When was the last time it was serviced, i.e. refrigerant pulled out, vacuumed down, and recharged?

 

Never. This is the first time the AC has not worked properly.

 

Probably a little low on refrigerant. Stick a meat thermometer in the front vent and blast the ac. Check your reading after having it running for a while. Reading when moving and on a stop and go traffic - Or just idling for a while. If its a 10+ deg difference. You are low. Mine blows 42 degree at idle. 40 when moving. In our 100+ degree weather.

 

The outside air was 90-degrees.

The FRONT is as cold as it's always been. It's plenty cold. No problems in the front.

The REAR is warm. It's not as hot as the outside air, but it's still plenty warm.

If the system is low on refrigerant, wouldn't the front and the rear BOTH be the same temperature?

 

Not necessarily. If it hasn't been serviced in the last 5-7 years the first thing I'd do is have that done. Even a properly sealed up system will lose an ounce per year so if that vehicle hasn't EVER been serviced then it could be 14 ounces low on refrigerant, or more. If it holds 28 ounces that means it's up to 50% low.

It needs to have what's left recovered, vacuumed down, and then have the proper amount put back in. Once they do that then it can be evaluated for proper operation, system pressures, etc. When they recover it they can tell you how much they recovered and tell you how low it was. That's where I'd start.

 

I am not sure if there is a blend door at the rear. I know the coolant goes straight to the rear without any control from the front heater valve. Maybe the evaporator heater motor? HONDA OEM 99-04 Odyssey 3.5L-V6 Evaporator Heater-Motor 79350-S0X-A51 | eBay If that is controlling whether you have heat or cold air, then the motor may have failed and stuck midway to heat. Hence no cold air. Try just blowing the heater in rear and see what happens.

 

A blend door back there is certainly a very good possibility. However, if the system has never been serviced then it's definitely low. Often times when refrigerant starts to get low the system will get colder before it gets warmer. That's because the system starts to freeze up. Then as it continues to get lower it will start to get warmer. On dual systems it can be a little more complicated on refrigerant flow, too. Additionally, low refrigerant means low oil to the compressor since the refrigerant carries the oil (think two-stroke engine on a weed eater or chain saw) to keep it lubricated. Most compressor failures are due to low oil which is caused by low refrigerant. Low refrigerant can also cause the symptoms described, just like a blend door can, so that's where I'd start. Start with a good working AC system and then track down any other electrical or mechanical issues that might also be there. That's the route I'd go if it were mine.

 

+1 on getting it recharged before doing anything else. Also, +1 on the point that low charge reduces lube to the compressor. If that compressor goes out due to lack of lubrication, a $100 problem quickly turns into a $1500 problem.

 

Do these vans have 1 or 2 evaporator coils? I.e., where is the rear air cooled? Is the rear air cooled by the evaporator coil located under the passenger-side dashboard? Or, is the rear air cooled by an evaporator coli located somewhere in the back of the van?

 

There is a rear evap coil. There would almost have to be a blend door or you wouldn't have any temp control to the rear unit, only fan speed control. Personally I would lean toward a rear blend door issue IF the rear is blowing HOT as if it is blowing air through a hot rear heater core, but by your description that is not the case. I too would start by getting the system properly charged and see where that takes you.

 

I suspect the rear expansion valve. It might be clogged.

I also suspect the rear heater & cooler motor assembly (the blend door motor).

I will investigate both.

 

The valve repair and refill are good to do, but that still might not fix your problem. So while you're waiting for parts ...

Have you run the built-in HVAC diagnostics yet? Ignition switch ON, Fan switch OFF, etc.? That might tell you something.

Any progress on looking into those items you mention above?

I think the rear blowing hot might be a good clue. If you have AC turned off, but temp set to cold (this should close the heater control valve at the rear heat exchanger, and the AC compressor clutch should not engage, so AC is not active anywhere), how hot is it back there? Then can you make it hot/cold by adjusting temp with the AC still turned off? If so, that would suggest your rear heater valve, etc. is working fine.

