these things connected or related? Is my voltage regulator on my alternator going bad? Coincidentally, a year ago shortly before the blower motor went, I also noticed my radio lost a lot of its reception. It went from picking up lots of channels to picking up only like two, and the volume wouldn't go that high. Is it maybe the bad blower motor compromising the entire electrical circuit for the AC system due to it running more inefficiently and drawing more power? Is my condenser bad? Should I check to see the pressures in the compressor to see if it's leaking? Should I replace the entire engine side AC wiring harness for good measure as well?

Any help would be appreciated. Sorry for the novel, I have a hard time consolidating. Lol

Compressor is fairly new, btw, replaced in 2018 and the car has only seen 2-3k miles of driving since then. I know these new re-manufactured A6 axial compressors are pretty dodgy at best in their quality and reliability, so maybe that's what's going on? The system is converted to R134A, for what it's worth.

[Warren Seale]

The compressor is running without cycling on and off due to the fact that it now has a larger heat load since the blower motor is working.  When there is no air flowing through the evaporator, the heat load is small so the refrigerent pressure is reduced low enough to cycle the compressor on and off.  Just remember that when the temperature across the evaporator and condensor is high the refrigerent pressure will rise.  The compressor will engage once the low side is above ~30psi.

[NY81TTASE]

Because you mentioned the radio...I've read that if the diode at the compressor harness plug is bad, the voltage feeding back can affect the radio. Take a look right behind the comp pulley at that 2 wire (green and black) connector. Many times there is a diode there. Mine cracked in half
In the electrical section here,,there is a sticky with the ac schematic.  Gm called the diode a rectifier.

[nUcLeArEnVoY]

Because you mentioned the radio...I've read that if the diode at the compressor harness plug is bad, the voltage feeding back can affect the radio. Take a look right behind the comp pulley at that 2 wire (green and black) connector. Many times there is a diode there. Mine cracked in half
In the electrical section here,,there is a sticky with the ac schematic.  Gm called the diode a rectifier.

Yeah, I replaced the extension harness with a new one which also included replacing that connector. Maybe I should check the radio now to see if it works! 

[nUcLeArEnVoY]

The compressor is running without cycling on and off due to the fact that it now has a larger heat load since the blower motor is working.  When there is no air flowing through the evaporator, the heat load is small so the refrigerent pressure is reduced low enough to cycle the compressor on and off.  Just remember that when the temperature across the evaporator and condensor is high the refrigerent pressure will rise.  The compressor will engage once the low side is above ~30psi.

I see... and since even though the blower is working now, it is working very poorly and is not exchanging the heat as efficiently as it should...

[NY81TTASE]

Yeah, I replaced the extension harness with a new one which also included replacing that connector. Maybe I should check the radio now to see if it works! 

When I was searching for that new connector I kept finding ones with no diode. I cant imagine why they sell it like that if it's needed. Just double check yours has it. It probably isn't your issue but it may help stop another issue from happening

[firebirdparts]

First off, I support soldering the high speed wire.  That's a good thing to do.

Second,

When the compressor is DISCONNECTED, and I turn on the AC to energize the solenoid, it gets around 14.1-14.2 volts as expected and is able to push the throttle lever to around 820-840 RPMS (my base idle is around 700). Then when I reconnect the compressor and then turn the AC on, the solenoid's received voltage goes down to around 13.1-13.2, and it no longer has the ability to move the throttle lever. Is this normal?

That's a pretty sharp pencil.  Both those numbers are perfectly reasonable voltages for the system.  I suppose that the solenoid, being 40 years old, might need to be replaced.  I think it should work.  I think it should work at 12 volts.

Third, the compressor on a 79 should cycle off when the evaporator pressure gets too low (too cold).  leaks will not make it run.  It needs to shut off when you drive, because the compressor will be turning 5 times as fast.  That's how it limits the amount of refrigeration to prevent feezup on the evaporator.  Sounds to me like you've got a full charge, and really, I think that part is good.

I suppose there was originally a capacitor on the blower motor that would have affected the radio reception, but not the volume.  I think the volume is more of an old age issue with the radio or the speakers.  There's also a cap on the fusebox.  We typically never replace them, and of course they're getting old.

[nUcLeArEnVoY]

First off, I support soldering the high speed wire.  That's a good thing to do.

Second,

When the compressor is DISCONNECTED, and I turn on the AC to energize the solenoid, it gets around 14.1-14.2 volts as expected and is able to push the throttle lever to around 820-840 RPMS (my base idle is around 700). Then when I reconnect the compressor and then turn the AC on, the solenoid's received voltage goes down to around 13.1-13.2, and it no longer has the ability to move the throttle lever. Is this normal?

