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Dad's Motorhome - Oh oh! :-)

marvin-miller on Sun July 26, 2009 8:02 PM User is offline

Year: 2000
Make: Monaco
Model: Diplomat
Engine Size: 315 ISC
Refrigerant Type: 134a
Ambient Temp: 26c
Pressure Low: 15-20
Pressure High: 150
Country of Origin: United States

Hi Folks!

I've got a strange one here and it's my dads :-0

Diesel pusher motorhome, compressor is in rear on engine, condenser, drier and electric fan on the side at the rear.
Expansion valve at the front as well as some sort of evap freeze detection device that cuts power to the compressor when the evap gets too cold.

The history on this A/C is spotless. No repairs ever been done. It used to blow 8c (46.4 f) but now only blows 16c (60.8 f)
I recovered, evac'd & re-charged to ensure that it's full of 134a - it is.

Re-test, system still blows 16c (60.8)

I bypassed the heater core - it's not getting hot water - same temps.
Gages read 15-20 & 150 @ 26 ambient (78.8)

Mileage is about 75,000 K (46,000 miles)

I can't figure out why it doesn't blow colder air. It used to and now it doesn't.
I think I've eliminated everything except the compressor and the expansion valve.

Usually up here at this ambient I'd typically see the high side at more like 200 psi but as far as I can see the system is working well - it's just not cold enough!

Can the TXV's go south? It looks like both senser bulbs are in their place (they are really gooped in there).

Any ideas? I'm a bit mystified as to why it no longer blows colder air...


Best & Thanks;

Dougflas on Sun July 26, 2009 8:25 PM User is offline

electric fans do fail. Spray water on condenser and see if temps drop. Measure temp drop across condenser input lines to output lines. Do high side gage readings vibrate? If so, that may be weak compressor. Readings seem like low charge but you say you recharged it. Depending upon where the gages hook up, I say still low charge.

marvin-miller on Sun July 26, 2009 9:49 PM User is offline

Hi Dougflas!

Thanks for the reply and those are good suggestions. I too thought it might be low (which was why I recovered) but I put exactly 52 oz into a 750 micron vacuum. The condenser fan is very powerful and was sucking a ton of air over the condenser.

There's a separate capiliary line/relay that serves to measure the evap temperature. The idea behind it is if the evap freezes then it cuts the compressor. I bypassed that to check to see if the temp dropped more but only got 1 degree less so I assumed it was working OK.

While I was in there I noticed that the evap/heater core box was loose! It's a pretty cheap plastic cover and one of the bolts had broken the case in top corner. As it turns out there's too few bolts holding the chamber against the firewall (poor design) and this led to about a .5" inch gap between the firewall and the cover.

I put a big washer on the bolt and this served to suck the box back up more/less flush against the firewall. I then used strip caulk to re-seal the entire top half. I then re-checked the temperature and that brought it down to 8.8c and also brought the heat side up to +40c so it looks like it's cased - at least as far as performance goes.

The one thing that I noticed that's different now from the last time I checked it is that it takes a long time to go from full hot to full cold. Previously it would take about 1 minute to bottom out on the temps but now it takes at least 5. Maybe there's more wear on the compressor but at least now the temps are very close to what they were when I last looked at it.

The loose case cost 9c on the cold side about the same (or more) on the hot side. The neat thing is, it wasn't leaking that much air but it sure made a difference in the performance. I wonder, if I add a few more ounces if it would cool better? As mentioned, up here with this ambient I usually see more like 200-225 on the high side.

If I add more refrigerant, how should I tell when to stop? My charging station will do it in .5oz increments if necessary. Should I be aiming for a higher high side? The stock charge from the M-Home vendor is 52 oz.


Best & Thanks;

marvin-miller on Sun July 26, 2009 9:58 PM User is offline

Wait a second, I just had a brainstorm (ouch!)

Perhaps the reason the high side is about 50-75 PSI lower then what I usually see in cars is because of the condenser location and the fan that's on it.

The condenser is located on the side at the rear of the motorhome so it's away from engine heat and it has a really skookum fan on it (you can actually hear the fan over and above the noise of the diesel engine).

