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Old 06-10-2022, 07:14 PM   #15
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With the engine running (or at least ignition on if that normally starts the compressor) you should get 12v within that fuse holder and neg probe to chassis ground. That's pos probe touching unpstream contact within fuse holder, neg probe to ground. You could use a test light for this. You're testing whether voltage actually gets to compressor assembly. There may be a fuse farther upstream.

A good rule is to replace a fuse even if it looks and tests ok. It may open under load.

If you sit in your toad with the engine off and press the brake pedal do the brake lights come on?
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Old 06-12-2022, 12:02 PM   #16
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Looks like there is power to the compressor. With the engine running, I'm getting about 14.5 volts at the fuse. With the engine off I'm getting about 13.2v.

Brake lights on toad come on with engine off.

Does this suggest a failed compressor or is there something else that would keep it from running?
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Old 06-12-2022, 02:21 PM   #17
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Be sure to check the ground. Otherwise I suspect failed compressor or pressure switch.

On your toad, a wire runs from the cold side of the stoplight switch up to the umbilical, to your MH, then up front to the indicator light. Also there's a ground from your toad to the MH. Places to check are the connector at the front of the toad, the connector to the mh and where the wire connects to the light. Have someone stand on the toad brake until you find the problem.
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Old 06-14-2022, 10:46 AM   #18
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Prior to our current problem, the compressor would start almost every time we started the motorhome. It would start whether parked at our home or on the road before the toad was connected. So I don't see how or why wiring or other components of the toad would have a bearing on the compressor starting up.

Not sure where the pressure switch is located or how to check if that is possibly the problem. From reading the manual it's part of the proportional valve. As I recall, from a previous problem, it's located closer to the front. Not sure if I can access it while the slide is out but will take a look. These 79 year old bones do not like getting under the motorhome.
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Old 06-14-2022, 01:26 PM   #19
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Found the proportional valve. The ground wire is easily contacted with the volt meter probe. But the power wire goes directly into the valve. Other than cutting the wire which I don't want to do, I'm not sure how to test the valve. Will a test light work? If so what do I need to touch?
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Old 06-14-2022, 01:41 PM   #20
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If you open the link I provided to the compressor listed on Etrailer there are some pictures of the unit.
On the 3rd picture it shows the back side of the air tank and compressor. There are 2 blue wires going to the pressure switch. Basically what this does is control the start/stop of the compressor as the air pressure in the tank goes down/up. One what to test the compressor is to just jumper these wires together, this will close the connection and the compressor should run.

Or you can use a volt meter, and check for power on the terminal block first, trace the wires to the pressure switch and then check the other wire coming off the pressure switch.

There is also a relay mounted next to the terminal block. This is probably what provides power to the unit when you turn the key or start your RV. There should be power going to it at all times and then when you turn the key or start the relay closes and power will go to the terminal block.
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Old 06-14-2022, 03:08 PM   #21
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The proportioning valve is what controls the release of pressurized air; When it's closed pressure should rise in the system until the pressure switch turns it off. If the proportioning valve was bad the compressor would run all the time and air would come out all the time. If it was acting normally the compressor would run until pressure was reached, then shut off. If it was failing closed the compressor would run but no air would ever come out.

The only thing that controls the pressure switch is pressure. When pressure reaches a set point (around 125psi) the pressure switch turns the compressor off. A bad pressure switch could make the compressor run all the time or not turn on at all.

The proportioning valve is simply releasing air metered by brake fluid pressure. With higher fluid pressure it meters more air. My old system had a mechanical plunger valve pressed by a treadle under the brake pedal. Worked fine until it didn't and couldn't be replaced, so I had to go with the modern proportioning valve.

The toad light wiring we're discussing has nothing to do with the compressor. Two completely separate issues.
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Old 06-15-2022, 03:18 PM   #22
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Quote:
Originally Posted by jacwjames View Post
If you open the link I provided to the compressor listed on Etrailer there are some pictures of the unit.
On the 3rd picture it shows the back side of the air tank and compressor. There are 2 blue wires going to the pressure switch. Basically what this does is control the start/stop of the compressor as the air pressure in the tank goes down/up. One what to test the compressor is to just jumper these wires together, this will close the connection and the compressor should run.
Please explain how to jumper the wires together.

Quote:
Or you can use a volt meter, and check for power on the terminal block first, trace the wires to the pressure switch and then check the other wire coming off the pressure switch.
Is this what is show in picture #7?