That's a pretty sharp pencil.  Both those numbers are perfectly reasonable voltages for the system.  I suppose that the solenoid, being 40 years old, might need to be replaced.  I think it should work.  I think it should work at 12 volts.

Third, the compressor on a 79 should cycle off when the evaporator pressure gets too low (too cold).  leaks will not make it run.  It needs to shut off when you drive, because the compressor will be turning 5 times as fast.  That's how it limits the amount of refrigeration to prevent feezup on the evaporator.  Sounds to me like you've got a full charge, and really, I think that part is good.

I suppose there was originally a capacitor on the blower motor that would have affected the radio reception, but not the volume.  I think the volume is more of an old age issue with the radio or the speakers.  There's also a cap on the fusebox.  We typically never replace them, and of course they're getting old.

Okay, so two dudes that are straight up gurus with AC are telling me the compressor not cycling off at idle in this Florida heat is normal, so I guess that's okay. While I'm driving the car at speeds, obviously the revs are higher so I can't tell if the compressor is running or not while I'm driving down the road, so I'm not sure about that. I'm guessing it's not if you say it shouldn't be.

In your experience, is the idle up solenoid usually not supposed to have the power to move the throttle lever on its own? Because that's what I'm reading at most sources in that they are designed to "hold" the throttle at the set speed after you goose it. The one I have on there now IS brand new from QuadrajetParts.com. At 14.0+ volts, it will move the throttle lever while at the 13 that it receives with the AC compressor running, it will not move the throttle lever.

The only part here that I know is still bad is the blower motor. Like I said, even though it's working again after I switched out that compressor extension harness and new resistor, it's weak as heck and makes a groaning noise. So I'll definitely get to replacing that. For all I know, it could be the whole problem here, since I know the blower motor is integrally wired into the AC circuit in these cars, and if it's bad, that means it's inefficient and may be overdrawing power and compromising the entire circuit.

But then what about my running hot issue? My base idle is 700 RPM and if I turn on the compressor and then blip the throttle so the idle solenoid plunger can extend all the way and "hold" the throttle at a higher idle, it still sounds like absolute crap. I'll need to hook up my Innova timing light on it to see for sure what speed it's idling at while it's under compressor load, but its sounds as though it drops to low 600's, high 500's in RPM. The engine gets so shaky that I can look at the harmonic balancer and it almost looks like it's wobbling. It shouldn't be doing this, since I'm sure that I adjusted the idle solenoid so that it engages and idles at my normal 700 while under compressor load and at around 850 while not under load. I'm guessing the overheating is simply because of the strain from the compressor and that I need to raise the idle speed more while it's engaged so that coolant can circulate more efficiently. I doubt that inside the one year I didn't use the AC, the condenser went bad... plus it's blowing cold air out the vents in the cabin (although weakly, because of the bad blower), so the condenser should be fine. Fan clutch is brand new, btw. Any ideas as to why the engine is getting so hot only with the AC running, and how much of a rise in engine temp I should normally expect with a running AC? I know my cooling system is up to par: new aluminum rad, 160 stat, new fan clutch, new water pump, coolant still nice n' green, and it'll idle until doomsday at a solid 180 degrees and barely ever exceeds 185-190 while driving... so how is an AC compressor piling on an extra 25-30 degrees of temp so fast?

[b_hill_86]

I don’t have much to offer you except if you are wanting to determine if and when your compressor is cycling on and off while driving I would think it wouldn’t be too hard to tap into either the compressor trigger wire or solenoid wire , add a fuse and run a temporary light into the passenger compartment that would illuminate when the compressor kicks on. Just a thought

[NY81TTASE]

I just went through the manual section on carbs but I have the 80 manual. There are two types of solenoids..the idle speed control  (isc), which is  what you have, and idle load compensator (ilc). They are actuated differently but serve the same purpose. (The ilc uses vacuum.)
The isc setting says "turn the ignition on but do not start engine. On vehicles equipped with AC actuated solenoid, AC switch must be on.  Open throttle to allow solenoid to extend, close throttle. Disconnect wire at solenoid,. Solenoid plunger should drop away from throttle lever.
Connect solenoid wire. Plunger should move out and contact throttle lever. Solenoid may not be strong enough to open the throttle, but the plunger should move." .....voltage should be between 12 - 15 volts.

I'm not sure how you can test what voltage the solenoid is drawing besides what it is getting. The manual said if the plunger doesn't move either way with 12 -15 volts..replace the solenoid.

