DPD Air Conditioning for type 2 bus? FAQ

[skills@eurocarsplus]

your pressures sound ok to me

look at this chart

http://acprocold.com/faq/r-134a-system-pressure-chart/

were it me, I would ignore the condenser temps and focus on the pressures. this is going to be a case of "wing it" and be solely dictated by your pressures to know if you have a full charge.

one thing you want to make sure of is that your compressor doesn't short cycle. overcharge will trip the blow off on the compressor. when that happens, it doesn't (usually) bleed off pressure till the system stabilizes...it will trip and just about empty the system

it will be interesting to see if you have any issues with the evaporator icing over. this generally isn't an issue when the evap is in the same housing as a heater core, but your system is a total recirculating system and doesn't draw in fresh air.

does your receiver/dryer have a site glass? check, because on the bock there is a dimple for the window that usually gets painted over when they make them. any bubbles means you can give it another sniff till the bubbles go away
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[Amskeptic]

[equinox]

"I've looked at Gilmore Enterprises already and all they have for the late bay's is the under dash setup. That's fine for the front passengers, but I want to cool the whole cabin."

This is one thing I never understood about buses...

A/C didn't come with them and only supplied by 3rd parties, but everything I seen to retrofit used standard automotive compressor on the engine, which has very low power as-is.

A/C is REALLY only needed for sleeping on scortching and hot days. My idea is to cut a hole through the top, especially of a later Type 2, like my 1976/77 and install a conventional RV-type retrofitted electrical A/C unit. Again, most sleeping at night and usually camped out, so use the camp site electrical....just a thought.

[69BahamaYellow]

[69BahamaYellow]

[69BahamaYellow]

Also forgot to add I'm making all my temperature measurements with an infrared thermometer, so my condenser pipe temps are probably not the most accurate because of the small surface area, but my evaporator vent temps, receiver drier surface temp, and condenser fan air temps should be accurate.

Based on today's measurements and test drive, I can't decide if my system is over charged, or if I've got too much sub cooling (ie condensers working too good), or if everything is just fine, given the unconventional multi condenser design and the very long hose lengths connecting everything. If I just ignore my measured data and look at how well the system is cooling the bus overall, I'd have to say it's running perfectly well. The compressor drags very little off the engine, and it's quiet and runs very cold. Per Equinox's earlier point, I can't understand either how VW could design a vehicle with such poor ventilation, but at least dpd figured out how to make a system that works well in a bus. Since dpd was based in San Antonio, TX, they know how to design for very hot climates. Even though my system is running 134a instead of 12 refrigerant, my improved condensers and modern compressor probably more than make up for the less efficient refrigerant.

I'd still love to get some feedback from the AC experts on the forum

[airschooled]

Hi there,

I was aimlessly browsing around today, and I found exactly what I was looking for. As an idle RPM compensator, try a purge valve from some American Monstrosity. O'Reilly Auto part # CP561. It should kick your idle up 200-300 RPM when tied into the AC clutch electrical circuit. I'm not sure if it would need its own relay, or if the clutch relay would also be able to handle the load.

I found it on luftvagon's Microsquirt thread on Itinerant-Air-Cooled.

http://itinerant-air-cooled.com/viewtopic.php?p=199577#p199577

[69BahamaYellow]

thanks for the tip on the purge valve Asiab3. That's exactly what I had in mind, and powering it off the clutch "engage" signal, just like you said. I've got plenty of power capacity in my clutch circuit for the purge valve, since my current power distribution panel is "overkill"

It's built this way, so the condenser fans can be powered off a combination of trinary pressure switch (in the receiver drier) and even a refrigerant temperature switch on separate circuits, so that the condenser fans don't have to cycle so much. I'm not sure the aftermarket condenser fans are up to the quality standards of OEM, so a lot of cycling may ultimately kill them. So far, I'm just powering all 4 condenser fans off the clutch circuit so if they make it through the summer, I may just keep it this way and maybe simplify the power distribution to just a 2 relay mini-weather protected fuse box. I figure with 4 fans, there's built in redundancy, and who knows, these things may surprise me and last longer than I think. After all, they are pretty quiet, for the volume of air they move, and these are the same fans that are used on a lot of RV's and specialty vehicles.

[rugblaster]

You made this A/C your bitch. I installed these DPD air conditioners back in the 70's at the dealership I worked at. I hated everyone of them. I wore out three air drills and I don't know how many hole saws. If they had cordless impact drivers back then, I would have been in heaven. If I recall, they paid us 8 hrs. flat rate for the install. One could get close to doing it in 8 hours, after about the 10th one you installed.

