A/C only works when driving!?

Hey all,

Just got my '87 305 TBI with factory A/C (pretty sure it's had 134a put it in at some point) on the road last fall after a long rebuild.

The A/C worked good when I bought the truck in late 2020. I kept the system fully intact during the next few years as I fixed up the truck. I've used the A/C here and there since starting to drive it on a regular basis at the beginning of this year and it's worked great.

But just yesterday a strange problem developed where it cuts off completely when I come to a stop. As in, no air flow at all. (Not like it's out of freon where the air still blows but it's warm.) But once I start moving, the A/C comes back on. Air flow returns and it's cold.

IDK a/c systems very well (never had a functional one on any of my old vehicles before), but the condenser seems to be sweating profusely and I just started hearing a "tic-tic-tic" sound coming from the compressor. (It wasn't making any sounds until yesterday.)

strange issue. but the blower is separate from the compressor. does it happen in any setting (vent, defrost, heat)?

I didn't experiment with the other settings. I'll have to try those next time I go for a drive.

First off, that's a really cool air cleaner lid! What holds it down without a wing nut?

As for the AC... so it quits blowing AND it gets warm? Or you don't know the temperature since it's not blowing?

Not cold while stationary is often a lack of airflow though the condenser. Like a failing fan clutch. The high side pressure will rise, the AC will become highly inefficient and if the pressure rises too high, the pressure switch will kick the AC clutch off. It's very hard on the compressor.

If it stops blowing when stationary, it might be a lack of engine vacuum at idle. Assuming the fan is still running, you just can't feel much air coming out the dash vents?

bucket said:

First off, that's a really cool air cleaner lid! What holds it down without a wing nut?

As for the AC... so it quits blowing AND it gets warm? Or you don't know the temperature since it's not blowing?

Not cold while stationary is often a lack of airflow though the condenser. Like a failing fan clutch. The high side pressure will rise, the AC will become highly inefficient and if the pressure rises too high, the pressure switch will kick the AC clutch off. It's very hard on the compressor.

If it stops blowing when stationary, it might be a lack of engine vacuum at idle. Assuming the fan is still running, you just can't feel much air coming out the dash vents?

Click to expand...

My first thought was high pressure switch kicking it off. Not a squarebody chevy, but my '91 cherokee would do that when driving down the highway. All of a sudden the air flow would stop, and as soon as I let off the throttle it would start up again. I never figured out why, but I suspected it was the high pressure switch turning off the system.

bucket said:

First off, that's a really cool air cleaner lid! What holds it down without a wing nut?

As for the AC... so it quits blowing AND it gets warm? Or you don't know the temperature since it's not blowing?

Not cold while stationary is often a lack of airflow though the condenser. Like a failing fan clutch. The high side pressure will rise, the AC will become highly inefficient and if the pressure rises too high, the pressure switch will kick the AC clutch off. It's very hard on the compressor.

If it stops blowing when stationary, it might be a lack of engine vacuum at idle. Assuming the fan is still running, you just can't feel much air coming out the dash vents?

Click to expand...

I didn't think the vents closed fully in off (like a loss of vacuum). wouldn't air still be flowing out somewhere (I think mostly the floor)?

but that's a good to listen for the fan vs feeling for air flow. help diagnose the motor or the vacuum pod system.

There's zero air coming out of the vents when stopped. And then the air flow starts happening and builds as I get up to speed. I did reach up and found no air flow at the defrost vents. But I didn't think to reach down to see if anything was coming out of the under dash vents. I'll try that next time I take it out.

I'm pretty sure it's got a good clutch on the fan. It feels right (like "medium resistance" when you spin the fan with the engine off). It's held the engine right at 195º since day 1. Can I start the engine, have someone put it in gear while I look at the fan and see anything? I'm sure it'll be spinning. Just not sure if I'll be able to tell if it's spinning enough.

What about the new "tic-tic-tic" noise I heard coming from the compressor? Is that a clue? (I heard this when I got back home and popped the hood as the engine was running.)

@Ricko1966

No air flow,out the vents the compressor tic,tic is a separate issue. It also isn't the fan or fan clutch,or freon charge. It's either loosing vacuum to the heater controls or loosing power to the fan motor. Make sure the plug gotta go, dentiest pulling me into chair

RaysnCayne said:

There's zero air coming out of the vents when stopped. And then the air flow starts happening and builds as I get up to speed. I did reach up and found no air flow at the defrost vents. But I didn't think to reach down to see if anything was coming out of the under dash vents. I'll try that next time I take it out.

I'm pretty sure it's got a good clutch on the fan. It feels right (like "medium resistance" when you spin the fan with the engine off). It's held the engine right at 195º since day 1. Can I start the engine, have someone put it in gear while I look at the fan and see anything? I'm sure it'll be spinning. Just not sure if I'll be able to tell if it's spinning enough.

What about the new "tic-tic-tic" noise I heard coming from the compressor? Is that a clue? (I heard this when I got back home and popped the hood as the engine was running.)

Click to expand...

Is the blower motor on when stopped? Yes= It's a vent issue (vacuum and flaps). No=It's a motor or voltage supply issue. Anything temperature related will only affect the compressor operation via the hi/lo switch. Engine temp/fan clutch is irrelevant.

Maybe I read it wrong, but I read it as it quits working, then when driven, the AC comes back on AND it starts blowing again.

So is the real problem it quits blowing? And since it's not blowing, you assume the AC isn't on? @RaysnCayne

bucket said:

Maybe I read it wrong, but I read it as it quits working, then when driven, the AC comes back on AND it starts blowing again.

So is the real problem it quits blowing? And since it's not blowing, you assume the AC isn't on? @RaysnCayne

Click to expand...

he said he could hear the compressor cycling at idle, so seems like it is working? although if it is short cycling, it might be low on charge.

First check to make - with the vehicle running, put the HVAC on heat or vent.
Turn the blower on high - does air come out in the right locations? This tests the blower motor and the high speed relay and the fuse for high speed.
Turn the blower on next click down from high - does air come out?
This tests the blower motor and the resistor pack, and the HVAC fuse.

Run the same tests in A/C position to see if the fan works, and also listen for the click as the compressor cycles on and off.

Sounds like your blower has stopped working, when you’re driving you’re feeling passive fresh air coming in maybe. Try all settings at idle without aircon.
With no airflow across the evaporator the compressor isn’t going to have a good time, and is likely the cause of the ticking noises.

Sorry for posting and ghosting.
But I couldn't replicate the problem until yesterday. It was the first really hot day since that day in May when it first happened that I had a chance to drive it. I turned the A/C on as I left my driveway. Got down to the end of the street and it's like I flipped a switch and shut it off. I tried it on all the setting (low, medium, high) and nothing came out of the vents. But as soon as I hit the gas to leave the intersection, the air came back on and was blowing cold. And it operated fine a few mins later when I came to a stop a couple times at the end of that drive.

But then today, it worked just fine leaving the neighborhood and arriving at my destination.

It is my daily, but that's very relative since I work from home and only need to drive it 2-3x a week. So I kinda feel fine about it - in that it appears to work most of the time. But at the same time, it's clearly got something wrong, but what??