Then set AC off, and set the rear temp back to cold, let it reach steady state. Ambient air blowing out, right? Now turn AC ON, so the compressor clutch engages and the AC is "working." Now does that make the rear air hotter? If that is the case, I will guess the problem is with the rear expansion valve. Basically the compressor compresses the freon, making it very hot, it cools in the condenser, then it is pumped to the rear expansion valve (as well as the front AC system, which sounds like it is fine). At the valve, there is an orifice that should maintain high pressure upstream and allow expansion on the downstream side. This expansion cools the freon and allows the liquid - vapor phase change. So the upstream side of the expansion valve should be hot and the downstream side should be cold.

But if there is a problem in the valve, either a blockage that does not allow much flow, or if the orifice is too big to force the expansion/phase change, there will be little or no cooling. There may actually be HEATING since you're pumping the hot (cooled by the condenser, but still hot) compressed freon through the rear evaporator.

If that's the case, you'll need to fix the rear expansion valve, in addition to the other stuff you're already planning to do. One thing to consider is that, if you do have a valve problem, it will affect your pressure readings (perhaps meaning the single-AC charts are needed, vs. the Dual-AC ones). And if you do need to replace this valve, the whole system will need to be evacuated, etc., so the MasterCool special tool is not so necessary.

Since you found a leaking Schrader valve, you definitely need to replace that and you need to refill the system, so most of what you're already doing is still needed.

In addition to the tests above, if you can access the rear expansion valve, it may be as simple as feeling the upstream and downstream pipes for temperature. Upstream should be hot, downstream should be cold.

 

Stick to KISS! Get the system charged to a known capacity to avoid going down rabbit trails! Low charge is the most common issue with A/C systems, and can present itself as poor cooling in either front or rear individually.

Of course, if you put an a/c manifold gauge set on and measured high/low pressures and vent temperature at a given ambient temp, you'd have a lot more legitimate diagnostic information. Low pressure on both low and high sides would give further evidence of a low charge. Your theory of clogged expansion valve would present itself as high pressure on the low side with normal pressures on the high side. Such diagnostics is most deterministic with a known good charge level though. Marginal charge level, air in the system, etc, can confuse the diagnostics.

 

Hello webbch,

Excellent advice! Great information. Many thanks!

 

If I find a manifold gauge set that I can use, how do I know which Schrader valve is the high pressure and which Schrader valve is the low pressure?

 

High pressure will be color coded RED. Low pressure = BLUE. Also, on R-134a setups, the connectors are physically different sizes so you cannot hook them up to the wrong port by mistake. And there will be a YELLOW hose in the middle of the manifold gauge set for attaching the can of new refrigerant you'll be adding.

Also, the high pressure pipe will be smaller diameter than the low pressure, and if your AC works at all, the high pressure pipe will be hot to the touch, while the low pressure will be cold (and maybe insulated).

Schrader valve - yes, these will have Schrader valves, just like on your car or bike tires. Except different materials to be chemically compatible. But it is only the Schrader **core** that is similar between the two. So don't go looking for something like what you've got on your tires. The core/pin thing in the center will be the same, but the part surrounding it will be different.

And the Schrader valves are a fairly common failure point that will leak. They're also pretty easy to test /adjust. You can get a valve tool for about $2 at a bike store, auto store, or kmart, etc., which will allow you to unscrew / tighten / etc. the valve core. I'd get one of those, and practice on a bike tire to see how the core unscrews. You won't want to remove the core on the AC (although there are special tools to allow this without losing refrigerant), but you can maybe loosen it just a hair and reseat it, noticing how loose it was. Also, some tests on re-seating the spring-loaded valve may help - e.g., pull it tight with small pliers to make sure it is seated. You can put your finger over the valve to see if pressure builds up before / after any of this - to see if the valve itself was / is leaking.

I have a Snap On Halogen detector (fan-powered sniffer) which is amazing at detecting AC leaks in real time, non-invasively. I've seen China-brand ones on eBay for reasonable prices. May be a good investment.

 

EDIT (10-minute limit blocked me) - after re-reading your original post, the stuff I said about leak-checking the Schrader valves is only relevant if your problem ends up being low refrigerant, and you need to find a leak. Unless you already have a leak, you might not want to go looking for one, since you might create one.

