Dash AC compressor troubleshooting

Topic: Dash AC compressor troubleshooting (Read 2855 times) previous topic - next topic

Dash AC compressor troubleshooting

April 11, 2025, 05:38:14 pm

Starting to track down why I'm not getting 12v to dash AC compressor. Already determined that my freon is good. Jumped 12v to compressor and it engaged, though didn't run it long enough to see if cold air came from vents...probably should have.

When checking voltage does the motor need to be running or just key on?

Schematic shows the relays and fuses but isn't clear where they're at. Looks like some are back in motor bay and some up front 🤷‍♂️

Re: Dash AC compressor troubleshooting

Reply #1 – April 11, 2025, 05:48:25 pm

On my coach, the relay for the compressor is located on the frame, right next to the isolator.

YMMV

jk

Re: Dash AC compressor troubleshooting

Reply #2 – April 11, 2025, 05:51:50 pm

If your coach is the same as mine, there are two bosh relays under the bed. One is for the AC compressor
and the other is for the condenser fan.

Re: Dash AC compressor troubleshooting

Reply #3 – April 11, 2025, 05:54:54 pm

Quote from: oldguy – April 11, 2025, 05:51:50 pm

If your coach is the same as mine, there are two bosh relays under the bed. One is for the AC compressor
and the other is for the condenser fan.

Some pics here Update Isolator/Auxiliary Start Panel

Single-click writeup here Isolator/Auxiliary Start Panel

Re: Dash AC compressor troubleshooting

Reply #4 – April 11, 2025, 09:49:34 pm

Dash cooling switch, relay on isolator panel, low pressure & high pressure switches on refrigerant hoses: all have to close circuits to let 12v flow to compressor. Use meter to check things out. If your engine compressor's side evaporator coil fan on street side, turns on with dash cooling switch that helps diagnose circuit. Coach engine does not have to be on to test, except the compressor will only rotate with engine running.

Re: Dash AC compressor troubleshooting

Reply #5 – April 11, 2025, 10:18:56 pm

Quote from: Barry & Cindy – April 11, 2025, 09:49:34 pm

Dash cooling switch, relay on isolator panel, low pressure & high pressure switches on refrigerant hoses: all have to close circuits to let 12v flow to compressor. Use meter to check things out. If your engine compressor's side evaporator coil fan on street side, turns on with dash cooling switch that helps diagnose circuit. Coach engine does not have to be on to test, except the compressor will only rotate with engine running.

The fans on the evaporator do not come on also.

So start at the rotory switch behind dash? The one that rotates to turn on AC (Max AC) or the one that rotates for temperature control?

Re: Dash AC compressor troubleshooting

Reply #6 – April 11, 2025, 10:22:53 pm

Quote from: Barry & Cindy – April 11, 2025, 09:49:34 pm

Dash cooling switch, relay on isolator panel, low pressure & high pressure switches on refrigerant hoses: all have to close circuits to let 12v flow to compressor. Use meter to check things out. If your engine compressor's side evaporator coil fan on street side, turns on with dash cooling switch that helps diagnose circuit. Coach engine does not have to be on to test, except the compressor will only rotate with engine running.

I am getting 12v to the relay on isolator panel, but not out of the relay to compressor (Wire B32 on my schematic), maybe I'll just try replacing that relay first. Doesn't the AC have to be on for it to show 12v coming out of the relay going to the compressor or do I have my wiring mixed up?

Re: Dash AC compressor troubleshooting

Reply #7 – April 11, 2025, 10:54:32 pm

And just so I know what I'm looking at, is this the low or high pressure switch?

Re: Dash AC compressor troubleshooting

Reply #8 – April 12, 2025, 12:12:47 am

On my 270 the compressor relay failed and caused my compressor to stay engaged while parked with the ignition switch OFF, draining down my start batteries. In trouble shooting my large power drain I found my compressor clutch to be very warm to the touch and that put me on to the relay. $8.00 at ORiely's. Mine is located near the batter isolator. Your's may be in a different location but that relay is where I would go first...