I always had the impression the York compressors put such a load on the engine it probably made them run to hot, at least in this locality.

Good job.
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[69BahamaYellow]

[69BahamaYellow]

Took the bus out for a really thorough AC Performance test today, and I've got to say, I'm really impressed with the overall results. Before I started testing, I hooked up my gauges and measured the static pressure and took ambient air temperature readings with my new contact temperature probe and infrared as follows:

Static Pressure = 91PSI
Ambient Air Temp Between 76degF and 80degF (Probe and IR
Temp inside bus with windows closed between 99degF and 100degF
Time 12:45PM / Sunny Skies / High Humidity / Bus in direct sunlight

The following static pressure/temp chart confirms gages and temps correspond pretty close

Next I hooked up my Engine Analyzer to get an accurate reading for engine RPM and Battery Voltage.

Engine Idle speed is 1000 RPM with compressor running, temp setting on Max Cold, all windows down and all fans on max speed. Under these conditions, battery voltage reads 13.1 Volts. At anything above 1200 RPM it goes to 14.4 Volts and does not vary. Next I took some High and Low side pressure readings at different engine RPM and also recorded condenser #1 Inlet pipe temperature (as T1I) and Condenser #3 outlet pipe temperatures (as T3O) with my contact temperature probe and giving about 30 seconds for temperatures to normalize. Here are the results of that test:

RPM----1000--------2000--------3000--------4000
Hi Pres-175PSI-----190PSI------192PSI------195PSI
Lo Pres-25PSI------16PSI--------12PSI-------10PSI
T1I-----156degF---163degF-----173degF----173degF
T3O----92degF-----91degF------92degF-----95degF

So from this data, the high side pressures look pretty good, as the following chart indicates they are normal, but the low side looks to be too low

I was tempted to add a little more refrigerant to get the low side readings up, since that is what I was doing when I got up to 60 oz (5 cans) of refrigerant on the initial system charge. When I was charging, the low and high side pressures were climbing at the same rate, so I stopped adding refrigerant when the high side readings got up to the correct range from the chart above. Thinking some more about my theory that the very long hose runs from the compressor, through the condensers & evaporator, then back to the compressor should make the high side readings look higher and the low side readings look lower, I also thought about adding some more refrigerant to "split the difference"; in other words get the low side to maybe 5-8 PSI below the normal range and the high side 5-8PSI above the normal range, but I ran across another chart that uses the sub-cooling method to determine the "correct" refrigerant charge for automotive AC Systems, and this chart seems to indicate I'm already overcharged. The way it works is you measure the condenser outlet pipe temperature and compare that to the high side gage reading and the point where those lines intersect represents your system charge:

I'm not sure if it makes a difference whether you have 1 or multiple condensers, when using this chart, but in theory multiple condensers in series should act like a single condenser (only bigger). It is possible that because my condensers are installed at 30 degrees from horizontal, I've got saturated liquid leaving one condenser and flashing back to vapor when it enters the next. When you also throw in that I've got 2 condensers with fans forcing air through them and 1 that just relies on forward motion of the bus for air flow, all the assumptions from the above sub cooling chart may be thrown out the window. No matter what, I've got pretty efficient condensing going on to take the refrigerant temps from 173degF down to 95 degF when the ambient air temps had crept up to 87 degF when I recorded all the temperature readings


I also used the temperature probe to measure differences across the evaporator and condensers as follows:

Evap Inlet Air Temp: 83-84 degF
Evap Outlet Air Temp: 60-61 degF (all vents are within 1 degree)

Condenser #1 Inlet Air: 100 degF / Oulet Air Temp: 112 degF
Condenser #2 Inlet Air temp: 87 degF / 96 degF

I think I had some air recycling hot condenser exhaust air back around the frame rails into the inlet on the #1 Condenser when these measurements were taken, since the breeze had picked up a little

Final test of the day was to take the bus out for a drive to see if I could get the evaporator to ice up. At 4:30PM the outside temp was 95 deg, sun was blazing hot, and humidity still high. With all the windows now closed, inside temps measured at chest level started coming down quickly from over 100degF to 85degF in maybe 10 minutes. That may sound like a long cool down time, but when the vents are blowing 55 deg de-humidified air right on you, the surrounding air in the rest of the bus doesn't feel hot at all. To measure my vent temps, while I was driving, I stuck the contact probe in one of the vents and put the meter in my cup holder so the display is easy to look at. It's odd that my IR temp gun was showing vent temps at 35degF, while the contact probe measured 55degF, but I think contact probe is probably a better indicator of overall air temp vs the IR temp showing me surface temps.

