A/C gurus - a quick question

[strotter]

I'm looking at the weather forecasts, thinking about August, and I figure it's time to get the A/C system functional. I've got:

> a condenser out of a 280 (direct bolt-in, in place)

> a GM-style Harrison 635 compressor (out of an '89 Firebird, in place)

> a GM accumulator (also out of the Firebird, in place)

> its' pressure cycling switch (in place).

The Helms manual says the GM system is a "CCOT (cycling clutch orifice tube system", which uses a orifice tube (and resulting pressure drop) to expand the refrigerant, and it cycles its compressor to keep the evaporator just above freezing. So I also need a expansion tube immediately in front of the evaporator. Understood.

What I need help is with the evaporator itself. I found an A/C system last year in a 240 at Pick'n'Pull, and snagged the evaporator housing and evaporator. Below is a picture of it:

 

 

You're looking at the downwind side. This thing fits into a housing that wraps around the fan housing in the cockpit and feeds air into the heater intake. When the A/C system is on, it feeds cold air.

OK, now my question is about the brass, mushroom-shaped deal on the intake (?). I believe it's a temperature-sensitive valve. There's no mention of any such valve in the Helms manual. I think I need to remove it to make this thing compatible with the rest of the system.

 

Am I correct?

[Dan Juday]

Don't remove it. That's a TXV, better than a fixed orifice. It's likely for an R12 system but you can get another power head/sencing bulb for it to convert it to 134a.

[Dan Juday]

Opps, closer look at the picture. Looks like that power head is not removable. Drop me an e-mail Scott. You're only 30 minute away, I'd be happy to help you with this.

Dan

[technicalninja]

I call that critter an expansion valve- It is replaceable and it is for R12 system. it is also still available cheap. This style of valve was used in aftermarket systems for thousands of different makes and models. the rest of your parts are also set up for R12 but changing to the GM style orifice tube is a LOT of work as the passages in the evap core are different for the two different valves. I would use everything out of the engine comp of a 75-78 280z and have two custom hoses built to adapt the 280 stuff to the 240 evap. I would also get a new expansion valve as the original ones rarely lasted more that ten years and a new receiver dryer.

If you can afford it stay with the R12. Its thermal characteristics are much better that the 134 and it is much less likely to leak. Both 134 evap and cond cores are 50 percent more dense than 12 cores. the manufactors had to do this just to get the systems to the point that customers would not complain about them. Stuff that has had 12 and the 12 oil in it usually does not work well when converted (oil between the two systems is vastley different).

Having the evap core so far away from the center vents also reduces the systems performance, makes that side of dash harder to work on, and effects passenger foot area.

I am working on a retrofit of the complete 280 system into the 240 using original dash and HV/AC control head. Will post this mod but not soon as other stuff is coming first for me.

The 280 system has a cool adjustable theromstat rod and I can get 33 degree center vent temps out of this system in hottest day of year (I am in Texas) freeway cruise/recirc/low blower speed.

The original 240 evap should have also come with a rotary temp switch (fit in little pod on drivers side of console) and has a sensing bulb (long copper/brass tube) that has to be properly installed in evap core. It needs to be pushed into center of core about 3/4 depth of core. You should put some sort of heat sink compound on it to assist in temp transfer of core to probe interface. Rout the tube carefully through under radio area forward of console. Try to keep it off of other things (especially things that get hot). This helps keep the signal from the core pure.

This rotary temp switch is used to cycle the compressor. It usually has two spade terminals that should show continuity when sensing bulb is above freezing (32 F) and no continuity below. The compressor is cycled to keep evap from going below 32 deg F and freezing up.

One interesting note is that all early Z firewalls have the later factory mount locations already center stamped in the metal. I've always wondered why Nissan did not factory A/C the pre 74 1/2 cars. It is obvious they were thinking about it.

The temp sensing bulb from the expansion valve should be insulated fully (using the black tar stuff). This valve works as a varible orifice in place of the GM fixed orifice. Colder the core get-less freon is allowed through which lowers the low side pressure causing even colder temps. This set up will provide a colder core that the G.M. set up which shoud have a much more stable low side pressure reading but never reaching freezing temps. G.M. used this set up mainly for cost reasons. The inline orifice tubes require a very specific mount location in the evap high side line at a specific distance from the core. Probably difficult to re-create in an early z car.

Good luck- hot days with no A/C reduce fun in a street car.

Rick

[Dan Juday]

Good info Rick. All the A/C I do stays in one place. While auto A/C is pretty simple it helps to know the idiosyncrasies.

 

R12 is very freakin expensive here in Cali. And I'm sure Scott will want to use the GM compressor as it is an easy bolt on with his motor. If I read you right we should expect about a 30% decrease in capacity with 134a?

 

Can you recommend a blend that might work better?

Dan

[waynekarnes]

Dan, apparently Vintage air has a set up, that works or is adaptible for the 240z.

 

Carl Beck, on the izzc is running one on his 240z.

 

drop him a note on the forum z mailing list ( classicz.com )

 

list@240z.org

 

that is the e mail address

 

if you do not subscribe to the list, ask him to respond to your e mail address.

 

you can tell him i sent you.

 

he says that the vintage air set up works very well.

 

he is running a stock 6.

 

wayne

[waynekarnes]

oh, tell him you have the blue z with the flares. the one Alex posted on the mailing list, from the Rio Vista Hybrid Z meet.