Re: Dash AC compressor troubleshooting

Reply #9 – April 12, 2025, 12:44:51 am

Often, the low-pressure switch is on the input hose to the compressor and high-pressure switch on output hose. These switches are inline (series) and both must be closed for compressor to close the clutch coil. Your photo seems to show a 2^ndpressure switch closer to compressor. Often a single wire goes from one pressure switch to the other pressure switch and then to the compressor clutch coil. All relays often have constant 12v input from a battery connection that only flows from relay to compressor through both pressure switches, AFTER another 12v source from dash triggers relay to close. So take this in mind when metering relays. One relay to compressor and another similar relay to evaporator fan on streetside panel. Both relays probably get the same trigger voltage from dash. Often temperature rotary modulates how much the heater hose valve opens with a non-electric push/pull wire, but could also turn off compressor when too cold, but not usually. Coaches are not all the same. Look behind dash. The knob that turns compressor on is usually an on/off electric switch, but could be dash-temperature sensitive. Metering the AC switch will let you know when & if 12v is flowing to isolator panel relays. Be nice if you had the wiring diagram, and if you get this diagnosed, you will be much further along on the road to understand and maintain your 12volt part of the motorhome. Before you replace relays, why not temporarily put 12v to the trigger wire on the relay, or put 12v on the output of the dash AC switch. You could also measure the relay trigger connection voltage to see if you see voltage when dash switch is closed and ignition on. Thinking backwards, keep in mind that IF dash AC switch is closed, it means that the switch output is connected to switch input, which is connected front panel relays, which are connected to ignition relay output. In this scenario IF the ignition is off, your 12v on relay's trigger connection could flow back to every load that is on the ignition relay. Disconnecting wire to dash AC switch or keeping this switch off, would be the way to test.

Re: Dash AC compressor troubleshooting

Reply #10 – April 12, 2025, 10:50:16 am

I replaced the relay but didn't fix it.

Whats the best way to check the pressure switch, continuity?

Re: Dash AC compressor troubleshooting

Reply #11 – April 12, 2025, 11:34:29 am

Quote from: Barry & Cindy – April 12, 2025, 12:44:51 am

Often, the low-pressure switch is on the input hose to the compressor and high-pressure switch on output hose. These switches are inline (series) and both must be closed for compressor to close the clutch coil. Your photo seems to show a 2^ndpressure switch closer to compressor. Often a single wire goes from one pressure switch to the other pressure switch and then to the compressor clutch coil. All relays often have constant 12v input from a battery connection that only flows from relay to compressor through both pressure switches, AFTER another 12v source from dash triggers relay to close. So take this in mind when metering relays. One relay to compressor and another similar relay to evaporator fan on streetside panel. Both relays probably get the same trigger voltage from dash. Often temperature rotary modulates how much the heater hose valve opens with a non-electric push/pull wire, but could also turn off compressor when too cold, but not usually. Coaches are not all the same. Look behind dash. The knob that turns compressor on is usually an on/off electric switch, but could be dash-temperature sensitive. Metering the AC switch will let you know when & if 12v is flowing to isolator panel relays. Be nice if you had the wiring diagram, and if you get this diagnosed, you will be much further along on the road to understand and maintain your 12volt part of the motorhome. Before you replace relays, why not temporarily put 12v to the trigger wire on the relay, or put 12v on the output of the dash AC switch. You could also measure the relay trigger connection voltage to see if you see voltage when dash switch is closed and ignition on. Thinking backwards, keep in mind that IF dash AC switch is closed, it means that the switch output is connected to switch input, which is connected front panel relays, which are connected to ignition relay output. In this scenario IF the ignition is off, your 12v on relay's trigger connection could flow back to every load that is on the ignition relay. Disconnecting wire to dash AC switch or keeping this switch off, would be the way to test.

Here's the schematics of what i have on my coach...so when you say apply 12v to trigger wire on relay (to compressor), do you mean wire E59? That one is already showing 12v.

Re: Dash AC compressor troubleshooting

Reply #12 – April 12, 2025, 07:54:11 pm

To check if low pressure switch is preventing compressor from running because of system pressure is too low or has failed to close even when pressure is okay, just jump the two wire together on the switch to bypass the pressure switch. Check continuity by removing two wires and measuring switch to see if contacts are open or closed. Low pressure switch is normally open and closed when minimum or higher, system pressure is sensed. High pressure switch is normally closed and open when pressure gets too high. I cannot find wire E59 All coach models are not wired the same for dash air control. Your coach if not been modified by previous owner and is using the printed wiring diagram, controls condenser fan separate from compressor. Relay ID: 30-from-battery, 87-to-compressor-or-fan, 85-ground, 86-from-control-switch. Put 12v on Relay-86 for a moment should make fan run or compressor coil pull-in.

Re: Dash AC compressor troubleshooting

Reply #13 – April 13, 2025, 11:27:54 am

Quote from: Barry & Cindy – April 12, 2025, 07:54:11 pm

To check if low pressure switch is preventing compressor from running because of system pressure is too low or has failed to close even when pressure is okay, just jump the two wire together on the switch to bypass the pressure switch. Check continuity by removing two wires and measuring switch to see if contacts are open or closed. Low pressure switch is normally open and closed when minimum or higher, system pressure is sensed. High pressure switch is normally closed and open when pressure gets too high. I cannot find wire E59 All coach models are not wired the same for dash air control. Your coach if not been modified by previous owner and is using the printed wiring diagram, controls condenser fan separate from compressor. Relay ID: 30-from-battery, 87-to-compressor-or-fan, 85-ground, 86-from-control-switch. Put 12v on Relay-86 for a moment should make fan run or compressor coil pull-in.