Given that in a car the condenser is in the engine compartment and it has to cope with much higher temperatures then maybe that's why the high side on this one is considerably lower?

Best & Thanks;

Dougflas on Sun July 26, 2009 11:21 PM User is offline

If it's a TXV system, charge is not too critical. Try adding an 1oz or 2 oz. The drier should store the extra.

NickD on Sun July 26, 2009 11:54 PM User is offline

What engine speed are you using to record pressures. Typical automotive is 1:1 engine to compressor, assume this diesel with low top rpm speeds is stepping up the speed of the compressor. In any event, you want the compressor to run at about 2,000 rpm for actual pressure readings. Ambient temperature is another factor, prefer more like 30*C, 26*C isn't too bad, but low side pressures of 15 to 20 are quite low. With a high side of 150 psi you are showing a good ratio to low, wouldn't suspect the compressor.

The bottom line is that you want to supply the TXV with pure liquid at the rated rpm, and with a motorhome, wouldn't count on weights, difficult to find two identical, even off the production line, can be longer lines that require more refrigerant to get that liquid. Easiest way to confirm that liquid is with a sight glass, I have a very similar system in mine as your dad's, it has a sight glass, did you look for one? But to use it accurately, have to have warm ambients, I like 30*C, compressor speed at 2,000 rpm, AC on of course, the windows open and the blowers at max. Then you can charge to leave just a tiny trace of bubbles. Can install one if you like, in the liquid line feeding the TXV. Sounds like you have a long walk between the evaporator and the compressor, where are your service ports.

Without a sight glass, you can feel sight pulsations in the liquid line, idea is to charge until those pulsations disappear, but requires a very fine feel to do this. Also your evaporator inlet and outlet temperatures, but your output is much warmer than the inlet, should be about the same, and both cold. at 26*C I would shoot for about 25 psi on the low side, never really depend on the high side for charging, but want to be darn sure it's not going through the roof, at your ambient 220-250 psi can be normal.

With a TXV low side pressures tend to remain more constant as the ambient temperature decreases as the valve closes, if it drops rapidly like a fixed orifice, then you know you have TSV problems, same with increasing ambients, valve opens, so the low side does not rise as fast as an FOT.

I also have that thermal switch in my evaporator, matter of fact, posted a circuit I designed some time ago, use normal diodes to get rid of those leaky capillary tubes, my old one completely leaked out, and a solid state compressor coil driver to get rid of relay contact problems. It's only purpose in life is to cut off the compress if the monitored evaporator fin temperature drops to 0.5*C to prevent ice buildup on the fins, then switches the compressor back on again when the evaporator warms up to 3.4*C.

From your readings with low low side pressures, you are feeding foam to the evaporator, and foam does not cool very well, but you sure don't want to overcharge it. Does your system have a HPCO switch? Would be in series with your evaporator temperature sensor switch. My didn't have, but with a huge condenser and fan, wasn't worried about it, if I hear a belt squeak, would just shut it off., Suppose I could have added one, maybe next year.

SpinRite on Mon July 27, 2009 1:30 AM User is offline

Originally posted by: NickD
...I also have that thermal switch in my evaporator, matter of fact, posted a circuit I designed some time ago, use normal diodes to get rid of those leaky capillary tubes...I'd like to try your circuit -- can you recall the subject of that post, so I can search for it?

marvin-miller on Mon July 27, 2009 2:46 AM User is offline

Hi guys - thanks very much for the replies especially Nick as you seem to have the same unit - or seen one just the same!

The engine speeds we were testing at were at fast idle which on this unit is about 1200 RPM
Agreed on the site glass, I believe there is one on the receiver drier but that's at the rear of the unit. The service ports are right at the evap on the front of the unit :->
Pressures on the gages were basically constant with no fluttering - as I mentioned, they looked good to me other then that the high side was a bit lower then the typical I see around here at this time of the year. I figured (later) that condenser efficiency might have something to do with that because it's a very efficient system (from what I can see)
The thermal switch on the evap sounds identical - it's also wired in series with the High Pressure Cut-Out switch that's located nearby. The thermal switch seemed to be working fine - shorting it only resulted in about a 1 degree temp drop or so with compressor being 'stuck' on. Originally I thought it might be cutting out early causing the higher then normal temps but the leaking evap box seems to have been the culprit. Sealing it up resulted in an 8 degree temp drop and a cooresponding increase on the heat setting.