 

Car OFF: Left gauge 94 PSI; right gauge 80 PSI.
Car at IDLE + AC on HIGH: Left gauge 150 PSI; right gauge 20.
Car at IDLE + AC OFF: Left gauge 57 PSI; right gauge 60 PSI.

Black and greasy around high-side Schrader valve. When the gauge set was removed, the high-side Schrader valve was bubbling. Diagnosis = the high-side Schrader valve is leaking.

If I can tighten the Schrader valve so it no longer leaks, what pressures do I need to achieve when I add some oil and refrigerant?

 

Was the "Car OFF" reading taken last, after the engine had been running during other tests? That would explain why the pressure had not equalized - takes some time. Also, when the compressor is not running, whether the engine is on or off, the pressure readings will increase with increasing ambient temperature (the temperature in the engine compartment where the AC tubes, etc. are.

That MasterCool kit nitely2 referred to is good. I have the extended version of their kit, which I've used, I think on my '99. But before getting a kit like that, etc., I'd first see if you can tighten / re-seat / adjust the leaking valve as I mentioned in a previous post. Also, you may as well do that before adding Freon.

The numbers you list are a little low. Here are the pages from the manual (for 1999). Be sure to follow the exact specs of how to run the test before you worry about the numbers. E.g., 1500 RPM. Hmm, how to do that without passengers in vehicle ... well you might need to bend the rules a little.

 

Wait a while after AC gauge removal and spray soapy water on the ports and see if its still bubbling.

I used this tool to change the shrader valve on my low port. https://www.amazon.com/dp/B0015KH93K?tag=vs-auto-convert-amazon-20 I got the valve core from AZ a $10-kit. After the repair, I added plain 134a refrigerant ( got a case from sams club) Depending on ambient temp, you should be somewhere around 30-35 psi on low side. You only charge on the low side. While still watching your high side. You don't want that to go way over either. No more than 300 psi?(need to look in my notes) You also want to know the temp on the vents. So put a thermometer there. BTW, you want to get the ac reading while its running for a while. You will notice that after 15-20 mins or so, it will change. Specially after adding a small can of 134a. It will go up and then settle down. Good luck.

 

I tried reseating and tightening the Schrader valve (loosen it, then tighten it) several times. Didn't work; still bubbling.

I tried pulling, pushing, and rotating the pin in the valve (with ultra-small needle nose pliers). Didn't work; still bubbling.

Now, calling around to parts stores to see if anyone carries a no-refrigerant-loss valve replacement tool. If I can't find one locally, then I'll go online.

Thanks for the info re the Mastercool tool.

 

Hello nitely2,
How did you get the tool to grab the valve so the valve could be pulled out (after unscrewing the valve)? Wouldn't the valve fall out of the "teeth" of the tool?

 

Short answer ... it is a snug fit between the "teeth" and the valve, so the valve core just stays in there once fully unscrewed.

You can test for the correct snugness of this fit before doing the actual valve core removal - the rod with the "teeth" on the end can be place on the valve core all by itself before the rest of the sealing apparatus is used. If too loose or tight, you can bend the teeth slightly to get a good fit. That part of the Schrader valve is not a high precision dimension. Then after removing the old core, do the same test and adjustment with the new core so it does not fall out as you try to insert it. (but that last thing did happen to me before I figured this part out - it can be recovered if you're careful)

 

Hello oldskewel,

The "Car OFF" reading was taken first. The van had been driven earlier in the day, but had been sitting for about 3-hours (this is when I took the "Car OFF" readings.

 

Well that seems a little weird, so you might want to keep it in mind. After sitting for 3 hours, the pressure within the system should have equalized, making the high and low pressure readings the same. It might indicate a blockage somewhere. Or, could it be that some gas vented when you were attaching the gauges?

EDIT - I just read the previous suggestion on the rear expansion valve. Definitely something to consider if you think those "Car OFF" measurements are good.

Also, with the compressor not running, you've basically got a system with liquid + gas refrigerant in there. As the temperature of the system (the tubes, the liquid, the gas, etc.) goes up with the temperature of things in the engine bay, the pressure should go up too. So it seemed odd that the pressures after it was sitting for 3 hours (and presumably cooler) were higher than with the engine idling and AC off. I guess it's possible that having the hood up let things cool down and the idling was not creating much heat.