Wire E59 was at the very bottom of the picture of the wire diagram and its feeding 12v to the compressor relay. So I'll put 12v to the wire that is leaving the relay (86) and feeding compressor and I should hear compressor coil pull-in correct?

Re: Dash AC compressor troubleshooting

Reply #14 – April 13, 2025, 12:05:24 pm

I have replaced my condenser fan twice since I bought the coach. Have you checked to see if it spins freely and then put 12Vs to it? Pretty easy to check.

First time MOT replaced it and second time I bought it from MOT. I did need to make a couple of brackets. It was not a direct replacement.

Rich

Re: Dash AC compressor troubleshooting

Reply #15 – April 13, 2025, 02:55:59 pm

Quote from: Rich Bowman – April 13, 2025, 12:05:24 pm

I have replaced my condenser fan twice since I bought the coach. Have you checked to see if it spins freely and then put 12Vs to it? Pretty easy to check.

First time MOT replaced it and second time I bought it from MOT. I did need to make a couple of brackets. It was not a direct replacement.

Rich

I'll definitely put that on my list to check, will the compressor not kick on if the condenser fans don't work?

Re: Dash AC compressor troubleshooting

Reply #16 – April 13, 2025, 06:59:48 pm

Wire 59 that goes to the 86 terminal provides 12 volts to the relay that then closes and puts the connection between 30 and 87 which should put 12 volts to the compressor clutch.

Re: Dash AC compressor troubleshooting

Reply #17 – April 13, 2025, 07:02:12 pm

See the picture in this post. These are the two circuit breakers that feed power to the relays. They are on a shared power source, so check that there is incoming power there first (the bar connects the two together on the feed side). If you have power in, verify that you have 12V on the output side of each breaker too.

Air condition breakers

Re: Dash AC compressor troubleshooting

Reply #18 – April 14, 2025, 12:29:47 am

Update:

So I found a disconected blue wire back where all the relays are that are connected to the fan cage. It is the wire I've been looking for...I jumped 12v to it and the compressor and condenser fan kicked on. The problem is I have no idea where it goes. As you all know that's a rats nest back there and I must have knocked it loose when I had it all apart to replace the defroster fans.
One thing my wiring diagrams don't have is a schematic of all those relays back there, specifically the ones screwed to the back of the fan cage...has anyone ever ran across that? I'm pretty sure that wire connects to one of the relays back there, hoping its one I can actually reach, some of them are buried where I can't reach them.

Re: Dash AC compressor troubleshooting

Reply #19 – April 14, 2025, 06:41:24 am

When you say "fan cage", what are you referring to? Got a pic?

Re: Dash AC compressor troubleshooting

Reply #20 – April 14, 2025, 09:58:02 am

Check both relays and see if 4 wires are connected if one has 3 wires that's where you need to connect
the blue wire to.

Re: Dash AC compressor troubleshooting

Reply #21 – April 14, 2025, 11:47:25 am

Quote from: WS6_Keith – April 14, 2025, 06:41:24 am

When you say "fan cage", what are you referring to? Got a pic?

The one behind the cabinet in front by the driver, the fan cage that holds the defroster fans. It has relays and vacuum switches for the vents.

Quote from: WS6_Keith – April 14, 2025, 06:41:24 am

When you say "fan cage", what are you referring to? Got a pic?

Hopefully it's not one of the ones all the way on the bottom, really can't get to those.
This picture is when I had it pulled out, but not so easy to access now that's it's back together.

Re: Dash AC compressor troubleshooting

Reply #22 – April 14, 2025, 11:51:28 am

Quote from: oldguy – April 14, 2025, 09:58:02 am

Check both relays and see if 4 wires are connected if one has 3 wires that's where you need to connect
the blue wire to.

There's way more than just 2 relays in there

...and a few of them I can barely see much less get my hands in there...

Re: Dash AC compressor troubleshooting

Reply #23 – April 14, 2025, 11:53:12 am

That looks like the SGM module:

The selected media item is not currently available.

Re: Dash AC compressor troubleshooting

Reply #24 – April 14, 2025, 12:01:58 pm

I may have answered my own question with the photo I posted above.

The blue wire i highlighted in red is the wire in question, and by the length of it it looks like i may have found the relay.
Those upper ones aren't too hard to reach so hopefully I can get it back in. What makes it hard is that it looks like it's the back spade of the relay not the easy one in front. 🤦‍♂️

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