 

he doesn't care a whole lot for modified cars, as most are cobbled together.

 

he absolutely loves your car. says if one is going to modify a 240z, yours is the perfect example of the way it should be done.

[technicalninja]

30 percent sounds right to me or maybe a little more.

The problem will be R134 works at higher pressures than R12 and the R134 molecule is much much smaller than the R12.

R134 requires "barrier" style hoses to stop leaks. Standard (R12) hoses will work its just I get tired of constantly recharging system to keep max cooling. Lose just a little R134 and the system gets warm. Never needs a full can to get it back to temp. Can be annoying.

Because of the higher pressures the compressor (used) is stressed harder that its ever been and its internals and seals have been soaked in R12 oil which is NOT compatable with R134. R134 compressors are more "robust" internally and designed to work with operating pressures 20 to 30 percent higher than R12.

Another problem is that the R12 condensor will not remove enough heat from the R134 and high side pressures and temps will be much higher after core than with a proper R134 core. Increased pressure and temp going into evap equals less cooling inside car.

In a much smaller way the evap core has the same probelm as the condensor. It was not designed to work at the higher pressures and is not as efficient as a R134 evap. Not as great a temp change occures across core and the lower side pressure rises accordingly. With a gas in a sealed system tempature is pressure.

All these factors work against trying to retrofit R134 into a R12 system.

The increased pressures and tiny R134 molecule make things leak like you would not believe! A system that holds R12 fine can leak R134 like there is no tomorrow.

I have always wanted to build a complete a/c system using the R134 parts (except the expansion valve) but using R12 as the referidgerant. Talk about cooling power! It would probably never leak and the compressor would last a long time.

One of my good friends owns an Auto A/C shop (mainly American stuff)

and he loves the R134 systems as they are more prone to serious failures which means work for him.

 

A complete retro-fit new system (with all R134 parts) will work well and should last a good long while. I do not believe it will last as long as if it were set up for R12 and I know it will not cool as well.

It gets waaay hot here during summer and vent temps below 45 degrees are important to me.

My 98 Infinity (glorified Maxima) has a well designed R134 system that works pretty good but cannot compare with my Mom's old ratty 88 Maxima (all original ac stuff except reciever dryer) or my 78 Z systems.

My Infinity's mileage is over 130k and it's system has never been worked on so R134 systems can last. They just need to be properly designed.

Exceptions Always Exist and I am sure that some one here has made a junkyard system function well.

I would not change an R12 system to R134. If I had to use the R134 I would get one of the complete replacement systems for R134.

 

A rumor I heard is that DuPont recieved the patent for R134 just after their patent for R12 expired. R12 is still available dirt cheap in most other countries and is still released at a massive rate into the atmosphere.Strange how the R12 destruction of the atmosphere is no longer a news story. I will always wonder if it was all a money game.

 

On blends- I will never use them and I refuse to work on any system that has had blends in it. Neither will my A/C shop owning friend. Blends gone bad call for complete system replacement. Customers are never satisfied with repairs to blend systems and many blends use hydrocarbon bases (butane, propane, heptane) BANG-tane.

Avoid blends if possible.

 

Hope this helps- I know it probably doesn't but its better to know the pit falls that are possible before you start your modification.

All of you parts will work fine with R12. Bite the bullet. Use R12 and the system will last longer and get colder. I can commonly source R12 for about $30 pound. I have seen R12 go pretty cheap on E-bay (be careful-probably lots of scammers).

 

Do not know how R12 licensing is done now. When I got my license (first month it was needed to purchase R12) it was an open book test that was sent to my house in the mail and the booklet that test was based off of had all of the answers and came with the test. $10 if I remember correctly. If you could read English you could be certified and legally purchase R12. It was such a JOKE to me.

 

Good luck on you system. Stay with R12

Rick

[strotter]

Lately I've been learning a lot about refrigeration systems...

 

To me, I don't really care if the system is R12-based or R134-based. I'm thinking I've got this big-assed compressor from a Pontiac, driven by a big-assed motor, and a pretty-big condenser (with air sucked through it by a big-assed fan though); the only thing I have that's little is my evaporator (no snickering here, please). But I do have a big-assed fan sucking air through it (thanks for the tip Owen!). I'm thinking that the small cockpit of the Z won't need a whole lot of BTU's to keep cool. Yes, it gets warm around here, too (weeks at a time over 100 deg, 105-110 not unusual).

 

The only thing that has me concerned is the "TXV" valve Dan (Magazine Cover Guy) mentioned earlier. He has kindly offered to help me get this thing functioning, but I don't want to show up at his place with a big stupid grin and a pile of crap; I want to have everything ready to go, get in, get out, mission completed. I've been hunting for a new TXV valve, so far without success.

 

Also: digging through the Helms manual for my donor car (a Firebird), and comparing it to the manual for a 260z, I've noticed that the systems are set up differently: for the GM, the cycle goes ("SV" is "service valve"):

 

Pump->Condenser->SV->SV->Expansion Valve->Evaporator->Dryer->Pump

 

For the Datsun, it goes like

 

Pump->Condenser->Dryer->Expansion Valve->SV->Evaporator->SV->Pump

 

Am I reading something wrong here? Also, the Datsun has service valves on either side of the evaporator. The GM seens to have a pair of them in the "liquid line" (the line from the condenser to the expansion valve).

 

A/C is hard.