Another measurement that may help is if you have a multimeter with a thermocouple (many have that, even fairly cheap ones), then getting measurements of the cold air coming out of the vents would give you useful info on how it's doing before and after any repairs or adding Freon.

All the warnings given by John Clark are good advice. So proceed with caution if you dare. The AC system must be kept very clean, so for example that grimy valve should have been cleaned carefully before opening. And yes, air in the system can be a problem. So you'll need to think carefully about what is in the manifold, tubes, can, etc. and make sure any air is purged before it has a chance to get in the system. So far, you're fine since the system is pressurized and just taking measurements will let gas out, but not air in. The other major warning to consider is to not overcharge the system. More is not always better. Careful monitoring of the pressures and the outlet temperature might help you avoid overcharging.

As far as adding oil, my recollection is that typically you can buy pure R-134a in cans of like 12oz, or you can buy a 4-oz can containing 2-oz of refrigerant and 2-oz of oil (get the right kind, of course). Then those are both charged the same way. When charging Freon, you want the can upright so you don't hydrolock the compressor - upright will just charge vapor; upside down would charge liquid. When charging the oil/Freon mix, you want to shake it up, and then tip the can back and forth so the oil all comes out. When charging the Freon, the can will get cold due to the expanding gas. This will have the effect of reducing pressure in the can, possibly significantly slowing charge rate. An easy solution is to get a little tub, like a coffee can, filled with hot water. Then put the can in there, which will keep it warm/hot even as the adiabatic gas expansion causes cooling. So with the can staying hot, the pressure will stay high, and charge rate will stay good.

 

Its fairly easy. Look here...

Just be careful. Don't forget to lock/close the system first before pulling out the old valve core slowly.

 

Here is the instructions from the mastercool website....https://www.mastercool.com/wp-content/uploads/2014/12/58531-INST.pdf

 

Please wear safety glasses when working on the AC.

 

Wow...this thread has gone a long way today!

I don't think it hurts anything to check your own pressures with a gauge set. However, there is absolutely no way to accurately charge your system using a gauge set. First, the system isn't built to do it that way. If it's low it needs to be evacuated of the remaining refrigerant, vacuumed down, and then recharged with the amount of refrigerant on the sticker under the hood. That's why they put the sticker there. If you could do it with gauge pressures you wouldn't need the sticker and manufacturers definitely wouldn't spend the money to put the sticker on there.

Second, even if you could do it with gauges alone you still have to be really careful not to allow any air into the system. It's easy to do it accidentally by not properly purging hoses, etc. Also it only takes a turn of the wrong valve to run into real problems. Even if you do everything right you can still have major problems. I had an o-ring burst on the high side fitting of a cheap Harbor Freight set of manifold gauges, with the compressor running, which vented a ton of refrigerant and oil until I could get it turned off--very dangerous in and of itself. Instead of evacuating the system I tried to recharge it by using gauge pressures. A month later the compressor started making noise...less than a year later the compressor failed and I had to replace the entire system with exception of the evaporator core which I had to flush. Fortunately, this was on my older 98 Nissan and not on my Ody. Lessons learned? Don't use cheap gauges and never attempt to recharge the system using gauges. I had overcharged it which stressed the compressor. It took a while for it to go bad but it did finally fail.

It sounds like you found your leak. I would take it to a shop and just have them service it. Let them know the valves are leaking and need to be replaced (many AC shops replace them as a matter or routine at service because they all leak.) It'll cost you $100-150 at most and no risk of damage to your system.

 

Rear expansion valve! Had same issue last year, fixed it by replacing the rear expansion valve. Good luck!

 

This is always a possibility but, unfortunately, the only way to come to that conclusion is to first have a full refrigerant charge and then diagnose with gauge pressures. If there is any problem in the system then attempting to charge with pressures will never work and you'll likely overcharge the system and ruin the compressor because the system pressures don't even have a chance of being within spec no matter how much refrigerant is in the system.

 

On my 06' Touring Odyessy, the front center vents had issues. Left one blowing cold air and the right side blowing hot. It seems that a relay switch in the a/c temperature control circuit was stuck. So it may also be the electronics.