After 30 minutes of highway driving at 70MPH, I pulled over to get in the back seats to make measurements. The very back (3rd row) seat air temp was 74 degF and you could feel the cold air blowing all the way back there. I then measured the evaporator air temps and got 74degF going in and 50degF air coming out, with no signs of evaporator icing. I opened the door to confirm that the condensate drains were still dripping a steady flow of water (which they were).

The blast of hot / humid air that I got when I opened the driver door after the highway drive also has me thinking I'm satisfied with my refrigerant charge level. I had to turn the temperature switch up on the way home, because it was getting too cold! In 95degF heat, I'd say that's a pretty good confirmation that these old dpd units can be made to work well

[scrivyscriv]

Thanks for the inspiration to get my own DPD unit working!
It's been sitting in the shed for several years now as removed from my first bus... Never pulled it out until yesterday.

I'm working on the bracket and idler to mount a Sanden 508 right now.

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Dünkelgrügen 1967 Java Green bug thread
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If you're ever in the Memphis area, you are welcome to stop by for advice and help.

[69BahamaYellow]

Getting a few PM's on how the DPD is working, so here's an update:

Temps in Central TX are near 100 deg F. now, and the overhead DPD system rocks! I think the secret is that it's blowing air down on your passengers instead of up like the Ice and Gilmore systems. Those systems may be great also, I just don't have experience with them. The experience with mine is that vents are blowing air right in your face, so you get cold air in just a few seconds, and because the hot air in the rest of the bus rises up to the evaporator, the system cools the whole bus down to the mid 70's within about 10 minutes (and that's starting with an interior that's almost 130 degF in direct sunlight). It's really impressive (as good as anything I've experienced in a modern car), and the bus is now a pleasure to drive in summer!!

Seeing how well the DPD works, I'd definitely go this route again. I think all the condensers I used helps out a lot too. I've not had any issues with the engine running hotter with the system on vs off. The Denso compressor I used is also another important factor, since it gives way more refrigerant mass flow rate than the original Yorks. Very important for cooling large volumes. Not sure if I'd change a thing. Well one thing I'm going to do is replace my system power relay with a delay on relay. with an instant on relay, my fans come on just about the same time as I hit the ignition switch (pulling around 50 amps away from my starter). I was hoping that getting a start signal off the AFM would not let the fans start until the engine was running, but apparently there's enough airflow through the AFM during startup that it kicks on my system before the engine starts. A delay on relay should stop that, so I can just leave everything turned on while I'm running errands, doing a lot of stop / start cycles.

[busdaddy]

Trigger it off the charge light.

Good to hear it's working so well, sounds awesome.
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[69BahamaYellow]

[tootype2crazy]

I called Ed @ Gilmore Enterprises and he told me not to bother with the overhead DPD units, of which I have 3 in very nice condition. I was bummed when he said so but this thread proves him otherwise. I'm gonna do it. A few modifications and it will work great.
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[69BahamaYellow]

Well, I don't blame them for telling you to ditch the DPD, in favor of their evaporator, since new parts are readily available for that unit, and its one less variable to deal with, when sizing everything to work together.

From my perspective, the only negative I've seen with the overhead DPD is you can hit your head on it, if you don't have your seat slid far enough forward, but having the air blowing right where you want it and cooling the whole bus far outweighs the downside.

If your bus is similar to mine, I think you'll be pretty happy with the DPD system, just by following a similar recipe for components that I described in this post.

[69BahamaYellow]

Now that air temps are consistently at 100+ deg F, I've noticed the bottom side of the engine to body foam seal gets really hot after a drive from all the hot air coming out of the heater valves (this air exits at around 270 deg. F), which explains why VW created the elaborate heater dump tubes to get all that hot air out.

In the process of trying to make my own heat dump tubes, I came up with a simpler solution with a piece of $7 2inch x 18 inch aluminum pre-heat tube. I cut this into 2 pieces (1 for each side) and just tucked the open end of this tube into the space between the exhaust U - tube, so the hot air just blows out below the engine.

So far the exhaust pipe has not melted the aluminum tube, and my foam engine seal is now much cooler after a drive.

[69BahamaYellow]

Found this delay timer on eBay for $6 (its the blue box in this picture beside the double relay/resistor pack). It's description is a delay on operation relay, 12V, 3 seconds fixed, 1 amp. Just used it to interrupt the start signal to my AC system for an extra 3 seconds to let my engine fully start up before the AC system kicks on. What an improvement this makes for day to day use. I just leave everything set, just the way I want, and just start/stop the bus as I normally would.

I think the suggestion of using the alternator warning light would work about the same, but I'd still need a special relay to turn on only after the warning light went off.

[busdaddy]