2nd Gen Non-Engine/Transmission A/C observations ....weird things happening

[The patriot]

So my A/C has been kind of not making the grade these last few weeks.
I bought a vent thermometer and came up with 70` deg out of the dash vents. Definitely no good.
It's been 95`ish here regularly for a few weeks now. Plus humidity.
So, I put in 1lb on the right side (low side) and it's now about a 40 degree drop from ambient. Back to the face.
This is what I think is a little strange.
At a steady 60 mph I'm getting say 55 degrees out of the dash vent on high.
When I stop a few minutes, say for a red light....the temp actually goes down! ???

What I'm guessing that's happening is, the engine coolant is flowing slower through the heater core, so it's letting the temp go down a little ( 3-5 degrees)
What else can it be?

PS, I may bypass the heater core next year to see if there's an appreciable drop with it out of the system.

 

[madmanmanick]

Gwen the truck is going down the road there may be more airflow across the condenser, but since the condenser is right next to the radiator and inter cooler, some of that heat lowers the efficiency of the condenser. When you stop at a stoplight, the engine goes to idle, the engine cools down just a little and therefore puts less of a heat load on the coolant system and the inter cooler. This makes it so the condenser is able to remove more heat from the system and you get a momentary increase in efficiency. Hope this is helpful.

 

[mweiman]

Possibly a touch overcharged that at high rpms it is building too much pressure and kicking out. I assume you don't have a set of gauges but maybe try revving the engine up while sitting with the doors open a/c on high when it is hot out. Watch to see if the compressor starts cycling in and out when it is revved up. Just a thought.
Matt

 

[BigPapa]

Gwen the truck is going down the road there may be more airflow across the condenser, but since the condenser is right next to the radiator and inter cooler, some of that heat lowers the efficiency of the condenser. When you stop at a stoplight, the engine goes to idle, the engine cools down just a little and therefore puts less of a heat load on the coolant system and the inter cooler. This makes it so the condenser is able to remove more heat from the system and you get a momentary increase in efficiency. Hope this is helpful.

Please correct me if I'm misunderstanding what you're saying here. You're saying that his problem is caused by heat transfer from the radiator, through the CAC, to the condenser, all while air is flowing 60mph through the three in the opposite direction?

 

[fest3er]

Please correct me if I'm misunderstanding what you're saying here. You're saying that his problem is caused by heat transfer from the radiator, through the CAC, to the condenser, all while air is flowing 60mph through the three in the opposite direction?

I'd sooner believe the PCM is misinterpreting the TPS, disabling the A/C because it thinks the go pedal it at/near full fueling. But when the pedal is at idle, the PCM allows the A/C to work.

 

[The patriot]

Possibly a touch overcharged that at high rpms it is building too much pressure and kicking out. I assume you don't have a set of gauges but maybe try revving the engine up while sitting with the doors open a/c on high when it is hot out. Watch to see if the compressor starts cycling in and out when it is revved up. Just a thought.
Matt

That may be, but I charged at idle. Yes, I have no gauges.

 

[The patriot]

I'd sooner believe the PCM is misinterpreting the TPS, disabling the A/C because it thinks the go pedal it at/near full fueling. But when the pedal is at idle, the PCM allows the A/C to work.

I dont have an TPS. It's a stick shift.

 

[mweiman]

That may be, but I charged at idle. Yes, I have no gauges.

It could still be overcharged at idle. Try running it sitting with the rpms up and see if the compressor kicks in and out just to check.
Matt

 

[The patriot]

It could still be overcharged at idle. Try running it sitting with the rpms up and see if the compressor kicks in and out just to check.
Matt

Might be a day or two.
It's not as hot as it was, and I'm in the middle of doing my throttle linkage PM.

 

[BigPapa]

I was talking to our A/C consultant here at the school bus shop. He said it sounds like it is over-charged.

 

[The patriot]

OK, thanks.
It apparently has a very minor leak, so it should be within spec next summer.

 

[BigPapa]

He basically said that it sounds like it's building too much pressure at speed and the high side switch cuts the compressor out, but at idle it doesn't build the high pressures so the compressor runs more.

He said it should cool best when diving at highway speeds due to the increased RPM and the air moving through the condenser.

That said, all this he's talking about is on max A/C, recirculate. If not on recirculate, the outside air can affect the vent temps quite a bit.

All of that's way over my head. I'm no A/C tech.

 

[The patriot]

I always run it on max/ reserc...I like it as cold as it could be.
One of my old cars used to blow so cold it would come out as a frost breeze.
That was many years ago but I believe it was coming out of the vent @ 38-39 deg. IIRC