Quote:
There is also a relay mounted next to the terminal block. This is probably what provides power to the unit when you turn the key or start your RV. There should be power going to it at all times and then when you turn the key or start the relay closes and power will go to the terminal block.
Will there be a click like a solenoid?
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Old 06-15-2022, 03:36 PM   #23
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Quote:
Originally Posted by johnhicks View Post
The proportioning valve is what controls the release of pressurized air; When it's closed pressure should rise in the system until the pressure switch turns it off. If the proportioning valve was bad the compressor would run all the time and air would come out all the time. If it was acting normally the compressor would run until pressure was reached, then shut off. If it was failing closed the compressor would run but no air would ever come out..
I'm likely confused at this point but is there a bad solenoid with the proportional valve which would prevent it from working?

Quote:
The only thing that controls the pressure switch is pressure. When pressure reaches a set point (around 125psi) the pressure switch turns the compressor off. A bad pressure switch could make the compressor run all the time or not turn on at all..
Is it possible the compressor is still pressurized?


Quote:
The toad light wiring we're discussing has nothing to do with the compressor. Two completely separate issues.
If they are separate, any thought on why the dash LED does not illuminate when I press on the brakes. All the toad lights work when the umbilical is connected.

I have pulled both ends off the umbilical and checked to make certain they are making good contact. I also have continuity on all six wires. The one thing I wonder about is whether or not the socket is dirty. We were doing a lot of BLM desert type camping with a ton of dust. I've got some sprays for electric but not sure it's OK to spray the sockets.
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Old 06-15-2022, 05:01 PM   #24
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A jumper can be as simple as taking a piece of wire and touching each ends across the pressure switch. If there is power going into the pressure switch on one side and you jump across the switch the compressor should run if the pressure switch was the problem.



I used to carry a small compressor to top off my tires but it had a pressure switch that cut it out to fast and it didn't build a high enough pressure, by jumping the switch the compressor would run while I filled the tires. I eventually bought a small pancake compressor that would build to 150psi.



The relay may click but it might be hard to hear. Here is a "typical" relay wiring schematic.



Do you own a multimeter? Do you know how to use one?

Working on any type of of type power can be dangerous so if you aren't familiar with electrical and/or testing it might be time to call in some help.
Attached Files
File Type: pdf relay diagram.pdf (68.5 KB, 2 views)
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Old 06-15-2022, 06:00 PM   #25
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You can carefully open the compressor drain valve to bleed off pressure, if any. You could just leave that valve open to confirm that the compressor isn't running at all with ignition on and zero pressure. I believe you said you had voltage at the compressor.
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Old 06-17-2022, 02:01 PM   #26
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A jumper can be as simple as taking a piece of wire and touching each ends across the pressure switch. If there is power going into the pressure switch on one side and you jump across the switch the compressor should run if the pressure switch was the problem.
If leads are imbedded, do I need contact with bare wire to do this?

Quote:
Do you own a multimeter? Do you know how to use one?
Limited use to measure volts and check continuity.

Quote:
Working on any type of of type power can be dangerous so if you aren't familiar with electrical and/or testing it might be time to call in some help.
Agree. If I could find someone to look at it I'd have them fix. What are the risks of jumpering two wires and how can it be done safely.
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Old 06-17-2022, 02:09 PM   #27
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You can carefully open the compressor drain valve to bleed off pressure, if any. You could just leave that valve open to confirm that the compressor isn't running at all with ignition on and zero pressure. I believe you said you had voltage at the compressor.
On my compressor at the house, there's a valve that I can loosen that will release air. Is there a similar valve on the BrakeMaster air compressor? It looks like there is one on the opposite end of the drain valve.

I do have voltage if I did it right. One probe to lead and one to ground.
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Old 06-17-2022, 02:10 PM   #28
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Use you volt meter and check to see if you have 12 volt coming to the compressor at the terminal block. With the key on and engine running there should be power there.



If not your problem is down stream from there toward the chassis. There should be a fuse somewhere in the system to protect the compressor and wiring, make sure it is good. Check the wiring going into the relay, there should be voltage going in and voltage coming out with the key and engine on. If not the relay may be bad.



If you have power to the terminal block then move your testing up stream. Follow the wire from the terminal block to the pressure switch. Use the volt meter to check for voltage across the pressure switch with the valve on the air tank empty so there is no pressure which should turn the compressor on. If you don't have power across the pressure switch that may be your problem.


If the pressure switch is sending power to the compressor and it isn't running it may be the motor.
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