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Trying to figure out why AC warning light is on
Hi all: Trying to figure something out, and wanted to see if anybody has run into it or wanted to give it a little thought with me.
When I got the bus, the prior owner had done a “work around” and jumped the front junction box 54 to 88. With the jumper on, the AC system works as expected. Pressures are good, condenser/evaporation fans work, etc. The issue this jumper causes is that the AC compressor turns on on both the AC and heat side, which makes sense when I look at the wiring diagram and how the rocker switch works.
When I take the jumper off, I get the AC warning light and the clutch does not engage. So I got the wiring diagrams to figure out why they had put the jumper on it and fix whatever they avoided fixing.
That is where I am stuck and looking for any help reading the diagrams as I am trying to figure it out myself before heading out on a trip.
When I look at the diagram attached, I see that 88 connects the compressor clutch to the drivers heater fan control module. I also see that the same wire connects to the relay that is connected to the AC warning light which explains why when 54 is jumped, providing 88 power, the light goes off and when I remove the jumper the light goes on. So, if I am reading this correctly, the question then is why doesn’t 88 have power without the jumper wire. I am wondering if it could be the fan control module or the drivers rheostat, but am not confident in my interpretation of the wiring diagram.
Appreciate any advice or thoughts you can share. Meanwhile I will continue to think about it and maybe do some more testing on the bus in the morning.
Thanks as always!!
Mike Harper
2004 Marathon XLII -
9 Replies
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I will post the solution once I figure it out, but thought I would post my current analysis/thinking in case it triggers someone’s experience.
Based on my understanding of the diagram, when AC is turned on, power should go to post 52 to the high pressure switch (NC) and low pressure switch. I verified with AC on, I get voltage at post 52 and 54 as expected in the diagram. I would have thought it would pass through the low pressure switch, energize the clutch, turn off the AC warning light and send power to post 88. What is confusing me at the moment is that the diagram shows the low pressure switch normally open which would block that flow. I tested all of the pressure switches with my ohm meter and all are at normal state (closed or open) per the diagram so on the surface they seem to be good, but I don’t have anything else yet to put my finger on.
I guess I am stuck at what normally gets power to post 88 and the clutch? They jumped around whatever that is but I can’t yet see on the power diagram what to go after.
Hopefully I will get some more insight today as I ponder it some more. If anyone has experience with the pressure switches or this system appreciate any thoughts.
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I am not sure if this helps your thinking but Normally Open and Normally Closed are opposite when you think of Electricity and fluid. A close Circuit in the electrical world means Connected, while in the fluid/Air space a closed circuit means it is shut off closed/not connected.
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Mikesharper
Think of the low pressure safety switch this way. When it is in you hand it is open because the pressure is less than 15 psi.
Now, in the real world, when it’s installed in a normal healthy system it closes at greater than 30 psi and won’t open again until the pressure drops below 15 psi. -
There is another normally open pressure switch on the accumulator, 130-170 psi. Closes at >170 psi, open again below 130.I’m don’t know if this is part of your issue?
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Don, Ahh now I think I get it. So even though it is normal open, in the system under working pressure it should be closed making the path for the power. The diagram showing normal open does not mean under normal operating condition but open if not in the right pressure range? So in essence, given that it is open now (with system charged) it would indicate a bad switch?
If I have that right, maybe it is as simple as swapping the low pressure switch. Seems like that is an easy thing to do and give it a try. Make sense?
Thanks – I feel better I might be getting closer to the answer
1984 96" with 8V92TA
(remodeled Liberty Coach)
Greenwood, MO -
Low pressure safety switch on the compressor of our 96 Liberty 40XL/95 chassis w/Prevost full bus OTR ac/heat = Prevost pt# 452406
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Thanks found that as well and ordered it today. Fingers crossed
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Installed the low pressure switch today and it is working as expected.
Thanks for the education on how the pressure switch works. I was assuming what was shown on the diagram was the working state versus the default state.
Problem solved.
Thanks again!!
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