On Saturday while driving down the interstate (steady at 70mph), I noticed that the air from the A/C was starting to get a little warmer. I figured that it's time to recharge the A/C system as I've got a small leak that no one has been able to find and I have to periodically recharge. I had been running on Max A/C with the fan in the second position. When I noticed it getting warm, I switched over to normal A/C and it didn't change much as far as temperature. Now for the strange part...when I pulled off the interstate I had to stop at a couple of traffic lights. While sitting in traffic I noticed the A/C started blowing much colder again. It seemed strange for the A/C to start blowing colder when I was stopped, considering I didn't have much air flow over the condenser coil at that point.

So what else would cause this behavior? I haven't had a chance to check the charge on the cooling system yet, but I thought there might be another explanation.

 

That is a wierd one, -------------the only thing that comes to mind right now would be loosing vaccum and the blower/mix door partially opens while driving, mixing in warm air with the cold air, then at idle it has enough vaccum to keep the blend door shut.

This is strictly a guess because I haven't worked on enough diesels as compared to gassers, which this scenario could occur on a gasser.

 

I didn't consider that option...can anyone speak to the feasibility of this happening?

 

is it still acting up. all the time

 

Hard to say, most of my trips during the week are around the city so I'm in stop and go all the time. I only notice it when I get on the highway and run for a while, which I won't be doing till Wednesday night when I head down to Houston. I'll have to see how it behaves then. This is the first time I really noticed it happening, but most of my other trips lately have been when the outside temp is in the 70s, so it's easy to cool. It was in the upper 80s the other day when I felt it happening.

 

i know when i would drive my buddies 99, when i was on the highway and it would downshift out of OD it would blow warm until it shifted back???

brett

 

You might try changing the belt and make sure it is tensioned properly. Had the same problem on multiple cars. But, I had been driving them to the airport, parking it for a couple of days, then driving it back. Like disc brake rotors, a day or two of humidity and they have surface rust. Same thing on the pulleys but the belt starts getting a burnish on it from slippage that you never see or hear. The compressor puts a lot of load on the belt as well so it is probably the first thing you would notice. Anyway, worth a try.

 

Thanks ajg.

I'm heading down to Houston tonight, so we'll see how it performs. It's been doing just fine around town this week, blows just as cold as ever.

In following some of the other A/C threads, I've also saw some talk about the evaporator freezing up. I didn't notice a decrease in airflow like they mention, but I'll watch out for that tonight as well.

 

OK, now I'm puzzled. I ran down the highway a little piece today to see how the A/C would perform. When I get up to speed, you can feel the air get warmer. When you slow down or stop, back to cold. This seems just backwards since I would expect it to be cooler when moving since I have more airflow over the condensor.

I checked pressure on the low side to see if I needed R134, and the pressure was right in the middle of the normal zone.

So what would cause this behavior, or what else can I check for?

Thanks.

EDIT: Also, when I had the gauge on it, the needle never moved and the compressor did not cycle on/off. I left it hooked up for about 5 minutes and never saw a bit of movement.

 

dosnt sound to be in the a/c, might be something to do with the heater core.

 

What was the low side pressure at idle, I know you said in the middle, but do you know the actual pressure? Also check for icing on the lines before your oriffice tube, could have a restriction, don't know it you can stop fast enough to get out and open the hood to check for icing though. I really don't think that's it though, I would think it would ice up at idle to.

Also did you check for the blend door opening up on you, or better yet, is it vacuum operated?

 

I'll have to check my gauge, but I believe the reading was 35psi. I'll run outside and check the gauge here in a bit and post back. I'm also going to look for any vacuum line issues under the hood, just in case.

 

The reading was 35psi.

No noticeable vacuum leaks under the hood and I you can tell the blend door is moving when you change temps while parked. So it appears to be working, going to check whether it's vacuum operated in the FSM right now.

 

According to the '01 FSM, the blend door is not vacuum operated. It is electrically operated. So I guess that rules out the possibility of the blend door mixing warm and cold air incorrectly while moving.

Also, in looking at the chart of low side pressures in the FSM, my pressure was slightly low based on the ambient temp. It was around 85 earlier when I tested, and my reading was 35psi. According to the temperature and pressure chart, with an ambient of 80*, my low side pressure should be 40-50psi and my output temp should be 55-60*. The output temps seem about right (didn't measure with a thermo).

But I'm still puzzled why things go bad once I'm moving.

 

Since we've ruled out the blend door...what else relating to heater core would cause this? Maybe it's time to block flow to the core and do some testing. Is this possible without stopping the whole system (need to look at the plumbing).