No low pressure

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chvet73
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No low pressure

Postby chvet73 » Wed Jun 13, 2018 9:27 pm

I’m working on a 1999 Class A Safari Motorhome.

I have static low pressure of 30. As soon as I start it up my low pressure goes to 0. High is 150.

This is a situation I have not encountered before in my limited a/c experience.

Any suggestions on what to check would be greatly appreciated.
JohnHere
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Location: South Carolina Upstate

Re: No low pressure

Postby JohnHere » Thu Jun 14, 2018 8:58 pm

Lots of questions for you to help us diagnose this.

Is the static pressure on the HP side also around 30 PSI? If so, the system is virtually empty of refrigerant.

Did you start the engine without turning on the A/C? If so, how did you measure the operating pressures? If the LP side is zero, does the compressor even engage on its own? If not, did you jump it? If jumped, did you measure pressures using a good manifold gauge set, engine running at 1,800 RPM, windows open, blower on high?

What is the ambient temperature? Vent temperature? Are the under-hood fans running? Is this a factory dual-evaporator CCOT/TXV system?
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bohica2xo
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Re: No low pressure

Postby bohica2xo » Fri Jun 15, 2018 8:46 am

The 1999 Safari is built on the GM P32 or Workhorse chassis. HT6 compressor.

Coach builders did all sorts of things with the dash A/C If they used mostly the GM P series parts it has an orifice tube.

Which engine is in this unit?
chvet73
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Re: No low pressure

Postby chvet73 » Fri Jun 15, 2018 12:34 pm

First thanks for your replies.

This Safari is built on a Magnum chassis. It is a diesel pusher powered by a Cat diesel.

Static HP was 150. Ambient temp was around 90.

I have a cheap set of gauges that had served me well up till now. But I have a better Mastercool set arriving today from Amazon.

I ran tests at high idle.

Tomorrow I will check out everything else that was asked. I have spotted a connection on the condenser that looks oily. So I’m suspicious of that. The other condensor connection looks good.

I’ll report tomorrow what else I find.

Thanks again for your input.
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bohica2xo
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Re: No low pressure

Postby bohica2xo » Fri Jun 15, 2018 10:41 pm

Pushers are always a handful with A/C. Is this the dash A/C you are working on?

The long lines & odd configurations that are manufacturer specific can lead to some odd pressure readings. Are the service ports both back at the compressor?

Sometimes you will find a low side port near the evaporator as well.
chvet73
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Re: No low pressure

Postby chvet73 » Sat Jun 16, 2018 10:16 pm

To clarify this is the dash air I’m working on. Service ports are at the front of the vehicle. I did discover a single port not far from the compressor. I assume it’s on the low side. I have been using the front ports.
With my new gauges I got readings of 83 on both high and low. Those were static and running. I also checked at idle and 1800 rpm. No change in readings. The output temp at the vents was 82. Temperature today was much cooler around 72 outside the coach.
This leads me to believe that the compressor is the issue. The compressor does engage and does not make any weird noise. Compressor appears to be a Sanden.
So once again any advice would be appreciated.
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bohica2xo
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Re: No low pressure

Postby bohica2xo » Sun Jun 17, 2018 8:07 am

Does the evaporator have a TXV?

The compressor may not be bad. Which model Sanden is it?

I would probably recover the system to see how much chage remains, and inspect the expansion device. I would also open the suction line at the compressor. You want to see a little oil film in the suction line at the compressor. Oil level & return to the compressor cn be an issue in a pusher.
ice-n-tropics
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Re: No low pressure

Postby ice-n-tropics » Sun Jun 17, 2018 5:18 pm

I would remove 6 bolts on the rear of the compressor and the rear head and inspect for a loosened bolt for the retainer of the discharge valves.
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chvet73
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Re: No low pressure

Postby chvet73 » Sun Jun 17, 2018 10:50 pm

Can’t find the tag on the compressor. It’s proably on the side I can’t see. It does have a TXV. one other thing I observed is if you shut the high side gauge down as if your going to charge it the low side pressure drops to zero. High side does not change. Also notice that when I disconnected my gauges the low side was cold to the touch. Unfortunately I do not own a recovery system.
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bohica2xo
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Re: No low pressure

Postby bohica2xo » Mon Jun 18, 2018 8:19 am

I just re-read your posts. A test with new gauges gave the same pressure on both sides, I should have caught that. But your last post says:

if you shut the high side gauge down as if your going to charge it the low side pressure drops to zero. High side does not change. Also notice that when I disconnected my gauges the low side was cold to the touch.


Yes I bet the low side line was cold. The gauge set was being used as an expansion device.

The handwheels on the gauge set should both be CLOSED when evaluating a system. The high side valve should remain closed whenever the gauge set is connected to an operating system with pressures above 50 psi on the high side.

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