I think you guys are on the right track in that it's probably under-charged a bit. Your rationale behind the motorhomes not being identical makes sense, and the truth is, even though this motorhome has been remarkably good in design and components, I still run into areas where you can see that the engineering is being done by folks with only 1/10th of the experience that someone like GM has :-) So I suspect that the 52oz spec is probably under by a bit.

At this point I think we're probably going to leave it as the brunt of the problem seems to have been caused by the leaking evap box. One thing sticks out in my mind though - it's not pulling the temperature down as fast as it used to. It can literally take 5 minutes to go from 30c to 8.8c (which I got as the lowest temp today).

I guess there's only so much you can do. I have my car's cycling switch (CCOT) set low enough to freeze the evap under just the right conditions. I did this because I wanted it to blow as cold as possible and if I need to change the mode to vents for a few moments to defrost the evap then so be it - the cold is worth it! Today it nearly froze me out of the car with an ambient of 30c You know it's working good when your ears hurt and you find you're clenching your teeth :-) Not bad for a 1988 GM orifice tube system :-)

Best & Thanks;

Edited: Mon July 27, 2009 at 2:48 AM by marvin-miller

NickD on Mon July 27, 2009 12:19 PM User is offline

You mean after all that typing, you are not going to increase your low side pressures? Still say it's taking a long time to cool down, can't do that to your dad, we will leave you out of his will.

Easier for me to repost than to find, this could have been 2-3 years ago. This shows the wiring in my unit with the probe in the vents.

And this is the circuit diagram, used the old thermal switch case for the mounting.

Made R3 accessible externally, rest was on a peg board, tight sealed with silicone glue, with AC running full blast measuring fin temperature, set R3 when the fin temperature got down to 33*F, hysteresis in the circuit is fixed, compressor will click back on at about 37-39*F.

In my application, had the 12V there coming from the climate control, only when in the AC position, and the wire going to the compressor coil, so just tapped into the compressor coil ground wire and brought that back as the switch didn't need that, this circuit does. Switching the climate control to other than the AC position or killing the ignition switch shuts the circuit off. Used an 1/8" bass tube to mount that string of diodes with insulation on the connecting leads so the brass would not short the diodes out. And plugged the ends with silicone glue.

Since I lost the gas out of my old thermal switch, contacts were always closed and the evaporator would ice up, doesn't cool very well when that happens. Did look around for a replacement switch, couldn't even find anything close, so made one.

mk378 on Tue July 28, 2009 9:58 AM User is offline

Your low side is too low for that evaporator temperature. Assuming properly charged, there is a restriction somewhere, such as the TXV. You are measuring high side downstream of the drier so it is probably OK. Again this is assuming properly charged. It can take a lot just to fill the long lines from the engine compartment to the front of the bus. After ruling out everything else, it is reasonable to accuse the TXV and replace it.

You don't need to worry about evaporator temperature control until the system is actually able to cool the evaporator to the freezing point. The 1995 Honda Civic contained a nice solid-state evaporator temperature control unit if you don't want to build one. It must be used with a relay though.

NickD on Tue July 28, 2009 4:50 PM User is offline

Problem I ran into with automotive evaporator sensors, tube is way too short, needed something that was about 24" long, couldn't find the original part, so had to play with it, since I had the parts laying around, was the easiest way for me. But I do not feel this is Marvin's problem, if his evaporator froze up or his compressor wouldn't run at all, then would be worth looking into. A tall ice filled glass works good for testing.

Do not feel Marvin has a restriction, if anything his TXV is wide open with those lower pressures, feel his main problem is lack of refrigerant, as both his low and high side pressures are low.

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