[nUcLeArEnVoY]

I just went through the manual section on carbs but I have the 80 manual. There are two types of solenoids..the idle speed control  (isc), which is  what you have, and idle load compensator (ilc). They are actuated differently but serve the same purpose. (The ilc uses vacuum.)
The isc setting says "turn the ignition on but do not start engine. On vehicles equipped with AC actuated solenoid, AC switch must be on.  Open throttle to allow solenoid to extend, close throttle. Disconnect wire at solenoid,. Solenoid plunger should drop away from throttle lever.
Connect solenoid wire. Plunger should move out and contact throttle lever. Solenoid may not be strong enough to open the throttle, but the plunger should move." .....voltage should be between 12 - 15 volts.

I'm not sure how you can test what voltage the solenoid is drawing besides what it is getting. The manual said if the plunger doesn't move either way with 12 -15 volts..replace the solenoid.

The plunger does move on it, it's just when the AC compressor is engaged, it doesn't have the strength to move the throttle lever by itself. You have to give a little gas to lift the throttle lever out of the way, allowing the plunger to fully extend and then hold the throttle lever at higher RPM to compensate for the compressor load. But then, I guess that's how it's designed to work, clearly. Even the 1978 service manual states that you have to lift the throttle lever so the plunger can fully extend. I just find it weird that one volt makes the difference between the plunger moving and not moving the throttle lever under its own strength.

Maybe I just need to readjust it.

[NY81TTASE]

Yes, its weird that it does that. Maybe try another one?

[firebirdparts]

I lived through the 70's, so I should know, but I just never really worried about whether that solenoid would lift the throttle or not.  I think it should, but I can't say I ever surveyed all the GM cars we had back then.  I have a couple here I guess I could experiment on.  I just now looked through the 78 manual but I don't see anything on it in the A/C section.

Normally it wasn't a big deal, with a daily drive you turn the A/C on about May 15 and then it's pretty much on for the duration.  The choke had to be set anyway, so you didn't even notice.

[Grand73Am]

This is what my 79 with 403 does. I have the original idle solenoid installed, connected, and adjusted, on the original q-jet. When idling, I turn on the a/c, and the idle rpm drops a bit. But if I push the accelerator pedal slightly, the idle solenoid pushes it back to the original idle rpm, and stays there. When I turn off the a/c, the idle goes up slightly until I push the gas pedal slightly, and idle goes back to original idle.

I don't know if it was supposed to do it that way originally, but that's how it is now. I'm satisfied with it. There's only about 100 rpm drop when the a/c is on, before the solenoid bumps it up with the push of the gas pedal. I'm using the smaller Sanden style compressor that comes with a Classic Auto Air underhood upgrade kit, so it doesn't draw down the engine as much as the big factory A6 compressor would. 

[Jeremy]

Keep in mind that when the AC is running a lot of hot air from the condenser is getting pulled through the radiator.  Cooling will be affected.  Some things I did on mine was to seal up all of the holes I could in the core support and around the condenser and radiator.  This forces all of the air through the two pieces.  It made my car less prone to drastic heat rises with the AC on and I got better AC performance as well.

[Grand73Am]

^^^ Good suggestion

[cyber104]

I just went through the manual section on carbs but I have the 80 manual. There are two types of solenoids..the idle speed control  (isc), which is  what you have, and idle load compensator (ilc). They are actuated differently but serve the same purpose. (The ilc uses vacuum.)
The isc setting says "turn the ignition on but do not start engine. On vehicles equipped with AC actuated solenoid, AC switch must be on.  Open throttle to allow solenoid to extend, close throttle. Disconnect wire at solenoid,. Solenoid plunger should drop away from throttle lever.
Connect solenoid wire. Plunger should move out and contact throttle lever. Solenoid may not be strong enough to open the throttle, but the plunger should move." .....voltage should be between 12 - 15 volts.

I'm not sure how you can test what voltage the solenoid is drawing besides what it is getting. The manual said if the plunger doesn't move either way with 12 -15 volts..replace the solenoid.

The plunger does move on it, it's just when the AC compressor is engaged, it doesn't have the strength to move the throttle lever by itself. You have to give a little gas to lift the throttle lever out of the way, allowing the plunger to fully extend and then hold the throttle lever at higher RPM to compensate for the compressor load. But then, I guess that's how it's designed to work, clearly. Even the 1978 service manual states that you have to lift the throttle lever so the plunger can fully extend. I just find it weird that one volt makes the difference between the plunger moving and not moving the throttle lever under its own strength.

Maybe I just need to readjust it.

Recently I wrote a lengthy dissertation about this issue a few threads down in this section I believe.  What you're experiencing is exactly why I installed a manual switch.  Have a look at my post - maybe it will help you understand the idle up solenoid a bit better.